Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 202341 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 641 PM EST Mon Feb 20 2017 .SYNOPSIS... High pressure will build over the region today and remain in place through at least Tuesday. Next chance for rainfall is later Tuesday night into Wednesday as a weak cold front traverses the region. Relatively mild but unsettled weather continues until late in the weekend into early in the weekend as a stronger cold front from the Ohio Valley moves in. Temperatures will continue to trend above normal for much of the workweek. && .NEAR TERM /THROUGH TUESDAY/...
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As of 344 PM EST Monday...Beautiful late-February sensible weather conditions this afternoon across the central Appalachians and NC/VA Piedmont Region featuring wall-to-wall sunshine, light southerly winds and dry air. That has led to rather dry RH`s in the 15 to 20 percent range being common. Subsidence exists both at the surface and aloft, with surface ridge extending from a broad anticyclone centered over western Quebec with pronounced mid-level ridge aloft over the Alleghenies and eastern Cumberland Plateau region. Well to the west over the southern/central Plains into the Mississippi Valley is a deep mid-level trough and associated surface cold front, leading to a meridional feed of deep moisture primarily focused along/west of the Mississippi River Valley. For Tonight: Deep-layer ridge axis to build over the forecast area this evening. Intervals of high clouds will advect northeast across the forecast area as the deeper moisture remains well to the west. At the surface though, it appears an northeasterly onshore flow/weak CAD signature develops for our Piedmont/foothills counties later tonight per indications from the 12z NAM and the 12z GFS. I`ve introduced a slightly greater degree of cloudiness in these areas, more for the after-midnight period, and also reduced low temperatures a little more in these areas as well. Lows project a bit milder (in the mid/upper 40s) for the Mountain Empire into Mercer County in more of a southerly return regime, with lows dropping into the low 40s out in the Piedmont and eastern Southside areas due to the aforementioned CAD potential. For Tuesday: Look for a continued increase in mid to high clouds areawide as mid-level ridge axis shifts eastward and an increase in moist southerly low-level flow. Given the amplified meridional nature of the mid-level flow regime, in addition to how dry the low- level air mass is, any preceding showers ahead of the weakening cold front hold off until the evening at earliest. So I`ve maintained a dry forecast. However, given likely cold air damming signature that only moderates slightly with a light southerly flow, it introduces some question as to how mild especially eastern and southern sections get. For these areas - lower NC piedmont, central VA Piedmont into the Southside - I have trended toward the lower end of the guidance temperature envelope, in the mid 50s to near 60. With more of a southerly flow and milder 850 mb temps (+8 to +10C vs +4 to +6C further east), forecast temps in the mid 60s.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 344 PM EST Monday... We will be dealing with a wedge of high pressure banked up against the east slopes of the Appalachians through the middle of the week. Synoptic forcing is weak, but as weak isentropic lift rides over the wedge with some upper short wave energy from a zonal northern stream and a closed low slowly migrating eastward along the Gulf coast, we can expect a good amount of cloudiness along with some showers. The best chance for precipitation will be Tuesday night into Wednesday as the main vorticity axis moves over the wedge. Temperatures on Wednesday will be cooler with precipitation in the wedge, though still well above normal. The surface high responsible for the wedge will drift eastward and off the coast for Thursday, giving a southerly component to our winds and boosting temperatures further above normal. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 344 PM EST Monday... As noted by WPC long range models were in decent agreement at the synoptic scale Friday and Saturday with the GFS becoming the outlier Sunday and Monday. West Virginia, Virginia and North Carolina will be in the warm sector with well above normal temperatures Friday and Saturday ahead of the strong cold front. 850MB temperatures Saturday night in the 0 to -8 range but back up just above zero by Monday morning. Low level jet and cold advection will lead to gusty winds behind the front but neither contributes enough that gusts would approach 40 kts. Pattern remains active Monday with a surface low over the southern plains that moves east and brings a chance of precipitation back to the Mid Atlantic region. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/...
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As of 640 PM EST Monday... High confidence in light winds, medium confidence in VFR cigs this TAF period. Latest satellite images show a decent canopy of cirrus overhead with more upstream. While guidance indicates this cirrus will remain over the region through tonight, there are some notable thin spots. Believe the overall coverage of high clouds will be enough to hinder radiational cooling and prevent another round of late night fog development. If thin spots become more pronounced this may have to be revisited, especially at KLWB. GFS is very keen to develop some low clouds associated with the wedge as it surges in overnight. The NAM and most other guidance is not nearly as bullish with low level moisture and late afternoon visible satellite images revealed little in the way of low clouds upstream. Believe the low levels will moisten a bit overnight and some orographic effects on Tuesday as flow becomes more southeasterly may aid low cloud development, but will play more of a compromise and keep all sites with VFR cigs, save for a tempo to MVFR at BCB. It will be quite difficult to discern any low clouds beneath the cirrus canopy tonight so will have to monitor obs to keep tabs the situation. Expect winds to remain light through the period with no significant gusts. Extended Aviation Discussion... A continued lowering/thickening of cloud cover expected through Tuesday evening hours. Showers and likely sub- VFR to develop Tuesday night into early Wednesday with an axis of showers preceding the weakening cold front arriving from the west. Expect showers to decrease Wednesday afternoon and night while leaving residual MVFR cigs in place. Later Thursday into Thursday night, low confidence of patchy light rain returning to the area, with cigs and vsbys remaining mainly VFR except for ocnl MVFR across southern and western sections. Friday, moderate confidence in precipitation trending more showery, as the low level jet increases in advance of a strong cold front. Expect a mix of MVFR and low end VFR ceilings and mainly VFR visibilities. Surface wind gusts of 20 to 30 kts possible across the higher terrain. Band of showers with the front to arrive Friday night into early Saturday followed by drier weather and increasing northwest wind by afternoon behind the front.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...AMS/MBS LONG TERM...AMS/MBS AVIATION...AL/MBS is the U.S. government's official web portal to all federal, state and local government web resources and services.