Arctic Airmass to Bring Taste of Winter
A strengthening ridge will build into Alaska early this week, setting the stage for an Arctic air intrusion into the Lower 48 later this week and next week. Downstream of the Alaskan ridge, a mid-level low/shortwave will slide down Canada's west coast into the Pacific Northwest mid-week. A subsequent feature, coming in behind the first, will act to ensure that a large-scale trough remains across the western U.S. through this weekend.
The Arctic air has already begun to spill into Montana, as the state, along with parts of Wyoming, Idaho, and much of North Dakota, brace for a multi-day winter storm. In fact, snow has already developed across Montana, and many areas will pick up 1-2 feet of new snow.
The southward and eastward progression of the Arctic front will be rather slow at first. Most areas east of the Rockies will experience above-to-well-above-average temperatures before the front begins to push across the northern and central Plains Thursday-Friday.
Figure 1: Friday, October 27, 2023 – High temperature Forecast
With deep-layer southwesterly flow well-established ahead of the front, areas from the Southern Plains, mid-Mississippi Valley, and even parts of the Midwest will remain under a flooding risk this week, with moderate-to-heavy rain expected.
Figure 2: October 25-30, 2023 – 5-Day Rainfall Forecast
The Arctic air mass will struggle to push past the Mississippi River before this weekend. But as the Alaskan ridge de-amplifies a bit and slides eastward this weekend, the western U.S. trough will also progress eastward. That'll 1) allow conditions to dry out in the central U.S. and 2) the stubborn Southeast ridge to begin breaking down.
A couple northern stream features will help reinforce a trough over the eastern U.S. early next week, finally allowing the colder air to make it into the New England, Mid-Atlantic, Ohio Valley, Tennessee Valley, and Mid-South just in time for Halloween.
Figure 3: Monday, October 30, 2023 – High temperature Forecast
The Southeast will remain the only holdouts, but Alabama, Georgia, the Carolinas, and northern Florida will finally have a strong cold front push through either on Halloween or the day after. So, it's a bit of a toss-up as to whether those areas have a toasty Halloween or not. But before then, some days will feel more like summer or early spring than what you'd expect in mid-autumn. Also, don't expect any beneficial rainfall until that ridge breaks down.