Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 210230 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1030 PM EDT Thu Oct 20 2016 .SYNOPSIS... A cold front will slowly move into the region from the west tonight, then exit the area to the east late Friday morning. This will spread showers and possibly a few thunderstorms across the Appalachians and central mid Atlantic region later tonight into Friday. Strong gusty winds behind the front will usher in much colder air for the weekend, and some wet snowflakes are possible at the higher elevations west of the Blue Ridge. && .NEAR TERM /THROUGH FRIDAY/...
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As of 915 PM EDT Thursday... Have scaled pops back quite a bit over the next few hours. Short range and longer range synoptic models really do not bring the bulk of the precipitation into the CWA until 10Z or later, mainly after 12Z. The exception is an area of scattered showers developing out ahead of the main area across eastern WV southwest into northeast TN and far southwest VA. This activity largely driven by daytime heating and instability and will likely go away in the new couple of hours, per HRRR and NSSL WRF. Also confined mention of thunder to areas west of I-77, mainly during the next 3-4 hours before instability wanes to 400 J/kg of CAPE or less. Temperatures across the Piedmont have cooled a little quicker than areas further west where it is cloudy. Essentially a reversed pattern from normal at this time of evening. Feel that Piedmont temperatures will level off as clouds move in that direction, and may actually increase slightly before the night is over as clouds overspread that area as well. The net result will be min temps fairly close to what was previously advertised. The other consideration is max temps for Friday. Latest GFS/HRRR suggest that it could still be well into the 70s across the eastern parts of the Piedmont during the afternoon Friday. As of 330 PM EDT Thursday... Expect quiet weather into this evening before the cold front approaching from the west can start to push some showers into the region. Guidance continues to trend slower with this system and the bulk of the precipitation looks to remain west of the Blue Ridge until well after Midnight. However, there will be a tight gradient to the precipitation so a few miles either way will make a big difference as to whether locations far west of the Ridge making it into some decent rainfall earlier tonight. Steep lapse rates aloft will also make for the possibility of some embedded thunder especially far west, with the thunder chances decreasing overnight. The front looks to be right along the Ridge by daybreak, but the best forcing will be shearing off to our north. This is expected to weaken the precipitation considerably as it progresses east of the Ridge but some redevelopment of convective elements will be possible in the piedmont early in the afternoon. Conditions will improve as the front departs, but upslope winds will keep scattered going west of the Ridge through Friday afternoon. Winds behind the front will shift around to the northwest and favorable momentum transfer indicated in model soundings will make for quite gusty conditions, especially at the higher elevations from the Blue Ridge westward. Expect the winds to be on the increase after daybreak and continue blowing throughout Friday.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... As of 330 PM EDT Thursday... This portion of the forecast to be dominated by sharp upper trough rotating across the mid-Atlantic region with attendant cold front pushing well off the east coast east Friday night into Saturday. Lingering rain and few high elevation snow showers will persist into early Saturday with a diminishing trend as the day progresses. 850mb temps fall below 0C Friday night and should allow for some snow or mixed precipitation in the elevations above 3000 ft. as noted. Strong winds may well be the most notable feature of this system as deepening low over the northeast increases pressure gradient through Saturday. Wind gusts over 30 mph will be common through this period. In addition some of the coolest temperatures of the season to date are expected although that is not saying much given the extraordinary persistence of above normal temperatures over the past six to eight weeks. Lows Saturday morning upper 30s to low 40s and similar on Sunday morning. No widepspread frost or freeze is anticipated as clouds and wind Friday night and wind Saturday night persist enough to maintain mixing and poor radiational conditions both mornings. The lows in fact will be right around the long-term normals for the 3rd week in October. Saturday will feel quite chilly perhaps owing to clouds/wind and highs from mid-40s west to near 60 east. Those are 5-10F below normal. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 330 PM EDT Thursday... Surface low pressure and associated upper system will be located over southeast Canada on Sunday with the flow across the lower 48 beginning to flatten out into a zonal pattern. This will promote a rather dry week ahead with few temperature extremes. A mostly dry front will push south across the area on Monday with only the slightest chances for some light precip in the far northwest. Not until about late Thursday does another front organize over the midwest and begin advancing eastward. This front looks to arrive early Friday with timing not too dissimilar to tonights frontal passage and again with best dynamics and deeper moisture to our north. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... As of 750 PM EDT Thursday... Clearly the main focus through this TAF valid period will be on a strong frontal system and negatively tilted upper trough slated to cross the region overnight and early Friday. Timing has slowed down on all models and this is in good agreement with current radar analysis. Thus, while some pop up showers are expected across southwest VA/southeast WV over the next few hours per HRRR and NSSL WRF, the main line of convection is not expected to reach the western part of the CWA until just before daybreak, into the KBCB/KROA area in the 12Z-13Z time frame, and into the Piedmont between 14Z and 16Z. With this in mind, there should be little no potential for any thunder as all instability of significance is gone within the next few hours. Having said this, it is still possible to get a rumble of thunder from elevated instability, but not worth putting in the TAFs at this point. Thus, cigs/vsbys are largely expected to remain VFR over the next few hours, with BKN-OVC cigs AOA 040 mainly west of a KBCB-KLWB line, with cigs AOA 050 further east across the Piedmont. Starting around 09Z, -SHRA will arrive in the western parts of the CWA at which points cigs will lower into the MVFR and then IFR range by 12Z with vsbys dropping into mainly the MVFR rain in -SHRA. Cigs will drop into the MVFR range further east after 12Z accompanied by a period of -SHRA and MVFR vsbys as well. The initial line of showers/convection will lift out of the eastern areas by early afternoon, but upslope showers will continue across the western mountains. While there may be a brief break in the showers western areas during the early afternoon, there is clear evidence of a deformation zone and moisture wrapping back into these areas as the upper trough takes on a negative tilt and rotates through the region during the afternoon. At times, given the site setting at KBLF, expect the cigs to drop to observation height and the cigs to drop below 005 and vsbys to drop to LIFR. Further east across the Piedmont, cigs should improve to high end MVFR or low end VFR after 18Z with no further restrictions to vsbys at that point. Until the front passes daybreak or later, winds will be mostly SSE-SSW 4-8kts, highest in the western areas. After the frontal passage, winds will become WSW-WNW 10-15kts with gusts 22-30kts. Winds will likely be stronger on Saturday with better mixing and insolation than what we will see on Friday. Medium confidence in cigs and vsbys throughout the TAF valid period. Medium to high confidence in wind direction/speed throughout the TAF valid period. Extended aviation discussion... A return to VFR conditions is expected for most areas Friday night into Saturday. The exception will be western parts of the region where a healthy northwest upslope flow, and lingering low level moisture, will prolong an IFR/MVFR ceiling during this time period. Even the mountain areas will become VFR again for Sunday and Monday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...MBS NEAR TERM...MBS/RAB SHORT TERM...PC LONG TERM...DS/PC AVIATION...RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.