Monday, 11:55 a.m.
The first two-thirds of October have been warm - VERY warm. Most locations from Chicago to New England and from the Great Lakes to the Southeast are running at least 3 degrees above average so far this month, and many in the mid-Atlantic states back into the Ohio Valley are in the 4- to 7-degree range above normal.
That's about to end.
One cold front is now moving across the Great Lakes and Ohio Valley, and it marks the first real drop in temperatures. Chicago, ahead of the front on Sunday, soared into the mid-60s in some locations, while this afternoon temperatures will stay in the 40s. The drop won't be quite so sharp in New England and the mid-Atlantic states between today and tomorrow for any number of reasons, but that doesn't mean the drop won't happen. It will just happen in a series of smaller steps, as there are plenty more disturbances and fronts in the pipeline.
Let's examine the image below for a moment:
This is the 12z Oct. 21 NAM surface forecast for Tuesday morning. From it we can pick out a few important features. The first is the aforementioned cold front, arcing from the surface low at the southern end of James Bay into the Adirondacks, then down the Appalachians into Tennessee. Most of the showers associated with it from Pennsylvania on south will wither on the vine, so to speak, having virtually no impact on the weather across southern New England and the mid-Atlantic tomorrow. There may be some clouds around to limit temperatures in these areas. Otherwise, it will be mild with little to no rain.
The next upstream feature is a developing surface low in the mid-Mississippi Valley. And, yes, it will act very much like a true winter feature better known as an Alberta Clipper! As with the true winter counterpart, this little low will have most of the steady precipitation associated with it along and to the north of the storm track. Boundary layer temperatures are likely to be warm enough to support mainly rain overnight, but don't be shocked if there is a slushy inch of snow in parts of Iowa and northwestern Illinois overnight from this little storm! More on that feature in a moment.
The third feature of note is the low along the Alberta-Saskatchewan border. It is still another upper-level disturbance coming over the top of the mean western ridge, and it is poised to further carve out the deep upper-level trough over the eastern half of the country this week.
Let's examine the 700mb moisture 24 hours later, or Wednesday morning:
Notice the conglomeration of moisture from Pennsylvania into southern New England. That's associated with the low scooting across the Ohio Valley tomorrow, then redeveloping off the mid-Atlantic coast tomorrow night. As it trucks eastward, a cold rain will fall across the Ohio Valley that can mix with wet snow before ending. Then, as the storm starts to crank up somewhat off the coast, a heavier rain will develop in the mid-Atlantic states and head into southern New England later tomorrow night and Wednesday morning. The speed of this feature should be such that while the daylight hours of Wednesday may be cold and rainy in Boston, it should dry out in time for Game 1 of the World Series. However, when the first pitch is tossed, it will likely be in the 40s with a stiff breeze blowing out of the northwest.
As the rain moves through, it may mix with wet snow before ending on Wednesday, particularly in the higher ground of Connecticut and Massachusetts.
Getting back to the moisture forecast for Wednesday morning, you can also see some hang back moisture in western Pennsylvania, and that's another little feature that will generate some instability rain or rain and wet snow showers across Pennsylvania and upstate New York Wednesday before it darts across New England with little to show for it Wednesday night.
That third system diving southeastward out of Canada will then bring a bit of rain and some wet snow through the Dakotas tomorrow night and more of the same in Iowa into central Illinois Wednesday and early Wednesday night. It will have less moisture to work with, but some unlucky spots might get a couple of inches of wet snow. Then, as it moves eastward Wednesday night and Thursday and crosses the Ohio Valley and mid-Atlantic states, it will generate a fair amount of clouds, but more sporadic precipitation. Unless you are inland from the Great Lakes and up in elevation, you won't see much snow for this particular pattern, just plenty of chill.
Yet another one of these disturbances will be dropping out of Manitoba into the Upper Midwest Friday and Friday night. It may pull slightly warmer air into the Midwest ahead of it, but it will be followed by a reinforcing push of chilly air from the Midwest Friday night and Saturday to the Northeast by Sunday.
Now, if you're a fan of late October warmth? You need to head west. That's the one place that, by and large, will be above normal for most of the next week. That could change somewhat next week.
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