Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 509 PM EDT Friday Apr 29 2016 Synopsis... high pressure will remain across the southeast region...before a cold front pushes in from the northwest Saturday night. This front will become stationary across the Carolinas and northeast Georgia through Tuesday. Canadian high pressure will then build into the area and persist through Friday. && Near term /through Saturday/... as of 500 PM EDT Friday...only minor tweaks to hourly grids...but nothing substantive to warrant any product updates. Otherwise...dry and unseasonably warm weather will continue through tonight... as cirrus gradually increases and perhaps thickens a bit downstream of miss valley convective complex. Min temperatures tonight will once again average about 10 degrees above climatology. By Saturday morning...an area of surface high pressure centered near Hudson Bay...and extending over much of eastern Canada and the northeast Continental U.S. Is expected to force a back door cold front through the middle-Atlantic and through much of North Carolina by Sat afternoon. As usual...exactly how far southwest this boundary penetrates into our forecast area is the subject of much uncertainty at this point. Increasing low-level southerly flow developing in response to large scale height falls to our west will result in weak isentropic lift north of the boundary...which could support some precipitation development by late in the day. Of larger concern will be what happens in the warm sector Sat afternoon. It appears appreciable moisture/clouds will hold off until late in the day...while dewpoints will be on the increase throughout the day. Meanwhile...respectable middle-level lapse rates of around 6.5 c/km are expected across the region Sat afternoon. Warm sector instability is therefore expected to become pretty respectable by the end of the day...with SBCAPE likely reaching the 1500-2000 j/kg range across the western half of the area by late afternoon. Meanwhile...as a middle-level short wave ridge pushes northeast of the area...weak large scale lift will increase...with short term guidance depicting the approach of a short wave trough. As this occurs...at least scattered convection should develop along the boundary and across the high terrain by late afternoon. Probability of precipitation in the 40-60 percent range (highest across the west) will be featured during this time. Wind fields will be increasing throughout the day...with deep layer shear increasing to 35-40 kts by the end of the day. While helicity will be relatively weak in a large scale sense...enhanced helicity will exist along the surface boundary...possibly supporting a weak tornado threat with any cells crossing the boundary. && Short term /Saturday night through Monday/... as of 200 PM EDT Friday...the short term forecast period kicks off Saturday evening with broad upper ridging over the southeast...while an 500 mb cyclone rotates over the Central Plains. At the surface...cyclogenesis is expected beneath said 500 mb low with a cold front extending southward through central Texas...while an east/west oriented warm front extends across the Tennessee/Ohio valleys. Meanwhile...high pressure over New England will likely weaken and slide out to sea effectively forcing The Retreat of a weak cad wedge over much of northeast Georgia and the western Carolinas...especially as the warm front surges north. Thus...am expecting ongoing convection to continue through the early/middle evening hours amongst the building warm sector...while weak upglide could allow for rain showers north of the front. Moving along...the warm front will have pushed through the entire forecast area by Sunday thereby allowing the region to sit firmly in the warm sector as the associated cold front crosses the MS River Valley. Guidance continues to favor convective initiation through the day as heating leads to destabilization. Steep low/middle level lapse rates and abundant dry air aloft yield forecast soundings that are somewhat impressive with convective available potential energy approaching/exceeding 2k j/kg with deep layer shear in the 25-30kts range. Therefore wouldnt be surprised to see scattered/numerous thunderstorms and rain with some cells reaching strong/severe limits on Sunday with large hail and damaging winds being the primary threats...with localized heavy rainfall as a secondary threat. Monday looks fairly similar to Sunday however with a bit weaker shearing and steering flow albeit still fairly unstable ahead of the approaching cold front...therefore isolated/scattered thunderstorms and rain looks possible over region. In summary...the forecast features widespread elevated chance/likely probability of precipitation on Sunday...lowering to chance levels for Monday. Temperatures through the period will hover a few degrees above normal on average. && Long term /Monday night through Friday/... at 230 PM Friday, on Monday night zonal upper flow will be over the southeast USA, while an upper trough will extend from eastern Canada to the midwestern USA, and an upper ridge will be over western Canada and the western USA. The upper trough amplifies over the eastern USA by Wednesday, progressing closer to the East Coast by Friday while the ridge upstream reaches the plains. At the surface, a cold front will depart the western Carolinas and northeast Georgia on Monday night, moving off the Carolina coast by Tuesday morning. The western end of this front stalls near the Gulf Coast. Moist flow over this boundary will support continuing precipitation over our area. On Wednesday a surface wave moving along the front will reach the lower Savannah River valley, and moist flow ahead of this system will keep precipitation going over our area. Precipitation will finally end late on Thursday, as another front moves over the southern Appalachians from the northwest and brings drier air into our area. Temperatures will initially exhibit a reduced diurnal range due to moisture associated with the stalled front and approaching surface wave. Temperatures will fall to slightly below normal in the wake of the second cold front. && Aviation /21z Friday through Wednesday/... at kclt and elsewhere...no changes to the kclt taf for the 21z amend. Otherwise...VFR conditions are expected to continue through the period...with gradually increasing/thickening high-level clouds expected...and possibly increasing middle-level clouds by the end of the period. Otherwise...winds are expected to become light west/SW at all terminals by mid-afternoon...before becoming light/variable by late evening...then picking up out of the east/NE Sat morning. Outlook...unsettled weather/occl convection/restrictions are expected to return from late Sat afternoon into early next week...as a front becomes stalled near the area. Definitive drying does not appear likely until at least mid-week. Confidence table... 21-03z 03-09z 09-15z 15-18z kclt high 100% high 100% high 100% high 84% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 100% high 100% khky high 100% high 100% high 100% high 84% 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. Complet hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: Www.Weather.Gov/gsp/aviation && Climate... records for 04-29 Maximum temperature min temperature station high low high low ------- ---------- ---------- ---------- ---------- kavl 89 1970 46 1999 63 1956 28 1967 kclt 91 1888 48 1999 65 1994 33 1973 1991 1914 kgsp 91 1917 47 1999 67 1975 32 1992 && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...sbk near term...jdl/tdp short term...cdg long term...jat aviation...jdl/tdp climate...

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