Area Forecast Discussion
Issued by NWS Blacksburg, VA

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260 FXUS61 KRNK 221117 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 617 AM 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 430 PM EST Saturday... 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 /10Z SUNDAY THROUGH THURSDAY/... As of 1150 PM EST Saturday... Poor flying conditions expected during the TAF valid period with widespread IFR/LIFR in low clouds,fog and rain. This evening into tonight, one area of rain has moved on to the northeast, but light rain, drizzle, fog, and log clouds will be left behind throughout the night with deterioriating conditions. Expect ceilings and visibilities to drop to or remain in the IFR to LIFR range at most TAF sites. Second wave of steadier rainfall then looks to build from the southwest during the morning hours Sunday probably arriving after daybreak, with coverage of rain covering a larger portion of our forecast airspace. There may be a break in this activity during the afternoon, but another more significant wave of rain and possibly thunder will arrive during the evening. These rounds of rain are associated with a deep upper low moving slowly through the southeast states currently responsible for several waves of severe weather in the deep south. Due to low forecast confidence in specific airports being affected by thunder, did not mention in the TAF at this point, but the best shot would be at KDAN. Expect flight categories to remain mostly IFR to LIFR through 00z Monday due to low ceilings, with visibilities 3-6 SM in rain briefly heavy at times. Winds generally east-northeast to east-southeast through most of the TAF period at speeds of 4-7kts, increasing and becoming gusty at KBLF during the evening hours. 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. && .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...PM SHORT TERM...DS LONG TERM...JH AVIATION...AL/KK/RAB HYDROLOGY...AL/PM

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