Area Forecast Discussion
Issued by NWS Blacksburg, VA

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423 FXUS61 KRNK 211033 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 533 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 /10Z TUESDAY THROUGH SATURDAY/...
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As of 520 AM EST Tuesday... Overall VFR looks to continue this morning with lack of any lower cloud canopy attm. However will need to watch over the next few hours to see if southeast upslope generates any strato- cu within the developing wedge as seen via the latest GFS solution. For now plan to bleed in VFR strato-cu cigs from southwest to northeast through the day with perhaps a period of MVFR along the Blue Ridge around KBCB/KROA later this morning. Otherwise keeping overall VFR in place under mainly mid/high clouds and southeast flow that could produce gusts to 20 kts at times over the mountains, espcly KBLF. Moisture will deepen from west to east overnight with a continued lowering/thickening of cloud cover expected through this evening. Showers and likely sub-VFR to develop later this evening over far western sections and spread east into early Wednesday preceding a weakening cold front arriving from the west. Best shot at seeing lower IFR conditions will be across the mountains into tonight, then southern sections into early Wednesday where low cigs and at least MVFR vsbys in showers/fog are most likely. Otherwise MVFR cigs to VFR/ocnl MVFR vsbys overnight through Wednesday morning. Expect rainfall to decrease Wednesday afternoon into Wednesday night while leaving residual sub-VFR cigs, and ocnl MVFR vsbys in place until possibly seeing some slow overnight improvement, but quite uncertain at this point. Extended Aviation Discussion... Weak warm front lifting back north Thursday into Thursday night may lead to added shower redevelopment before the residual wedge exits on Friday. This could cause sub-VFR to persist in low clouds and spotty rain into Thursday night. Area will slide in between weak low pressure off the southeast coast Friday and the next stronger upstream cold front that looks to arrive by midday Saturday. This should allow for some improvement in cigs/vsbys within the warm sector Friday as the low level jet increases. Expect a mix of MVFR and low end VFR ceilings and mainly VFR visibilities during the day with surface wind gusts of 20 to 30 kts possible across the higher terrain. Band of showers including a period of sub-VFR to arrive late Friday night into early Saturday with the front, followed by drier weather and increasing northwest winds by afternoon behind the boundary.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...RCS LONG TERM...AMS/MBS AVIATION...AL/JH/MBS

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