Area Forecast Discussion
Issued by NWS Greer, SC

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898 FXUS62 KGSP 270727 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 327 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will cross our area today, then stall across Midlands and central North Carolina tonight through Wednesday. An upper level low will drop south into the Ohio Valley on Wednesday and linger there through the end of the week. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM EDT Tuesday: Mesoanalysis has placed the cold front pretty close to the TN/NC state line, with a band of showers developing along it. Most hi-res guidance/CAMs seem to want to break all of this apart over the mountains and while that may be the case the area of convection in general does seem to be expanding. Made some updates to the near-near term to account for this. As for the bigger picture however, we`ve been advertising this approaching cold front and parent upper low for several days now, so the synoptic overview should be no surprise. The initial movement of the front into the area will stall as a weak shortwave on the southeast edge of the upper low lifts rapidly northeast, but then the main trough axis will dip toward the Deep South later today as the closed low pushes toward the Ohio Valley overnight. With the brief slowdown/stalling of the front, this will allow the low-level southerly flow to bring one more surge of warm/moist air to the area, with another day of above-normal temperatures for the area. CAPE values should increase to upwards of 1500 J/KG again today across the foothills and piedmont and as the upper low approaches, deep-layer shear is no longer negligible, albeit still very low. An isolated strong storm cannot be ruled out. Luckily with the additional synoptic forcing the storms will be moving fairly quickly, but if storms manage to develop over areas that received heavy rainfall yesterday (read: Charlotte) localized flash flooding could still be an issue. PW values at AFD time remain anomalously high, pretty close to the daily max values and at least at the 90th percentile, so will keep that heavy rain/flash flooding wording in the HWO. The front will slowly push off the mountains tonight but in the wake of the smaller shortwave impulse and before the main kicking mechanism comes into play, it will have a hard time really getting through the area. The surface front and associated wind shift itself are progged to push through by the end of the period, but the subsequent CAA will not be quite as strong just yet. With this, have ended up bumping up tonight`s low temperatures a few degrees (same with dewpoints) to address this. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY/... As of 300 AM EDT Tuesday...A large upper low will drop south into the southern Great Lakes on Wednesday, then continue to wobble around the Ohio Valley thru at least Thursday night. The 00z suite of operational models shows good agreement on the center`s track, and even the major vort lobes rotating around the vortex. Significant height falls ahead of the low should keep backed deep-layer SWLY flow atop the CWFA. The models continue to trend slower with bringing the dry slot in from the west. So the eastern third of the CWFA now looks to support at least isolated to scattered convection Wednesday afternoon. The NAM soundings, in particular, show over 2000 J/kg with around 40 kts of 0-6km bulk shear. So convection may become organized, but mainly push east of the area by late afternoon. The new Day 2 SPC Outlook has a marginal risk along and east of I-77, with a decent chance that some of that area may be upgraded to a Slight later. In any case, PoPs were bumped up in the east to chance. Temps were also bumped up within the warm advection SWLY flow. Wednesday night and Thursday...the first major spoke of vorticity will pivot thru the southern Appalachians, and provide westerly upslope showers along the TN border overnight, then possibly across much of western NC on Thursday with daytime heating. The steep mid level lapse rates under the low could result in enough instability for thunder. Will continue the chance PoPs north to less than slight chance in most of the Upstate and northeast GA. Temps are expected to cool off to below normal Thursday, but models have trended warmer, as flow remains out of the SW ahead of the low. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 315 AM EDT Tuesday...The models have converged on the handling of the upper low for late in the week thru this weekend. The low is expected to slowly drift north from KY into OH Friday and Saturday, then turn northeast into the eastern Great Lakes Sunday and Monday. This track should bring a dry slot into the CWFA and basically keep it there. So other than some initial westerly flow upslope showers along the TN line, the medium range forecast is dry. Temps are expected to be coolest Thursday night into Friday, about a category or two below normal. Then temps rebound to slightly above normal for the weekend through Monday. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... At KCLT: Convection has almost completely moved out of the area with just some lingering -SHRA. This will continue to wane over the next hour, after that the concern will be development of restrictions. Expect at least MVFR to develop shortly, lowering to IFR and possibly LIFR. Should see slow improvement after sunrise, but with the approach of the front expect to see another round of TSRA this afternoon. Best chances look to be between 22-00z tonight and ended up with short prevailing TSRA but will continue to reevaluate that with new data. Winds remain generally S but with the front approaching may see brief backing ahead of it, and then just beyond the end of the period winds will begin shifting W. Elsewhere: Convection has moved out of the areas, though another line is developing along the mountains with the front that may impact KAVL overnight. Bigger concern will be widespread IFR conditions, with confidence not quite as high across the Upstate TAFs as for KAVL/KHKY. Improvement after sunrise and with the front moving into the area should see another round of SHRA/TSRA, and handled this with a combination of PROB30/VCTS/prevailing TSRA. Mainly S winds slowly veering SW, and for KAVL/KHKY should see winds shift NW toward the end of the period. Outlook: Cold front will slowly cross the region today. Dry high pressure builds in slowly behind the front on Wednesday, which should bring VFR conditions in most places through the end of the week. Confidence Table... 07-13Z 13-19Z 19-01Z 01-06Z KCLT Med 70% High 84% High 100% High 95% KGSP Low 45% High 96% High 100% High 98% KAVL Med 64% High 80% High 96% Med 63% KHKY Low 57% Med 73% High 100% High 98% KGMU Low 55% High 91% High 100% High 100% KAND High 85% High 96% 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 && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...ARK NEAR TERM...TDP SHORT TERM...ARK LONG TERM...ARK AVIATION...TDP

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