Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 230803 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 403 AM EDT Tue May 23 2017 .SYNOPSIS... A weak stalled front over northern North Carolina will remain in place today as ripples of low pressure slide northeast along the boundary resulting in added rainfall into tonight. Yet another area of low pressure will arrive midweek with added showers likely for Wednesday and Thursday. Weak high pressure will follow this system for Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 245 AM EDT Tuesday... Convection along the NC/VA border and points south continues to evolve slowly north into the developing wedge north of the residual front that remains across the far southern counties attm. This in advance of main shortwave energy that will round through the southeast states within strong southwest flow aloft this morning before exiting with a final surface wave passing offshore this evening. Increasing low level easterly flow along with a surge in PWATS to over an inch south/east beneath a 6-8 hour window of good upper diffluence, supports widespread showers/rain through much of the day as overrunning of the boundary persists. However exactly where the axis of heaviest rainfall sets up still iffy given potential for more convection to the south and overall lighter rates per more stratiform rainfall over our region. This supported by the latest HRRR that has multiple rounds of steadier rainfall while keeping heavier totals to the south and west. Latest flash flood guidance has become much lower over the far south given rounds of heavier showers in the past few days and where higher QPF totals appear likely into this evening. Thus have opted to hoist a flood watch across the VA/NC border counties into tonight given ongoing deeper convection and likely lingering light/moderate rainfall later on. Just how far north some of the elevated/embedded convection gets key to heavier amounts elsewhere espcly east so something to watch. Otherwise trend of ramping pops up to likelys up to Highway 460 on track through daybreak, followed by likely to categorical coverage for rain at times through the day all sections. Will be much cooler within the developing wedge with potential for temps to stay in the 50s in spots today, with only low/mid 60s at best most locations. Expect rainfall to linger espcly eastern half this evening before seeing drying aloft kick in behind the departing wave overnight. This should allow pops to trend back to chance or lower through the night as the region slips in between this exiting system and the next upstream closed low that will be approaching by morning. Will remain cool with lows overall in the 50s as the wedge remains. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... As of 300 AM EDT Tuesday... Any reprieve in rainfall will be short as a strong upper low across the Midwest teams with associated surface low pressure to drive another axis of deep moisture ahead of a cold front into the area Wednesday. Appears enough of a remnant wedge will linger ahead of this feature to limit instability and convective development, but good isentropic lift and dynamic support will make for increasing rainfall and some possibly some embedded thunder from lift over the stable layer. Also, there may be a very narrow window just ahead of the front, espcly far southern sections where the wedge has weakened that may allow for just enough surface based instability to support deeper convection in a highly sheared environment. Other issue will be with another round of heavy rainfall that this time looks to occur mainly over western sections with higher rainfall rates possible within elevated convection. This combined with rainfall from today could prime things up even more that another watch may be needed. Thus will address in the HWO for possible flooding into Wednesday evening behind the first event today. Bands of showers rotating around the upper system may linger into Wednesday evening before some semblance of a dry slot aloft arrives overnight. This along with loss of heating and best lift passing to the north/east should allow for decreasing pops overnight. However may not be until late before coverage wanes given wrap around potential far west and diffluence elsewhere ahead of the cold pool that will still be west of the mountains through daybreak Thursday. Should be a little milder Wednesday given strong warm advection aloft and a gradual breakdown of the wedge which supports highs mostly in the 60s. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 330 AM EDT Tuesday... Active and very unsettled weather to continue through the period. A deep 552dm upper low will be overhead early Thursday, rotating northeast to off the Mid-Atlantic coast line by Friday morning. With the upper low over the area Thursday, expect scattered to numerous convection with steep lapse rates and ample low-level moisture in place. After an active day Thursday, convection will diminish fairly quickly after sunset Thursday evening as the upper low lifts off to the northeast. Friday, will bring a brief break from the active weather as the area is briefly sandwiched between the departing upper low and yet another upper low digging southeast from the Northern Plains. Can`t completely rule out a few afternoon showers/thunderstorms across the western mountains, but in general pops are below 15% for much of the CWA during the daytime hours Friday. Saturday, a baroclinic zone develops across the area as the upper low to the northwest very slowly sags southward into the Midwest. This pattern brings about a concern for more heavy rainfall from convection as well as a greater severe threat than we are seeing with the near term/short term convection. The first round of convection will arrive into the region Friday night, then redevelop along the baroclinic zone during the day Saturday. Another round is likely Sunday as the upper low shifts slowly eastward. Both days there appears to be at least some threat for severe with strong instability along the baroclinic zone. SIG SVR values are in the 20,000 to 25,000 range on several models both Sat and Sun afternoon. Temperatures will be warmer and closer to or even slightly above normal during the extended periods with highs 70s west to lower 80s east and lows in the 50s west to the 60s east. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... As of 1250 AM EDT Tuesday... Will see a gradual deterioration in flying conditions overnight as scattered convection to the south combines with added moisture heading north to bring a quick lowering of cigs through daybreak. Rain should start spreading northward overnight and cigs will start to sink to MVFR or lower, especially along/east of the Blue Ridge. The rain will become steadier and possibly heavier late tonight into Tuesday from KBCB/KROA and points east. Should also see areas of fog with mountain obscurations likely as well into Tuesday. This pattern will stick around through the end of the taf period, but rain will be lighter over KBLF/KLWB. Extended Aviation Discussion... The chance of precipitation continues Wednesday and Thursday with sub-VFR conditions associated with any of the precipitation. Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more precipitation and sub-VFR conditions. && .HYDROLOGY... As of 345 PM EDT Monday... An inch to inch and a half of rain is expected tonight and Tuesday. Amounts will have to be monitored as some locations along the Virginia/North Carolina border, especially Carrol, Patrick and Henry Counties in Virginia and Stokes, Rockingham, and Caswell Counties in North Carolina. FFG along parts of the southern Blue Ridge was in the 1.5 to 2.5 inch range. More rainfall is expected Wednesday and Thursday, which could lead to small stream and river flooding. Models continue to favor areas along/east of the Blue Ridge, possibly resulting in another 1 to 3 inches of rain. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...JH LONG TERM...RAB AVIATION...AMS/JH/WP HYDROLOGY...AMS/JH

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