Tuesday, 11:45 a.m.
We in the meteorological community tend to have a few things that we rely on for pat answers or explanations for things. One of them is the jet stream, that fast-flowing ribbon of air at higher altitudes of the atmosphere. It is typically found in the mid-latitudes, but it can be found at relatively high latitudes, especially in the warm season. At times it will split into a least two distinct streams, with one of the two usually at lower latitudes and tapping into tropical moisture.
In recent weeks, the main jet stream has been zipping along. Oh, there have been some very distinct and deep bends in the jet stream that have allowed some cold air to come flowing south and east across the country, at times deep into Texas and across the Gulf Coast into Florida. In each instance, though, there has been absolutely nothing to hold the upper-level troughs in place, and they've easily advanced downstream off the East coast and out over the Atlantic. Look at the NAO, which is, in many ways, a direct measure of whether or not there is blocking downstream:
Most systems of late, be they storms with attendant cold fronts or just plain cold fronts, have been ushered off the East coast with relative ease. Look back at the one that roared off the East coast yesterday. A week ago, some of the computer guidance had it as a Tuesday event. In reality, it went from west of Chicago at 18z Sunday to off the mid-Atlantic coast in less than 24 hours!
Why? A progressive flow, that's why. In other words, the jet stream is just whizzing along, and there's nothing to stop it. If you look back at the modeling, the one model that has been performing rather poorly, to be kind, is the European model. The GFS has been far superior, and other models, such as the Canadian, have also done fairly well. Given their relative performance, I find it hard to believe there would be much trust place in the European model, at least through last evening. Perhaps some improvements in the model being unveiled today will mean a better track record in the coming days!
Let's look at the next front that will sweep across the country late this week into the start of the weekend, and I'll do so by first examining the upper-level flow pattern first. Here's the 12z Nov. 19 NAM 500mb forecast for Thursday evening:
There are several things to note in this image. First of all, we find a strong upper-level low just southeast of Newfoundland. The upper-level disturbance bringing all of the clouds and the coldest of the air across the Northeast today will carve out an area of low pressure in the Northwest Atlantic tomorrow and in the process become detached from the main jet stream flow. It will also pull some moisture in from the remains of Melissa churning through the North Atlantic. That said, this upper-level low will be underneath the main flow of the jet stream late this week, and it is a fast-moving flow at that.
You will also note a subtropical ridge over the northwest Caribbean, a feature that leads to upper-level ridging over the Southeast later this week. That, in turn, sets up a broad west-southwest flow aloft from the southern Rockies toward the Northeast.
That west-southwest flow comes around a feature dropping into California, an upper-level low that is also becoming detached from the main jet stream flow in the West. That's a system that for a time will get buried over the Southwest late this week and this weekend and could pose a problem for pre-holiday travelers next week as it eventually opens up and comes out across the country.
Finally, we can pick out a couple of distinct disturbances in the main jet stream flow. One of them is moving across Saskatchewan and Manitoba and the Dakotas. Another is coming out of the Yukon Territory toward northern sections of British Columbia and Alberta.
These latter two features are the ones of most importance. The fact there are two of them, one hot on the heels of the other, will play a critical role in the ongoing forecast. The 12z NAM appears to be catching on to the northern branch and its 'heavy hand.' Here's its latest surface map projection for Friday morning:
Low pressure would appear to be over southern Ontario, east of Lake Huron at this point, with the front stretching southwestward from there to north-central Texas. And the second feature is causing low pressure to moving across Saskatchewan, with another chunk of arctic air behind it coming out of the Northwest Territories.
Now it should be noted that the 12z GFS does not appear to have this trailing wave in its 12z forecast update. However, it drives this same leading cold front to the New England and mid-Atlantic coasts by Saturday morning!
I have not seen the latest European model forecast, but if the past is any indicator of the future, then it will probably still have the front coming through the East later Saturday, which just seems out of touch given the fast progressive flow across the country these days and out into the North Atlantic.
One thing appears to be certain - regardless of the exact timing, the air behind the front will be the coldest we've seen yet so far this fall, and will generate a lot of lake-effect and upslope snows from around the Great Lakes to the spine of the Appalachians this weekend, and it will allow ski operators at many resorts from the Great Lakes to the Appalachians and into the Northeast time to make some snow before Thanksgiving.
Much of the country from the Plains to the East Coast will have several more days of very cool weather. Meanwhile, Odile is threatening Arizona and New Mexico with flooding rains later this week.
A strong storm in Quebec combined with a very large high near Montana will result in sweeping changes across the Ohio Valley into the mid-Atlantic and Northeast in the next 24 hours.
The latest surge of heat and humidity into the Midwest will spread to the East tomorrow but be trimmed back to the South later this weekend by the passage of a cold front.
Summer's heat and humidity will still have a period of time to sizzle the rest of the week into the start of the weekend, but a fall-like air mass will invade the northern Plains and Midwest this weekend.
Even though fall is but three weeks away, summer heat and humidity will have a full run this week from the Plains to the East Coast.
The steamy weather from the Mississippi Valley to the East Coast today will be trimmed only slightly behind a cold front tomorrow into Wednesday. It will heat up again late this week.