Thursday, January 26, 2012

A slow-moving storm and looking toward the weekend

It's been a while since I last posted on the blog as we've scrambled to recover from last week's winter weather here in Seattle.  I had a grand total of four inches of snow at my place, topped with a quarter inch of freezing rain, topped by another two inches of snow.  All in all, a pretty good storm--enough to close down the university for a few days.  However now we're returning to more seasonable weather--periods of rain with highs in the 40s.

A different kind of storm has been slowly making its way across the southern US these past few days.  Here's this morning's radar mosaic over the southeast:
A line of strong storms with some very heavy rain is moving across the area.  This is really the same line that moved across east Texas and western Louisiana yesterday, spawning a few tornadoes in the process.  Here is a map of the Storm Prediction Center storm reports from yesterday.  The red dots indicate tornado reports and the blue dots indicate severe wind reports.
I've been surprised at how slow-moving this storm has been, particularly since organized lines of storms like that tend to move eastward pretty quickly.  However, the upper-level structure shows that this surface low is being supported by a cut-off low aloft.  Here's yesterday morning's 300mb analysis:
And here's this morning's 300mb analysis:
Not a whole lot of change.  The center of the cut-off low at 300mb has moved east from west Texas to more over central Texas, but that's about it.  We call this a "cut-off" low because it's basically cut off from the main jet stream.  Note the strong jet streaks across the northwest and the northeast, but the 300mb low is well south of that stream.  Without those strong winds moving through, cut-off lows tend to move more slowly and stay around for a while.

Interestingly enough, the upper-level forcing from this trough really isn't communicating all that well with the surface.  Here's this morning's GFS surface analysis:
There is no strong surface low analyzed over the southeast--just a diffuse area of lower pressure along a baroclinic zone (a zone where there is a strong temperature gradient).  Cool (but not particularly cold) continental air is somewhat organizing behind this baroclinic zone as a cold front, and as this air encounters warm, moist air brought up from the Gulf of Mexico, we're seeing storms.

This storm looks to slowly lift northeast over the next 24 hours.  Here's the GFS forecast surface map for tomorrow morning:
The cut-off low aloft is actually forecast to begin folding back into the main flow today and, as that brings the surface low north, the surface low will begin interacting with that strong jet streak over the northeast that we saw in the 300mb map above.  This extra support aloft looks to deepen the low and really sharpen that cold front.

There are some other interesting mountain-driven features on this forecast surface map as well.  Notice the big, sprawling high-pressure area centered over Washington state. With clockwise (anti-cyclonic) flow around that high, you can see that it's bringing winds from the northwest to southeast over the Rockies in Montana, Wyoming and into Colorado, and it's also bringing winds that are trying to come from the east over the Cascades and Coastal Ranges in California.  In both cases the air is descending from high mountains down to either the lower Great Plains or out to the Pacific Ocean. When air descends down mountain ranges, it tends to warm up (sometimes rather significantly) and that lowers its pressure.  You can see that, as a result of this, there's a trough of lower pressure extending down the east side of the Rockies (or, rather, over the northern and central plains) and also an inverted trough of lower pressure extending up the coast of California.  See how there is that series of upward bulges in the pressure contours along the west coast?  That's areas of lower pressure within the larger sprawling high pressure.  This is still considered a trough--an area of lower pressure--even though it's not pointed in the direction we normally expect a trough to be.  You'll sometimes see this phenomenon (particularly on the west coast) referred to as a thermal trough.

Anyhow, moving ahead in the forecast, on Friday evening a quick Alberta-clipper type low is forecast to move through the upper midwest, bringing a shot of snow.  Here's Saturday morning's forecast surface map:
Behind this low is colder air, but not frigid--here you can see temperatures only forecast to be in the teens and low 20s for much of the upper midwest.  Certainly cold, but not extremely so.  Much colder air is pooling to the north in central Canada, but longer-range forecasts keep that cold air to the north throughout the weekend.

Wednesday, January 18, 2012

More snow in Seattle? This morning brings a new look...

A lot can change overnight.  Yesterday evening's model runs were increasingly pessimistic about how much snow would fall in the Seattle area.  However, this morning's projections are looking more on the optimistic sie for seeing 4-6 inches of snow in the Seattle area.  So what changed?

In my last blog post I talked about how the location of the surface low moving in off the coast would be critical in determining the amount and duration of the snow event in Seattle. Too far north and we'll rapidly transition to rain.  Too far south and we won't get enough moisture brought north to overcome the large amount of cold dry air at the surface over Seattle.  Yesterday, the models were bringing in the small surface low into the coast at or just south of the mouth of the Columbia River--this was a bit too far south to get the maximum snow in Seattle. We'd see some snow, but not much.

At least, that's what last night's 00Z models were saying.  But then something changed overnight--the surface low got closer to land.  And as the surface low gets closer to land, it enters our observation network--it gets closer to our offshore buoys and weather stations on the shore.  As such, we get a lot more data about the strength and position of the surface low the closer it gets to shore.  We can use that data to improve our model analysis of where the surface low might be.  This is done through a process called data assimilation.  Most of my research revolves around improving this very process.  So what did data assimilation tell us about the low overnight?

Below is a map from our experimental real-time ensemble Kalman filter system (a type of data assimilation system) here at the University of Washington.   This system uses the power of ensemble modeling to really give us a good idea of the amount of uncertainty in a particular forecast, then uses that uncertainty to help determine how observation will impact the model. This is an increment map of sea-level pressure from 6Z last night (10 PM).  You'll see the familiar sea-level pressure contours in black.  This is what the model system thought the sea-level pressure field was before we assimilated data.  The color shadings show the impact of assimilating all the data we have--nearly 10,000 observations of wind, temperature, pressure and moisture from that entire area--into the model system. Anywhere you see red, that means that the observations wanted to increase the sea-level pressure there.  Anywhere you see blue, that means the observations wanted to decrease the pressure there.
The observations wanted the model to decrease the pressure on the eastern end of the pressure trough extending out to the east from the large low center in the central Pacific.  This isn't too much of a surprise--we knew that a surface low was going to form there anyhow.  Now let's go ahead three hours to 9Z last night (1 AM).  Here's the adjustment map for that time:
Notice that by this point the model indeed formed a small surface low in the center of that pressure trough--it responded to those observations.  But now look at the location of this next pressure adjustment.  The observations want to decrease the pressure to the north of that developing surface low!  This is to say, the observations want to pull that developing low further north.  This kind of pattern continued all morning, with the observations wanting to pull the surface low slightly further north.

Now, our operational deterministic models that I show here don't include the data assimilation analyses from our experimental EnKF system.  However, the initial and boundary conditions for their 12Z runs this morning do come from a larger model (the GFS) that does have a lot of data assimilation included.  So, when looking at the 6-hour forecast of where our 4km WRF wants the surface low to come ashore this afternoon, we see this:
It has brought the surface low back up north a little bit, now coming ashore just north of the Columbia River.  Even though the change in the position of the low is subtle, it has huge ramifications.  This brings the warmer, moister air closer to Seattle and increases the amount of snow that could fall.  Here's this morning's model 24-hour snowfall projections ending at 12Z tomorrow morning:

Remember last night the models had backed off to maybe 1-2 inches in the Seattle area.  This morning's forecast shows 4-6 inches (with an interesting convergence-like band over the Kitsap Peninsula and Seattle...).  Quite the change overnight.

And, since this morning's 12Z run (which is based on data from 4 AM), our ensemble Kalman filter (EnKF) system has continued to run.  Here's the 15Z (7 AM) adjustment of sea-level pressure due to observations:
It shows the low analyzed off the mouth of the Columbia River (still offshore) but there is still a strong adjustment signal that wants to lower the pressure to the north of the low (and raise it a little to the south).  This points to the low continuing to be pulled northward.  Hopefully it doesn't get pulled too far northward--then we'll end up with a lot of rain tonight.

So, the forecast is still developing, but I have pretty high confidence that we'll see several inches in Seattle today.  Not only are these short-range forecasts, but now that the low is close to the coast our models can take advantage of all of our observations here and really nail down where the low is going to be.  I also already have two inches or so at my place, so yes--I have confidence.

Tuesday, January 17, 2012

What's the deal with the snow forecast for Seattle?

As much of the nationwide media is now proclaiming, there is a potential for a major winter weather event in western Washington tonight and tomorrow.  Here's what we're looking at.

Snow showers continue over western Washington this afternoon as cool, moist air streams in off the Pacific.  Portland and the northern Willamette valley are really getting hammered.  Here's the latest composite radar image for the region.
However, the latest surface map shows that the temperatures around Portland are several degrees above freezing and most of what is falling is rain.

Showers will continue tonight, but the big snow event won't be until late tonight and into tomorrow as a relatively weak surface low approaches the coast.  Here's the 27-hour forecast from our 12Z initialized, 36 km WRF for tomorrow morning:
Note the really strong pressure and temperature gradients shaping up along the coast for tomorrow.  There's very cold air in the interior of British Columbia and relatively mild, moist air being brought up from the south behind the surface low.  So how does the interaction between these two air masses lead to a snow event?

As the surface low moves in, there will be a strong pressure gradient across the mountains--high pressure associated with the cold air inland and the lower pressure associated with this warm frontal wave off the coast.  This pressure gradient is going to draw down cold air near the surface out through gaps in the mountains and down into the lowlands.  Meanwhile, the warmer, moist air being brought up from the south will tend to lift over the cold air.  That lifting will cool the moist air, clouds will form, and precipitation will fall out into the cold air near the surface.  Here's a diagram that hopefully will help explain that setup:
That is a Google Earth perspective looking at the Olympics from the Cascade Mountains east of Seattle.  As the low pulls down a wedge of cold air out of the mountains to the north, warm moist air circulating around the low will move in from the south and rise over this low-level cold air.  The result is snow, and a lot of snow if the boundary between the leading edge of the cold air and the warm air to the south doesn't move much.

However, locating that boundary between the cold and warm air is critical for determining where the snow will fall.  If the warm air moves too far north, Seattle will be too warm for snow and we'll get a lot of rain.  If the cold air pushes too far south, the warmer, moist air will be precipitated out by the time it reaches Seattle and we'll see less snow.  This is all related to the placement of the surface low.  Here's two scenarios:

First, let's say the surface low stays further north--approaching the coast from the Strait of Juan de Fuca, perhaps.  In this case, warm, moist air will be brought up into western Washington.  However, because there is that nice gap between the Olympics and Vancouver Island (the Strait of Juan de Fuca), the surface low will draw most of the cold air straight west and very little will filter south into the Seattle area.  This would turn into an all rain event for Seattle.  I show this setup below.
Now let's consider another case.  If the low pressure center moves toward shore just a few hundred miles to the south, some things change.  We still will have warmer, moister air brought from the south around the low and into western Washington.  However, the cold air being drawn out of the mountains in Canada will be pulled more southwesterly out of the Cascades--and straight at the Olympics.  Some of the cold air will then get pulled down through the Puget Sound lowlands in addition to wrapping around the Pacific side.  This setup is much more favorable for snow in Seattle--we get that cold air near the surface that we need to support snow:

And it looks like our models so far are favoring a southern route.  Here's last night's 00Z forecast from our 4km WRF for where the surface low would be at 18Z on Wednesday (Wednesday morning).
Notice that the surface low was forecast to be right off the mouth of the Columbia river.  The pink horizontal line across the map marks the freezing line at 925mb (near the surface) anywhere north of that line should be cold enough to support snow.  Definitely a snow event for Seattle.  Here's the corresponding 24-hour snowfall accumulations that were forecast by tomorrow evening over western Washington:
This placed 6-8 inches of snow over the Seattle area--very impressive snowfall totals.

But, wait...this was last night's model.  What about this morning's 12Z run?

This paints a slightly different story.   Here's this morning's model surface forecast for 18Z Wednesday (the same time as the surface map above):
The low pressure center (and the freezing line with the cold air behind it) has been moved slightly further south--it's not much, but it's enough to make a dramatic difference in the final forecast snowfall amounts:
Now the model is only forecast 2-4 inches over the Seattle area, with a sharp decrease to the north to almost no snow north of Everett.  Since this morning's model run moved the surface low and cold air south, the most efficient precipitation production also moved to our south.  Thus, the model precipitation amounts have come down.

This change in precipitation amounts is very sensitive to the exact location and timing of the arrival of warm air and the low pressure center on the coast.  One thing we do know is that with this kind of model trend (moving the colder air further south), it should definitely be cold enough to support snow through the duration of this event.  We're not as concerned about the transition to rain at the end anymore.  The big question is still what final precipitation amounts will be.  The low may end up moving slightly further north and increase the snow amounts again--we'll have to wait and see what tonight's round of models have in store.

Even with the slightly diminished amounts, we're still looking at a major snow event late tonight and tomorrow in the Seattle area.  It looks like the only questions now are in the details.

Friday, January 13, 2012

Quick verification of yesterday's snow and turning to the Pac NW

Yesterday's snowstorm across the upper midwest is well on its way to moving out of the area.  I thought I'd take a quick minute to look at what the final snowfall pattern looked like and compare it to the model outputs I was showing yesterday.

First, final totals from around the Chicago area are now in.Most areas got at least three inches of snow, with totals up to 8 inches being reported in some areas of the Chicago suburbs.  Here's an analysis map from the WFO Chicago webpage of 24-hour snowfall amounts as of 7 AM today.

Let's compare this to the forecast map I showed yesterday.  The time periods don't match up, so the amounts won't be the same.  However, I'm more interested in the geographical distribution of the snow.
There's actually remarkably good agreement in the extent of the areas being forecast to have snow.  The model is correctly leaving snow out of the Detroit area, and the western edge of the 0.1 inch or greater snow line matches up well with the 1-2 inch line on the analysis.  Also,the areas of the most snow--from northern Michigan and eastern Wisconsin back down through northern Illinois--also agree fairly well.  I'm rather impressed with this model forecast.

However, I want to turn my attention now to the potential for snow where it's more unusual--in the western Washington lowlands.  A shortwave trough aloft is moving down the west coast of Canada, bringing with it cooler air and a setup that's very favorable for snow in the lowlands.  Here's the 500mb setup now:
A large, deep, cold trough is sitting in the Gulf of Alaska.  This trough has brought some of the very heavy snows that Anchorage and southern Alaska have been getting over the past few days.  The same trough is not done yet--by Sunday morning it's forecast to have moved south over the Pacific northwest.  Here's a 48 hour forecast:
The cold air accompanying this trough will linger for several days--through at least the middle of next week.  With westerly flow both aloft and near the surface, the air moving in will contain some moisture as it moves in off the Pacific.  However, the models are all forecasting low-level temperatures near or below freezing to persist.  With "moist" air moving in over sub-freezing temperatures, there definitely is a potential for snow.

And the models are really starting to go a little crazy with this.  Here's our UW WRF 12-km forecast of 24-hour snowfall accumulations ending Sunday night:
Definitely some snow creeping down into the lowlands.  That scale shows around an inch in the Seattle area on Sunday.  There also looks like there may be some convergence zone enhancement to the north of the city.    The mountains are definitely getting hammered.  However, if you look at the breakdown of the precipitation, most of the lowland snow is coming from several scattered showers.  Here's the next 24-hours of accumulation ending on Monday night:
Interesting pattern here that puts a lot of snow to the north and to the south of the Olympics.  This snows 2-3 inches falling over Tacoma, and another inch over Seattle.   Once again, though, this isn't an organized, massive snow event--it's more just snow showers. Here's the next map ending on Tuesday night, though we're starting to get beyond the model's predictive confidence...:
More convergence zone enhancement to the north of the city, and lots of snow in many places of the lowlands.  Another 1-2 inches forecast for the Seattle area.  And finally the map ending Wednesday night:
Wow--4 inches forecast for the Seattle area.  If you sum that all up, that's a forecast potential for up to 8 inches if you believe the longer range forecasts.  But Sunday is just two days away, and the last several model runs have consistently pointed to at least a small amount of accumulating snow in the Seattle area.

There's a lot of factors that could complicate this.  Water temperatures are still above freezing in the sound, so low-level temperatures will have that to compete with to maintain snow all the way to the ground.  There's also concern that the westerly flow that's bringing moisture in from over the Pacific may spend too long over the water, warming the air enough that we don't get below freezing. Also, with this concern about the temperatures, any light accumulations will probably melt off during the day, meaning that we won't see too much on the ground at once.  My main concern is just how many days we're facing the potential for snow.  Even with small amounts, the constant threat of snow could cause some problems, particularly for commuters.

We'll have to watch this.  The coastal radar and the dual-pol capabilities of the radars will be put to good use next week.

Thursday, January 12, 2012

An update on the snow forecast for the upper midwest tonight

As I talked about in yesterday's blog, the first significant snowfall event of the season is underway across much of the upper midwest.  The areas around Lake Michigan look to be the hardest hit with this storm in terms of snowfall amounts today.  Here's the latest regional radar image:

Lots of precipitation falling throughout the area, and it's pretty much all snow once you get west of central Indiana.  Notice the banding of the precipitation coming off of the eastern side of Lake Michigan and into western Michigan--that's evidence of strong lake effect enhancement going on there.  However, as we'll see in a moment, it's a bit of a tricky situation going on there...

Here are some of the snowfall totals as of around 12:15 this afternoon (Central Standard Time) (from the WGN weather center).

Lincolnshire 1.0 inches
Crystal Lake 1.2 inches
Woodstock 1.0 inches
Romeoville 1.8 inches
Downers Grove 2.1 inches
Machesney Park 2.4 inches
Montgomery 1.5 inches
Yorkville 1.2 inches
Rockford 2.0 inches
O'Hare 0.9 inches
Oak Brook 1.4 inches
Midway Airport 1.4 inches
Batavia 1.5 inches
Poplar Grove 2.0 inches
Aurora 0.5 inches
Arlington Heights 1.0 inches

I supplement this with the reports from the Milwaukee forecast office for southern Wisconsin:
Milwaukee  0.9 inches
Beaver Dam 2.1 inches
Madison  2.1 inches
Oregon 1.5 inches
DeForest  1.8 inches

The big winners so far seem to be points west of the city, including my home territory of the Rockford area and its suburbs which are approaching 2-2.5 inches so far.  I have seen a few scattered 4+ inch reports coming in from eastern Iowa, but I haven't found a compiled list from the Davenport forecast office yet.
** Just got a list from the Davenport office of a few observations there***
Dubuque  3.3 inches
Cedar Rapids  4.8 inches
Stockton  3.5 inches
 I'm also waiting to see what the Grand Rapids office reports about lake effect snow amounts on the Michigan side.

So how much more snow are we going to get in these areas?  Often we turn to high-resolution, short-range models to try and figure that out. One of the newest of these models is something called the High Resolution Rapid Refresh, or HRRR. This model was initialized at 16Z this morning (10AM CST) and forecasts out for the next several hours.  Here's the HRR forecast for total snowfall accumulation from 10AM through 3PM this afternoon:
You can see that the HRRR is highlighting a band from the south side of Chicago back through north central and into northwestern Illinois as the area with the most snow (disregarding the snow related to the cyclone further east over eastern Ontario).  A few isolated spots in central Michigan and central Wisconsin are also highlighted.  One thing I initially found peculiar about this image is that it's forecasting little to no snow accumulation over the Grand Rapids area in southwestern Michigan.  Surely one would assume with that lake-effect-style banding coming off the lake that they'd be getting buried with snow.  But, if you check the latest surface map...
You'll see that in that area of southwestern Michigan the temperatures are above freezing-- 37 to 40 degrees --  with rain being reported.  An onshore component to the wind, combined with the fact that that area is somewhere very close to the surface low pressure center is helping keep the low-level temperatures there warm enough to support rain.  So, what started out as my mild disbelief of the HRRR model turns into a mild respect for its ability to pick out that particular phenomenon.

Just to keep following up on this, it turns out that the Grand Rapids radar is indeed upgraded to dual-polarization.  We can check the correlation coefficient product from that radar to get an idea of whether or not what's hitting the ground is rain, snow, or a rain/snow mix.  Here's the latest correlation coefficient image.
The radar is right in the center of the image.  Anywhere you see white on the image it indicates that the radar returns were highly correlated--this means that what the radar is seeing in those areas is all rain or all snow.  Since the radar beam gets higher above the ground the further it moves from the radar, the large areas of white at a distance from the radar are also pretty high off the ground.  This probably means we're looking at all snow up there (since it gets colder aloft).  But, right near the radar we see a transition to the pinks and oranges.  Those indicate lower correlations, which points to a mix of rain and snow.  This means that as we're getting closer to the ground (closer to the radar) the snow falling from above is melting--becoming a mixture.  And right near the radar there's a small area where we're all white again--so, that could mean right at the ground we've suddenly switched to all rain  (if you believe the signal that close to the radar, but those surface observations seem to confirm this).  It's very close to being snow, but it's just warm enough near the surface to melt that snow into rain.  This illustrates one of the great benefits of having a dual-pol radar--we can use it to monitor the transitions between rain and snow.  The more that pink and orange melting layer shrinks in closer to the radar, the lower the snow level is becoming.

So now that we've verified that the HRRR is actually being somewhat accurate, let's press a little further with our forecast.  Here's the forecast snowfall accumulations between 10AM and 6 PM CST tonight (including the evening commute...):

That area from the south side of Chicago westward still looks to be getting the brunt of things.  In the previous map they had accumulations of about 2-3 inches, and hear they're starting to get into the 3-6 range. So that's saying that another 1-4 inches of snow are forecast to fall between 3 and 6 PM local time.  Also notice that the model does start to show some accumulations in that area of southwestern Michigan--the model is forecasting it to cool down enough for that snow to finally make it all the way to the surface in the Grand Rapids area.

Another complicating factor will be the winds.  Here's the HRRR forecast for the winds at 4 PM CST tonight:

This is showing 20 knot winds or so around the Chicago area and points west.  The color coding indicates the wind speed.  Notice the winds are stronger out over the lakes where this is less friction.  They also weaken significantly over the bulk of Michigan as that area is pretty much right underneath the low pressure center.

With lots of snow falling, strong winds and darkening skies, visibility is going to be an issue for the commute home tonight across northern Illinois.  Take all necessary winter driving precautions and take it slow.

If you'd like to look at graphics from the HRRR yourself, they can be found at:
(Note: if there don't appear to be a lot of images available, you can change the initialization time to the previous hour from the drop down box at the top of the page.  You can also use the region drop down box up there to select a different region to zoom the map in.  I used the Great Lakes region for the maps I showed here.)

Wednesday, January 11, 2012

Snow on the horizon

In my last blog post I talked about the evolution of the upper-air pattern for the rest of this week.  I noted that there looked to be a shortwave trough forecast to dive south from Canada and later strengthen as it approached the east coast.  It looks like this is indeed what's going to happen, and as this trough brings in colder air, winter weather advisories are going up across the upper midwest.  Here's the NWS watch/warning map from this morning.  The dark blues and purples in Wisconsin, Illinois, Iowa and Missouri (and also New England) are all winter weather watches or advisories.

So how is this cyclone forecast to develop?  This morning, the shortwave trough aloft was analyzed over the northern plains in the following GFS analysis 500mb map.

Our shortwave trough that brought the heavy rain to Texas is also still hanging around, bringing more rain and possibly some severe weather to the southeast today.  As the northern trough digs down further, it's going to run into this southern trough and it looks like the two will merge--eventually.  Here's the 24 hour forecast for Thursday morning.
The northern trough is forecast to dig south and strengthen considerably, while the southern trough races ahead and moves up the east coast today.  Notice the two still haven't merged yet.  However, by Thursday evening, we have this...
There's still a subtle remnant of that southern trough off the coast of Maine, but for the most part the jet streak surrounding the northern trough has both strengthened and merged with the jet streak associated with the (formerly) southern trough.

This interplay between the two troughs has interesting implications for the organization of this developing cyclone at the surface.  I find the GFS analysis of surface pressure this morning somewhat unorganized...
...but at the same time the upper-level forcing remains weak.  Notice on the first 500mb map I showed above that the northern trough doesn't have that strong of a jet streak associated with it yet.  That limits the amount of upper-air divergence that's available to support a developing low at the surface.  What we do see on this morning's surface map is a cold front getting its act together across the high plains.  Very cold surface temperatures--in the teens and single digits--highlight a pool of cold air right behind a strengthening pressure gradient and pronounced wind shift from northern Minnesota back through eastern Nebraska and the Texas panhandle.  However, remember that we still have that southern trough out ahead over the southeast.  That trough itself helped bring cooler air down south before this new front even got started.  That makes it harder to develop a strong temperature gradient at the surface if cooler air was already in place.  In fact, the warmest surface temperatures are far to the southeast over Georgia and Alabama--far from this developing cold front.

As the cold air behind this developing cold front gets closer to that very warm air in the southeast, the surface temperature gradient will increase and, with it, the strength of the winds aloft (as we saw in the 500mb maps above).  Here's the forecast surface map for 24 hours later (Thursday morning).
The surface lows associated with the northern and southern troughs are very close to merging.  The cold front, now shown from Indiana back down the Mississippi River to western Louisiana, is much sharper and more organized.  By 12 hours later...
...the cold front has continued to move eastward and the temperature gradient has become MUCH stronger--the cold air is finally interacting with the 55+ degree Fahrenheit air that the southern trough had been keeping ahead of it for so long.  So, the presence of this other trough to the south and the east really helped delay the formation of a strong surface low by keeping the warmest air in the lower atmosphere well out in front of the developing cold front for as long as possible.  By looking at the 500mb map above for this last time (Thursday evening), you can see that just as the front at the surface has really gotten strong, so too have the winds aloft.  And with a strong jet streak aloft, that helps give us the divergence needed to deepen and organize the surface low.  Likewise, in the last surface map, you can see that we finally have one low center over the Detroit area.

Another consequence of having this southern/eastern trough out ahead is that not only does it keep the warmest air away, but it also keeps much of the moisture to the east as well.  Here's the forecast 3-hour accumulated precipitation at 12Z Thursday morning:

This is before that low center really got organized.  Remember the cold front at this point extended from Indiana down the Mississippi River. However, the main precipitation area is well to the east--associated with that southern/eastern trough!  Remember that our main low center will develop over Detroit 12 hours after that image, and pretty much all of the precipitation is to the east of that area where there is strong warm advection.  There is a light snow band that looks to be developing from Wisconsin down through Missouri, but the real snow doesn't look to start coming until after the low centers have merged on Thursday night.  Here's the forecast 3-hour accumulated precipitation for 00Z Friday (Thursday night):

It's pretty incredible how much the pattern changes.  Once the surface lows merge, air from the warm conveyor belt to the east (originally controlled by that eastern trough) can finally get wrapped up behind the western trough and its associated cyclone.  That air has its origins over the Gulf Stream off the east coast--it has a fair amount of moisture in it.  So we see that it's not until after the lows merge that heavy snow really gets going behind the low.

Since this precipitation is out behind the low center (and consequently behind the cold front), we're pretty confident it will be snow.  A quick check of the forecast 1000-500 mb thickness for this time shows that most of that precipitation is north of the blue "critical" thickness line, indicating that it's likely all snow.
Much colder air, and finally a snow-worthy system seem to be in store.

But what about on the west coast?  Seattle is also looking at a possible lowland snow event this weekend as well.  Long-range forecasts show a strong, cold, 500mb trough moving southeast from the Gulf of Alaska this Saturday and Sunday.  Here's the forecast 500mb heights, temperatures and wind valid early Sunday morning (9Z).

With cold air moving in, there also looks to be just enough moisture to get some snow in the lowlands.  The models have been putting out sporadic lowland snow from Sunday through Tuesday, but the placement is anything but certain.  Here's the latest 12km WRF run's 24-hour snow accumulation valid 12Z Tuesday morning.

Lots of snow for the mountains seems pretty much a guarantee for this event.  You can see some tendrils of snow leaking down into the lowlands and the Seattle area.  We may even see some convergence zone snow following the cold frontal passage on Sunday or Monday.  It's still pretty far away, but it definitely bears watching.

Monday, January 9, 2012

Relief for Texas, cool weather ahead

A cut-off low over the desert southwest is slowly churning eastward through Texas and into the deep South.  In response to the low-level cyclonic motion generated by this low, southerly winds have helped pull lots of moist air out from over the Gulf of Mexico and into Texas.  The result?  Heavy rains today throughout much of the state.

According to the Houston forecast office, they're expecting 1-3" on average for most of their forecast area with locally heavier amounts up to 5" today.  That's a lot of rain, particularly for an area that still is in a large precipitation deficit.  Here's the latest national drought monitor image (from January 3rd, released January 5th):

Much of Texas, particularly south Texas, and parts of Oklahoma are still in "extreme" to "exceptional" drought. Hopefully today's rains will help alleviate some of that...

Let's look at how the upper-air pattern is going to change over the next few days.  Here's the current 500mb picture from this morning's GFS analysis:
You can see the cut-off low centered over the El Paso area.  By Wednesday morning, the low is forecast to have move eastward across the deep South.

However, notice that there's a shortwave trough that's beginning to dig in from up in Canada.  In this 48-hour forecast the trough is centered over southern Manitoba back through Eastern Montana.  This trough is forecast to deepen pretty significantly over the following 48 hours.  The little cut-off low in the south is forecast to move up the east coast, helping to bring in warm air from out over the Gulf Stream.  As the shortwave trough digging in from Canada in the above image moves eastward, it's going to encounter the rather strong baroclinic zone (warm-cold temperature gradient) left behind by the little cut-off low over the east coast.  Remember that upper-level winds are strengthened by strong horizontal temperature gradients below.  As such, that shortwave trough really starts getting strong by Friday morning:
This could mean some pretty crazy weather in the northeast on Thursday and Friday of this week.  We'll have to keep an eye on that.

But notice what is happening to the overall upper-air pattern--a strong ridge is building over the west coast, while large-scale troughing is occurring over most of the US east of the Rockies.  This would be considered the longwave pattern, as opposed to the shortwaves we usually talk about.  Shortwaves are the smaller troughs embedded in the bigger, longwave pattern.  Here the longwave pattern has shifted to ridging over the east Pacific and troughing over the central US.  This is actually a very typical La Nina type of upper-air pattern.

And if we're getting into a typical La Nina type pattern, all of those long-term weather predictions about the type of winter to expect should start coming into line.  The pattern is forecast to become even more established by Saturday:
Still a big ridge in the eastern Pacific with troughing over the eastern US.  Notice the location of the jet stream--down the Rockies and then out eastward over Texas and the south.  Since the winds aloft are connected to temperature gradients below, the jet stream often marks the boundary between cold, arctic air to the north and warmer, subtropical air to the south.  With this large-scale troughing in place, we can anticipate much cooler temperatures across much of the central US by the end of the week.  Here's the forecast 1000-500mb thicknesses (a proxy for temperature throughout the lower atmosphere) on Saturday morning:
Anywhere north of the solid blue line is usually cold enough to support snow.  You can see that everywhere except for the southern-most tier of states is in the snow regime.  We're talking high temperatures in the teens and 20s for parts of the northern US by the end of the week.  Looks like winter is going to happen this year after all.