Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 251147 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 747 AM EDT Sat Mar 25 2017 .SYNOPSIS... High pressure off the coast will bring one more day of fair weather and mild temperatures to the region today. However, a series of low pressure systems will move out of the Mississippi valley for the second half of the weekend and into the first part of next week. This will bring a good chance for showers and thunderstorms to the area through Tuesday, though temperatures will remain well above normal. && .NEAR TERM /THROUGH TONIGHT/... As of 300 PM EDT Saturday... High pressure surface and aloft will remain off the coast as a closed low slowly approaches from the west. This will bring increasing clouds as southerly winds bring increasing moisture into the region. Forcing is largely absent so any significant chance for precipitation will hold off until later Saturday night. Temperatures will remain well above normal with highs today in the mid/upper 70s east of the Blue Ridge, lower 70s to the west. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 400 AM EDT Saturday... Active weather pattern expected through the period with a kinematic zonal flow embedded with several vigorous upper short waves or closed lows. In fact there are five such systems slated to track across the U.S. from west to east over the next seven days. Thus, there will be numerous and frequent chances for rain. As instability increases, the threat for thunderstorms, some even strong to severe, also increases through the period. Given that all of the systems are of Pacific origin, temperatures will remain mild and well above normal through the period. The first upper system to bring precipitation to the area will arrive Sunday morning. While this will be a potent system as it tracks through the Southern Plains and into the western TN Valley today, it will be lifting north into the Ohio Valley and weakening, as well as slowing in forward speed, as it arrives in our region, thanks to a downstream upper ridge in the western Atlantic. Although shear is fairly decent and dynamics are notable, instability is marginal at best. Would expect rain showers to arrive in the western parts of the forecast area around daybreak Sunday, then translate east through the morning into the afternoon. Likely to categorical pops are warranted. Enough instability is indicated to warrant mentioning isolated thunder west of the Blue Ridge during the afternoon where the air mass will be more unstable. This first system will lift north of the region Sunday evening/Monday morning, but leave an increasingly moist and unstable air mass in its wake. Monday, the CWA will be located between upper lows/troughs. However, the air mass will be moist and unstable. With forcing increasing during the afternoon/evening in advance of the upstream short wave, would expect some scattered showers and isolated thunderstorms to pop up across the forecast area. Nothing organized is anticipated. By Tuesday, the next in the parade of troughs/upper lows moves into the region from the west during the morning hours then into the Piedmont by afternoon. Instability and moisture are significantly more abundant as this system arrives. In addition, the more kinematic nature of this second/open wave system combined with notable instability warrants the mention of thunder areawide during the afternoon/early evening Tuesday. Fortunately, upper-level wind fields/hence shear, appear to be relatively weak and weakening further with time as this system shears out toward the northeast U.S. So while a few stronger storms cannot be ruled out, the severe threat appears low with this system, but does appear to be somewhat greater than with the Sunday system. QPF for the two events should average from just under an inch in the west to about 1/2 inch across the Piedmont. Given relatively dry weather in the recent past, this should not cause any hydrologic issues. As noted above, temperatures will be unseasonably warm through the period with lows mainly in the 50s and highs in the 60s west to the 70s east. These temperatures are around 20 degrees above normal for lows and 10-15 degrees above normal for highs. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 420 AM EDT Saturday... Behind the Tuesday system, we will see a brief respite from the parade of storm systems as an area of high pressure from Canada filters southward into the area stabilizing and cooling the air mass. This should lead to a relatively precipitation free and slightly cooler period. Cannot completely rule out some light showers due to weak ripples in the flow aloft. Temperatures and dewpoints will drop about 10 degrees from the Mon-Tue period. By Friday, the next, and fairly potent, upper low tracks toward the area. However, there is considerable discrepancy between the GFS and the ECMWF in the configuration and timing of this system into the eastern U.S. The GFS depicts more of an open wave reaching the CWA as early as Thursday night moving east of the area by Friday afternoon. Meanwhile, the ECMWF is much slower, deeper, and further south with a cutoff upper low lifting it slowly north-northeast into the Ohio Valley by late Friday. The difference between the two models can be accounted for in noting the strength and blocking of an upstream ridge across the Atlantic. This upstream ridge is much stronger per ECMWF, which also depicts a notable wedge down the east side of the Appalachians as the closed low lifts north into the Ohio Valley. The ECMWF would result in a much wetter and stormier pattern for the CWA, although it would not arrive until Friday as opposed to Thu night per GFS. For now, have generally broad brushed the area with high chance to likely pops beginning Thu night and continuing into Friday. In addition to the differences noted above, there are also massive differences between the two models regarding instability and the potential for severe, with the ECMWF much more noteworthy in this category. For now, have included a chance for thunder without any further details. It will likely take several runs for the models to resolve their differences on this system. Another system is slated to follow by the end of the weekend or the beginning of the next week, so the active pattern is expected to continue into the next week. Temperatures will be closer to normal Wed-Thu, then begin to trend back to above normal levels Fri-Sat. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... As of 730 AM EDT Saturday... VFR conditions should continue for all TAF sites through 00Z. However, extensive mid and high clouds are overspreading the area, so it will be BKN-OVC all day with a mid-level deck in the 050-070 range and a high level deck in the 200-250 range. Moisture will continue to increase overnight/early Sunday as a vigorous upper-level low slowly approaches from the TN Valley on its track into the OH Valley. The continued moist south to southwest flow ahead of this system will bring lower MVFR clouds/ceilings to all TAF sites overnight/early Sunday. Showers should not arrive until mid to late morning Sunday, just beyond this TAF valid period. Current thinking is that we will only see a low stratus deck early Sunday and not any fog, but some MVFR in fog cannot be completely ruled out early Sunday. Winds will generally be S-SW through the daytime becoming S-SE after 00Z. Look for speeds of 5-10kts through 00Z, mostly 5kts or less after 00Z. Some low end gusts are possible once again today across the Piedmont, but are not likely west of the Blue Ridge. Medium to high confidence in ceilings and visibilities throughout the TAF valid period. Medium to high confidence in wind speed and direction through the TAF valid period. Extended Aviation Discussion... An active weather period with a series of vigorous upper-level low pressure areas tracking from west to east across the U.S. during the week. Showers and a few thunderstorms will overspread the area Sunday with the first system with widespread sub-VFR cigs/vsbys in clouds and precipitation. Monday the area will be between systems, but a moist/unstable air mass will remain in place and widely scattered showers and isolated thunderstorms are possible. Areas of sub-VFR cigs/vsbys are also possible, especially during the morning hours, in low clouds and fog. The next upper-level low will arrive Tuesday with another chance for showers and thunderstorms along with periods of sub-VFR conditions. Wednesday-Thursday may bring a period of improved aviation conditions as a weak cold front temporarily pushes the deeper moisture south of the area. However, by Friday the next in the series of upper-level systems arrives with another chance for showers, thunderstorms, and sub-VFR conditions. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...MBS NEAR TERM...MBS SHORT TERM...RAB LONG TERM...RAB AVIATION...MBS/RAB

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