Area forecast discussion National Weather Service Blacksburg Virginia 422 am EDT Tuesday may 21 2013 Synopsis... low pressure aloft will drift east of the region this morning. A large low pressure system will track from the Central Plains into the Great Lakes by Thursday. This system will push a cold front through the area Thursday...followed by high pressure on Friday. && Near term /through tonight/... as of 410 am EDT Tuesday... Radar showed isolated showers in north central North Carolina this morning...moving southeast. Have kept chance of precipitation out of the forecast for this morning. Did leave in areas of fog. Short wave was over the forecast area early this morning. Most models have feature moving into eastern Virginia by this afternoon. RUC and WRF models suggest most likely location for thunderstorm development will be along the southern Blue Ridge this afternoon. Then in eastern Kentucky and West Virginia...moving into our southeast West Virginia counties and through the northern County Warning Area this evening. Do not envision more than scattered coverage this afternoon or tonight. 850 mb winds turn to the northwest briefly afternoon following the short wave then back to southwest for tonight. Northwest flow and 850 mb temperatures around +16 will result in decent warm up for the foothills and Piedmont...especially in areas that have sun this morning. Guidance looked reasonable for afternoon highs. && Short term /Wednesday through Friday/... as of 300 am EDT Tuesday... Closed upper low and sharp trough over the Midwest will make slow eastward progress against strong ridge off the Atlantic coast but eventually will shove a surface front through the County Warning Area Thursday afternoon. Until then area will remain under southerly flow with generally abundant moisture but weak to disorganized forcing. May see a pre-frontal squall line across the Ohio Valley Wednesday afternoon which will likely be weakening as it approaches far western County Warning Area Wednesday evening/night. Still maintained at least high chance probability of precipitation in the mountains ahead of any possible line Wednesday afternoon where differential diurnal heating and upslope enhancment will be focus for most likely activity with lower chance probability of precipitation across the Piedmont. Some storms potentially strong with cape approaching 1000 j/kg but limited shear should preclude much in the way of severe storms. Again could be with locally heavy rainfall as precipitable waters remain in the 1.2 to 1.5 inch range ahead of front and storms could be slow- moving. Probability of precipitation diminish overnight Wednesday with loss of heating but renewed convective activity likely on Thursday with passage of surface front. Not looking for much in the way of severe as best support well to north of County Warning Area but a few strong storms and more pockets of heavy rainfall possible. Probability of precipitation decreasing Thursday night and Friday as drier northwest flow takes over. Left chance probability of precipitation Friday as cooler air aloft enhances instability lessening trend. Mild temperatures will predominate ahead of the front with 80s Wednesday and even Thursday but substantially cooler weather expected behind the front for Friday and beyond. && Long term /Friday night through Monday/... as of 345 PM EDT Monday... Both the GFS and European model (ecmwf) are delivering a good intrusion cooler air by 12z Friday with 850 mb temperatures slipping to +5c. The upper trough is forecast to slide across New England on Friday with cool northwest flow prognosticated from the Great Lakes to the middle Atlantic. Cooler northwesterly flow also suggests the air will be dry...thus have removed all mention of precipitation for the weekend. That said...as high pressure passes ovhd Saturday and rolls off the East Coast Sunday...moisture could sneak back in the picture as early as Sunday afternoon. At this time...will go with an optimistic forecast and just advertise increasing clouds for Sunday...and hold off on next opportunity for showers until Monday. Both Saturday and Sunday mornings look cool with early morning lows in the 40s in the mountains to the lower 50s in the Piedmont. If winds decouple and skies remain clear...some of the sheltered mountain valleys may slip into the upper 30s. && Aviation /08z Tuesday through Saturday/... as of 145 am EDT Monday... Many observations have not been transmitted since 04z/midnight due to an outage in National communications center. Until data flow is resumed...have added...amend not schedule...to the end of the tafs. Expect areas of fog and stratus overnight. IFR to LIFR ceilings are likely by 09z/4am. Locations that had rain on Monday will have fog...including roa and Dan. A short wave over the area tonight moves into eastern Virginia today. Isolated to scattered thunderstorms will develop again this afternoon. Probability at any one Airport is too low ton include in the taf at this time. The region stays in a warm...moist airmass through Thursday. Patchy fog with MVFR visibilities is likely each night and each afternoon and evening there will be scattered thunderstorms. Next front approaches on Thursday with more showers/storms developing. High pressure and better flying conditions are expected on Friday behind the front. && Rnk watches/warnings/advisories... Virginia...none. NC...none. WV...none. && $$ Synopsis...air mass near term...air mass short term...pc long term...PM aviation...air mass