Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 538 PM EDT Tuesday may 31 2016 Synopsis... the remnants of tropical system Bonnie will slowly lift northeast along the Carolina coast over the next couple of days. Moisture will increase ahead of a weak cold front approaching from the northwest on Friday. The front will stall out over the area over the weekend, with a stronger cold front moving through the region early Monday. && Near term /through Wednesday/... as of 530 pm: tclt and kgsp radar continued to indicate sct to numerous storms across the County Warning Area. This activity should continue well into the evening hours, developing over areas that have not been worked over so far today. Pulse storms may produce brief severe hail through the remainder of the daylight hours...then svr threat should decrease. I will update the forecast to increase pops across the forecast area. The sky grids will be populated with satellite data. As of 220 pm: deep convection has broken out over portions of the NC mountains, with an appreciable cu field covering much of the Piedmont. An area of congested cumulus just north and NE of Spartanburg has now also produced some heavy showers. Taking the County warning forecast area as a whole, our various convection allowing models are not handling the coverage very well, mostly because they have been overdone with coverage all day. Based on the development so far, and what happened yesterday under similar circumstances, the current activity is likely to expand mainly southward. Isolated cells however are expected over the eastern Piedmont zones and Savannah River valley before the end of the day. The storms so far have produced just heavy rain, and while profiles don't support a particularly great damaging wind threat, the weak shear implies storms will remain in pulse Mode and an isolated large hail event or downburst is plausible. Lapse rates are expected to remain unchanged tonight, with a similarly weakly forced environment as we experienced Mon night into early Tue. The low level flow is expected to veer, which could provide some slight upslope forcing and showers to linger into the early morning. I have handled this trending pops down more slowly than they would on a typical diurnal curve. Despite the easterly flow, low stratus looks unlikely (particularly given that it was overforecast by guidance for today). Min temps will be a few degrees above climo on account of partial cloudiness from convective debris, and humid conditions. Overall the pattern will have changed little by Wed aftn. Another round of diurnal thunderstorms is expected, mainly forming over the mountains and moving into the foothills, but with isolated to scattered cells developing over the Piedmont as well. Weak wind profiles still point to pulse storms and associated threats. Max temps will be a degree or two above climo. && Short term /Wednesday night through Friday/... as of 145 PM tuesday: flat ridging builds over the southeastern US through the period. Weak low level easterly flow remains in place across the area Thursday as the remnants of Bonnie slowly move north along the NC coast. With warm temps and copious low level moisture in place, moderate instability will develop once again. Expect numerous convective coverage to develop across the mountains with high end scattered coverage elsewhere. Low level flow becomes southwesterly on Friday as a weak cold front moves into the area from the northwest. Low level moisture remains quite high which keeps moderate instability across the area. Expect convective coverage to be similar if not slightly greater than Thursday. Shear remains weak even with the approach of the frontal system. However, expect a few severe storms to develop each day given the instability and convective coverage. Isolated flooding is also possible with slow moving storms both days. Highs rise from a couple of degrees above normal on Thursday to around 5 degrees above normal Friday. Lows will be up to 10 degrees above normal. && Long term /Friday night through Tuesday/... as of 210 PM EDT tuesday: the medium range fcst picks up at 00z on Saturday with relatively flat upper ridging over the southeast and a nearly stationary southern stream upper low over Texas. Over the next 24 to 48 hours, steep upper ridging amplifies over the western Continental U.S. And broad upper troffing digs down over the Great Lakes. The upper trof axis is expected to be east of the fcst area by the end of the medium range on Tuesday as the upper ridge spreads farther east. At the sfc, the long range models are coming into better agreement wrt the evolution of the sfc pattern. The pattern remains fairly stagnate and weakly forced on Sat and most of sun as a stalled cold front lingers over the region keeping deep lyr moisture and healthy amounts of instability in place. A stronger cold front approaches the County warning forecast area on Sunday and is expected to move thru the fcst area sometime late Sunday into early Monday with drier and cooler air overspreading the region in the front's wake. As for the sensible fcst, I kept solid chance to low end likely chances for numerous showers and ts on Saturday and Sunday. Drying is expected during the latter half of Monday with dry conditions for Tuesday. Temps will start out just above climatology and cool on Sunday and Monday. && Aviation /22z Tuesday through Sunday/... at kclt: there is potential for a thunderstorms and rain to impact the field this afternoon and early evening. Some meso model runs depict clustered storms moving thru the kclt area, though based on these models' poor performance so far today, this appears too unlikely to warrant a mention. A tempo may be needed in subsequent amendments, but most likely all impactful convection will remain west of the field. Low VFR cu will persist into early evening, with some stratocu or altocu seen overnight. Guidance that suggested restrictive stratus would move overhead this (tue) morning was incorrect, and under nearly the same pattern the same guidance is making the same prediction for Wed. I will maintain VFR fcst. Winds remaining in NE quad and light. Elsewhere: scattered +shra/thunderstorms and rain are expected to grow in coverage thru mid-aftn, gradually moving S-southeast off the Blue Ridge. Tempos are included accordingly at kavl/kgsp/kgmu. A chance does exist at khky/kand though not great enough to warrant a tempo. Some showers will linger into tonight though likely not having much impact. Some patchy fog is expected to develop over the mtns early Wed am, particularly where +ra falls today, and perhaps at kand. Winds will be mainly SW at kavl and NE elsewhere, going light/vrb overnight. Outlook...remnant low from former tropical cyclone Bonnie is expected to move very slowly up the coastal Carolinas over the next several days, maintaining increased precip and morning stratus chances at kclt. Otherwise, patchy fog chances continue at kavl each morning, with scattered afternoon/evening rain showers/thunderstorms and rain increasing in coverage throughout during the week. Confidence table... 21-03z 03-09z 09-15z 15-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 83% high 100% kavl high 100% high 100% high 90% 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 schedule 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...jpt near term...Ned/Wimberley short term...rwh long term...jpt aviation...Wimberley

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