Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 221808 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 108 PM EST Sun Jan 22 2017 .SYNOPSIS... A warm front south of the area will continue to be the focus for periods of rain today. 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. && .NEAR TERM /THROUGH TONIGHT/... As of 954 AM EST Sunday...Composite radar imagery reveals very light radar echoes arcing east-southeastward across the southwestern part of the forecast area. It`s now clear that model guidance QPF fields for the Today period are vastly overdone, as northward advection of deeper moisture content (PWATs in excess of 1") has been confined ("robbed" is probably the best way to call it) to ongoing stronger convective complex extending along the Carolina coastline into southern GA/FL Panhandle. Nearly all CAM model output suggests that ongoing light precip band is all that our forecast area really sees until tonight, as dry punch moves in aloft from the eastern TN valley. Only the 12z HRRR shows any showery precip increasing again by 21z into the North Carolina high country, with the greater slug of QPF associated with warm frontal zone that advects northward after dark. So I`ve followed that consensus and have essentially nowcasted that area of light precip northeastward through the afternoon. Patchy drizzle and fog apt to precede and taking place behind this light precip band as low-level inversion associated with wedge/CAD remains in place. This is a high-PoP, low-QPF scenario and I`ve also reduced QPF down significantly for today as best I could. Little significant warm-up is anticipated today and earlier idea of undercutting MOS guidance looks good. Therefore, I`ve made no alterations to temps with this update. Previous near-term discussion issued earlier this morning follows... Upper closed low over the Red River Valley (TX/OK) border will move across Arkansas today, and into the TN valley by this evening. A 995 mb surface low will move east, deepening to near 985 mb as it reaches the southern Appalachians tonight. A warm front which extends east of the low will move north into our region today. An area of rain will move north along the warm front, crossing the forecast area from southwest to northeast. Elevated convection will result in some intermittent higher rainfall rates embedded within the area of rain that passes through the forecast area today. Rainfall amounts between 7A-7P are expected to average between 0.25 to 0.50. As for thunderstorms, it appears the greatest threat for storms today will remain south of the forecast area across the Deep South and possibly as far north as TN/NC. Forecast elevated CAPE for our CWA approaches 100 j/kg across our southern CWA later this morning, but coverage is just not enough to warrant mention of thunder (at least not for the daylight hours) in the today portion of the forecast. Temperatures today will change very little (maybe a 3 to 5 degree rise), the forecast area spending most of the day on the north side (cool side) of the warm front. For Tonight: The surface low will have deepened about 10 mb on its trek from Arkansas to the border of TN/NC, resulting in a second wave of rain which will cross the forecast area during the overnight. Upward vertical motion will significantly increase courtesy of the upper low, forecast elevated CAPE increasing to around 500 j/kg. This increase in elevated instability is enough to support mention of thunder in the forecast for tonight, this elevated deep convection enhancing the rainfall rates and resulting in periods of moderate to heavy rain. The storm prediction center has maintained a Marginal/5% severe and Slight/15% severe across our southeastern CWA for tonight, which primarily impacts to our Piedmont NC counties into the southside of Virginia due to strong vertical wind profiles. However only very marginal if any surface-based CAPE will exist this far north latitude with much of the already modest instability being elevated. Potential is there for thunder but given the limited amount of instability due to the low level stable air over the forecast, not thinking we`ll see anything severe within the confines of the RNK CWA. A conditionally greater risk for stronger cells remains well to the south from central NC to FL and as reflected in SPC`s Day-1 Convective Outlook with implications for Supercell Storms and Tornadoes. Our greatest hazard for the central Appalachians looks to be potential for heavy rainfall. Models suggest upwards of an inch, maybe two due to the embedded deep convection. There will be a period tonight when the wind flow is out of the southeast, so it`s not out of the question for some locally higher rain amounts to occur along the eastern slopes of the Blue Ridge per added rain efficiency from the upslope wind flow. See the Hydro section which speaks more to the hydrologic aspect. Temperatures tonight will change very little...although could sneak up a degree or two per the encroachment of the warm front across the NC Piedmont. && .SHORT TERM /MONDAY THROUGH MONDAY NIGHT/... As of 330 AM EST Sunday... On Monday, a closed upper level low will make gradual progress northeast across our region. Various model guidance solutions are converging on a solution that passes the center of it over the southeast part of the area. This trajectory will allow for the best chance of precipitation along the western and northern flanks of the region in alignment with the associated deformation zone. By Monday evening, the center of the low will be lifting northeast out of the region, and northwest winds on the backside of the system will start to increase across the area. This will help maintain upslope precipitation across the mountains with areas east of the Blue Ridge starting to see less cloud cover as subsidence increases. Enough cooler air may reach the higher elevations between southeast West Virginia and the Northern Mountains of North Carolina for a mix with, or change to, light snow showers. The bulk of the precipitation across the mountains will remain as rain showers. Gusty conditions are expected across the area by late Monday night into Tuesday as northwest 850mb flow increases to around 40 to 45 kts. Numbers are subject to change, but the latest indications are that gusts of 25 to 35 mph will be common across the mountains, with the highest elevations approaching 50 mph gusts. This most likely locations for the strongest wind gusts is still the Northern Mountains of North Carolina, north into the Grayson Highlands of southwest Virginia. Across the Piedmont, gusts of 15 to 25 mph will be more likely. Upslope rain/snow show showers will continue during the day Tuesday, but coverage and intensity will decrease as the day progresses thanks to drier air entering the region. Tuesday night into Wednesday, and upper ridge will move east of the region, allowing for low level winds to transition to being southwest or west. No precipitation is expected Tuesday night into Wednesday. By Wednesday night, a cold front will be approaching from the west. To our south, guidance varies to the degree which a disturbance moves northeast within the southwest flow and adds a tongue of moisture and instability in advance of the front. The GFS offers a solution is more robust for precipitation across our area as compared to its European counterpart. Our forecast will reflect an average of the two, and offer a drier version of the GFS. Temperatures during this portion of the forecast will continue to remain very mild for this time of the year, averaging around fifteen degrees above normal. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 330 AM EST Sunday... Return to more winterlike conditions will ensue by weeks end with a cold frontal passage by Thursday ahead of a broad longwave trough that will drop south into the region and persist into next weekend. However latest guidance not nearly as strong or deep with this feature making for more Canadian type air vs. much colder thickness seen yesterday that was supportive of Arctic air. However will see persistent cold advection develop by Thursday with weak passing shortwaves gradually helping to bring in colder air aloft by Saturday. A band of showers possible along/behind the front Thursday into Thursday night although iffy given only sheared southern energy. Therefore will only include a low pop shower mention including possible western snow showers overnight. Otherwise will evolve to mostly upslope driven clouds and periodic light snow showers far west, to mainly clear/sunny east Friday into Saturday, under a rather dry and more westerly flow driven environment. Highs mostly 30s to around 40 mountains to mid 40s east into the weekend. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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As of 106 PM EST Saturday... 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 continue to maintain generally IFR to LIFR categories mainly due to low ceilings with intermittent periods of MVFR/IFR mist. Conditions remain more or less status- quo until a strong warm front advances from south to north generally after 01z. Expect a period of at least light to moderate rain (visbys 4-6 SM) across the TAFs, although rain may be heavy at times/IFR-type visby at DAN between 02-06z. Limited potential for thunder and due to low confidence, 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, leading to potential for LLWS as well. Easterly winds stand to increase to near 10-12 kts at all terminals with gusts to 20-25 kts at BLF, LWB and BCB through early morning. Warm frontal rainband then pivots north-northwest after 08z Monday. Steadiest rain, light to moderate, to then focus and west of the Blue Ridge, with intermittent/showery rains at LYH and DAN. 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 BCB/LWB late in the TAF period. Low to 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.
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&& .HYDROLOGY... As of 530 AM EST Saturday... Periods of rain will impact the Blacksburg hydrologic service today and tonight, lingering through Monday. 24 hour rainfall amounts,ending 7AM Monday are expected to range from 1 to 2 inches, with isolated heavier amounts along the Blue Ridge. Current expectation is for gradual 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...PM NEAR TERM...AL/PM SHORT TERM...DS LONG TERM...JH AVIATION...AL HYDROLOGY...AL/PM is the U.S. government's official web portal to all federal, state and local government web resources and services.