Area Forecast Discussion
Issued by NWS Blacksburg, VA

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982 FXUS61 KRNK 222349 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 649 PM EST Sun Jan 22 2017 .SYNOPSIS... A strong area of low pressure over the Mid Mississippi valley will approach from the west tonight bringing a period of moderate to heavy rain. This low pressure system will slide slowly east across across the area Monday before exiting Monday night. In the wake of the low pressure system, a period of gusty northwest winds are possible for the North Carolina high country and the Blue Ridge. Temperatures trend closer to seasonal levels by later in the week. && .NEAR TERM /THROUGH MONDAY/... As of 337 PM EST Sunday... Conditions are relatively quiet as of this writing as a persistent surface wedge/CAD has kept us overcast and cool (though still above normal); however, that stands to change abruptly tonight and extending into much of the short-term period. Regional surface analysis depicts a developing low (now at 992 mb in central Alabama); its surface warm front extending eastward into coastal Georgia/SC with a frontal occlusion south of the surface low just west of Talahassee FL. Surface cyclone is projected to deepen further and advance northeast tonight into the southern Appalachians, and move more slowly east into the eastern shore of Virginia/North Carolina Monday. Aloft, pronounced closed upper low over the lower Mississippi Valley will shift northeast into western North Carolina by morning, eventually becoming nearly vertically stacked with the surface cyclone. Slow northeast progression is anticipated into eastern Virginia from 12z Monday thru 00z Tuesday. For Tonight: Active period with several aspects to consider, including rainfall/hydro aspects, breezy east to northeast winds at elevation and potential for elevated thunder tonight. Regarding the rainfall aspect, warm front/occlusion pivots northeast over top of lingering wedge late tonight and into early overnight. Most of the higher resolution CAM models and coarser-res global models indicate that the eastern two-thirds of the forecast area stands to get brushed by at least steady rain on the northern "comma-head". This rainfall will probably sharpen owing to strong 850-700 mb frontogenetic forcing apparent in the 12Z GFS. Given 40 kts of easterly flow tapping into higher PWATs over the western Atlantic and the GFS indicating marginal but still noticeable instability (elevated instability with MUCAPEs up to 150 J/kg), I`ve added heavy rainfall wording into the forecast for an area from the Virginia and North Carolina piedmont into the southside of Virginia area for a part of the overnight. It`s such a narrow corridor of higher moisture content, though, that I didn`t extend it further west than those areas, but even areas to the Blue Ridge stand to see steady light to moderate rain and slight chance for Thunder. But the risk is slight in every sense of the word and we are not expecting even stronger thunderstorms at this point given the meager degree of instability. In fact, SPC has trimmed back its earlier areas of Marginal/Slight Risk across parts of our forecast area to indicate more general thunder, which seems reasonable. Steadiest rain doesn`t get into the Mountain Empire northeast into southeast West Virginia and the Alleghany Highlands until well into the overnight as the precipitation shield pivots back northwestward along an inverted trough extending north from the upper low. Precipitation gradually becomes more showery east of the Blue Ridge as that process proceeds. So by the morning commute, the steadiest rain would be confined to our western zones. Wide range in forecast QPF amounts ranging from up to a tenth in western Greenbrier county to near 1.5 inches along the North Carolina high country up into the Virginia Piedmont. Rainfall rates should generally be steady/gradual, but potential for slightly higher rates tonight in the southside of Virginia and Virginia/North Carolina Piedmont. Specific to winds - easterly winds stand to increase both at the surface but particularly at elevation. East-northeast low level jet of up to 50 kts progresses up the spine of the Blue Ridge, with gusts up to 35 mph above 3000`. At this point, given the potential for moderate rain and that it is not a common trajectory for strong winds, opted to hold on any wind headlines due to these easterly winds. Under overcast, lows will generally not fall all that much and I`ve generally warmed temps from MOS by several degrees, into the 40s to low 50s. For Monday: Steady light rain will be ongoing across the western portion of the forecast area (Blue Ridge westward) with high- Likely to Categorical PoPs through much of Monday, focused by an axis of mid-level deformation on the northern side of the upper low. Precipitation character east of the Blue Ridge should stay more showery however as a dry slot on the eastern side of the upper low pushes northward across that portion of the forecast area. Though the NAM offers some leftover instability east of the Blue Ridge, I`m not as convinced such will be realized as it will take a break in the clouds to really tap into that instability. Rainfall amounts Monday are greatest west of the Blue Ridge (areal amounts of a quarter to half inch) with up to a tenth of an inch east. By late in the day as surface low exits into the Piedmont, winds begin to shift from light and variable to northwest with an increase in speed, and may turn gusty at elevation late in the period. Temps range from the upper 40s to the middle 50s across the forecast area. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 337 PM EST Sunday... Stacked low pressure system will be exiting east and offshore Monday night with lingering precip mainly of the wrap around/upslope variety espcly western mountains with decreasing coverage east overnight as subsidence/downslope develops. Will again have the higher likely pops west of the Blue Ridge along whats left of the deformation zone early on and strong northwest upslope through early Tuesday. Appears may finally get cold enough aloft under the passing upper cold pool and cooling boundary layer to support at least some elevation snow showers espcly BLF-LWB corridor and points west. However wont go as cold as the latest Nam as it has a much colder conveyor belt scenario per the low tracking farther south to start. This still supports an inch or so of snow northwest ridges before moisture shallows out from above by Tuesday afternoon when will cut pops to just slight pops. Other lingering concern again with the potential for very strong northwest winds in the wake of the system Monday night into Tuesday afternoon as the pressure gradient ramps up on the back of exiting upper low. Latest GFS still showing 45-50 kts at 850 mb with a period of strong subsidence and lowering inversion levels overnight. This in conjunction with local scheme program values supports advisory criteria along the Blue Ridge from TNB to ROA so may expand mention in the HWO. Quiet weather returns Tuesday night into Wednesday as strong shortwave ridging shifts east ahead of the next upstream cold front. Very warm temps aloft under this feature likely to bring the last of the mild days for a while Wednesday with potential for all spots to break 60, and perhaps near 70 Piedmont given westerly flow, dry air and sunshine. Next cold front enters from the west Wednesday night signaling the end of the long duration thaw as cooling 850 mb temps chase the boundary east overnight. Some solutions attempting to link some southern stream moisture up along the boundary overnight similar to the GFS, while others tending to keep jet moisture sheared and separate. Again think the GFS is overdone but for consistency sake will leave in some low pops overnight. This mostly rain showers until perhaps very late far west where colder air could catch departing moisture with a brief period of snow showers before upslope kicks in. Lows mostly 30s west to low/mid 40s east. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 337 PM EST Sunday... Pattern change to overall broad longwave troughing over the region will begin to take shape Thursday in the wake of a passing surface cold front by early in the day. This will lead to a prolonged period of cold advection under west/northwest flow to the east of the main surface high situated over the Rockies through next weekend. However appears the overall scenario to remain on the quiet side with mainly shallow moisture resulting in upslope driven periodic snow showers espcly northwest sections into Sunday. Appears amounts to be limited though given more westerly trajectory and only shallow moisture beneath the inversion, so overall only chance pops this far out along the western slopes. Otherwise should see the typical split from mainly sunny/clear skies east of the ridges per downslope, to clouds far west, and at times out to the Blue Ridge pending the passage of weak impulses. With the broad trough becoming rather flat and 850 mb temps roughly above minus 10C, only expecting a return to normal or slightly below normal temps with highs 30s/low 40s mountains, to low/mid 40s east, and lows in the 20s. However will feel quite a bit colder after all the recent warmth, given a persistent blustery northwest wind under the lingering gradient between low pressure well north and the high well to the west. Appears coldest could occur during the weekend when thickness drop a bit so trended toward the colder earlier EC mos as the GFS MEX values remain warmer. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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As of 649 PM EST Sunday... Poor flying conditions will continue across the terminals through much of the TAF period. Only very slight improvement is expected by later Monday. Wedge/CAD conditions will continue generally IFR to LIFR categories mainly due to low ceilings with intermittent periods of MVFR/IFR rain. A strong warm front advances northward across the region tonight into Monday. Low pressure will consolidate off the Virginia coast by Monday evening. Expect a period of at least light to moderate rain (visbys 4-6 SM) across the TAFs, although rain may be heavy at times with IFR VSBY at DAN overnight. Because any thunderstorms will be hindered by the stability of the Wedge. Decided to limited potential for thunder, and have left out of the TAFs. Nevertheless, IFR to LIFR conditions should continue as the rain overspreads. Along/ahead of the warm front, easterly winds increase to near 40 kts above 3000ft. Light easterly winds will increase to near 10-12 kts at most terminals with gusts to 20-25 kts at KBLF, KLWB and KBCB through early morning. On Monday morning, Warm frontal rainband pivots north-northwest. Steadiest rain, light to moderate, to then focus and west of the Blue Ridge, with intermittent/showery rains at KLYH and KDAN. Slow improvement anticipated Monday with conditions primarily IFR to MVFR near the end of the TAF period. Easterly winds veer around to southeast/south 4-6 kts most terminals, but become west to northwest at KBCB/KBLF/KLWB late in the TAF period. The timing of the improvement of ceilings appears a little slower on this evenings model run. Medium confidence in ceilings and visibilities throughout the TAF valid period. Medium confidence in wind speed and direction throughout the TAF valid period. Extended Aviation Discussion... The slow moving and deep upper low will continue to affect the area Monday with waves of rain, but the thunder threat should have moved well east of the area by Monday. Expect sub-VFR conditions to continue into Monday, but the activity will likely become more showery in nature as the center of the upper low drifts overhead and to the north of the region. The upper low will finally move northeast of the region Tuesday resulting in improving conditions, especially east of the Blue Ridge. Gusty northwest winds are expected Tuesday as surface low pressure deepens across the northeast U.S. High pressure will finally spread into the region by Wednesday. A band of MVFR showers possible along/behind the front Thursday into Thursday night. West to northwest flow will continue into the Weekend with upslope driven periodic snow showers in the western mountains. .
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&& .HYDROLOGY... As of 326 PM EST Sunday... Periods of rain will impact the Blacksburg hydrologic service tonight into Monday from a complex frontal system taking shape over the southern United States. A warm front moving in tonight from the south will allow for a widespread period of rain. Areas south and east of the Blue Ridge stand to see the greatest potential for heavier rainfall rates overnight tonight. The rain band then pivots northward where rainfall intensity is expected to be more steady and lighter. Rainfall should be primarily focused from the Blue Ridge west during the day on Monday, with more intermittent/showers along and east of the Blue Ridge. Forecast rainfall amounts tonight through Monday night range from 1 to 2 inches. There have been modest river rises occurring today, and the expectation continues to be a gradual rise on main-stem river rises with faster/greater response on smaller rivers/creeks, along with some potential for ponding on roads and in areas of poor drainage. The only river forecast point projected to reach or go just above flood stage (Minor Flooding) is the Dan River at South Boston. Since this is the only river forecast point and there is still considerable uncertainty as to how much, if any, we will exceed 2 inches of rainfall (which is what it would take to cause flooding), will not issue a Flood Watch at the present time. Worst case would be to have this 2 inches fall in a shorter period of time or the upper low slow its forward progress resulting in a longer duration of upslope flow with rain amounts exceeding the 2 inches. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/KK HYDROLOGY...AL/PM is the U.S. government's official web portal to all federal, state and local government web resources and services.