Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 231426 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1026 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 1020 AM EDT Tuesday... Moderate to heavy rainfall mainly east of the Blue Ridge earlier today has moved out of the area, causing streams and creeks to run near bankful and drainage into the mainstem Dan river has prompted flood warnings from Danville through South Boston. There is a lull in the action at the moment, but the frontal boundary is lingering in our far eastern counties and the upper jet structure continues to generate large scale ascent as additional waves slide northeast along the front. Meso guidance and an early glance at 12Z data coming show a resurgence in rainfall with precipitation moving up from the southwest this afternoon. The flood watch continues to look on target and no changes will be made. Previous discussion... 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 /14Z TUESDAY THROUGH SATURDAY/... As of 645 AM EDT Tuesday... Will see a gradual deterioration in flying conditions from south to north this morning as bands of light to moderate rain shift north. However appears the majority of the heavier rainfall will stay in the KDAN/KLYH corridor this morning. This will result in sub-VFR along/east of the Blue Ridge with ocnl IFR/LIFR cigs and periodic MVFR to IFR vsbys mainly around KDAN. A bit more uncertainty exists from KBCB westward where cigs will be slower to lower within the wedge with perhaps KBLF-KLWB staying VFR until late morning. Expecting all to fall below VFR this afternoon with widespread IFR possible as moisture deepens and another round of rain slides northeast. Steadier rain should taper off this evening leaving IFR/MVFR cigs and vsbys in fog espcly after midnight. Another low pressure system will arrive Wednesday with additional showers and storms likely espcly in the afternoon. Sub-VFR conditions should again accompany this area of rainfall with better chances of thunderstorms across southern and perhaps eastern sections Wednesday afternoon/evening. Extended Aviation Discussion... Rainfall should become a bit more showery by Thursday with sub-VFR mainly associated with the showers while periods of afternoon VFR possible outside of the convection. Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more showers and thunderstorms including sub- VFR conditions. && .HYDROLOGY... As of 405 AM EDT Tuesday... 1 to 2 inches of rain with locally up to 3 inches can be expected especially across southern and eastern sections today into tonight. FFG along parts of the southern Blue Ridge remains in the 1.5 to 2.5 inch range with the ground nearly saturated in spots from heavy rainfall over the past couple of days. This additional rainfall through tonight combined with locally heavier rainfall rates due to showers and storms could result in possible flooding of streams and creeks. Thus a flood watch has been issued for counties along and south of the VA/NC border excluding the NC mountains into tonight. Flooding along the lower reaches of the Dan River could also occur tonight into late week pending the amount of rainfall today and subsequent runoff. More rainfall is expected Wednesday and Thursday, which could lead to additional small stream and river flooding. Models continue to favor areas along/west of the Blue Ridge, possibly resulting in another 1 to 3 inches of rain which could pose more widespread flooding issues in the wake of heavier rainfall from today. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flood Watch through late tonight for VAZ016-032-043-044-058. NC...Flood Watch through late tonight for NCZ003>006-019-020. WV...None. && $$ SYNOPSIS...JH NEAR TERM...MBS/JH SHORT TERM...JH LONG TERM...RAB AVIATION...AMS/JH/WP HYDROLOGY...AMS/JH

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