Tuesday, 11:55 a.m.
It has been fascinating to watch the pattern evolve over the past week! The onslaught of arctic air has been well advertised by the models, though the details of how it is coming about have been somewhat elusive. Several days ago, there was little hint of this snowstorm unfolding across the mid-Atlantic states and into New England, but now it's becoming a borderline blizzard in some areas or will be tonight and tomorrow morning in some areas. I will readily admit I did not see a storm of this magnitude coming a week ago, and even yesterday when it was obvious it would be a significant storm, I was still hesitant to call this a big storm. Egg on my face, because it is a big storm!
Part of the reason for the bigger storm and bigger accumulation may be tied to the slowness in its development. Even as of late morning, the surface pressure is still in northwestern North Carolina, as you can see from the latest surface pressure analysis:
This low has been slow in moving out of the Midwest and across the Ohio Valley over the past 24 hours, and the evidence of that is in the slow arrival of the arctic air in the Midwest yesterday. The models were suggesting that the lowest temperatures would, indeed, occur after 12z yesterday, but it was delayed by a few hours in some locations from even that time frame. That resulted in some 5- to 10-degree temperature busts for forecast lows (and even daytime highs, for that matter). The cold wasn't so much denied as it was delayed. It has gotten every bit as cold in Minneapolis and Chicago this morning as was forecast, but it took several hours longer to get there.
The same has been true in the East this morning. The arctic front has just inched its way southward overnight and this morning, and temperatures remain well above forecast highs even at this late hour! In fact, along the Delmarva Peninsula, the wind has turned onshore, and that has kept temperatures well above freezing southern Delaware on south.
Another interesting aspect about the air mass that is slowly being displaced is that it was really mild yesterday where there was any sunshine! Look at the highs from Monday:
If you looked back at the setup yesterday, there was a surface high pressure area over the Gulf of Mexico and a wave of low pressure in the Great Lakes. That created a decent surface pressure gradient south of the arctic boundary, and that made for a well mixed atmosphere that allowed temperatures to rise very nicely where there was any decent sunshine. What helped out in many of these areas from the central Plains to the mid-Atlantic was a lack of snow cover, so the air wasn't artificially cooled from below. Remember this, because I have a feeling it will become an issue with temperature forecasts over the next couple of weeks as these waves or discharges of arctic air ebb and flow.
As cold as the air mass is moving across the Midwest and Ohio Valley as well as the Northeast today, the one that will follow it is arguably colder. One comparison is on 850mb temperatures. Here's the morning NAM analysis:
Here's what that looks like tomorrow morning:
If you look just north of Minnesota though, there's another surge of arctic air about ready to descend into the Midwest and Great Lakes. A surface low will roll across Minnesota into Wisconsin later tomorrow. The surface flow ahead of that wave will become south to southwest, and that will allow temperatures to rise in advance of the cold front attached to the storm. It will still be colder than normal, but for many areas there will be a nice little bump tomorrow into the Ohio and Tennessee valleys, as well as in the mid-Atlantic states on Thursday.
However, look at the breadth of the cold Thursday afternoon as the next front reaches the East Coast:
With this wave, of course, there will again be some snow, with a couple of inches across part of North Dakota today, Minnesota this afternoon into tonight, and Wisconsin into Michigan later tonight into tomorrow. There will also be some snow tomorrow in the Ohio Valley, generally an inch or less in most places, then not very much survives over the Appalachians into the East.
Now look at the setup on Friday afternoon:
That doesn't look too much different from yesterday in many areas! High pressure along the Gulf Coast, with low pressure to the north (this time, largely well north of the Great Lakes), and a pretty tight pressure gradient from the lower Great Lakes and Ohio Valley into the Tennessee Valley. The air mass in the southern Plains will be relatively mild, so as the winds pick up and the sun shines, at least some of that mild air will be mixed into the Tennessee and Ohio valleys Friday.
The mild air will sneak into the East late Friday or Friday night into Saturday ahead of the next cold front, and temperatures may well get higher than you think would be possible given the pattern. However, the source region of the air mass in the East ahead of the front is a warm one, so with any sun and good mixing, it looks to be pretty mild.
That said, that next front means business, and another arctic blast roars in behind it - through the Midwest into the Great Lakes Saturday, then farther downstream on Sunday. Look at the 850mb temperature forecast for Sunday morning:
Now, if the European model is right, and it has support in the ensembles and certainly in the Canadian model, still another arctic wave of low pressure will move across the Ohio Valley and Great Lakes into the Northeast late Sunday or Sunday night into Monday morning, followed by perhaps the strongest discharge of arctic air in the pattern to open up next week. That said, don't be surprised if the air mass south of that boundary ends up milder than forecast for a day.
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