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.

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