A couple of things I will be watching this evening
- How the cold air interacts with the developing cyclone
The million dollar question (literally) is going to be how the cold air interacts with the developing cyclone. If the cold air plunges more south than southeast, then I would expect the ejecting surface low pressure (and associate low-to-mid-level lows) to move with a more northerly trajectory. If the cold air surges southeast, then the lows will eject with a more eastward component. This has huge implications for who will see the heaviest snows in Oklahoma and southwest Missouri.
The reason for watching how the cold air interacts with the cyclone is the result of how and where the low-level cyclones develop — which on the eastern edge of the “cold-dome”. I strongly believe the surface lows (and associated other lows) will ridge along the edge of the cold dome. Thus, if the eastern extent of the cold air has a more north-south orientation, this is how the surface low will move, placing central Oklahoma in a longer period of heavier precipitation. If the cold air orientation is more southwest to northeast, then expect the heaviest snow to shift east of central Oklahoma, as the surface development will be farther east.
- How the upper-level jet streams interact
Nearly all numerical weather prediction models are forecasting extreme precipitation amounts overnight and through the day tomorrow. It’s a bit perplexing considering the dry ambient environment. This dryness is most likely being overcome by the strong upper-level divergence associated with the ageostrophic response to a couple of a southern and northern jet streak. This strong upper-level divergence results in strong low-level convergence across portions of Oklahoma that will most likely rapidly transport moist air currently in place across eastern Texas, southern Arkansas, and Louisiana. When and where these upper-level jet streams do couple will play a major role in deciding where the heaviest precipitation bands are located.
- How does the precipitation shield evolve overnight
The last several runs of the operational GFS and NAM develop rapidly accumulating snows across eastern Oklahoma with strong low-level frontogenetic forcing. This is in spite of a +0.5C to +1.5C warm nose aloft. This would seem to favor more sleet than snow. However, if lift is strong enough, this might quickly be overcome resulting in 3-4″ per hour snow rates that are currently forecast. Farther west, the 12 and 18 UTC NAM are developing a more classic deformation zone across a large part of central Oklahoma during the late morning and early afternoon hours tomorrow. This deformation zone is more typical of substantial Oklahoma snow falls than the strong, low-level frontogenetic forcing described previously. This large deformation zone is the reason why the NAM has a large area of 6-10″ snow accumulations to the west of the 12-18″+ forecast across eastern Oklahoma.
Also, if a large squall line develops across central Texas and races east too quickly, the squall line might use up a lot of the moisture that is poised to be drawn northward into this cyclone. This would decrease snow totals across portions of Oklahoma, Kansas, and southwest Missouri.
- The evolution of the temperature vertical temperature profile
I hinted at this in a couple of the previous bullets, but how the vertical temperature profile evolves will be crucial in determining snowfall amounts. If a strong warm-conveyor belt does develop overnight, warm air might hang on longer than forecast across portions of Oklahoma. This might lead to an extended period of sleet which would cut down on snowfall totals considerably. However, 1-2″ of sleet, coupled with 2-4″ of snow would be just as bad, if not worse, than a pure 8-12″ of snow.