Monday, January 31, 2011

Quick Comparison of This Event with the January 1999 Blizzard

In reading a lot of the forecast discussions from WFOs across the upper midwest, I noted several references comparing this coming winter weather event to the blizzard of January 2-4th, 1999.  Though I apparently lived right through the middle of it, I honestly didn't remember this particular event.  So, I thought I'd see what all this hubub was about and quickly, qualitatively compare the two events.

The initial synoptic setup is remarkably similar.  Take this morning's 12Z 500mb analysis from the GFS:
Fig 1 -- GFS 500mb geopotential height and wind analysis at 12Z, Monday, January 31, 2011.
Note the two features I noted in my last post that will combine to produce this powerful storm--a shortwave digging out of southern Alberta into nothern Montana and another in the desert Southwest over Arizona.  Compare this to an archived 500mb analysis from 12Z on Friday, January 1st, 1999.
Fig 2 -- NOAA Difax 500mb N. America geopotential height analysis from 12Z, Jan. 1, 1999. 
There are indeed some eerie similarities between the two analyses.  There were also two shortwaves analyzed before the 1999 storm, though these were slightly further east--one was coming out of southern Saskatchewan while the axis of the other was in New Mexico.  Though these features are slightly further east, the forecast for the current event will quickly catch this week's shortwaves up and even speed past the timing of the 1999 event.  But more on that later...

Back to this current event, twenty-four hours later (12Z on Tuesday morning), the GFS forecast has a well-defined, if somewhat east-west expansive, trough across the central part of the country with embedded short waves.
Fig 3 -- GFS 24-hour forecast of 500mb geopotential height and winds valid 12Z, Tuesday, February 1, 2011.
Comparing this to the 1999 event 24 hours later, we see that the analyzed 500mb trough didn't seem to have these embedded shortwaves and was more north-south oriented.  This is an important difference--the lack of tilt in the 1999 event slows down the progression of the trough and associated surface features.
Fig 4 -- NOAA Difax 500mb N. American analysis of geopotential height valid 12Z, Saturday, Jan. 2, 2011.
Even with these differences, however, both analyses feature at least some form of trough axis moving through central or eastern Texas with an attendant jet streak around the base of the trough.  Both of the exit regions of the jets would lie in the mid-Mississippi valley, enhancing the divergence aloft there.  But the timing and orientation of these two features is still different.  Here's the surface pressure forecast for the GFS at 12 hours later--00Z on this Tuesday night.
Fig 5 -- GFS 36-hour forecast of 500mb geopotential height and winds valid 00Z, Wednesday, February 2, 2011
Compare that to the surface analysis from the 1999 event at 24 hours later--12Z, Sunday, Jan. 3 1999.
Fig 6 -- NOAA Difax surface analysis valid 12Z, Sunday, Jan. 3, 1999.
BOTH analyses have a deep surface low located in the Memphis/southern Illinois region.  However, the GFS (and most of our other) model forecasts bring that low there a full 12 hours faster than the January 1999 event.  By the time we move the full 48 hours out in the GFS forecast for this current event, the low pressure center at the surface has already moved into Indiana and begun to occlude.
Fig 7 -- GFS 48 hour forecast of surface pressure and temperature valid 12Z, Wednesday, Feb. 2, 2011.
We don't see the occlusion on the 1999 storm until sometime before 72 hours out:
Fig 8 -- NOAA Difax analysis of surface pressure (and fronts) for 12Z, Jan. 4, 1999.
So by the time this low occludes, our current forecasts for the storm this week move things along 12-24 hours faster than the 1999 storm.

There are cold air intrusions in both events, however the observed 20-25 degree Fahrenheit morning lows in Oklahoma by the time of occlusion in the 1999 event are outstripped by the single-digit lows being forecast by the GFS for this week (NOT that we trust the GFS surface temperature forecasts...).  This hints at a stronger baroclinic zone and much colder air behind the front.  This becomes important because if we look at snowfall totals from the 1999 event--
Fig 9 -- NOAA Difax analysis of observed snowfall on the ground at 12Z, Jan.3, 1999.
--we see that while the upper midwest saw large amounts of snow (17 inches at O'Hare in Chicago on that list to the right...), the snow amounts taper off to the southwest.  Only a few inches were reported at places in Missouri and none in Oklahoma.  Contrast this with the blizzard warnings extending all the way into central Oklahoma with the event tomorrow--12-18 inches of snow are expected in parts of Oklahoma.  From this, we have to conclude that there is more moisture being forecast with the current event and/or the temperatures were colder further south, supporting more snow all the way into Oklahoma.

Interestingly, according to records at Wiley Post Airport in Oklahoma City, on January 2-3, 1999, snow and rain were both reported in the observations, but no measurable accumulation of either seems to be recorded.  The high temperature also hovered in the low to mid 30's with overnight lows in the low teens after the event. Not near the cold weather we're looking at according to current forecasts...

So, in conclusion, there are some general similarities in the evolution of both the 1999 blizzard and this week's event.  However, this week's storm will move much more quickly, have much colder air behind it, and bring wintry precipitation further south.  Up north, however, the proxy seems to be pretty good--if a storm of that intensity (and perhaps even more limited moisture) in 1999 could bring 17 inches of snow to Chicago, projections in that range would not seem to be out of the question for the upper midwest with this week's storm.

Finally (on a slightly technical note), I just want to leave with a cross-section image generated on the College of DuPage website from this evening's soundings.  This cross section runs from Albuquerque in the west to Nashville in the east.  We can see that "warm conveyor belt" starting to ramp up with that bulls-eye of moisture (the green contours) in an area of southerly winds to the east.
Fig 10 -- Cross section from Albuquerque, NM (left) to Nashville, TN (right) showing contours of potential temperature (red), mixing ratio (green), theta-e (yellow) and wind barbs valid at 00Z, Tuesday, Feb. 1, 2011.  From the College of DuPage website.
And so it begins...

Also, for all of you reading this at the University of Washington--I will be giving tomorrow's weather briefing in 310C at 12:30 PM and will be going into a fair amount of detail about this event during the presentation.  Feel free to come by...

Sunday, January 30, 2011

Two short waves, one big storm, Chicago blizzard?

It's been a week or so since I last posted on here.  The annual meeting of the American Meteorological Society was here in Seattle last week and needless to say I was very busy during that time.  But now that's done and my schedule can resume something more normal...

Today's National Weather Service summary map looks pretty spectacular:
Fig 1 -- NWS watches and warnings as of 2300Z, January 30, 2011.
The darker purple from Montana down through the high plains and into southern Wisconsin is all winter weather advisories.  The pinks through much of Missouri and eastern Kansas, Nebraska and Oklahoma are winter storm watches.  And what about that bright green blob surrounding Chicago?  That's a blizzard watch for the Chicago CWA.  Milwaukee is also issuing blizzard watches at this time.  I quote the following from Milwaukee's latest Winter Weather advisory:


Pretty powerful words.  So what's the setup for this?

Right now, there is a highly amplified ridge over the west coast with broad scale troughing aloft across the eastern half of the country:
Fig 2 -- GFS 12Z analysis of 500 mb geopotential heights and vorticity on Jan 30, 2011.  From the HOOT website.
Note two individual shortwaves are evident in the west embedded in the larger-scale flow.  One is the nearly cutoff shortwave in southern Oregon and northern California.  The other is further north on the British Columbia/Alberta border in Canada.  This northern shortwave is located in the region we usually see "Alberta Clipper" type storms form.  It's somewhat unusual to have two shortwaves stacked on top of each other like that. Latest model forecasts show these two shortwaves merging together over the plains by late Tuesday:
Fgi 3 -- GFS 60 hour forecast of 500 mb geopotential height and vorticity valid 00Z, Wednesday, Feb. 2, 2011.  From the HOOT website.
Cold air in association with the northern shortwave is forecast to spill down into the plains as that shortwave moves south.  We can see a blob of colder temperatures moving down across the northern plains at 850mb on Monday:
Fig 4 -- GFS 30 hour forecast of 850mb temperatures and geopotential heights valid 18Z, Monday, Jan. 31, 2011.  From the HOOT website.  

Of course, there's already very warm air to the south in Texas and along the Gulf Coast.  As the cold air moves south, this is going to increase the temperature gradient across the central part of the country.  You can already see how quickly the 850mb temperature changes from Nebraska into Oklahoma on the image above.  This increasing temperature gradient means two things:

  1. Frontogenesis is going on at the low-levels--a cold front is most likely developing from Iowa down through Oklahoma at the time above.
  2. Based on those thermal wind arguments (yes, those again...), the increasing temperature gradients below translate to strengthening winds aloft parallel to the forming front.
Do we see winds increasing aloft in this area?  We do indeed see a jet streak strengthening in that area--and all the way back down into west Texas.
Fig 5 -- GFS 36 hour forecast of 300mb winds and geopotential height at 00Z, Tuesday, Feb 1, 2011.  From the HOOT website.
Quite the dynamic pattern here.  Not only can we see those two shortwaves starting to merge together, but look at how amplified that ridge along the west coast has become.  It even has a north-south oriented jet streak!  That's pretty rare.  We also have two jet streaks that are roughly east-west oriented--one somewhat associated with the exit region of the northern trough and the other associated with the exit region of the southern trough.  If we look at these in terms of our four-quadrant jet streak model:
Fig 6 -- Same as figure 5 but annotated with the four-quadrant model showing regions of convergence and divergence associated with jet streaks.

We see that divergence regions of both of these jet streaks almost "coincide" over the same region.  This is a form of a phenomenon known as "jet streak coupling", where the combined effect of multiple jet streaks can really enhance the divergence (or convergence) aloft.  Of course, with this huge amount of divergence going on aloft, the pressure at the surface is going to fall rapidly:
Fig 6 -- GFS 60 hour forecast of surface (2m) temperature, MSLP and wind barbs valid 00Z, Wednesday, Feb 2, 2011.  From the HOOT website.
There's a strong surface low forecast by the GFS to move into southern Illinois and continue trekking northeastward into Michigan.  Look at the tight pressure gradient forming across the upper midwest and into the southern Plains.  This translates to very strong northeasterly winds across the upper midwest on Wednesday.  So if this model forecast were to verify, blizzard-level winds would indeed be possible on Wednesday across northern Illinois and southern Wisconsin.  The reasoning behind this blizzard watch begins to make sense.

But what about snowfall amounts?  How cold will it get?  We'll take a look at the models again tomorrow sometime and see how things are continuing to shape up.  For now, just be on the lookout for a strong winter storm during the middle of this week.

Friday, January 21, 2011

Update on Seattle Weather This Weekend with Ensembles

Quick post to just update the outlook for the weather here in Seattle this weekend.

Interestingly, the 500mb analyses this morning really don't pick up much on the shortwave we were forecasting (on Tuesday) to move through last night.  Neither the RUC 500mb analysis:
Fig 1 -- RUC 500mb analysis of geopotential height (contoured) and dewpoint depression (shaded) at 12Z, Jan 21, 2011.  From the HOOT website.
Nor the initialization of the 00Z UW WRF ensemble last night:
Fig 2 -- UW WRF Ensemble 500mb geopotential height mean (contours) and spread (shading) at 00Z, Jan. 21, 2011.
Seem to have much of a shortwave signal at 500mb near the Pacific northwest.  However, this morning at 850mb we do see a weak trough moving through the area:
Fig 3 -- UW WRF ensemble 12 hour forecast of 850mb geopotential height (mean) and spread (shaded) for 12Z, Jan 21, 2011.
All this is saying is that this shortwave trough came onshore shallower and weaker than we had thought on Tuesday.  However, it still provided enough lift to kick off some rain.  This morning it was raining in Seattle and the UW ensemble model confirmed this:
Fig 4 -- UW Ensemble WRF 18 hour forecast of probability of measureable precipitation in the previous 3 hours, valid 18Z, Jan. 12, 2011.
However, the rain has now ended in Seattle and looks to be gone for a while.  The same ensemble model run shows slight precipitation chances mostly confined to the mountains tomorrow, though a few showers could pop up in the lowlands:
Fig 5 -- UW Ensemble WRF 45 hour forecast of probability of measureable precipitation in the previous three hours at 21Z, Sat., Jan 22, 2011.
That blob of precipitation probabilities in the central Puget Sound region has pretty low probabilites--at most, 50%.  The location suspiciously looks like a Puget Sound convergence zone event (something else I saw potentially hinted at back on Tuesday).  If this is so, the fact that convergence zone precipitation is usually in a very narrow band whose location can drift might explain that broad brush of low probabilities in that area.  Hopefully we'll stick with the odds and stay dry.  It will probably be somewhat cloudy, though.

On Sunday things no longer look as clear as they used to:
Fig 6 -- UW Ensemble WRF 69 hour forecast of probability of measureable precipitation in the previous three hours at 21Z, Sun., Jan 23, 2011.
On Tuesday we saw the highest precipitation amounts being forecasted on Vancouver Island and this probability forecast confirms that.  However, some of the ensemble members are bringing showers down into the Seattle area.  In the graphic above, it's only a 30% chance of measureable precipitation for the Seattle area, though that increases the further north you go.  I note this really says nothing about the intensity of the precipitation, just the probability that rain will occur.  Often in Seattle we just get very light mists or showers which really aren't enough to slow you down...

Anyhow, I have to get back to work and preparations.  But as people start making their way out here, I just though I'd let people know a bit of what to expect...

Tuesday, January 18, 2011

A (Promising!) Long-Range Look at Seattle's Weather This Weekend (for all you AMS visitors heading in...)

As many of the meteorologist readers of my blog probably well know, the American Meteorological Soceiety's annual meeting for this year is being held next week in Seattle.  Many people I know will be flying into town this weekend to attend the conference.  Some (including me) also have plans to do some sightseeing and enjoy all that Seattle has to offer this weekend before the conference starts.  However, as any Seattleite will tell you, the best time to enjoy Seattle is when it's sunny and warm out.  Which, in the middle of winter, is a hard type of weather to find.  So what are we expecting for this weekend?

There's actually rather good agreement in terms of the overall synoptic pattern in our global models, which gives me greater confidence in the forecast overall. Let's start by looking at the setup on Thursday.  Seattle is currently (and will be for the next few days) under a trough of sorts that will linger through Wednesday and Thursday:
Fig 1 -- GFS 18 hour forecast of 500mb heights (contoured) and winds (shaded) for 06Z, Wednesday, Jan. 19, 2011. From the HOOT website.
This trough is contributing to several rounds of showers moving through the area, which will continue to exacerbate the areas already hit by high water and flooding this weekend.  The amplified ridge-trough pattern seen in the above image begins to flatten out by Friday.
Fig 2 -- GFS 72 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Friday, Jan. 21, 2011. From the HOOT website.
Notice that even in the more zonal (east-west) flow off the Pacific northwest coast, there is still a prominent shortwave indicated approaching the coast (annotated on the figure above with a red dotted line).  The ECMWF also picks up on this feature:
Fig 3 -- ECMWF 72 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Friday, Jan. 21, 2011. From the HOOT website.
The ECMWF solution brings the trough a little further south and slightly deeper.  Regardless of the exact depth and orientation of the trough, this translates to some cloudiness and rain on Friday.  Our local UW extended 12km WRF model picks up on these conditions Friday nicely.  First, the outgoing longwave radiation forecast (think a forecast infrared satellite image) shows the increased cloud cover:
Fig 4 -- UW 12km extended WRF 78 hour forecast of OLR for 18Z, Friday, January 21, 2011. From the UW Mesoscale Modeling Group.
And the 24-hour precipitation totals from Saturday morning show quite a bit of precipitation the day before (don't take these numbers too seriously--they're usually way off this far out).
Fig 5 -- 96 hour forecast of the previous 24 hours of precipitation accumulation from the UW extended 12km WRF at 12Z, Saturday, Jan 22, 2011. From the UW Mesoscale Modeling Group.
The highest precipitation amounts are on the coast and along the Cascades (that swath of greens and pinks going up the middle to right side of the image).  Note that there is a relative minimum of precipitation on the eastern side of the Olympic peninsula just left of center on the image.  This is because the prevailing low-level winds during this event will be out of the west and, as such, the eastern sides of the Olympic peninsula is getting rain shadowed by the Olympic Mountains.  If the deeper trough with stronger winds indicated by the ECMWF is closer to what will happen, then that rain shadowing may be enhanced and decrease the precipitation totals all the way out over Seattle on the eastern side of the sound.  It's a possibility...and it happens here every so often.

By Saturday, the trough will be moving out and a ridge looks to begin building in:
Fig 6 -- GFS 96 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Saturday, Jan. 22, 2011. From the HOOT website.
While this ridge will shift winds to the northwest in the low levels, the wind speed rapidly decreases.  The upwind pattern also does not extend too far north, so we're not expecting strong cold air advection behind the shortwave on Friday.  Clouds look to still be lingering on Saturday evening, but beginning to break up...
Fig 7 -- UW 12km extended WRF 108 hour OLR forecast for 00Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.
...and precipitation on Saturday will definitely be less than it was on Friday.
Fig 8 -- UW extended 12km WRF 120 hour forecast of the previous 24 hours of precipitation accumulation for 12Z, Sunday, Jan 11, 2011. From the UW Mesoscale Modeling Group.
Much of the rain will have stopped by Saturday morning and, in fact, the Seattle area shows no precipitation on Saturday in the above image.  That swath of greens to the north of Seattle could represent a Puget Sound convergence zone event (more on that in another post...) which is a fun local weather phenomenon that people may be interested in...

Anyhow, by Sunday that ridge has really built in.
Fig 9 -- GFS 120 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Sunday, Jan. 22, 2011. From the HOOT website.
Under such strong ridging aloft, subsidence would set in across all of western Washington and southern British Columbia.  As a result, few deep clouds, if any clouds at all:
Fig 10 -- UW extended 12km WRF 129 hour forecast of OLR at 21Z, Sunday, Jan. 22, 2011. From the UW Mesoscale Modeling Group.
And no real precipitation in the area, except out over Vancouver island and maybe a brief shower or two in Vancouver:
Fig 10 -- UW extended 12km WRF 144 hour forecast of the previous 24 hours of precipitation accumulation for 12Z, Monday, Jan 24, 2011. From the UW Mesoscale Modeling Group.
A look at a forecast surface map for Sunday shows light winds and temperatures in the upper 40s to low 50s (degrees Fahrenheit):
Fig 11 -- UW extended 12km WRF 129 hour forecast for 925mb temperatures (shaded, in degrees Celsius), surface sea-level pressure (contoured) and winds (barbs) for 21Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.
 So, I'd definitely bring a good jacket, but it looks like if anyone has plans to explore Seattle on Sunday the current forecasts are calling for it to be a pleasant day.  Probably the best you could hope for in the middle of winter, actually.  I'm suspicious with such light winds and under ridging aloft that fog might roll in in the mornings of Saturday and Sunday depending on the humidity levels.  A forecast for relative humidity Sunday morning does show greater than 75% relative humidity in much of the Puget Sound lowlands:
Fig 12 -- UW extended 12km WRF 120 hour forecast for Surface relative humidity (shaded) at 12Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.
But even if there is some fog, it will hopefully clear out by the afternoon.

Remember these are still long range forecasts--the potential for them to be inaccurate is rather high.  We're still out of range of the ensembles to get good probability estimates too.  The general pattern of the global models seem to agree, though, so my confidence in this forecast is higher than usual for this far out. So, for all those of you headed into Seattle this weekend, the current forecast looks very promising for a good beginning to the AMS conference!

Saturday, January 15, 2011

Something a little different--a look at Australian weather

I thought I'd look at an area of the world that has been receiving a lot of attention as of late, though for a slightly different reason.  Much has been said about the devastating floods in Queensland and the Brisbane area of Australia over the past few weeks.  It has brought worldwide attention to the meteorology of Australia and the ongoing rains that have contributed to the flooding there.

One thing we have to remember about Australia is that it is in the southern hemisphere and consequently the "shapes" of its weather patterns are different from what we in the northern hemisphere are used to.  For instance, while we're in the middle of winter, it's the middle of summer there.  Consequently, temperatures are rather warm:
Fig 1 -- Surface temperature analysis at 00Z, Jan. 16, 2011.  From the Australian Bureau of Meteorology.
Most of the country is shaded in orange colors, representing anywhere from 20-30 degrees Celsius.  That's from 68-86 degrees Fahrenheit.  Much more summer-like than winter-like. Of course, Australia is also positioned slightly closer to the equator than the continental United States.  Take a look at the latitude lines on the map above.  The northern tip of Australia almost gets up to 10 degrees south latitude.  That's only 10 degrees away from the equator.  By comparison, Key West, Florida, is around 24 degrees north.  So northern Australia is 14 degrees closer to the equator than the southernmost part of the continental United States.  Down on the island of Tasmania off the southeastern coast of Australia, they get up to almost 45 degrees south. Mainland Australia only gets down to almost 40 degrees south.  In comparison, much of the US-Canada border is at 49 degrees north--so slightly further north than Australia is south.  Even with these slight differences in position, both Australia and the US span similar bands of latitude and consequently see somewhat similar types of weather systems.  They just look a little different to northern hemispheric eyes.  For instance, here's the latest surface analysis:
Fug 2 -- Surface MSLP and frontal analysis at 00Z, Jan 16, 2011.  From the Bureau of Meteorology.
Let's examine a few features on this map.  One is the low pressure center directly south of Australia.  That's a respectable 984 mb low sitting there, but notice the cold front associated with that low.  It seems oriented somewhat oddly compared to northern hemisphere cold fronts.  We're used to seeing the cold front drag to the south or southwest of a low pressure center because of counter-clockwise circulation around the low.  However, two things are different in the southern hemisphere:

  1. The cold air is located to the south instead of the north.  So any cold air being dragged in with low pressure centers will come from the south.
  2. The Coriolis effect, which turns winds to the right in the northern hemisphere and explains why we have counter-clockwise flow around low pressure centers, turns winds to the left in the southern hemisphere.  This means that winds flow clockwise around a low pressure center in the southern hemisphere.
This provides a sort of mirror-image effect for typical cold-core cyclone structure in the southern hemisphere. Remember our basic structure of a low pressure center and fronts in the northern hemisphere:
Fig 3 -- Simple diagram of the structure of a northern hemisphere extratropical cyclone.  From Paragliding Weather website.
The general flow of the winds is counter-clockwise around the low.  Warm air from the south (toward the equator) is advected northward and we see its leading edge as a warm front.  Cold air from the north is advected in, swinging around from the west on the back side of the low.  We see the leading edge of this air as a cold front.  What about in the southern hemisphere?
Fig 4 -- Simple diagram of the structure of a southern hemisphere extratropical cyclone at the surface.  From the South African Climate Systems Analysis group.
Here we see the winds flowing the opposite direction around the low--clockwise.  Warm air from the north (since the equator is now toward the north) is this time advected southward along the eastern side of the low. Cold air from the south is advected northward on the western side of the low.  The result is usually frontal boundaries that generally lie to the north of the low (as opposed to fronts that lie to the south of the low in the northern hemisphere).  So this is why we see all those cold fronts extending mostly to the north of the lows in the surface analysis above.

Note how I kept saying "clockwise" and "counter-clockwise" in terms of the winds instead of "cyclonic" or "anticyclonic".  The term "cyclonic" is defined as the way the air moves around a cyclone (a low-pressure center).  So in both the northern and southern hemisphere, the winds around the low pressure centers are technically "cyclonic".  They may be going opposite directions from a clockwise/counter-clockwise perspective, but the term "cyclonic" definitely applies to both.

Anyhow, now that we're oriented a bit, it's kind of fun to see how this spin affects different things.  You may have noticed in the surface analysis above that there's a feature called Severe TC "Zelia" to the northeast of Australia.  This is indeed a tropical cyclone-- a category three, as a mater of fact.  They are referred to as tropical cyclones in the southern hemisphere--"hurricane" is only used in the Atlantic basin.  So "Severe TC Zelia" would translate to "Major Hurricane Zelia" if it were up here.  Here's the forecast track from the Australian Bureau of Meteorology (the BOM):
Fig 5 -- Track and forecast track for severe TC Zelia as of 00Z, Jan 16, 2011.  From the BOM.
Note that the storm is forecast to stay well away from the mainland and weaken to a depression before it reaches New Zealand.  This is good news--as a tropical storm hitting Queensland with all their flooding is the last thing they need right now.

One thing that's NOT different about mid-latitude weather systems in both hemispheres is their direction of movement--the prevailing jet streams (and consequently storm motion) are still from west to east.  So just as hurricanes in the Atlantic move away from the equator and then get pushed around until they're moving to the east, so too do tropical cyclones in the southern hemisphere move away from the equator (south this time) and get pushed to the east the further they get from the equator.  Pretty fun.

This cyclone has plenty of energy to work with, too--here's the latest sea surface temperatures off of Australia:
Fig 6 -- Sea Surface temperatures off of Queensland and the Coral Sea from Jan 15, 2011.  From the BOM
All of that red is 29 degrees Celsius--some 84 degrees Fahrenheit.  Very, very warm water.  No wonder this storm strengthened to category three strength.  But you can see that it's going to be moving over much cooler waters the further south it goes.  This helps explain why the storm is forecast to weaken as it moves along.

And finally, here's the latest IR satellite image over Australia:
Fig 7 -- IR satellite image from 532Z.  From the BOM.
Some of these features we've been talking about show up rather nicely on this image.  TC Zelia is on the upper right side of the image--it's the cyclone with bright white cloud tops and a very dense core.  Note the orientation of the swirls of all those low pressure centers--if you spiral from the outside in it is a clockwise spiral!  That low pressure center to the south of Australia also shows up rather clearly.  Notice that where the cold front was indicated on the northeast side of the low that we see most of the high cloud tops (the darker gray shades compared to the surroundings), indicating some deeper convection there associated with the front.

Remember what I said, though, about Australia being closer to the equator than the US?  This explains why the main storm tracks (associated with the south polar jet) are well to the south of Australia--there's that parade of lows spinning around in the ocean south of Australia, rather far away from land.  While the northern US would be hit by storms at those latitudes during our summers, Australia is just enough closer to the equator to miss out on most of those storm tracks.  However, Australia extends close enough to the equator to get caught up in the tropical rain belts.  Note the string of "cloud blobs" across the northern edge of Australia in the IR image above?  Those are all  tropical waves--the kinds of storms we'd usually see in the Caribbean.  Australia is close enough to the equator that those impact the northern part of the country.  In fact, on the surface analysis in figure 2, you can see the long dotted line running from east to west across the northern part of the continent.  This persistent trough of low pressure brings lots of tropical rain to that belt during the summer.

So that was a quick look at something different--weather in the land down under.  It's summer there, and I thought we could use a little break from our winter.  Back to the US for my next post, though...

Tuesday, January 11, 2011

Columbia Gorge Flows and Freezing Rain in Portland

There's a very active weather pattern at the moment, at least on the edges of the country:
Fig 1 -- Surface temperature (shaded), mean sea-level pressure (contoured) and winds at 21Z, Jan. 11, 2011.  From the HOOT website.
Not only is there a nor'easter building with the merging of those two low pressure centers in the east, but another very deep low pressure center is moving into the Pacific northwest.  For all of you high-pressure chasers out there, there's also that huge ridge of high presure stretching all the way from the Canadian prairies down into Texas (with lots of cold air underneath it).  Today I want to focus on some a certain peculiarity of this storm moving onto the Pacific northwest coast.  So, let's switch our perspective and take a look at a forecasted 850mb map for what's going on right now:
Fig 2 -- 12 hour forecast of 850mb heights (contoured), temperature (colors) and winds (barbs) for 00Z, Jan. 12, 2011 (4PM PST Tuesday).  From the UW 36-km WRF.
Note a nice low-height center west of Vancouver island.  We see that the cut-off low formed right along a baroclinic zone (an area with a strong temperature gradient) and its winds are doing a nice job of advecting warm air up from the south on the eastern side of the low.  So, at 850mb, there's a fair amount of warm air advection.  Let's zoom in and look at the surface/low(er)-level situation:
Fig 3 -- 12 hour forecast of 925mb temperature (colors), mean sea-level pressure (contoured) and winds (barbs) for 00Z, Jan. 12, 2011 (4PM PST Tuesday).  From the 4-km UW WRF model.
Note that we can see the strong pressure gradient (and equally strong winds out of the south to southwest) approaching off the coast.  The winds get a bit crazier on land due to the terrain effects.  One big thing that stands out on this map is the significant dome of high pressure sitting over eastern Washington and northern Idaho (roughly the Columbia River basin).  If you remember from one of my previous posts, I talked about how this area is surrounded by higher terrain on all sides which tends to keep air bottled up in that "bowl".  Here you can see that all that air bottled up is contributing to very strong pressure gradients.  On the ocean side of the Cascades and higher terrain, pressures have begun falling due to the approaching low off the coast.  But inside that "bowl" pressures have remained relatively high.  This is setting up an extraordinary pressure gradient between the interior basin and the coast.

I need to clarify my statement about there being no good way to get out of that interior terrain "bowl".  There is one way--through the Columbia River gorge along the Washington-Oregon border.  The Columbia River is an amazing river--its elevation is way down almost at sea level through most of its journey through eastern Washington and out to the ocean.  That is to say--it stays near sea level even as it flows through the Cascade Mountains.  That's pretty amazing.  It also provides one of the only ways out of the interior basin.

Because there's such a large pressure gradient built up, there are currently very strong winds pushing out of the Columbia gorge, particularly as it goes through the Cascades.  Here are the latest wind observations near Portland:
Fig 4 -- Surface observations from the Portland, Oregon, area at 22Z, Jan 11, 2011.
  Portland is almost right in the middle of this image.  The Cascade mountains would be oriented north to south along the right edge of this image.  You can see that strong winds out of the east are being reported right where the Columbia River is coming through the Cascades and the winds are easterly throughout this entire region (the northern Willamette Valley, if you want the actual name of this region).  This somewhat contradicts the surface pattern we would expect with a surface low off to the northwest--we'd expect winds out of the south or southwest.  But that strong pressure gradient across the mountains is pushing cold air out of the inland Columbia basin and down the Columbia River toward the ocean.

We can see this effect in forecast soundings for the Portland area.  This morning, there was a strong and deep push of cold air out of the Columbia Basin:
Fig 5 -- 3-hour forecast sounding for 15Z (7AM PST), Jan 11, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.
The above sounding is for Troutdale, Oregon, which is just northeast of Portland on the Columbia River.  Note that the above sounding is NOT on a Skew-T chart--the isotherms (temperature lines) are pointed vertically and not skewed to the right like we usually see.  The separation between the air being pushed out of the Columbia basin from the east and the southwesterly onshore flow from the west and southwest aloft is very clear.  Also notice how the temperatures in the layer near the surface where the winds are out of the east are cooler than the air aloft--this shows the difference between the cold air coming out of the interior basin near the surface and the warmer air associated with the Pacific low aloft.

Let's move along a bit in time:
Fig 6 -- 9-hour forecast sounding for 21Z (1PM PST), Jan 11, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.
This was six hours later, or earlier this afternoon.  Notice how the westerly and southerly winds aloft continue with easterly winds still coming out of the Columbia River gorge.  Of course, with easterly winds near the surface and southerly winds aloft, the winds are veering (turning clockwise) with height which indicates warm air advection going on.  This should be maximized where the wind direction is changing the most rapidly, or at around 950mb on the plot above.  And wouldn't you know--there's a bubble of warmer, above-freezing air that is forming right around that level. Amazing.

But now we should start to worry--all that warm air advection is also bringing in more moisture aloft.  In the first sounding, the air was saturated above 700mb.  Now the air is saturated above 850mb--so more moisture is moving in aloft and the level of saturation is dropping.  That warm air advection has also pushed temperatures above freezing just above the surface.  However, note how those cold easterly winds spilling out of the Columbia River gorge are still keeping the surface temperature very close to freezing.  This is a recipe for potential freezing rain.  Water would fall through the above-freezing temperatures just above the surface and melt into rain, but then hit the ground (which is at freezing or slightly below) and freeze.  The question is what particular factor will win out--the warm air aloft or the cold air persisting near the surface.  At this point,our saturation is pretty high off the ground--and according to observations it was not raining yet at this point.  As more warm air advection continues on top of that cold air coming down the Columbia River, the warm air will continue to mix down and erode into the cold air near the surface.  You can already see how much that cold layer (and the easterly winds) has shrunk between those first two soundings.

Six hours later:
Fig 6 -- 15-hour forecast sounding for 03Z (7PM PST), Jan 11/12, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.
This is later on this evening.  Warm air advection aloft continues and with it, more moisture.  Now our saturation level is down to 900 mb. There's a significant above-freezing layer that has built up above the easterly flow and cold Columbia basin air near the surface, but those easterly winds are still keeping the temperature near freezing at the surface.  Freezing rain is definitely a concern at this point.

Finally, by 4AM the next morning:
Fig 6 -- 24-hour forecast sounding for 12Z (4AM PST), Jan 12, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.
By the next morning, all that warm air advection has FINALLY mixed down to the surface.  All that warm air mixing down has warmed the temperature considerably--at the surface the air is now several degrees above freezing.  We're also saturated all the way down.  Except for that one stubborn barb near the surface, winds are out of the south to southwest all the way down to near ground level.  The precipitation falling at this point would be all rain.

So this is why Portland is on the lookout for potential freezing rain tonight.  It all has to do with its geography, sitting there along the Columbia River gorge. It's pretty fascinating to look at how local topography and mesoscale influences can radically change the forecast for a particular location.