Tuesday, 11:35 a.m.
The big picture today is one that will look quite a bit different by the end of the week. All you need to do is look at the pattern aloft this morning versus Friday evening, and you'll be able to see the significant changes that are soon to take place. Here's the 12z Dec. 3 NAM 500mb forecast for early afternoon:
Note the rather zonal flow across the country. The arctic air that had been in the East over the holiday weekend has largely been changed and/or modified, and the newly minted arctic air mass driving into the Northwest and northern Rockies is only gaining a foothold this morning. Compare that to the Friday evening forecast:
The development of a strong upper-level ridge over Florida late this week means they won't be seeing any kind of cold air any time soon. Furthermore, with that ridge becoming so established late in the wake the upper-level flow around will become quite strong from the southwest. Therefore, the front approaching from the Midwest through the Great Lakes and Ohio Valley will run into increasing resistance Friday and Friday night before finally running out of push. And, as I took the time to explain yesterday, there will be multiple waves along the front, and as any wave approaches from the southwest, it will generate more wind from the south and southeast ahead of it to push warmer air up through the East toward southern and eastern New England.
This warmth will have more or less free reign Thursday into the East, boosting temperatures into the 60s all the way to New York City, if not into parts of southern New England. Even on Friday in extreme southeastern New England and much of the coastal mid-Atlantic, the day will start mild with at least a threat of rain.
However, the cold will keep coming. At last check, it was 2 in Cut Bank, Mont., with winds gusting over 30 miles an hour and wind chills hovering around 20 below zero! The front has just come through Cheyenne, Wyo., in the past couple of hours, and it will reach Denver this afternoon. It will keep drilling southward tonight and tomorrow, reaching the Dallas area sometime tomorrow night.
As the cold air pushes the warm air out of the way, it will be very effective at dislodging the warm air in the low levels of the atmosphere, but it may take a little longer aloft. This will set the stage for trouble later this week as multiple waves are spit out of the western trough, move across the Plains and head northeastward from there. While rain and snow will largely be the main forms of precipitation through tomorrow, ice will become a bigger part of the mix Thursday afternoon and especially Thursday night into Friday as moisture is carried high over the low level arctic air.
The modeling is not going to be very helpful for individual forecasts, I'm afraid, as they assign different importance to any given wave, and, therefore, how fast they move and how much cold air arrives ahead of each one, or not. The big picture is easy to see, but the various strengths of individual waves is different.
For instance, the latest 12z model runs are different in many ways. The big ticket item is the strength of the second feature coming across the southern Plains toward the Mississippi Valley. The NAM is making much more out of it than the GFS is. The proof is at 850mb Friday afternoon:
The GFS has the same feature, but it's not nearly as impressive:
This leads to a flatter, faster wave on the GFS that allows more cold air to seep into the Ohio Valley and Northeast ahead of the wave, meaning a better chance of seeing snow and ice across the Ohio Valley and central Appalachians into central and even interior southern New England as it moves by Friday and Friday night.
That wave will move off the coast Saturday, and some cold air will make it to the New England and mid-Atlantic coasts, but not all of it. Look at the 12z Dec. 3 GFS 500mb forecast for Sunday evening:
The ridge isn't budging, and as a result, not all of the cold air can make it to the East Coast, and virtually none to Florida. By then, another wave will be coming out across the Plains. With a little more cold air in the way ahead of it, expect more snow ice issues from the southern Plains to the Ohio Valley, then into the interior mid-Atlantic and New England with time. It could even snow back to Chicago as the low organizes and moves by to their southeast Sunday night.
It does appear as if a bigger piece of the arctic air will make it to the Northeast and mid-Atlantic states behind that wave Monday night and Tuesday of next week, but not until then.
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