Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181814 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 114 PM EST Mon Dec 18 2017 .SYNOPSIS... Weak high pressure to our south with a passing upper level disturbance will contribute to variably cloudy skies today into tonight. Turning warmer into Tuesday ahead of a frontal system emanating from the Tennessee Valley, which will spread rain along and south of Route 460 on Wednesday. Cooler wedge conditions then set up for the rest of the workweek. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1250 PM EST Monday... The afternoon forecast update will reflect two features that impact each other. Cloud cover is dissipating more quickly and over a greater expanse of the region than previously forecast. The result is more sunshine than forecast, leading to higher forecast high temperatures. Have made adjustments accordingly based upon the latest surface observations and satellite imagery. As of 950 AM EST Monday... No major changes to the ongoing forecast, however, have increased the extent and duration of some patchy dense fog across the far western sections of the forecast area through noon. The latest forecast update will reflect this, and a Special Weather Statement has already addressed this. No other notable changes at this time. As of 632 AM EST Monday...Increased coverage of fog a bit in western sections given current satellite and METAR trends. There has also been some patches of mist which have formed in the Southside Virginia area. Lowest visibilities have been in southwestern Virginia and in southeast West Virginia though, where fog is expected to linger well into the morning under a canopy of OVC and a continued influx of shallow moisture. Otherwise, tried to better account for current temperatures and dewpoints. No other changes made attm. Previous discussion issued at 400 AM follows... We often mention how weather conditions can vary depending on what side of the Blue Ridge one resides. Well, that really couldn`t be more true than this morning. If you live along or west of the Blue Ridge into the Appalachians, you`re waking up to overcast skies/stratus and patchy mist/fog. Get out into the Piedmont and you`re getting into thinner sky cover. Influx of shallow moisture continues to be pumped northeastward from the Tennessee Valley, underneath westerly confluent mid-level flow. Relatively non- descript surface pressure pattern, with a weak high pressure area centered over northern Georgia. We`ll be looking at continued moisture advection today given the west-southwesterly component. For areas along and west of the Blue Ridge, BUFKIT forecast soundings off the NAM, RAP and GFS each reveal this low-level moisture will be trapped underneath dry subsidence layer aloft for a good part of the day. Meanwhile in the Piedmont/foothills of Virginia and North Carolina, skies should tend to clear with intervals of high clouds spilling over in upper-level southwest flow. Unfortunately am not all that optimistic on much clearing and warming temperatures this afternoon west of the Blue Ridge - if it happens, it would be late in the day. Would therefore expect to see overcast skies with periods of drizzle at times in the overcast. Mist or fog should continue into part of the morning until we can tap into west flow that may help to disperse. All in all a pretty dreary day seems likely for these locations. Though MOS guidance wants to push highs into the upper 40s to lower 50s here, I`ve cut these down by 2-4 degrees given the expected cloudiness to values in the mid/upper 40s. From Roanoke into the Piedmont and foothills, a more diurnal temperature trend is expected with highs reaching well into the 50s. May even be a spot 60 degree reading in Stokes, Rockingham or Caswell. Continued variably cloudy skies should be the rule into tonight, greatest furthest west. As an upper-level disturbance passes to our north through the overnight, that should enhance westerly flow as depicted by the 00z guidance. However warm advection occurs overnight and it`s unlikely lower-elevation locations mix GFS- modeled westerly 50-kt jet in that regime, but terrain above 3000` stands a fair shot at seeing breezy conditions (gusts to 35 mph). Light west to calm winds appear more likely in the lower elevations. Late in the overnight, will have to watch the northern NC Piedmont into Southside for patchy fog. NAM-based BUFKIT soundings at GSO reflect a potential fog layer developing that could advect northward, and these areas may also stand to radiate better. Lows should range from the mid/upper 30s in most locations, coolest in the Piedmont given better prospects for radiational cooling. With stronger winds/better mixing along the ridges, I opted for values in the mid 40s.
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&& .SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/... As of 400 AM EST Monday... A confluent flow will be in place across the Mid-Atlantic region at the beginning of the period with distinct southern and northern streams evident. The northern stream will continue to be influenced by a strong upper low/Polar Vortex anchored over Hudson Bay. However, this upper low has lifted somewhat north and west of its position last week as a discontinuous retrogression process appears to be in place with respect to the large scale pattern, with the upper trough gradually shifting westward through the extended periods. A notable strong subtropical ridge (eventually 595 dm!) will be anchored off the southeast coast generally attempting to expand northwestward into the weekend. Meanwhile, the southern stream will be quite active during this period with a classic wintertime strong, yet compact, highly kinematic disturbance evident in central TX at the beginning of the period tracking across TN/GA/NC through Thursday. The confluent flow in place across the Mid-Atlantic will limit the northern extent of the associated precipitation Tue-Wed with likely a sharp contrast in pops from north to south (i.e., below 15% far north to categorical/definite southern areas, especially the NC counties. In essence the disturbance will track almost due east from south central TN along or near the TN/GA and VA/NC border, exiting the coastal sections of eastern NC by Thu morning. QPF will range from zero or trace amounts in the far north, especially north of I-64 to 1/2 to 2/3rd inch in the NC counties and adjacent tier of southern VA counties. No concern with winter precipitation during this event as 850mb temperatures remain above freezing for most if not all of the CWA through the beginning of next week with the 850mb 0C isotherm remaining just north of the CWA through the period. The strong ridge just to our southeast certainly will contribute to the lack of winter weather threat during this period. Temperatures are expected to be well above normal most of this period. In advance of the upper-level disturbance Tuesday, temperatures will average some 10-15 degrees above normal with much of the CWA seeing balmy 60+ degree readings, even closing in on 70 in the eastern Piedmont. Cloud cover and precipitation will knock about 10 degrees off maximum temperature readings for Wednesday, with the passage of the disturbance, temperatures should fall another 5 degrees or so compared to Wednesday. Minimum temperatures will show the greatest departure from normal averaging 10-20 degrees above normal much of this period. Lows should be largely in the 20s at this time of year, but instead will be mostly in the 30s and 40s, with many parts of the CWA only falling as low as 32F during the period. && .LONG TERM /FRIDAY THROUGH SUNDAY/... As of 430 AM EST Monday... Quite a pattern change evident during this period as a strong subtropical ridge amplifies across the southeast and adjacent western Atlantic in response to the southeast Canada upper low retrograding westward into south central Canada with a deep, positively tilted upper trough developing into the southwest U.S. With time, significant discrepancies appear between the GFS and ECMWF as to how quickly an associated cold front will move into the eastern U.S., with the ECMWF slower than the GFS. With the GFS much more progressive, precipitation and cold air will reach the CWA much sooner than with the ECMWF, which keeps our region on the east side of the trough axis through Christmas, but by the same token is much wetter in the later periods, and also much slower to move the precipitation out of the area as well. It should be noted that there will be a sharp contrast in temperatures from the central U.S. air mass, which will show some true Arctic signs to the eastern seaboard dominated by the strong upper ridge and thus much above normal temperatures. Along and near the associated frontal boundary separating these two air masses, will be a corridor/conveyor belt of heavy rainfall. Model consensus, especially favoring the ECMWF, is that this will remain just west of our area for the most part. Certainly the potential for heavy rain and event winter precipitation will become a greater likelihood as we move into the week of Christmas. Otherwise, temperatures are likely to remain above normal as long as we are on the east side of the upper trough axis/frontal boundary/baroclinic zone. Based on the current extended models, especially the ECMWF, am not advertising any winter precipitation through the extended periods. If you were looking for a White Christmas this year, the odds are definitely against having such as it appears right now. && .AVIATION /18Z MONDAY THROUGH SATURDAY/...
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As of 1245 PM EST Monday... VFR conditions prevail across the area generally along and east of of the crest of the Blue Ridge. This trend is expected to continue through tonight into Tuesday morning. The exception may be some patchy IFR/MVFR stratus late tonight near KDAN. Likewise, some patchy MVFR fog may also develop overnight in this area nearest the river valleys. Across the mountains, the story is a bit different. IFR/MVFR ceilings were eroding this afternoon, but coverage is still expected to be greatest in areas between roughly KLWB-KBLF-KJFZ. As we head into and through the overnight hours, look for a bit of resurgence of in the cover over the IFR/MVFR ceilings, to an area east to near the crest of the Blue Ridge again. Some patchy drizzle will also be possible across parts of southeast West Virginia, near and west of KLWB. Winds will start to increase a bit this afternoon, especially in areas without, or with decreasing, cloud cover as low level mixing takes place. Some gusts, especially across and near the mountains, will reach 15 to 20 kts. Overnight, we expect a westerly low level jet to increase. While the mountain valleys and Piedmont region will remain decoupled, 15 to 25 kt gusts will be common at the higher ridge tops. There also may be some low level wind shear across the Northern Mountains of North Carolina where the increasing low level jet is not too far above the higher elevated mountain valleys of that region. On Tuesday, westerly winds will be generally a little stronger than those of today across the region once the morning inversion breaks. Cloud cover in the west will erode as Tuesday morning progresses. Extended Discussion... Sub-VFR conditions will return to mainly central and southern sections of the area on Wednesday as a disturbance heads east through Tennessee and North Carolina. VFR returns briefly on Thursday in advance of our next cold front that will approach the area on Friday. Look for flight conditions to gradually worsen during the day Friday, especially in the west. Sub-VFR conditions will be probable for most areas Friday night into, and through, Saturday with the arrival and passage of a potent cold front. Winds will also become gusty to very gusty immediate in advance and then post cold frontal passage. A distinct abrupt wind shift is also expected late Friday night or early Saturday morning across the area with the passage of the cold front.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL/DS SHORT TERM...RAB LONG TERM...AMS/RAB AVIATION...DS

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