Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 1012 am EST sun Nov 23 2014 Synopsis... moisture will increase rapidly in advance of a cold front that will move over the region on Monday. A coastal low is then expected to develop off the southeast coast and move up the Atlantic Seaboard on Wednesday and Thursday. High pressure will spread back over the area by the end of the week. && Near term /through tonight/... 1000 am EST Sunday update...latest radar trend indicates that widespread bands of moderate rain with isolated heavy rain were pushing northward across our County Warning Area as a series of upper disturbances ripple through the region in southwesterly flow aloft. Hence...have increased probability of precipitation to 90-100% range for entire County Warning Area this morning through the afternoon hours. Also...have update quantitative precipitation forecast per latest wpc guidance...suggesting 1.5-2 inches of rainfall across much of the County Warning Area with higher amounts over the Blue Ridge Escarpment. The latest 12z NAM continues to highlight persistent in-situ cad throughout the day...before eroding by around 00z-03z. Therefore...some degree of elevated buoyancy will have been pushed farther north into at least southern half of the County Warning Area by this evening. Hence...will continue to mention slight to low end chance for thunder mainly over the Piedmont. As of 640 am...a lull in the more widespread precipitation associated with warm upglide is now moving into the western third of the County warning forecast area...with the warm front now positioned from around Macon Georgia east to Charleston. There is some more intense rain falling along the front which may be robbing moisture from our area and accounting for the lull. Nonetheless latest guidance still supports probability of precipitation increasing from midday to late afternoon as the warm front nears and elevated instability comes into play. Temperatures are likely to remain nearly steady through middle morning due to wet bulb effects offsetting any solar warming. As of 315 high pressure centered off the Outer Banks will continue to move east today. However as a deep trough moves into the Mississippi Valley and is preceded by a southern-stream shortwave over the Gulf south...deep SW flow will provide ample moisture and warm air advection into northern Georgia and the Carolinas. Increasingly widespread rainfall will result over the County warning forecast area this morning...and a brief in- situ cad event will develop. Temperatures today will be 7 to 10 degrees below climatology under cloudy and damp conditions. It appears temperatures will be warm enough to preclude any ptype issues even in the high elevations. To our West Heights will fall through the day with the leading shortwave lifting from the Gulf Coast today to the upper Ohio Valley at daybreak Monday...with the larger trough not pushing much past the MS river by that time. The leading wave will bring a warm front through the area this evening which has some potential to erode the cad. However the GFS and especially the NAM suggest that The Wedge will remain entrenched at the surface through the night tonight...though very shallow. The NAM/GFS/sref all indicate elevated instability above The Wedge inversion along and behind the warm front and this is the primary reason we will include thunder in the forecast. A few sref members do retreat The Wedge boundary northward enough to bring SBCAPE values of under 100 j to about I-85. Even small buoyancy will be enough to cause some concern for damaging wind or a brief spin-up tornados due to the tremendous shear parameters that will be present. This due to the very strong low to midlevel flow forced by the shortwave in conjunction with backed near-surface flow. Deep layer hodographs are very impressive and looping...producing helicities of several hundred m2/s2 at their peak this afternoon. Effective srh plots are a more muted 100-200 m2/s2 on account of the minimal buoyancy at that time. By the time that sbcapes exhibit their small peak...the shear numbers are already past their prime. Storm Prediction Center has largely carved US out of any risk area in the day 1 graphic...but did include our srnmost zones in a 5 percent wind threat area...these areas seeing the best chance of destabilizing before the shear/helicity weaken. We will nonetheless be closely monitoring the convective situation this afternoon particularly south of The Wedge front. We have been watching hydrologic threats with this event for several days and thinking remains largely the same. The upglide itself is not especially impressive until the moisture advection and lift are aided by the shortwave this afternoon. This is when our rainfall rates will be greatest. Overall quantitative precipitation forecast numbers from various models are in the same ballpark. The fast motion of the low is largely responsible for keeping Hydro threats to a minimum...but the recent dry weather of course suggests soils have relatively high capacity. && Short term /Monday through Wednesday/... as of 320 am Sunday...a cold front will progress across the area at a rather deliberate pace on Monday...likely taking a good chunk of the day before to completely clear the forecast area. The NAM continues to insist on developing some rather substantial Cape Monday afternoon across the Piedmont. And while I do have doubts that the upper 60s dewpoints that the NAM is advertising will be stands to reason that instability will be more significant than that advertised by the GFS...which is probably sweeping the front across the area too quickly. Having said that... forecast soundings are quite warm in the mid-levels...and it/S not at all clear that air parcels will be able to overcome that... especially in the absence of significant forcing. Therefore...will continue to hold probability of precipitation down in the slight chance range (for showers) Monday afternoon. The bigger story will be the forecast soundings depict a very deep mixed layer...with as much as 60 kts (!) Of SW flow at the top of the bl. With only a 4-6 mb surface gradient advertised across the forecast area...I doubt we/ll realize anything approaching that degree of gustiness. Nevertheless...gusts as high as 40 miles per hour can be expected Monday afternoon. A return to cooler and drier conditions is expected by early Tuesday before uncertainty enters the forecast equation by Tuesday night and Wednesday. The main issue is with southern stream energy lifting out of the Texas...and across the southeast during this time frame. Each model has its own idea of how the pattern will evolve...particularly with how a northern stream wave dropping out of the northern rockies will interact with the southern stream wave/potential for phasing of the two streams. The European model (ecmwf) remains the most aggressive in keeping the streams separated through Wednesday...with the southern stream wave taking on a negative tilt orientation...and instigating strong cyclogenesis across the eastern Gulf Tuesday night...then tracking it very close to the southeast Atlantic coast. Meanwhile...the GFS is a little farther off shore and not as deep with the cyclone...while the NAM depicts nothing more than a weak wave off the coast by early Wednesday. It/S also somewhat disconcerting that there appears to be only a single sref member that is ec-like. Nevertheless...based upon the fact that even a GFS solution develops some precipitation into our eastern zones Tuesday night...will continue to advertise solid chance probability of precipitation across the NC Piedmont by Wednesday morning...with low chance as far west as the Blue Ridge. Will continue to feature mountain rain/snow possibilities...with generally all liquid outside the mountains should be stated that if the situation evolves in an ecwmf-like manner...significant snowfall will be possible across portions of the mountains late Tuesday night into Wednesday. && Long term /Wednesday night through Saturday/... as of 345 am...the medium range appears largely cool...dry...and a low amplitude long wave trough becomes established over the eastern Continental U.S.. the European model (ecmwf) depicts progression of a cold front into the region by the end of the week...associated with an area of height falls overspreading the Great Lakes. is not at all clear if sufficient moisture will be available to support precipitation chances over our area. && Aviation /15z Sunday through Thursday/... at kclt...warm air and moisture advection will gradually increase today bringing lowering ceilings and reinforcing cold air damming over the area. Guidance fairly solid on ceiling lowering to LIFR by late afternoon as rain continues and forcing peaks aloft with the passage of a short wave trough. The Wedge boundary is most likely to stay S of kclt until this evening...but isolated elevated convection could result in ts earlier in the afternoon. Best chance however is around sunset with the boundary nearing while some diurnal buoyancy still exists. Though some drying will occur Sunday evening...there is not a good scouring mechanism for the cad and only slight improvement to ceiling is expected overnight. Winds NE until going southeast this evening as wedge starts to weaken. Have included low level wind shear mention this afternoon and evening where suggested by model winds...with very strong flow present above the planetary boundary layer ahead of the shortwave. Elsewhere...trends much like kclt. Have included a low-end thunderstorms and rain mention in the late afternoon at the SC sites as they are most likely to be affected by activity near The Wedge boundary. Low level wind shear is included mainly in vicinity of the shortwave passage...but at kavl the flow remains strong enough tonight to include through 12z. Wind changes overall are similar in progression with the exception of kavl which will see southeast winds throughout the period. Outlook...brief drying follows a cold front Mon-Tue...with rain and/or wintry precipitation possibly returning Wednesday. More settled weather will return to end the week. Confidence table... 15-21z 21-03z 03-09z 09-12z kclt medium 69% high 97% medium 75% medium 67% kgsp high 94% high 100% high 80% medium 77% kavl high 94% high 89% high 86% medium 66% khky high 91% high 100% high 100% low 59% kgmu high 94% high 97% high 89% medium 73% kand high 94% high 97% medium 63% low 46% 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...jdl near term...joh/Wimberley short term...jdl long term...jdl aviation...Wimberley

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