Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 261413 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1013 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure centered offshore will maintain a moist southerly flow of air into the region for the next several days. Subtropical Storm Alberto moving into the Gulf of Mexico this weekend will move ashore and into the lower Mississippi Valley early next week. Tropical moisture will stream north into the region making for very rain efficient showers and thunderstorms. && .NEAR TERM /THROUGH TONIGHT/...
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As of 950 AM EDT Saturday... Seeing slow moving convection across the far west within an axis of weak convergence aligned along the western mountains between upstream troffiness and return of deeper tropical moisture per morning soundings. Latest analysis also showing a weak mid level wave riding up from the southwest to the north of the residual front and surface wave to the south. This supports some slow expansion of convection to the north and east with coverage perhaps slowed somewhat by lack of instability due to cloud cover. However latest short term solutions show quite a bit of shra/tsra expanding east under increasing southwest flow so keeping likely/cat pops through the afternoon. Weak steering still of concern with potential flash flood issues but given more localized nature and uncertainty including how far north/east heavier rainfall might materialize wont go with with a watch unless things become more widespread. Lowered highs a little in the west/south per more clouds/showers while keeping mainly 80s in the east. Previous discussion as of 430 AM EDT Saturday... Unsettled weather is expected through the Holiday Weekend as southerly wind flow brings increasing moisture to the Appalachians. Surface dewpoints in the 60s to near 70 along with PWAT values in excess of 1.50 inches will favor August- like conditions with high humidity and rain efficient showers/storms. The key word is showers, as it will not rain the entire time, but when it does rain, it will pour. Shower coverage today will be the greatest during the peak heating of the day with convergence along the mountains driving much of the lift to create the showers and afternoon thunderstorms. There is a subtle shortwave embedded within the southerly flow that may provide enough lift to help focus some of the deep convection into clusters, so will have to monitor this closely as this may help provide a focus for an area of localized heavy rain. The high resolution models suggest threat for the heaviest rain today will be just north of our CWA where the westerlies across the Ohio Valley and northern Mid-Atlantic converge with the southerly winds coming up from the southern Mid-Atlantic. For this reason WPC has highlighted areas north of I-64 within a Slight Risk for excessive rainfall in their Day-1 Outlook. There is a weak upper level trough moving east across the Great Lakes and Ohio Valley. This feature will maintain enough lift for showers to persist across the Virginias tonight, and especially north of I-64 where wind convergence will continue between the two flow regimes. Temperatures will trend 10-15 degrees above normal for the next several nights per the significant uptick in dewpoints. However, daytime highs will not be as extreme per the abundant cloud cover and shower coverage...yielding Tmax values of no more than about 5 degrees above the norm.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 400 AM EDT Saturday... The pattern will remain blocky and sluggish into the first part of next week with a shallow trof sliding by to our north and Alberto moving slowly along the eastern Gulf coast to the Mississippi delta by Monday. This will keep the Appalachians and central mid Atlantic region in a warm and very moist airmass for the next several days. Sunday looks to feature ample instability in a relatively weak wind field as the upper trof tries to push a washed out frontal boundary into the region from the northwest. This will add some synoptic scale lift to air convection and should yield good coverage of showers/storms especially across the northwestern two thirds of the area. Any severe threat will be small with more of a focus on potential hydro issues from training storms in an environment of precipitable water values in the upper percentiles. Sunday night into Monday will see some subsidence around the periphery of Alberto push in from the south along with slightly lower theta-e air. While there will still be showers/storms around, believe these limiting factors will suppress activity a bit and keep the preferred area for convection across the southern half of the area. Steering flow becomes very weak with little to move precipitation along and will have to watch for areas of prolonged persistent rainfall. A better push of moisture arrives on Tuesday as Alberto will be moving up into the lower Mississippi valley and throwing waves of energy in our direction, but the best forcing with this system remains well off to our southwest. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 400 AM EDT Saturday By mid to late week, remnants of the tropical system could bring tropical moisture and heavy rains to our region. Flooding potential will depend on the track of the system and how much rainfall is accumulated from the showers and thunderstorms earlier in the week. Expect Alberto to make its way into the Tennessee valley by Wednesday. The timing of the ECMWF was quicker and further north and west (Ohio Valley) compared to the GFS which was slower and more compacted (along the Appalachians. By Thursday and Friday, the low will open up and weaken as it moves northeast. There is a continued threat for showers and thunderstorms into the end of the week. Urban and small stream flooding risk will be highest early in the week with river flooding possibly by the end of the week depending on actual rainfall totals. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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As of 800 AM EDT Saturday... Increasing moisture from the south will bring increasing cloudiness to the Mid-Atlantic Region with finger of MVFR Cigs creeping north along spine of Appalachians. This moisture will combine with the heating of the day to produce numerous showers and embedded thunderstorms...esp during the afternoon and evening. Other than convergence from the mountains, showers/storms will lack any sort of organization and are expected to dissipate after sunset. Mean steering wnd aloft is fm 240 deg at 12 kts. Low level winds will be out of the SSE at less than 10 kt. .Extended Aviation Discussion... Wet pattern anticipated through the upcoming week with periods of at least diurnally driven sub-VFR conditions associated with deep convection. Moist low level southerly winds will also favor lower layers of cloudiness at night, especially along the spine of the Appalachians with potential for both MVFR Cigs and early morning MVFR visibilities from mist/haze. The tropical system over the Gulf of Mexico may begin to impact the region mid-week.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...JH/PM SHORT TERM...MBS LONG TERM...MBS AVIATION...PM

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