Area Forecast Discussion
Issued by NWS Blacksburg, VA

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133 FXUS61 KRNK 140922 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 422 AM EST Sat Jan 14 2017 .SYNOPSIS...
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A weak cold front has drifted south of the region and will remain south of the area until Monday at which time it will drift back north of the area. Meanwhile, a series of disturbances aloft will track over the region keeping clouds and periods of precipitation across the region for several days. Finally, a stronger cold front is slated to move through the region by the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 400 AM EST Saturday... Wedge front has drifted as far south as northern SC and into western NC. Models do not support it drifting much further south, but also now keep it south of our area until Monday, at which time a stronger short wave approaching from the west moves it back north. Temperatures have been rather slow to fall and as of this hour all areas remain above freezing. While, there is plenty of light rain drifting across the region per series of weak upper disturbances in zonal flow aloft, with temperatures still above freezing, freezing rain has yet to materialize. NAM and ECMWF still insist that temperatures will drop toward 30 degrees as far south as Craig and Monroe counties in the 12Z-15Z time frame, then hold there until mid-afternoon today. Historically, once a wedge of cold air gets in place, it is very difficult to dislodge. Thus, will not make any changes in the configuration or timing of the Freezing Rain Advisory at this point, although was strongly tempted to do so earlier seeing the reluctance of temperatures to drop below freezing. By the same token, was also tempted to pull out some of the southern counties, but the cold air has more efficiently spread down toward the New River Valley than the Greenbrier Valley, so will leave Craig/Monroe/Botetourt as is as well. If conditions fail to materialize or end earlier than 4 pm, which appears likely, day shift can cancel early or remove counties as needed. Otherwise, have used a NAM/ECMWF blend for the temperatures today as the GFS appears too warm again. Used more of a consensus model blend for tonight. All-in-all, a non-diurnal temperature trend is in order for today. With regard to precipitation, timing will be difficult, but all models suggest that another wave of rain will develop, mainly across western/northern areas later today, then focus back more into the TN valley overnight/early Sunday. At any rate, there will likely be drizzle when it is not raining, so have included drizzle throughout the CWA in addition to the light rain. Pops range from categorical west/north to chance south/east.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
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As of 315 AM EST Saturday... Initial low level wedge across the region to start Sunday will weaken as the flow turns more northwest behind a weak exiting wave offshore. This may bring about some drying across central/eastern sections for much of Sunday before another transient but weaker surface high starts to build in from the north again with yet another wedge ensuing. In addition another faint mid level shortwave will approach along the residual baroclinic zone from the west allowing more backing aloft to take shape. Latest forecast soundings suggest that this will be enough to saturate the column again with early low rain chances far west to start becoming more widespread from west to east by late Sunday. Highs perhaps a bit warmer espcly south and east given weak downsloping and possibly some sun. Otherwise staying on the cooler side of MOS with 40s north to low/mid 50s south. Weak wave aloft crosses the area Sunday night with possible drier air to the north aiding in cooling as the wedge deepens while being overrun by 850 mb warm advection. Models again differ on the degree of low level cooling within the wedge with the Nam again the coldest in showing a rather impressive cold nose just off the surface while other solutions keep boundary layer temps above freezing across the northwest. Since this CAD episode appears a bit weaker as the surface high is not quite as strong, may only leave in a brief freezing rain mention late Sunday night into early Monday in the LWB-HSP vicinity at most. Otherwise appears another rather low QPF event but enough coverage to go with some low likely pops northwest with at least chance elsewhere. Lows mostly 30s north to 40s south but more in line with the slightly warmer Mav/EC mos. Warm advection regime will finally start to win out later Monday into Monday night as more in the way of shortwave ridging takes shape ahead of the next upstream cold front. With the surface high shifting from eastern sections Monday morning to offshore by Monday night would expect the cooler 40s to linger east Monday with warmer 50s far west. Pops likely more scattered to isolated given less forcing and deeper moisture to the west with perhaps little shower coverage by Monday night when should be well into the warm sector.
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&& .LONG TERM /WEDNESDAY THROUGH FRIDAY/...
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As of 300 PM EST Friday... Frontal boundary lifts north into New York Tuesday and reaches Maine by Tuesday night as strong upper low moving across the central U.S. amplifies ridge downstream. Cold front approaches from the west on Wednesday and moves into the CWA on Thursday. On Thursday night into Friday, the boundary slowly moves north across the region. There are differences in timing of frontal systems with GFS faster than ECMWF. Leaned towards superblend for Medium range. High temperatures will run about 10- 20 degrees above normal Tuesday into Friday.
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&& .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... As of 650 PM EST Friday... Area of LIFR clouds and fog had spread into western Virginia out of Kentucky this evening. Patches of light rain has also reached southwest Virginia and southern West Virginia. Medium confidence that KLWB will drop at least to MVFR by 03Z/10PM. Areas of rain will continue north of an east-west oriented stationary front across Tennessee and North Carolina. As high pressure moves from the Great Lakes into New England, a wedge of high pressure will develop along the east slopes of the Appalachians and cold air will advance south along the east side of the mountains. Still some differences in the models but consensus was to follow cooler NAM. This keeps the colder air in a shallow wedge, mainly less than 2000 feet thick. A brief period of freezing rain will be possible in southeast West Virginia and across the Alleghany Highlands and southern Shenandoah Valley early Saturday morning, including at KLWB and KHSP. High confidence that steadier rainfall and any threat of FZRA will be after 06Z/1AM. Extended Aviation Discussion... A baroclinic zone will remain anchored across the region sandwiched between a strong upper ridge over the southeast states and an active jet stream from the southwest U.S. into the northeast U.S. Thus, an unsettled, wet weather pattern is expected to remain in place through the Wednesday. The frontal boundary may lift north enough by Monday for a brief period of mainly VFR, but a stronger front will arrive By Tuesday with more showers. Generally looking at MVFR much of the period, although periods of VFR as well as IFR-LIFR can be expected at times. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Freezing Rain Advisory until 4 PM EST this afternoon for VAZ018>020-023-024-035. NC...None. WV...Freezing Rain Advisory until 4 PM EST this afternoon for WVZ044-507.
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&& $$ SYNOPSIS...RAB NEAR TERM...RAB SHORT TERM...JH LONG TERM...JH/KK AVIATION...AMS/RCS

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