Area Forecast Discussion
Issued by NWS Blacksburg, VA

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108 FXUS61 KRNK 210828 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 328 AM EST Tue Feb 21 2017 .SYNOPSIS... High pressure along the Mid-Atlantic coast will shift east and offshore today ahead of a weak cold front approaching from the west. This front should slide east into the mountains overnight into early Wednesday before dissipating. High pressure will strengthen offshore for the end of the week ahead of a stronger cold front that looks to arrive from the west on Saturday. && .NEAR TERM /THROUGH TONIGHT/... As of 240 AM EST Tuesday... Strong upper ridging across the region will gradually get dampened a bit this afternoon as shortwave energy passes to the north later today in advance of the main upstream split flow troughiness tonight. This will continue to keep rounds of high/mid clouds advecting in from the west today overtop weak low level wedging that looks to linger espcly Blue Ridge east into tonight despite high pressure exiting offshore. However weak low level southeast flow around the surface high the main concern in regards to possible coverage of developing low clouds this morning and whether or not they persist given lingering dry air aloft. This could make for large temp differences as more insolation similar to the drier Nam would push highs well into the 60s most spots while the cloudier GFS looks a bit more realistic under easterly flow in holding spots near the eastern slopes a category or so cooler. Thus trended in that direction while bumping up highs slightly from previous with warmest values far west and across the southeast. Otherwise appears enough dry air in between the low level moisture and higher cloud canopy to preclude any pops today espcly with better lift still to the west per model consensus. Complex system to the west will basically split overnight with the main upper low heading across the Gulf coast states leaving a weak front and associated mid level shortwave to track toward the Appalachians late. Although widespread deep moisture limited, guidance showing a swath of higher PWATS and upper diffluence pushing east to just east of the Blue Ridge late. Appears this in combination with deeper southwest flow aloft and thinking that any deep convection with the upper low to the south should be far enough removed to avoid a cutoff of moisture advection ahead of the front late. Therefore running with a gradual increase in pops from chance to low likelys far west after midnight and overall chance coverage Blue Ridge and less out east. QPF still looking rather light heading east although could see a tenth or two western mountains where better southeast flow around the perimeter of the residual eastern wedge/subsidence could help lift a bit more. Expect overall uniform lows mainly in the 40s given clouds and light showers that should hold readings up some under the weak warm advection regime. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 240 AM EST Tuesday... A northern stream cold front, coming across the Ohio Valley, will washout by the time it reaches the Appalachian Mountains Wednesday morning. However, prefrontal showers and short waves being ejected north by a southern stream low will be able to advance east of the mountains as they get caught up in zonal flow Wednesday. The intensity of these showers will weaken as they move east and over a surface wedge. Showers moving east of the Blue Ridge will enhance the wedge over southern and southwestern Virginia and into the Carolinas. The parent high of this insitu wedge will drift east, but linger over the region into Thursday night. Rounds of warm moist air and insentropic lift coming over the wedge will keep low clouds and the chance for light rain/drizzle for area along and east of the Blue Ridge Wednesday night into Thursday night. With more clouds and precipitation expected and the models have a warm bias during insitu wedges, we have cooled temperatures just under guidance for Wednesday and Thursday afternoons. Even though we are dealing with a cold air damming event in late February, temperatures will run 10F to 15F warmer than normal. Also during these insitu events, models are to quick to have winds coming out of the south, therefore back them to more east-southeast Thursday and Thursday night. && .LONG TERM /FRIDAY 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 /08Z TUESDAY THROUGH SATURDAY/... 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. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...RCS 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.