Tuesday, 11:55 a.m.
On Monday's post, I noted the discrepancies on the models with regards to the weekend outlook from the Plains to the East Coast. Since then, the models have continually changed their stance on things, but it's pretty clear now that there will be two fronts coming into play going into the weekend. The first will generate most of the showers and thunderstorms, but the air behind it is simply dry, not necessarily all that much cooler. However, behind the second front, much colder air will get pulled down through the northern Plains and Midwest into the Great Lakes and Ohio Valley, then, eventually, into New England and the mid-Atlantic states.
I think you can see this in a couple of different ways. Look at the 12z NAM 500mb forecast for Friday afternoon (18z):
The trough coming into the East is rather weak, and it will weaken as it comes east from the Plains Thursday and across the Mississippi Valley into the Ohio and Tennessee valleys Thursday night. Trailing behind this lead trough will be another one, represented by an upper-level low cruising by the northern Great Lakes. Like so many fronts we have seen for the past few months, there's plenty of pretty cold air to tap behind it across Canada. The 6z April 22 GFS ensemble 2-meter temperature anomaly forecasts for Friday clearly show this:
Essentially the lead front is what we might term a 'Pacific' front - in other words, the air behind it is much more from the Pacific in origin, not the Arctic. Therefore, once the front passes, the rain ends, the clouds break, and the atmosphere will have ample time to become well mixed. Let's look again at the latest NAM forecast, only of 850mb temperatures for Friday evening:
If you mix that out, with several hours of sunshine, there's every reason to believe most places from Chicago to Cleveland on south will have little trouble reaching the 70s Friday - behind the initial cold front! I suspect those anomaly forecasts will have to play catch up over the next two days to get the right flavor of highs and projected anomalies. It should be noted that those highs may NOT translate to the mid-Atlantic on Friday, because there will be too many clouds around along with some showers.
Now, once the trailing upper-level low can swing the second cold front through, then much cooler air will be able to advance farther south and east. This means a cooler day throughout the Midwest and Great Lakes into the Ohio Valley. In contrast, it should be warmer in the mid-Atlantic with some sunshine and some form of a downsloping wind off the Appalachians. Here's the projected 2-meter maximum temperatures for Saturday from the 0z April 22 European model:
While it will get downright toasty in parts of the southern Plains and very warm in the South and Southeast, the cooler air will spread out behind the front. Basically, as implied yesterday, the middle ground of yesterday's model extremes will, indeed, be the place to be.
Sunday, the chilly air may be even more widespread as a large surface high over Hudson Bay builds southward into the Midwest and Great Lakes and sends a lot of low-level cold air with it. Then, as a storm develops over the eastern Rockies and western Plains, the stage will be set for a lot of overrunning. This will generate a lot of clouds from the central and northern Plains into the Ohio Valley, and those clouds will be thick enough to really put a lid on temperatures, especially if rain breaks out, which seems likely. Look again at the European model forecast of 2-meter maximum temperatures for Sunday afternoon:
My hunch is that come Sunday, that entire grid will be shifted a bit farther south, the gradient across the boundary will be tighter, and that the core of the warmth and chill will be stronger than forecast - in both cases! Basically, if you're in the warm sector, you're going to be quite warm, and if you're north of that front, it will be quite chilly.
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