Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 235 PM EDT Wednesday Aug 24 2016 Synopsis... high pressure will persist over the region for the next few days, allowing temperatures to climb back above normal. Another weak surface front will approach the western Carolinas from the northwest toward the end of the work week, with high pressure building back to our north over the weekend. Rain chances will remain below normal through the beginning of next week. Another front will approach the area by the middle of the next work week. && Near term /through Thursday/... as of 215 pm: a broken stratocu deck lingers over the wrn two-thirds of the cwfa, evidently being maintained by weak westerly upglide over the high pressure over the eastern Seaboard. The deck is becoming more cellular on visible satellite, but in some places is being replaced with diurnal cumulus as fast as it is dissolving. Isolated showers are also developing within the deck as destabilization occurs. Models have backed off their depictions of rain showers/thunderstorms and rain coverage over the high terrain, given the delayed warming, but there still is potential for widely scattered activity to develop there. The pattern remains largely the same tonight, with low level winds veering in response to a trough moving thru the northeastern conus, but still generating weak isentropic lift and bearing a fair amount of moisture. Patchy midlevel clouds thus are expected to persist, albeit not as uniform as what we experienced this morning. Guidance consensus develops fog in the mountain valleys and portions of the foothills before dawn, which in light of limited mixing today is plausible as long as the cloud decks remain broken. Min temps should be a tad above climo. For tomorrow, I was a little reluctant to really clear skies out quickly seeing how it developed this morning, seeing as the pattern will have changed so little. One difference is that winds will be more downslope, which could help speed erosion. Once again the mountains are expected to destabilize sufficiently to expect scattered convection, with the Piedmont still seeing the limiting effect of subsidence within the ridge airmass. So pops are limited to the high terrain. Max temps are expected to be near climo, a bit warmer than today with the expectation of less cloudiness. && Short term /Thursday night through Saturday night/... as of 215 PM EDT Wednesday...the late part of the week still looks relatively quiet as a mid/upper anticyclone/ridge moves overhead on Friday. This should keep some warmer air aloft and convective inhibition that will help to put a damper on the amount of deep convection. Thus, have limited the precip probability to the low end of the chance range and only over the higher terrain. Temps will climb back above normal. Starting on Saturday, uncertainty ramps up with regard to the effect of the anticyclone center moving off to the east and the position of a weak front drifting down from the north. The operational GFS is more responsive than the other models, perhaps because of a stronger southeast flow at low levels which brings in Atlantic moisture, while the operational NAM might be too dry because it drives the surface boundary to our S and actually has a wedge-like configuration by late afternoon. The official forecast still favors the higher terrain with a slightly below climo chance of precip, but allows for isolated storms to move out across the I-40 corridor in the afternoon and the rest of the Piedmont/upstate in the evening. Not sure yet how this will trend, but still expect precip chances to be diurnal in nature. Temps will depend on the ability of the shallow surface boundary to slip southward. The chance for severe storms looks low. && Long term /Sunday through Wednesday/... as of 230 PM EDT Wednesday...the extended forecast begins 12z Sunday with a large upper ridge over the southeast beginning to flatten. Model guidance diverges almost immediately with the handling of this ridge and subsequent features, so the forecast consists of a multi- model blend that is weighted towards the ECMWF, which has displayed better run-to-run consistency than the rest of the global model suite over the past few days. The upper ridge does maintain a warming and drying influence over the southern Appalachians into early next week, though it begins to decay somewhat Sunday and Monday. Pops were therefore kept below climo through early next week, and maximum temperatures were kept a few degrees above climo. A surface frontal boundary will begin to approach the area next Tuesday, though its timing and the amount of moisture it has to work with are dependent on the track and intensity of a potential tropical system that is being investigated by the National Hurricane Center. Most models now show some development of the system, and though the tracks of the system in the major models are currently quite different, it appears that no direct impact from it will occur in our area through the next seven days. The European model (ecmwf) allows for more moisture return into the southeast, so pops were elevated to near/just above climo by the middle of next week. Overall, little change was made to the previous medium range forecast, especially considering the large amount of uncertainty in global model solutions at the current time. && Aviation /19z Wednesday through Monday/... at kclt and elsewhere: offshore Atlantic high pressure will generally maintain southerly surface flow over the area thru Thursday. However, moist west-southwest flow atop this airmass has brought a persistent area of stratocumulus to the southern Appalachians and vicinity. These clouds are showing signs of breaking up and lifting at 18z, but will persist in some places thru evening. Sufficient destabilization is expected over the mtns to prompt thunderstorms in the vicinity inclusion at kavl. Winds aloft will veer and bring in additional moisture overnight. Low VFR cigs are likely to come and go throughout the period. Models depict only a small amount of restrictive clouds overnight, generally favoring the production of fog instead. With so much higher cloud cover present, and uncertainty about how much mixing/drying will occur in the low levels this aftn, fog mention is included only in tempo at kavl/khky; at kand nearby soaking rains earlier today will play a factor. Outlook: a deep ridge will remain in place over the southeast thru the weekend, keeping afternoon rain showers/thunderstorms and rain chances below normal each day, and generally confined to the mountains. However, sfc moisture will gradually return, and morning vsby restrictions will become increasingly likely during this time. Confidence table... 18-24z 00-06z 06-12z 12-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 97% high 80% high 91% kavl high 98% high 94% high 84% high 84% khky high 100% high 100% high 100% high 91% kgmu high 100% high 97% Med 75% high 91% kand high 100% high 100% high 86% high 88% 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. && $$ Synopsis...level near term...Wimberley short term...PM long term...level aviation...Wimberley

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