Millions of Americans across the Deep South will be facing some dangerous weather conditions tomorrow and Saturday. At the same time, the Midwest will be dealing with some snow, which while it won’t be historic, it’ll be disruptive. So, let’s get right to this.
With the PNA going strongly negative, and the AO and NAO being strongly positive, our pattern is not a favorable one for East Coast snowstorms. Rather, it’s more favorable for Midwestern storms and inland cutters, which I think we’re going to see plenty of over the next week or two. A negative PNA is very favorable for a trough over the West Coast with ridging over the East Coast, and indeed, as the 500 millibar height anomaly map from WeatherBELL below (Figure 1) shows, this is the case currently.
There’s no doubt that the pattern is highly amplified as both the trough and ridge are very deep. Temperatures out west this weekend are going to be below average (they will only get colder from here), while the east gets a big blast of warmth with highs in some places exceeding 30°F above average for this time of the year (Figure 2).
This sharp temperature gradient has led to baroclinic instability which will begin to churn some things up in the atmosphere over the next 12-24 hours. A surface low will form along this cold front which will provide some lift in the atmosphere.
Because there’s a ridge in the east, the clockwise wind flow will sweep across the Gulf of Mexico and turn up into the Southeastern United States. As these winds in both the lower and upper levels (Figure 3) of the troposphere blow across and above the water towards the shore, the air will soak up evaporated water due to the warm ocean and air temperatures and move it inland.
As the storms build up and move eastward, the low-level jet will be activated, as depicted by the high-resolution NAM 3km below (Figure 4), which will could have winds anywhere from 55-75 mph. This low-level jet will only add to the transfer of warm, moist air from the Gulf to the states.
With the low-level jet activated, wind speeds will decrease with height leading to speed shear and maybe even directional wind shear, which is a big red flag for tornadic activity.
As the storms pop up along a the cold front, they will be in a long vertical line. These systems are known as “Quasi-Linear Convective Systems” or simply “squall lines.”
The biggest impacted areas tomorrow will be from Dallas east to Jackson, Mississippi and from the Gulf Coast of Texas and Louisiana north to Missouri. The damaging wind and tornado threat is high, but flooding and hail are certainly not out of the question, and are in fact likely to occur as well. As a result, the NOAA SPC has put out a convective outlook with the slight, enhanced, and moderate categories all across the Deep South (Figure 5).
As the storms progress eastward on Saturday, the focus areas will shift to the Southeast corner. Areas from New Orleans northeast to Nashville, and southeast to Jacksonville, Florida are in the big impact zone (Figure 6). So, expect similar, but less severe conditions.
The heaviest rains will occur from the Gulf Coast north to the Great Lakes. The Mid-Atlantic and most of New England will not be significantly impacted. Rain totals could be on the order of 1 to 3 inches for many. Some isolated spots may see even more over the next two days.
Snowfall will also be an impact from our low as it tracks Northeast across the Mississippi, Tennessee, and Ohio Valleys this weekend. As the cold air infiltrates south out of the Canadian icebox, areas from Oklahoma to the Michigan Peninsula will see snowfall. Most areas across the Southern and Central Plains will see 3-5 inches of snow, give or take. Once you get north of the Illinois-Wisconsin border, snowfall amounts will increase significantly ranging from 6 to 12 inches.
If any of y’all are going to be impacted by this, please be safe. God bless!