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.
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.
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...
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).
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.
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.