Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200527 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1227 AM EST Tue Feb 20 2018 .SYNOPSIS... A warm front will lift north of the region overnight and a strong ridge will then set up off the southeast coast providing much above normal temperatures through the week. Chances for rain increase late Wednesday into Thursday. && .NEAR TERM /THROUGH TODAY/... As of 1216 AM EST Monday...Issued dense fog advisory in effect until 10 AM Tuesday for much of the Piedmont, mountains of NW NC and into the New River and Roanoke. Fog or mist has become prevalent across much of the VA/NC Piedmont as well as into the New River Valley. Fog should become dense at times in the Advisory area given influx of deeper moisture advection amid light southeast low- level flow, trapped underneath a near- surface inversion. NAM/HRRR and GFS LAMP visby progs lowest in the Piedmont and foothills areas, but hint at expansion into the Roanoke area overnight. It`s still open to question when fog may lift - BUFKIT soundings suggest the boundary layer may not mix out until the early afternoon. Will need to revisit with possible extension of the dense fog advisory later in time. Otherwise, no other changes needed attm. As of 235 PM EST Monday... The big weather story in the near term (and beyond) is in fact the development of a very big ridge off the southeast U.S. coast which all the global models have been consistently showing with somewhat varying amplitude for at least several days and short- term models now in agreement. WPC model diagnostic discussion favors the EC and EC ensembles for the strength of the ridge which is a little less extreme than the UKMET and CMC but still highly anomalous. Heights more typical of summer with 594 dm heights at h5 building over the Carolinas by midday Tuesday will allow for unusually warm temperatures expected both overnight and Tuesday (see Climate section below for details on possible records). 850 mb temps are near +15C by Tuesday afternoon and with more sun than today would not be surprised if some records highs are at least challenged, although at this point only Bluefield is currently forecast at a record for tomorrow. Overnight lows will not fall off much tonight especially in the east where lingering overcast impedes diurnal cooling. Patchy fog is likely tonight and some very light drizzle possible mainly along the Blue Ridge where ceilings may be quite low at times. Clouds will continue to diminish tomorrow with temps maxing out in upper 60s to low 70s, some 20 to 30 degrees above normal for this date. && .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... As of 200 PM EST Monday... Surface high pressure over Bermuda and a strong 595 dam upper level ridge just off the East Coast will provide a strong warm air advection pattern for Tuesday night and Wednesday. Meanwhile, a cold front will try to move eastward across the Ohio River Valley. Model guidance continues to show well above normal temperatures for Wednesday that could put records in jeopardy, which are further discussed in the Climate section. The only caveats going against this record warmth are cloud cover and a chance of afternoon showers. A slight threat of a thunderstorm exists over southeast West Virginia, but model soundings reveal quite weak instability with CAPE only reaching 100 to 200 J/kg. The latest GFS solution has a little more cloud and shower coverage than the NAM and ECMWF models. There is a consensus that some morning clouds could linger along the southern Blue Ridge until the afternoon, but temperatures could soar significantly once the clouds break up. The chance of rain will increase during Wednesday night and Thursday as the cold front tries to edge closer. Models portray the upper level ridge slowly flattening and sagging southward, but it will still resist the eastward progression of the cold front. Temperatures should remain well above normal for the middle of February, but highs should run a little cooler for Thursday due to surface high pressure passing over eastern Canada and beginning to wedge southward along the eastern side of the Appalachian Mountains. Eventually, the cold front will fold over and set up as a nearly stationary baroclinic zone over northern Virginia. Thus, this forecast keeps the highest probability of rain north of Interstate 64. Shower coverage should also increase west of Interstate 81 later in the afternoon. With the cool wedge in place by Thursday night, lows should be notably cooler compared to Wednesday night. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 200 PM EST Monday... The cool wedge should weaken as surface high pressure heads offshore on Friday. However, the latest GFS shows a slower weakening trend compared to the ECMWF model. Another area of low pressure will try to develop over the southern Gulf Coast as an upper level trough progresses eastward from the Rocky Mountains. More clouds and chances of rain seem likely for the upcoming weekend as this area of low pressure pushes eastward. Meanwhile, the upper level ridge over the Southeast should finally give way and heads toward Florida. That pattern change should free up the atmospheric flow to allow a cold front passage over the Mid Atlantic toward Sunday night. High pressure should allow drier conditions by Monday. Despite the overall trend of more clouds and rain, temperatures still should remain considerably above normal with no anticipated threat of freezing temperatures. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 1204 AM EST Monday... Areas of mist/fog have been common this evening across the Piedmont of VA/NC and as well up into the New River Valley. Expect most TAFs aside from Bluefield to see conditions ranging from IFR to VLIFR through at least 12z either from low ceilings or fog or a combo of both. Fog may be dense at times as influx of subtropical moisture reinforces the fog layer, trapped underneath low-level inversion evident in forecast soundings. Lowest confidence of fog/mist at Lewisburg, and indicated 4SM BR for a good portion of the overnight. It`s open to question when fog actually thins out. Typical fog climo would suggest a mid- morning burn off, but some indication from forecast soundings suggest the boundary layer doesn`t start to mix until around noon/16z, so it is possible that fog may linger later than presently indicated. There does appear to be a window of brief VFR/MVFR conditions during the afternoon. Atmosphere may destabilize and lead to spotty convective showers mainly west of the Ridge, ending by early evening. Sub-VFR stratus again begins to build in Wednesday evening late in the 06z TAF period. Surface ridge overhead has resulted in light to calm winds, helping to foster fog development. Would expect that to remain unchanged through overnight, with areas of low-level wind shear above 2000` where southwest low-level jet of 35 kts. Winds become south to southwest 5-8 kts by late morning/early afternoon into the evening. In terms of forecast confidence in the TAF period, moderate confidence on ceilings, low to moderate confidence on visbys and timing of fog erosion, and high confidence on winds. Extended Aviation Discussion... Scattered MVFR showers are expected Wednesday into Thursday with the approach of a cold front. This front lifts back north Friday with continued threat of showers and potential low clouds. && .CLIMATE... As of 330 AM EST Monday... Tuesday and Wednesday will feature well-above normal temperatures that may approach or eclipse daily record highs and daily record high minimum temperatures. It`s also possible that all-time warmest February minimum temperatures could be approached or broken in this period. Following are daily records, as well as the all-time warmest minimum temperatures recorded in the month of February, for our five climate sites with long-established periods of record: Tuesday 2/20/2018 Site MaxT Year HiMin Year BLF 67 2017 48 1994 DAN 74 1971 51 1949 LYH 76 1930 59 1939 ROA 75 1939 54 1971 RNK 69 2017 47 1991 Wednesday 2/21/2018 Site MaxT Year HiMin Year BLF 65 1996 54 1997 DAN 75 2011 55 1953 LYH 75 1930 50 1981 ROA 73 1930 48 1997 RNK 71 1986 49 1953 Warmest HiMin - February: Site HiMin Date BLF 57 Feb 5 2008 DAN 60 Feb 18 2008 LYH 59 Feb 20 1939 ROA 57 Feb 12 1932 RNK 52 Feb 16 1990 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Dense Fog Advisory until 10 AM EST this morning for VAZ013-014- 016-017-022-032>034-043>045-058-059. NC...Dense Fog Advisory until 10 AM EST this morning for NCZ001>006- 018>020. WV...None. && $$ SYNOPSIS...PC/WP NEAR TERM...AL/DS/PC SHORT TERM...PW LONG TERM...PW AVIATION...AL/DS/PC CLIMATE...AL

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