Area Forecast Discussion
Issued by NWS Blacksburg, VA

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703 FXUS61 KRNK 281744 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 144 PM EDT Sun May 28 2017 .SYNOPSIS... A frontal boundary stretching across Indiana, southeast to NC will shift northward today. as a low tracks into New York. A stronger cold front will move through the region on Wednesday morning. && .NEAR TERM /THROUGH MONDAY/...
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As of 100 pM EDT Sunday... Cu field starting to grow over the Blue Ridge south of Mabry Mill to the NC/TN border. Weak convergence boundary situated in this area, and anticipate showers and thunderstorms to fire up along/east of the divide in the next hour. Weaker showers further north where clouds have kept it more stable. Still expecting good insolation in the south with the boundary slowly moving north to produce a decent enough baroclinic zone/differential heating zone to fire thunderstorms after 2-3pm. At the moment high-res models coming in tend to favor more coverage over the mountains from NC to WV with less coverage east, Getting a little more wind aloft into WV should send any storms just north of east or northeast. Upper flow becomes a little more difluent ahead of next shortwave over VA so expect a better coverage north of Highway 460, but with better severe threat south of Highway 58 in VA, with more widely scattered coverage. Forecast updated to reflect this, but still no headlines at this time. Previous late morning discussion... Getting very concentrated area of moderate to heavier showers along the Blue Ridge north of Peaks of Otter, with isolated lighter coverage elsewhere. The 13z HRRR and RAP13 showed this fairly well, can hang onto a higher pop regime from Southeast WV to the Blue Ridge north of Roanoke, east to Buckingham through early afternoon. At the surface there is an area of convergence in the wind field over northern NC. Vis satellite indicated strong insolation taking place in the south, with heavier cloud cover north of Danville. This looks to be a place where differential heating and some upper vorticity advection takes place through early afternoon. However, 12z RNK sounding even modified is not too aggressive with severe threat today, in terms of an organized threat. Airmass is highly moist so will have to watch for downpours as low level flow is weaker and storms may get anchored. Severe mode today will be pulse to multicellular with isolated large hail or damaging winds. The front should act to focus these storms. SPC still has slight risk out. No flood watches at this time, see hydro section below for details. Forecast update to lower pops based on radar in most of the forecast area, and adjust sky cover to have more sun in the south per latest satellite trends and 12z NAM. Previous discussion from early morning... Large upper low will deepen over southeast Canada today and tonight and gradually back the mean flow over the Mid Atlantic region from the west to the southwest. Models showing a series of small short waves tracking through the southern stream around the base of this trof. Any of these features may aide in occurrence of precipitation. At the surface low pressure will be just off the southeast Virginia coast this morning along a warm front trailing into central Ohio. This boundary will provide some low level surface convergence in the Virginia piedmont this afternoon. By late today the low and front move northeast, with the models showing the front east of Farmville by 03Z/11PM. A weak cold front approaching from the northwest will reach the mountains by Monday morning. Amount of high clouds left over from the line of showers and thunderstorms across Tennessee and North Carolina will impact amount of heating this morning. Expecting enough sunshine to support MUCAPES in the 500-1000J/kg range this afternoon. No good consensus between models today on where most likely areas and times where there will be thunderstorms. Will have the highest probability when the instability will be largest, between 18Z/2PM and 03Z/11PM.
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
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As of 1110 AM EDT Sunday... With the potential for active weather again this afternoon, the short and long-term forecast was completed early, mainly using the 12z NAM, 09z SREF, 06z GFS and the 00z ECMWF. While still some question as to its exact position leading into the period, a poorly-defined northeast to southwest oriented surface cold front should extend from southern Delaware southwestward through the northern Tennessee Valley Monday night. Boundary will lie roughly along the 500 mb gradient between subtropical ridge over the Bahamas and broad cyclonic flow regime associated with the southern end of an upper low over the northern Great Lakes region. This is a setup does not typically encourage much southward progress of the baroclinic zone; as such, chances of mainly showers will remain focused across the Southside of VA southwestward across the northern NC Piedmont and into the NC foothills/High Country, as weak mid-level vorticity maxima propagate along the boundary. Prospect of thunder is low and appears to be more focused from the NC Triangle/Triad areas southward into Upstate SC. Will keep slight chance but it probably is slight in every sense of the phrase. Expecting a dry forecast outside of these areas; may need to watch for potential radiation fog in the dry areas conditional on winds staying light but confidence is not all that high. Cold front should have slipped just south of the forecast area by early Tuesday morning, due to cold advection and effect of rain- cooled air pushing the effective position south. This should lead to a mainly dry forecast at least initially. Some question between the GFS and the ECMWF on areal coverage of showers, with the ECMWF shows some additional light QPF on the northern side of the frontal zone associated with a weak vort max. I`ve therefore kept a low PoP/slight Chance for the NC foothills. The area where both NAM, GFS and ECMWF agree upon as far as shower coverage goes is in eastern WV and the Mountain Empire along a passing surface trough in westerly flow. Depicted PoPs here into the lower Chance range; wouldn`t discount idea of a couple thunderstorms but not thinking areal coverage of those would be all that great. Surface trough advances eastward with little fanfare into the evening hours with forecast trending dry for Tuesday evening. Broad longwave trough over the northern tier of states then digs southward towards the central Appalachians on Wednesday. A more potent mid-level shortwave trough in the west-northwest flow and associated surface front moves eastward Wednesday. Wind fields are rather strong ahead of the front, but poor low-level moisture, limited degree of instability and questions about trough timing cast uncertainty on potential shower/storm coverage and intensity. Due to this uncertainty, kept a generic slight chance PoP for showers and thunderstorms along and west of the foothills of the Blue Ridge. This could be a day with more active weather if greater heating/instability and better moisture can be realized but is definitely conditional. Temperatures through the period trend near to slightly above normal Monday night into Tuesday (highs in the 70s-mid 80s, lows in the 50s to near 60), then start to trend closer to normal toward Tuesday night and Wednesday.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... As of 1111 AM EDT Sunday... Heavily used a blend of guidance for this forecast period. Clearing of the front Wednesday night with increasing subsidence follows for overnight into Thursday. Thursday appears to be the next day with fully dry and mostly sunny conditions for the forecast area along with seasonable temperatures. Unfortunately, it appears that is short-lived, as a trough digs into the southwestern CONUS and sends weak vorticity maxima pivoting eastward across the central Plains, Ohio/TN valleys and into the Appalachians. Difficult to time any specific disturbances at this range but it does appear we return to a more unsettled weather pattern for late week into the weekend. Temperatures start out near normal, then begin to trend near to slightly above along with rising dewpoints heading into the weekend. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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As of 135 PM EDT Sunday... Looks like VFR will be the rule over the area today into this evening outside of any heavier convection. Latest sat/model trends favor a LWB-LYH line for sub VFR with shower and possibly thunderstorm threat, with less coverage to the south. Am confident enough to have tempo TSRA at LWB/ROA/LYH along this corridor for an hour or so between 18z-23z. Still some MVFR cigs at LYH but should be VFR soon. Appears showers/thunderstorms subside this evening, though a few showers or storms may still be present ahead of the cold front tonight in WV. With wet ground/moist low levels and some clearing have added fog in mainly LYH/DAN/LWB/BCB for 3sm, though lower is a possibility. Extended Aviation Discussion... Deep convection becomes less organized Monday into Tuesday with only hit and miss sub-VFR conditions. Perhaps late night and early morning sub-VFR river and mountain valley fog. Better organization of convection Wednesday with the passage of a cold front. Thursday is expected to be VFR and dry, with widely scattered showers or storms starting up again Friday.
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&& .HYDROLOGY... As of 1040 AM EDT Sunday... No flood/flash flood watches at the moment for us. Will have to watch the Blue Ridge north of Roanoke, as well as the Alleghanys, as high-res models favor this area to get showers/storms locked in over the terrain per weaker low level winds this morning. WPC already highlighting this area for possible hydro issues where the low level flow along the front works to keep showers slower moving and meandering along the boundary. Given isolated threat will not put a flash flood watch out at this point. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS/WP NEAR TERM...AMS/PM/WP SHORT TERM...AL LONG TERM...AL AVIATION...AMS/PM/WP HYDROLOGY...WP

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