Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 642 am EDT Friday Jul 25 2014 Synopsis... high pressure will build in behind a departing cold front on Friday lasting through the majority of the weekend. Another cold front is expected to reach the area from the northwest on Monday morning and into Monday afternoon potentially leading to scattered thunderstorms. High pressure will once again regain control on Tuesday lasting through the remainder of the work week with only minimal precipitation threats by weeks end. && Near term /through tonight/... as of 640 am...low clouds spreading in from the east or already in place across the mountains should lift or scatter out by noon. Otherwise... going forecast still looks on track. Updates mainly for current conditions. As of 305 am...slow moving cold front keeping enough low level convergence across the southern upstate to break the cap and produce isolated rain showers. These should end or move south of the area by daybreak as the front moves out of the area. Although the front moves south of the area...guidance is nearly unanimous keeping low level moisture across the area. In addition... low level flow becomes southeasterly leading to weak upslope flow. This is forecast to provide enough lift to tap the elevated instability and produce isolated to scattered convection along the Blue Ridge and west across the mountains given the model support...have changed the forecast to include pop in this area. There may also be some convection across the far southern County warning forecast area as that area will be close to the front. chance will be south of the County warning forecast area. Clouds will linger through the morning across much of the northern and eastern County warning forecast area...before mixing out by afternoon. This and the cooler air mass behind the front will lead to highs a little below normal. Convection should dissipate during the evening with loss of heating. However...cannot rule out some lingering rain showers over the Blue Ridge late into the evening or early overnight before the low level flow becomes southwesterly. Lows should be near normal. && Short term /Saturday through Sunday night/... as of 230 am EDT Friday...the short term period initializes Saturday morning with remnant front well to the southeast along the Carolina/Georgia coastline. Surface high pressure will quickly build under rising heights aloft. Despite weak Lee trough developing just east of the forecast area potentially providing a focus for afternoon convection...soundings indicate a substantial low/middle level cap in place. Not expecting any potential updrafts to break through the deep inhibition therefore kept the forecast dry for Saturday. Also kept forecast dry through Sunday morning as upper heights are slow to fall due to approaching upper trough and associated surface cold front. Still think area of greatest lift and lowest inhibition will remain tied to Lee trough redevelopment east of the i77 corridor therefore continue with dry forecast for the Piedmont region through the afternoon. Further West...Heights will begin to fall rapidly as upper short wave and associated moisture advect into the central/southern Apps. Models tend to agree on at least some precipitation pushing into the northern mountains by middle/late afternoon on Sunday...with chances increasing ahead of the approaching front through the overnight. Therefore...solid chance probability of precipitation are featured Sunday evening along the higher terrain of NC increasing to near likely probability of precipitation by Monday morning near the Tennessee border. Elsehwere probability of precipitation taper down sharply to slight chance levels over the NC/SC foothills and western most areas of the Piedmont. Temperatures through the short term period will gradually warm as southerly flow ramps up ahead of approaching surface trough. Expecting highs on both Saturday and Sunday to surge to above normal levels with middle 90s possible for southern portions of the forecast area on Sunday. && Long term /Monday through Thursday/... as of 315 am EDT Friday...the extended range period will begin with a rather active pattern on Monday morning. Strong height falls will be advecting in from the west as a cold front stretches along the appalachian chain from a parent surface cyclone across the eastern Great Lakes. Probability of precipitation will initialize at high end chance levels across the higher terrain as the prefrontal activity pushes into the region around or just before daybreak Monday. The entire complex will continue advecting east through the morning and early afternoon hours allowing for some diurnal destabilization ahead across the NC/SC Piedmont regions. There is still some uncertainty regarding overall magnitude of instability that will build ahead of the front as abundant sky cover will work to limit heating. That said...modest warm sector already in place will not need much heating for lapse rates to support substantial low level lift. Aloft drier air will be advecting in as strong west-northwest flow associated with trough axis pushes into the region. This will allow middle level lapse rates to steepen...potentially leading to a fairly unstable profile. Speaking of...latest GFS forecast soundings indicate nearly 1500j/kg SBCAPE and long straight hodographs producing deep layer shear in excess of 35kts suggesting multicell strong/severe thunderstorms. All that said...there is still plenty of time for the pattern to change slightly which could substantially alter the outcome. Thus...due to range in the forecast...kept probability of precipitation capped at chance levels for the entire region on Monday afternoon. Surface high pressure will build into the region behind the front for Tuesday and Wednesday leading to lower precipitation chances despite persistent upper trough remaining nearly stationary. Forecast will feature slight chance probability of precipitation across the high terrain through Wednesday morning amidst strong northwest flow. The surface ridge remains dominate through the remainder of the period as it repositions itself to the northeast and extends in a wedge like pattern along the Lee side of the Appalachians by Thursday. Surface flow will shift to more of an east to southeast component leading to potential upslope/upglide precipiation into Friday. Thus chance level probability of precipitation are once again featured through the remainder of the forecast along the high terrain with slight chance probability of precipitation highlighted elsewhere. Temperatures will gradually cool behind the frontal passage on Monday with highs topping out nearly a category below normal by Thursday for some locations. && Aviation /12z Friday through Tuesday/... at kclt...IFR stratus has spread across the airfield and will continue through middle-morning when a slow rise to low VFR by 18z develops. That said...moisture is shallow and ceilings could scatter out before lifting. Few to scattered low VFR clouds expected through the evening and overnight. Northeasterly wind becomes east-northeast this afternoon and evening then calm overnight. Right not expect any restrictions overnight. Elsewhere...low stratus is making a run from the east at the non mountain sites. Low VFR ceilings have developed and MVFR or even the IFR deck could develop. If so...should be a relatively quick return to VFR... generally before noon. Then VFR expected through the rest of the period. Northeasterly wind becomes Ely or southeasterly by late afternoon with generally calm wind overnight. Kavl will see MVFR or IFR this morning with low VFR developing by noon. Northerly wind becomes southerly for the afternoon...then calm overnight. Isolated convection will develop across the mountains...but chance too low for taf inclusion. Kavl could see restrictions from fog toward daybreak Sat. Outlook...Sat through Sunday morning should be quiet and VFR... except for the mountain valleys where early morning fog may occur. Scattered thunderstorms may return to the forecast area for Sunday afternoon...though more likely another cold front approaches and then moves through the area by Monday night. Confidence table... 10-16z 16-22z 22-04z 04-10z kclt high 83% high 88% high 100% high 100% kgsp medium 66% high 95% high 100% high 98% kavl medium 67% high 88% high 93% high 94% khky medium 66% high 88% high 99% high 100% kgmu medium 74% high 100% high 100% high 100% kand medium 72% high 93% 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: (must be lower case) Www.Weather.Gov/gsp/aviation_tables && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...cdg near term...rwh short term...cdg long term...cdg aviation...rwh

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