A potent mix of thermodynamic and kinetic forces will become colocated across the lower Ohio and Tennessee Valleys on Saturday to create conditions favorable for destructive, powerful supercells and potentially violent tornadoes.

A very powerful jet stream will work across this very unstable airmass with winds at the 200mb level in excess of 150 mph, as depicted by the NWP guidance. It is very rare to have winds of this strength/a jet stream/ directly across the convective layer and will add significantly deep layer shear aiding in ageostrophic imbalances helping to promote mesoscale ascent.  Typically, the mid latitude jet is well removed of the warm/buoyant air mass and rarely directly overlaps.  This could be one of the more unique events and we need to watch this very closely given the kinetic forcing in the upper levels of the troposphere in this upcoming event.

Guidance at this time suggests an early round of showers and non-severe thunderstorms will move through the Mid-South during the early morning hours, followed by partial mid-day clearing allowing for solar insolation and destabilization in the Ohio, Tennessee-Valleys. How these lead convective elements ultimately play out will likely define mesoscale outflow boundaries that will determine where supercells and tornadoes develop later in the day.

This midday-afternoon destabilization combined with a rapidly strengthening wind field at all levels will increase the severe weather risk as a strong upper level (300-200 mb) jet streak punches into the region during the afternoon with a jet wind maximum of nearly 150+ mph closely collocated with the storms

Convection with supercells and the risk for strong to violent tornadoes will develop in association with this powerful  jet/shortwave from the Ohio-Valley southward into Mississippi/Alabama and spread eastward.

The area at highest risk of tornadoes appears to me from Southern Illinois, Southern Indiana,  across Kentucky, Tennessee southward into Central Mississippi and Alabama.  The strongest tornadoes would likely occur in Southern Illinois, Southern Indiana, Kentucky and Tennessee closest to the best 0-1km shear maximum, 0-1km SRH and lowest LCL’s. The tornado risk may spread into western Georgia.

Forecasters and those living in these regions should closely monitor developments and SPC outlooks over the next few days.

On to the maps….

Update:  2/4/2014 03:00Z

Models have trended originally from a Miller B system to a strong Miller A system yesterday that brought heavy winter precipitation up the east coast from NC to Maine.

Today the models have largely backed away from any inland development and any phasing would take place off-shore and on Sunday.  The pattern I outlined below, I wrote about 3 days ago,  is not expected to materialize as the models had alluded it might.  The confluence area across the Eastern North America does not look like it will be very strong and the short wave coming into the U.S. is likely to be somewhat weaker than earlier projected and will not dig into the Southwestern U.S.

The end result is a much different scenario, with little or no cold air damming and possibly no precipitation.

2/1/14 23:30Z

The ECMWF and GFS deterministic & ensembles have been supportive of the synoptic setup that is favorable for a wintery, cold air damming event across NC/VA the end of next week with a mixture of precipitation, though predominantly freezing rain or sleet.

Having forecasted these types of events for many years as an operational meteorologist for Duke Energy, I had to often look for synoptic clues many days out rather than relying on explicit model output of things like 2 meter temperatures or precipitation type values that would indicate a significant icing event.

Models generally do not handle very cold air at low levels,  particularly well. Especially the global models as they have a smaller number of vertical levels near the boundary layer where a lot of the shallow very cold air exist with arctic air masses.  The global ensemble models have even less vertical levels than the deterministic models making them often  poorer choices for depicting things such as cold air damming events.

So while we are some 6-7 days from this potential event, there has been a consistent theme among the global models that makes me believe that a growing risk exist of winter weather and potentially a significant amount of freezing rain Friday into Saturday.

Historically, one of the primary mechanisms we have observed in NC that accompany significant icing events, is an upper level confluent flow across the Northeast or Eastern Canada that is maintained though much of an event.  This causes surfaces pressures to remain high and re-enforces cold air at low-levels down the Piedmont of Virginia & North Carolina.   Future model runs will need to be monitored to see if this trend is maintained.