Severe Weather Outlook for Friday and Friday Night March 2, 2012

A large trough is currently beginning to eject out of the western US. As an associated jet streak continues to round the base of this trough and as phasing occurs with a polar branch closed low over the northern Plains, the trough will take on a negative tilt over the central US. This will allow for a surface low pressure to intensify quickly as it tracks northeastward towards the lower Great Lakes on Friday.

There is relatively good model agreement on a sub-990MB surface low tracking into southern-central lower Michigan by early Friday evening, with a warm front extending eastward across Lake Erie and western PA by sundown.

In the interim, elevated convection is expected to develop and expand by the start of the period as a low level jet intensifies in the warm sector of the developing low pressure early Friday morning. As this jet noses into the TN and eventually Ohio Valleys, these elevated storms will likely pose a marginal large hail threat early Friday morning. Note the strengthening low level jet depicted by the GFS for 12z Friday:

The strong low level jet combined with the aforementioned deepening low pressure system tracking towards the lower lakes will pull a warm front northward. Much of this elevated convection should remain north of the warm front and will continue to pose a marginal hail threat through much of the day into the upper OV and perhaps the lower Great Lakes. Elevated convection will also develop in the warm advection over the Deep South, which is expected to track east-northeastward through the day. Some of this activity may become surface based, mainly well south of the Ohio River.

As the low tracks farther northeast, the strong low level wind fields will advect 55 degree plus dewpoints north into central and perhaps northern IN and OH with dewpoints of up to 65 degrees potentially extending into the upper TN Valley. A punch of dry and warm mid level air (EML) will likely result in sunshine in the warm sector during peak heating, especially south of the Ohio River, resulting in surface temperatures quickly rising well into the 70’s by mid afternoon south of the River…this warm and moist airmass is expected to yield MLCAPEs of 1000-2000 J/KG from southern IL/IN/SW OH south through the TN Valley into the northern Gulf Coast states with values of up to 500 J/KG reaching northern IN/OH.

By mid afternoon a 100+kt 500mb jet streak will begin nosing over the warm sector from TN points north…and an outstanding dual upper level jet structure will exist in this same area with 120-140kt jet streaks providing outstanding large scale lift:

This large scale lift combined with a sharp cold front advancing eastward will result in new convection quickly developing in the warm sector by mid afternoon that will begin racing ENE-ward on very fast mid level winds. Elevated convection over the TN Valley and northern Gulf Coast States may also become rooted to the surface by this time.

Given backed surface winds in the warm sector due to the deepening sfc low/especially over the Ohio Valley/lower Lakes resulting in sfc-500mb wind crossovers approaching 90 degrees of change and bulk shear in excess of 60 kts over the majority of the warm sector and approaching 100 kts towards and north of the Ohio River and the relatively favorable thermodynamic environment/MLCAPE of 1000-2000 J/KG especially from the Ohio River points south in tandem with the aforementioned large scale lift convection will likely rapidly become severe once it begins to fire/or become surface based.

Given the strong shear with 0-1km helicity values of 200-300 over much of the warm sector rotating updrafts will be likely initially…supporting a tornado and large hail threat along with the threat for damaging strait line winds. Sfc-500mb shear vectors will be out of the W-WSW across a cold front aligned SSW-NNE, which favors discrete storms. As the sun goes down do expect organization into a squall line along/just ahead of the cold front with a widespread damaging wind threat and continued squall line tornado threat given strong low level shear.

Expect severe threat to gradually decrease with time overnight as action presses eastward towards the Appellation Mountains.

As for placement of highest severe threats…believe greatest risk will lie from far southeast IL…far southern IN…and far SW OH points south through much of western/central KY, western/central TN into northern MS/AL. Believe highest risk for severe will be east of the Mississippi as aforementioned EML will likely cap the warm sector into Friday afternoon and favorable portion of 500mb jet streak will not begin to move over the warm sector until approximately 18-21z…at which point front is expected to be east of the Mississippi. Shear will be a bit weaker over the TN Valley/northern Gulf Coast States but moderate instability and what is still favorable shear for supercells combined with a less interrupted moisture source should result in a notable severe threat into northern AL/MS by late afternoon. North of the Ohio River instability will be more limited but shear will be favorable with very large hodographs and the strongest bulk shear. Main concern is convection that will be ongoing over TN/AL/MS through much of Friday morning and early afternoon may create enough subsidence to limit severe potential somewhat. However, given expected MLCAPEs of 500-1000 J/KG into central IN/OH combined with favorable shear believe a conditional severe threat will exist into at least central IN/OH. The farther north one gets the more questionable moisture return/instability will get but given very dynamic kinematic environment believe a marginal risk for severe will exist into northern IN/OH by late afternoon

Will go with a moderate risk for tornadoes across the central and lower Ohio Valley into the TN Valley given good potential for supercells. Given very large hodographs and expected instability in these areas a few strong/long track tornadoes will be possible, however given some questions about capping and updraft intensity given decent EML to try to punch through will not go with high probabilities.

Will also go with a moderate risk for damaging winds in a similar corridor and also extend this risk area into the northern Gulf States as instability and shear will easily support bowing line segments with significant wind damage risk as convection transitions into a more linear mode by late evening.

Severe weather probabilities…probabilities are SPC equivelents and represent the chance of a severe weather event occurring within 25 miles of any given point. For official forecasts…visit spc.noaa.gov and/or weather.gov …

Tornado probabilities…dotted = 10% or greater risk of F2-F5 tornado within 25 miles of any given point

Damaging wind probabilities…dotted = 15% or greater risk of 65+ kt gusts within 25 miles of any given point…

Hail probabilities…

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