Scientific Forecaster Discussion

000 fxus62 kgsp 212347 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 747 PM EDT Thu Mar 21 2019 Synopsis... cool and dry high pressure moves over our region from the Great Lakes on Friday and weakens as it moves over the Atlantic to start the week. Low pressure is forecast to track across our area early in the week then dry high pressure returns for Wednesday. && Near term /through Friday/... as of 740 pm: the second lobe of a merged shortwave across the mid-Atlantic region will cross the area this evening. The precip now ongoing over the eastern County warning forecast area is expected to depart to the east with the leading lobe, but as lapse rates and dynamic forcing increase under the second, showers may return to at least the mountains. Short-term guidance is still not especially keen on much development east of there, and pops are low as a result, but this is a trend to watch. Lightning strikes and showers of small hail or graupel could occur with any of the showers this evening, due to low wet-bulb zero levels, though at least the lightning may diminish with the loss of diurnal heating. Occasional mping reports and social media posts have revealed such, both upstream and locally. Dewpoints remain relatively low, so there is potential for rapid evaporative cooling with the onset of any heavier precip. Snow showers could begin to accumulate in the mtns where temps are near freezing already. Gusty winds are also evident invof the showers, again resulting from the evaporative cooling. Following the passage of the shortwaves, an associated cold front will push through the area. With precipitation quickly tapering off behind it, latest guidance continues to depict wrap-around moisture to linger along the Tennessee border through the early overnight hours. Thus with the infiltration of colder air, expect a mix of rain and snow showers, tapering off as snow showers early Friday morning. Elsewhere, expect conditions to remain dry. Will note, this system continues to lack abundant moisture, thus have kept with the trend of little in the way of quantitative precipitation forecast across northeast GA, the upstate, and northwest Piedmont. As for the mountains, expect any snow accumulations from the anticipated northwest flow snow scenario to remain confined to the higher elevations above 5000 feet, with 1 to 2 inches possible including the convectively enhanced amounts early this evening. For the lower elevations and into the valleys, little to no accumulation is expected at this time, as suggested by latest snow levels. Low temperatures tonight will drop into the upper 30s across the area, with mid to upper 20s to low to mid 30s across the mountains. Gusty northwest gradient winds are expected to pick up late this afternoon and into tonight behind the cold front, carrying through the rest of the near term forecast period. Overall, expect that sustained winds and gusts will fall just short of Wind Advisory criteria across the mountains. As for areas elsewhere, sustained winds will remain well below criteria, but still gusty. Thus, have kept with wording of gusty winds for the mountains in the current Special Weather Statement which highlights the potential for snow accumulations through the overnight hours for the higher elevations (below Winter Weather Advisory criteria at this time), as well as the winds. Into Friday, expect a reinforcing dry cold front to push through the area during the morning hours, with sfc high pressure building in behind. With nearly clear skies/plenty of insolation, expect high temperatures to fall just shy of normal. && Short term /Friday night through Sunday/... as of 235 PM EDT thursday: a reinforcing eastern trough will reinvigorate the northwest flow gradient over the southern Appalachians to keep windy conditions going well into Friday night. Some shallow low-level moisture could linger in the northwest upslope flow prone areas of the northern mountains, but probably not enough for a snow shower mention. There could, however, be some continued mountain wave cloudiness along and east of the Blue Ridge through daybreak Saturday. Otherwise, the pressure gradient will relax through the day on Saturday. Dry surface high pressure will build over the region Saturday and gradually migrate off the eastern Seaboard through Sunday. Below climatology temperatures on Saturday will rebound to near normal on Sunday. && Long term /Sunday night through Thursday/... as of 245 PM EDT thursday: there is now better model agreement on upstream height falls from the Midwest to the Southern Plains on Sunday night. The resulting positive tilt trough axis will move over the confluence of the Ohio and MS river valleys on Monday and likely cross the southern Appalachians Monday night. Meanwhile, forcing ahead of this feature begins to ramp up late Sunday night as left exit region divergence of the southern branch of the upper jet approaches the base of the southern Appalachians, coincident with the steady return of moisture from the southwest. The trough axis will push a cold front through the region from the northwest during the early part of the week, with better consensus on a narrower window of higher precipitation chances. Peak pops are expected mainly Monday afternoon and evening, and positive surface-based instability may support isolated to scattered thunder in the southern Piedmont ahead of the fropa late Monday. The cold fropa will then promote drying through Tuesday. Broad upper ridging will return from the west Wednesday through Thursday, with dry high pressure in place along the eastern Seaboard. Cool, Post fropa temperatures will slowly rebound toward climo by Thursday. && Aviation /00z Friday through Tuesday/... at kclt and elsewhere: a double-lobed passing shortwave will bring a period of rain showers to the area in the first few hrs of the taf period. Only kclt and kavl are expected to see operational impacts from these, and then only briefly (hence tempo). Some redevelopment could occur as the second lobe moves thru the area later this evening, which means khky is not totally out of the Woods, but confidence is low enough to omit from taf for now. Winds will shift to northwest behind the shortwaves as their associated cold front moves thru. MVFR level clouds will persist near kavl as a result of upsloping overnight, but a cig looks unlikely. Gusts will be the impactful wx for the remainder of the period. Cold temps aloft but relatively warm sfc conditions will permit deep mixing within a fairly strong pressure gradient, so a few gusts of 25-30 kt are likely across the area, especially at kavl. Outlook: with gusty winds tapering off across the mountains into Friday night, expect VFR to prevail into the weekend. The next weather system will affect the area by Monday, bringing some precip and chances for restrictions at that time. Confidence table... 23-05z 05-11z 11-17z 17-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 100% high 100% khky high 100% high 100% high 100% high 100% kgmu high 100% high 100% high 100% high 100% kand high 100% high 100% high 100% high 100% 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 && Fire weather... dry and breezy to windy conditions will redevelop on Friday, which could be a problem if precipitation late today isn't wetting enough to increase fuel moisture values. Saturday will be much less windy, but low relative humidity values develop once again. Expect afternoon relative humidity values around 25 to 30 percent in most areas east of the mountain on Friday and Saturday, and a touch higher on Sunday. && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...deo

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