Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 244 PM EDT Tuesday Sep 2 2014 Synopsis... a weak cold front will push through the region tonight followed by Bermuda high pressure Wednesday into Friday. Another cold front will sweep through the region over the weekend providing an additional focus for thunderstorms on Saturday and Sunday. Building high pressure will highlight the weather pattern for the start of the next work week leading to cooler temperatures and more stable weather. && Near term /through Wednesday/... for the remainder of the afternoon...scattered showers and thunderstorms are expected to develop and fan out across the foothills and Piedmont. Other storms over north Georgia should make their way NE into upstate SC through early evening. Expect some of these storms to go severe through afternoon with the environment rich in cape and dcape...which should contribute to hail and strong downbursts. Additional storms may move in from the west and northwest in the early evening along outflow boundaries from storms over the Tennessee Valley. This activity could be further enhanced by the arrival of a short wave and upper divergence from a passing jet streak. The short wave will move across the forecast area tonight and decelerate over the Piedmont on Wednesday. This feature will support the continuation of storms over the foothills and Piedmont through the evening hours. The NAM in particular keeps the storm chances across the clt metropolitan area until the early morning the forecast will be nudged in that direction of keeping a chance pop. The chance pop over the Piedmont of the Carolinas will be re-evaluated and could be expanded/extended based on convective trends later this evening. Low temperatures tonight should remain mild. Patches of dense fog are also possible where rain falls. On Wednesday...the models agree with stalling the remnants of the wave over the Piedmont to our south and east...yet still develop convection in the afternoon perhaps because of ample moisture at low levels. This seems like a decent bet over the mountains and along the eastern border close to the wave remnants...but there is uncertainty in between these two areas. The guidance paints a slight chance pop across the entire area while the ongoing forecast has a chance. Will keep the chance everywhere based on the latest sref for the time being. Temperatures should be a few degrees cooler based on increased cloud cover. && Short term /Wednesday night through Friday/... as of 230 PM Tuesday...on Thursday...middle level ridge will remain centered across the Carolinas and western Atlantic. Short range models indicate that moisture will lift north across the region during the daylight hours on Thursday. Moisture is likely increasing south of weak warm front lifting north across the Carolinas. Forecast soundings indicate that precipitable water will increase to 1.75 inches...with convective available potential energy around 1500-2000 j/kg. This environment should primarily support air mass thunderstorms and rain...forming over the mountains during the early afternoon...expanding east through the remainder of the afternoon. Pulse severe thunderstorms and rain appear possible across the County Warning Area. On Friday...conditions will change little from Thursday. Forecast soundings indicate the precipitable water will increase to 1.8 inches with broad low level southeast flow. Weak upslope flow will favor the higher elevations for thunderstorms and rain. Both days...I will forecast 30 probability of precipitation across the Piedmont to 40-50 across the mountains Thursday and Friday...high temperatures are forecast to range near normal. && Long term /Friday night through Tuesday/... as of 200 PM EDT Tuesday...latest global models continue to agree that an upper trough will progress eastward across the Great Lakes region Sat and into the eastern Seaboard by sun. This will push a trailing cold front into the southern Appalachians by late Sat. With increasing Atlantic moisture advection in southeasterly flow along with diurnal buoyancy...probability of precipitation ramp up into the solid chance range over the Piedmont and into the likely range over the NC mountains Sat afternoon. The front will then considerably slows down its Seward progression as it reaches the Tennessee/NC border line. Both the GFS and the European model (ecmwf) indicate that the frontal boundary will become stationary over the Piedmont by early sun before it eventually pushes south of the area by Monday. This will yield widespread convection ongoing Sunday into early Monday and thus have mentioned solid to likely probability of precipitation across much of the region during this period. Given the moist profile (pw values near 2")...good low level convergence and moderate instability...heavy rainfall looks to be the main concern att. Monday and Tuesday...probability of precipitation ramp down as the front remains south of the area and a wedge of surface high builds in from the NE. Temperatures will stay near normal Friday/Sat before cooling down considerably by Monday. && Aviation /19z Tuesday through Sunday/... at kclt...scattered cumulus developing across the Piedmont with bases expected to be around 050 through 21z or so. Prevailing wind should be SW outside of influence of nearby storms. Kept a tempo for thunderstorms and rain in a window from 21z to 01z based on a compromise between the WRF-arw and the hrrr. This will no doubt need further amending. The balance of the overnight should have VFR clouds and convective debris with a light wind...perhaps calm. If a thunderstorm moves over the airfield...the chance for fog around daybreak Wednesday will increase considerably. For was left out of the taf. Low convective clouds should redevelop with some boundary layer heating on Wednesday morning. Wind should have a northerly component because of a weak boundary moving through overnight. Any chance for showers/storms was left out for the time being...but it should be after 16z. Elsewhere...similar to kclt. All sites get a four hour tempo for thunderstorms and rain in the middle to late afternoon or early evening...except for kavl because of the ongoing storm at issuance time. That storm at kavl greatly increases the chance of fog at that site. Have limited the fog/stratus to IFR after 07z...but given the precipitation that has fallen...restrictions could begin much earlier and could drop down into the LIFR/vlifr ranges. Any other site that has rainfall will have an increased chance for fog early Wednesday morning. Left off precipitation chances after sunrise Wednesday for brevity. Outlook...generally VFR conditions are expected through the rest of the week...but restrictions are likely early each morning in the mountain valleys due to low clouds and fog. Diurnally driven rain showers/thunderstorms and rain are expected each afternoon and early evening...with the most coverage over the mountains. Confidence table... 18-24z 00-06z 06-12z 12-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% medium 65% medium 75% khky high 100% high 100% high 95% high 95% kgmu high 100% high 100% high 100% high 100% kand high 100% high 100% high 95% high 95% 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...joh near term...PM short term...Ned long term...joh aviation...PM

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