Scientific Forecaster Discussion

000 fxus62 kgsp 190257 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 1057 PM EDT Thu Apr 18 2019 Synopsis... an approaching strong cold front from the west will bring the return of showers and thunderstorms to the area beginning later today, with strong to severe storms possible on Friday. Cooler temperatures arrive behind the front for the weekend, with lingering showers across the mountains on Saturday. Temperatures will rise above normal again next week. && Near term /through Friday/... as of 1045 PM...the latest radar mosaic now shows the qlcs entering northwest Georgia. The northern half of the line has weakened quite a bit, and probably will weaken further, as it runs out ahead of the front and better instability. Meanwhile, guidance is way overdone with pre- frontal upslope-driven precip that was expected along the S/se- facing Escarpment. Have trimmed the pop thru the next few hours to mainly slgt chc, until the convective line enters around 5z. Otherwise, forecast looks on track. Looking at the synoptic setup, an area of low pressure centered near Lake Huron is carrying a cold front through the Ohio Valley and southwest to Texas attm. This is associated with a highly amplified pattern with a virtually full-latitude trough over the plains, and a deep ridge just off the East Coast. Moist southerly flow in our area, along with seasonably warm/moist sfc conditions, maintains a shallow cu field across most of the County warning forecast area. Pops will develop first along the Blue Ridge Escarpment, but expand to more of the area as the synoptic setup continues to develop tonight into early Friday. Overall, this setup continues to point to a significant high-shear/Low-Cape qlcs threat for a large chunk of the southeast. The shortwave will continue to dig and sharpen up across the Mississippi Valley overnight, with a closed upper low developing near Memphis by morning, with the whole trough shifting east thereafter. The resultant midlevel flow is strongly meridional as the trough crosses the region, which is important per hslc research. Surface-based instability is expected to be effectively nil for most of the night, but is shown to develop very quickly in the morning once diurnal heating begins. The tight gradient ahead of the trough will be associated with a 50+ low level jet in the early morning, and 40-50 kt 850mb winds persisting through the day, along with an amplifying jet streak and well-defined region of upper divergence traversing the area. 0-1km bulk shear will easily reach 40 kt, a value also representative of severe hslc events. Once the instability starts to develop, the best overlap between it, the llj, and dynamic forcing associated with the trough appears to be in the western cwfa; activity will become increasingly likely to form into a qlcs with time. Cam consensus shows this happening around midday in the central cwfa, propagating east of the County warning forecast area by late afternoon. Href output depicts a number of updraft helicity swaths during peak heating mainly along/east of a line from Hickory NC to Union SC. The threat mainly will be damaging wind but a couple of 1 to 2 inch hail events could occur in the eastern County warning forecast area if we do realize the 1000+ j/kg as depicted on the href mean and operational NAM. A few short-tracked tornadoes certainly will be possible at times through the day, but especially in the eastern zones. That said, the NAM/WRF family of guidance (which of course includes the href members) are a bit scarier than the GFS. The latter shows the 0-1km shear tapering off a bit following the diurnal loss of the llj, plus it is not as unstable. We have evaluated hydrologic threats for this event, and given the anticipated exceptionally high precipitable water values and robust moisture flux, at least isolated flooding will be a concern in the near term period. Perhaps the most likely concern (as usual) would be the south-facing Escarpment, with terrain anchoring and enhancement most likely to produce excessive runoff. However, with the event being relatively short duration, rises on the rivers of the French Broad basin still are not expected to push them to flood stage. For the remainder of the area, the forcing and moisture will produce very high rainfall rates, but storm motion is likely to be quick enough to keep the threat localized at best. Also as usual, the Charlotte Metro area may be the exception to this rule. All this said, the areas most likely to flood are small and isolated enough that a Flood Watch or Flash Flood Watch is not really appropriate. && Short term /Friday night through Sunday/... as of 230 PM EDT thursday: the numerical models feature excellent agreement on a 542 dm (and deepening) closed 500 mb low center approaching the southern Appalachians Friday evening. This should keep numerous showers going across the mountains as lapse rates steepen, with more scattered showers across the foothills and Piedmont. Snow levels will gradually descend to uncover the higher peaks around the smokies through the late evening and overnight hours. The incresingly stacked system will bring the base of the associated trough axis across the forecast area through Saturday. At least scattered showers will continue under the trough, especially over the mountains, where snow levels will continue to fall. Some accumulating snow is likely above 4000 feet, with a couple of inches possible above 5000 feet. There is very little pure orthogonal northwesterly upslope flow of moisture into The Spine of the mountains until late Saturday into Saturday night, and by then the low level moisture is becoming quickly shallower. Thus, do not anticipate any winter hazard products with the package this afternoon, but the high elevation potential in and around the smokies will bear close watching. As the low pressure system lifts slowly east through the mid- Atlantic region the latter half of the weekend, the cold advection northwest flow gradient will pick up a bit. The resulting breezy to windy conditions will help inhibit frost formation across the active zones (which is everywhere but the northern NC mountains.) Anticipate temperatures warming back into the 70s in many areas east of the mountains as insolation improves and downslope warming continues. && Long term /Sunday night through Thursday/... as of 245 PM EDT thursday: upper low over the mid-Atlantic will continue to lift north, with high pressure building in and increasing thicknesses. Temperatures will increase through the extended, 5 to maybe even 10 degrees above seasonal. A shortwave spinning around the upper low may bring some showery activity or even possible thunderstorms Tuesday afternoon, GFS a little stronger with this idea than European model (ecmwf). Next system gearing up over The Rockies will push off into the plains sometime midweek, but there are some very significant timing differences between the global models, so frankly opted to keep a fairly generic blend through the end of the period and into the new day 7. As a note to readers, depending on how near-term activity shapes up tonight, the extended forecast may not be updated until Friday afternoon. && Aviation /03z Friday through Tuesday/... at kclt and elsewhere: VFR conditions should continue thru the evening. Low-level moisture will increase within a developing southerly jet overnight, producing shallow showers and MVFR to IFR cigs. This will be ahead of a line of deep convection approaching from the west. The low-level jet will also be responsible for low level wind shear across the mountains, and possibly across the Piedmont sites as well in the roughly 07-16z time frame. There is still some uncertainty on the speed of the squall line/qlcs starting early Friday morning thru Friday aftn. Prob30 and prevailing thunderstorms and rain are based on best guess for now. Some guidance is a little faster and some (particularly the nam) is slower. Expect potentially strong wind gusts and torrential downpours with the storms. Precip should taper off behind the line late aftn, but some lingering moisture will keep a small chance of showers and MVFR to low VFR clouds around into Friday evening. Winds will be out of the S/south-southeast and ahead of the squall line and associated cold front, becoming gusty overnight, then shift to SW with continued gusts in its wake Friday evening. Outlook: improving conditions expected late Friday or early Saturday. Spotty restrictions cannot be ruled out over the mtns as an upper low passes by on Saturday. Confidence table... 03-09z 09-15z 15-21z 21-00z kclt high 100% high 100% Med 79% high 87% kgsp high 100% high 98% high 85% Med 77% kavl high 87% Med 73% Med 75% high 82% khky high 93% high 89% low 50% high 82% kgmu high 100% Med 79% Med 71% high 80% kand high 100% high 85% Med 79% Med 75% The percentage reflects the number of guidance members agreeing with the scheduled taf issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: Www.Weather.Gov/gsp/aviation && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$

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