Thursday, February 23, 2012

A "typical" winter storm

As most people in the central US are probably well aware, a powerful winter storm will be moving across the area over the next few days.  In fact, it's already pretty well developed.  Here's the surface analysis at 17Z this morning (11 AM CST):
We see a very well-developed low over western Missouri, with strong pressure gradients to the northwest and southeast of the low.  Strong pressure gradients mean strong winds, and already we've had damaging winds reported across eastern Colorado and western Kansas.  The tightening pressure gradient on the southeastern side of the low is going to help increase the winds out of the south, helping to advect very moist air out of the Gulf of Mexico and into the southeast.  Right now, the cold front doesn't look too impressive behind the low--in fact, as the low is analyzed now, it looks to be in the middle of the warm sector with only a weak frontal wave evident in the temperature field.  As more cool air pours down the high plains, though, this low should only become better organized.

Here's this morning's 500mb GFS forecast at 18Z this morning (12 PM CST).  Notice that the surface low over western Missouri is in a region between two jet streaks, though the streaks themselves are not too impressive.
However, as cool air continues to move down the high plains to the west of the low, it's going to encounter warm air in the 60s and 70s over Oklahoma and north Texas.  This is going to greatly strengthen the temperature gradient there, sharpening the cold front even further.  We see a response to this sharpening temperature gradient below in the 500mb wind forecast.  Notice how just 6 hours later in the GFS forecast at 00Z tonight, the jet streak over the southern plains looks much stronger:
As that jet streak strengthens, the upper-air flow becomes favorable to support the surface low deepening even more as it moves eastward across central Illinois.  By tomorrow morning, the surface low is forecast to be well-organized, with clear warm and cold fronts:

We're expecting a lot of different precipitation types with this storm.  First--snow to the north.  You'll notice on the surface map progression that north of an Iowa-central Illinois-Detroit sort of path the surface temperatures don't look to get much above the low 30s.  While surface temperatures would seem to be cold enough to support snow, we need to look at the temperature of the atmosphere above to be sure that the whole mass of air above is also cold enough to support snow.  We might suspect this is so, given that the jet stream shown in the maps above stays to the south of this particular line, and usually the jet stream is a good indicator of the separation between cold, polar air to the north and warmer, subtropical air to the south. But, we can still check the forecast for 1000-500mb thickness.  Here's the forecast for this afternoon:
The "critical" thickness line (the 5400m thickness line) runs right through southern Iowa, central Illinois, and into northern Indiana and Ohio.  North of that line, the atmosphere should be cold enough to support snow.  near and south of that line, we're looking more at rain.  So we can start to narrow down the areas we'd expect to see snow.  First, it's only cold enough north of this critical thickness line.  Secondly, we're pretty sure there will be a lot of lift, as a well-defined warm front and a surface low are moving northeastward from western Missouri into central Illinois.  The only remaining question is the availability of moisture.  This gets back to that strengthening pressure gradient on the southeastern side of the low that I mentioned a while ago.  As southerly winds increase to the east of the low, more moist air will be brought up from the south.  This air should rise over the warm front, cool, and reach saturation in a band just north of the warm frontal boundary.  We can see this in the GFS forecast for 700mb relative humidity for this evening:

Remember that at lower levels (near the surface), southerly winds are bringing up warmer, moister air from the south.  That air will only really rise once it encounters a mechanism that will force it to rise, like the presence of a warm front.  We saw that the warm front would be moving through central Illinois and the Ohio River valley by this evening.  And, sure enough, as we look at 700mb, suddenly we see a band of near 100% relative humidity just north of the warm front.  That's where that warm, moist air below has lifted over the warm front, cooled, and is now saturated.  This is the best area for precipitation to form, and it so happens that most of this looks to be in the area where it's cold enough to support snow.

In fact, if you look at the National Weather Service watch/warning map from early this afternoon, it outlines much of this same banded area with winter storm watches and warnings, particularly on the northern edge of the moisture:
The thinking prevalent in most of the forecast discussions I've seen is that by the time we get into southern Iowa and central Illinois, we'll be too close to that "critical" thickness line, and the atmosphere may not be cold enough to support snow.  Further north there's a better chance of snow.

As this storm moves east, there will also be the possibility of severe weather.  At the moment, that warm air advection to the east of the low is rapidly bringing the atmosphere throughout parts of the southeast to a conditionally unstable environment.  This is particularly true slightly further north away from the Gulf where the upper-air temperatures are colder, making the atmosphere more unstable.  However, until a strong lifting mechanism comes through, this conditional instability will remain untapped.

That cold front looks to quickly blast through the southern plains and is even expected to move across the Mississippi river by the evening.  How fast of a blast is this going to be?  Notice the large areas of pinks in Oklahoma and Texas on the watch/warning map above.  Those are Red Flag Warnings, indicating that they're expecting unusually strong winds accompanied by dry air. This is all the result of a fast cold frontal passage.

As that cold front moves into the southeast, a strong lifting mechanism will be present that can tap into that conditional instability.  It's no wonder that the Storm Prediction Center's day one outlook has a slight risk of severe weather for the Ohio River valley down into Tennessee and Kentucky.

So we get two different kinds of "severe" weather with this--snow to the north and thunderstorms to the south.

Thursday, February 16, 2012

Will winter every fully arrive?

Last weekend's massive high pressure system brought some cooler air to the middle of the country, getting us slightly closer to more winter-like weather.  Compare the temperature anomalies from the first week of February nationally (from NCDC)... the temperature anomalies from the second week of February.
This shows a significant change in temperatures across the central part of the country--from well above normal (over 15 degrees Fahrenheit above normal for much of the northern plains) to near normal or slightly below normal for much of the high plains by the next week.  That dome of cold, high-pressure air really had an impact.

Cooler weather allowed snow to fall across the central and southern plains, generating one of Oklahoma's few snowstorms even in a relatively warm year.  Here was the snow depth map as of the morning of the 14th (from NOHRSC):
A good 1-2" across much of Oklahoma, with higher amounts to the north in Nebraska.  Of course, now two days later the snow has almost completely melted.  Here is this morning's snow analysis:

Much of the snow is gone--even back up into the Chicago area. Warmer weather looks to be returning...

The longer-term forecasts also seem to be pointing toward keeping the warm weather around for much of the eastern US at least through this week.  Compare the following three GFS forecast maps.  One for 500mb at 12Z tomorrow:
Notice a trough over eastern Canada, but there is no well-defined jet stream or any particularly outstanding jet streaks.  A small cut-off low in far southern Arizona and New Mexico may lead to severe weather in south Texas later on Friday.  Here's the 500mb forecast map for Saturday morning:

Still not a very well-defined jet stream, and even the trough and ridge pattern is not well defined.  This is not the signature of any strong push of cold air coming out of Canada any time soon.  Finally for Monday morning:
Still nothing impressive.  There looks to be large-scale troughing over much of the western US, but there are no organized jet streaks around the trough.  This implies that the temperature contrasts in the atmosphere beneath are pretty weak--very little in the way of strong organized fronts, probably.  Without strong pushes of cold air from the north, temperature in the pre-existing air mass should have time to moderate and warm a little.  I'm expecting the temperatures to remain at or slightly above normal for much of the eastern US for the next few days.  Winter is still on hold...

Friday, February 10, 2012

A really big high brings winter back

A lot of snow and rain is currently impacting the eastern third of the country.  This morning's radar composite sums it up nicely.
Heavy rain for southern Louisiana with scattered heavy showers throughout the southeast.  Further north where it's colder, snow (enhanced by the lake effect) is being reported in the Chicago area and northern Michigan.

All this active weather...but where is the surface low?  Here was this morning's 12Z GFS surface analysis:
The only really organized center of low pressure is analyzed well to the northeast in eastern Quebec.  There is, however, a general trough of low pressure extending back through the Great Lakes and down into the southern plains.  This trough looks to be simply a consequence of being caught between two high pressure centers--one weaker high off the Carolina coast, and another, stronger, sprawling high pressure center in the Canadian Prairies.  Notice the very cold temperatures associated with this high pressure center--well below zero Fahrenheit a the surface this morning in parts of central Canada and the northern plains.  There's also a pretty sharp boundary between this colder air and slightly warmer air being pulled north in that low pressure trough. So, even in the absence of a strong surface low, we still have such a strong high pressure center that good enough thermal gradients are set up to help produce some significant weather.

This high pressure center looks to be here to stay--by tomorrow morning it is forecast to have set up shop in the central plains, bringing down much colder air than we've been seeing in the central and eastern US as of late.  Here's tomorrow morning's GFS forecast:

Notice the very strong temperature contrasts still on the leading edge of this high pressure center.  Furthermore, as this cold air moves down over the land surface, it encounters much warmer air over the open waters of the Gulf of Mexico and off the east coast.  This is setting up some very strong-looking temperature gradients that follow the coastline.  It also looks to be driving some cyclogenesis off the east coast.  Furthermore, with such a strong high pressure center, the pressure gradients are also very strong, and this means strong winds--you can see some 15-20 knot winds forecast over the upper Mississippi valley and into the southeast.  This means when the cold air arrives, it's going to arrive with quite the blast.

This high continues to linger into Sunday, according to the 48-hour GFS forecast:
Nos the strongest pressure gradients look to be across the middle Atlantic states.  Should be quite the blowdown.

With this huge high pressure center crashing the party, it looks like the weather pattern is finally turning to something more winter-like.  Seattle is once again back to having its rounds of rain with relatively cool weather, frigid temperatures are building back across the northern Plains, and with the presence of this high pressure center things look to dry out a bit from the rainier-than-normal conditions some places in the south and east have seen lately.  Perhaps these La-Nina-based seasonal predictions will come true after all...

Thursday, February 2, 2012

Another slow-moving storm forming in the southwest

Last week I was talking about a slow moving storm that was pushing through the southern US.  This week we're looking at another cyclone, currently developing over the desert southwest, that looks to slowly push eastward into the central plains into this weekend.

Let's start by looking at the GFS model 500mb forecasts.  At the moment, the Oklahoma HOOT site where I usually get my model graphics is migrating to another server and as such their output has fallen a bit behind.  For something completely different, I decided to use San Jose State's website for today's model graphics.  Here's the GFS 500mb height and vorticity forecast for 18Z today...about now.

Without going into the details of what vorticity means, one of the things it is useful for is tracing the location of shortwave troughs aloft.  For instance, you can see that in the base of the deep shortwave trough on the left side of the map above, the vorticity values are very high--those bright yellows, reds and purples indicate strong positive absolute vorticity, which is indicative of a cyclonic (counter-clockwise) horizontal shear of the winds.  We typically see such wind motions in a shortwave trough.  Let's use this to trace the forecasted path of this trough over the next few days...

By 18Z Friday, the trough is forecast to move eastward into the high plains.
Notice there still is a streak of high vorticity values associated with this trough.  Let's go another 24 hours out.  This is now Saturday at 18Z:
By this point, the trough has become much more poorly defined--looking at the 500mb height lines (the black contours), you may not necessarily think there is much of a trough there.  But, there is still a big vorticity maximum present in the central plains.  With the right temperature structure, this means that the flow could still be unstable enough to keep developing any surface cyclones.

So lets look at the surface forecast as this storm moves across the plains.  Here's the forecast for 18Z today at the surface (about now):
Looks like an area of low pressure trying to get its act together over New Mexico.  Notice that the path of the 500mb trough we saw above tends to follow the zone where there is a stronger horizontal temperature gradient at the surface.  This is not a coincidence--the steering winds aloft are controlled by low-level temperature gradients.  Also notice the very warm temperatures throughout the southern US and the southeasterly winds blowing from the Gulf of Mexico into the southern plains.  That indicates a lot of moist, warm air moving in, setting the stage for an unstable atmosphere.

Going forward 24 hours, we see that the surface low, while not very well defined on the map below, appears to be slowly moving eastward with the shortwave aloft.

While we can see the beginnings of a cold front in the Texas panhandle and New Mexico, the cold air behind it really isn't that deep, nor is the temperature transition really that abrupt.  Since we're looking at surface temperature here and the surface temperature naturally gets colder as you go up into the Rocky Mountains, the cold air behind the front is not as cold as you might think.  Furthermore, there's a strong pressure gradient developing in eastern Colorado that could help pull air down from the high mountains out onto the plains.  As air descends down the mountain slopes its pressure increases and it warms up--another factor working against a strong push of cold air.

And now the forecast surface map for Saturday:
Notice that even though our shortwave trough aloft had become somewhat less defined in the 500mb height field, the surface low pressure center has continued to consolidate (though it hasn't deepened that much.  However, in this image things don't look that good for the low.  The cold front is weak at best, and any attempt at a warm front is having difficulty getting through the Appalachian Mountains.  Still no sign of a strong push of Arctic air down from Canada, so temperatures will probably continue to be seasonably mild for much of the eastern US.

As far as precipitation goes, we're looking at showers and thunderstorms in the warm sector of this storm, which will generally stay in the southern plains and the deep south.  Lots of stratiform rain is expected to the north and east as the storm is able to pull that warm moist air up and around the low.  Some areas may see some snow, particularly in the central plains north of the low where the air is just cold enough.  Here's the forecast 6-hour precipitation accumulations for 6Z tonight:
That strong pressure gradient I mentioned over eastern Colorado will help increase the wind speeds quite a bit in that area.  Combined with the expected snow, this has led to blizzard warnings being posted for much of the west-central plains.

Moving to tomorrow, the precipitation comma follows the low nicely.
The purple contours on these maps are the 1000-500mb thickness lines.  Often we use the 5400m 1000-500mb thickness line as a separation between snow and rain.  We see here that this line runs from western Kansas up through Iowa and into the Chicago area.  While it seems clear that places like northern Colorado and western Nebraska will get all snow, the precipitation type forecast becomes a bit more difficult the further south and east you go.

On one final note, with such warm moist air coming off the Gulf into the southern plains, the Storm Prediction Center has slight risks for severe weather in Texas and Oklahoma tonight and tomorrow.  So, we may see a few strong thunderstorms with this storm as well.