Area Forecast Discussion
Issued by NWS Blacksburg, VA

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323 FXUS61 KRNK 300744 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 344 AM EDT Thu Mar 30 2017 .SYNOPSIS... High pressure will continue to wedge south along the eastern slopes of the Appalachians today before passing offshore this evening. Low pressure will pass to the northwest of the area tonight into Friday with a trailing cold front crossing the region by Friday night. High pressure follows this system with mainly dry weather expected over the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM EDT Thursday... Wedge continues to evolve across the region early this morning with satellite pics showing low clouds filling in along/east of the Blue Ridge with better cool advection entering from the north per MSAS. Latest short term models indicate a better surge in low level moisture advection through sunrise per deepening southeast flow with low clouds spreading over all except the far west. Also depicting some spotty light rain or drizzle developing by daybreak eastern slopes with stratus lowering, resulting in fog across the ridges this morning. Although this likely overdone, will leave in going low pops for light rain central sections early on, with coverage gradually shifting southwest to along the southern Blue Ridge into this afternoon as warm advection starts to increase above the low level cool pool. Otherwise looking at a much cloudier day in the wedge with perhaps a lot more sun far west and some breaks east/southeast as a bit drier air sinks in. High temps very trick with readings likely to not rise much from 12z values along portions of the Blue Ridge while possibly surging west of I-77 where will be on the periphery of the cool air. Mos spread remains large pending solutions depiction of the depth of the wedge and subsequent cloud cover. Think best to continue on the cooler side similar to the latest consensus and supported by the new Euro highs which is mostly in the 50s. High shifts offshore this evening while an area of strong upper level low pressure slides by well to the northwest by early Friday. This will shift a warm east toward the mountains late with an increasing southerly jet aloft coming in line with a rapid increase in upper diffluence from west to east espcly after midnight. Will initially be wedged within a stable environment to start with the CAD getting eroded from aloft western counties and out along the Blue Ridge by daybreak. Models again a bit faster while showing lots of convergence/lift over much of the region late as upslope precip becomes enhanced via approach of lead energy aloft before merging into the precip shield ahead of the front aloft to the west. This should spell periods of moderate to perhaps heavy rain overnight with likely more elevated/embedded convection that could run out east of the mountains by dawn. Some potential espcly if the initial wedge is weaker over the far southwest to see a broken band of deeper more surface based convection spill east right along the axis of strong shortwave energy aloft. Thus beefed up thunder chances south/west given shear and will mention in the HWO per latest SWODY1 slight/marginal outlook. Winds ahead of this feature may also be an issue along the western ridges as forecast soundings show a 45-50 kt jet just above the inversion later this evening. However with the flow veering more southerly think should stay below advisory criteria for the most part so wont headline for winds at this point. Appears QPF of an inch or more possible southwest sections including parts of the Blue Ridge through late tonight but should not be enough to cause flooding issues given rather dry conditions to start at this point. Otherwise running with a rapid increase in pops to high end categorical all sections overnight with lows 40s to mid 50s before rising some late. && .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/... As of 330 AM EDT Thursday... Looking at a rather complicated but also an active forecast for Friday. 00z model guidance has trended about 3 to 6 hours faster than prior model cycles with regard to early-day rainfall. Though specific spatial details and intensity are still somewhat fuzzy, given a potential earlier eastward shift of early-morning precip, a second/redevelopment of showers and storms appears possible Friday afternoon. Will attempt to describe these prospects below... Friday morning: This precipitation, likely to be a steady band of moderate rain, stems from overnight, weakened convective activity in the Deep South/Tennessee Valley. As upper low begins to deamplify, an axis of steady rain is indicated on a negatively-tilted trough axis. Strongly diffluent upper level flow should also accompany this band of rain. Though surface-based instability will be lacking given the time of day, elevated instability alone should be enough to at least include a chance of thunder with its northeastward advance. Effective-layer vertical wind shear magnitudes may be as high as 60 kts with at least some backed, southeasterly 2-m AGL winds shifting to low-level southwesterly. Wouldn`t think prospects for severe are all that great with this initial band given the marginal instability. Friday late morning and aftn: In the wake of the early-day rain/embedded thunder, a number of higher-resolution models including the 3-km NAM and ARW depict a window of potential redevelopment, primarily from the Blue Ridge/North Carolina foothills eastward. Shear profiles aren`t quite as pronounced as in the morning given more veered/southwesterly surface winds, and the coverage and strength of convection are contingent on afternoon cloud breaks/insolation. Forcing coming in with the upper low and cold front, and at least some colder temperatures aloft would support thunderstorm redevelopment. Instability is still rather modest at values around 500 J/kg or less, though could be greater if more peeks of sun were expected. It`s really during this period that would likely need to be more closely monitored for stronger thunderstorm development. SPC reflects this in their Day 2 Convective outlook, carrying a Marginal Risk/5% severe probability in the Piedmont area. In the grids, I`ve depicted this as a lowering from Categorical morning PoPs to high chance/low likely, ramping to likely-level PoP for the afternoon. Temperatures Friday are contingent upon any cloud breaks; I did raise highs up into the mid 60s for most areas except low 60s for western Greenbrier County. Rainfall amounts Friday are roughly a third to half-inch with potential for greater in isolated spots due to convection. Friday Night: Upper low moves across West Virginia and off the coast late. Any instability-driven showers/storms dissipating and/or having already shifted eastward. Northwesterly component to low- level flow supports a period of upslope cloudiness and rain showers in the Alleghanies and southeast West Virginia. Clouds should begin to scatter out elsewhere, with lows in the mid 40s to around 50. Saturday/Saturday Night: Ridging builds in from the northwest with cold thermal advection occurring through the daytime hours. Mainly sunny skies with a northwesterly breeze. Highs top out in the mid 50s to low 60s from the Blue Ridge west, but are some 10 degrees warmer east of the Blue Ridge aided by downslope warming. Sunday/Sunday Night: Ridge builds to our east during the day, with southwesterly warm advection occuring in Smyth and Tazewell counties. Mostly sunny skies should again predominate boosting highs into the 60s to low 70s. However, later Sunday into Sunday night models indicate that we again return into a cold air damming regime. Increasing cloudiness with easterly onshore flow in the evening would be the result. Essentially, Sunday night may be a carbon copy of tonight. Started to trend lows a bit warmer using some of the warmer raw guidance, which supports lows in the upper 40s to low 50s. && .LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 315 AM EDT Thursday... Lift will then steadily increase along with stronger overrunning late Monday into early Tuesday as the main wave passes to the west and a warm front pushes north into southern sections Tuesday morning. This will bring another round of at least light/moderate rain espcly Monday afternoon into early Tuesday before the warm front passes north per latest wetter ECMWF. Convective threat again focused along the retreating warm front and perhaps within the warm sector Tuesday pending timing and subsequent heating ahead of the trailing cold front Tuesday afternoon. System exits Tuesday night into Wednesday with shortwave ridging returning, allowing for a period of drier weather for middle of next week. High temps to stay above normal, mainly 60s mountains to low 70 east, except cooler within the wedge Monday when some 50s could occur. Lows to start out on the cool side, mostly 40s, before rebounding under clouds/warm advection to mainly 50s into midweek. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... As of 1245 AM EDT Thursday... Cloud cover blossoming across the region espcly east of the Blue Ridge late tonight and expect the canopy to become more widespread through early Thursday morning. Confidence is high that cloud bases will continue to lower as low level moisture increases under deepening southeast flow. Areas closest to the Blue Ridge can expect ceilings to drop into the MVFR or even IFR range by early Friday, with low end VFR to ocnl MVFR common farther east across the Piedmont, and farther west into parts of West Virginia. Patchy light rain or drizzle will also develop in the preferred upslope regions with visibilities VFR to MVFR through the forecast period within drizzle/fog today. Any sub- VFR ceilings will likely remain at those levels through the remainder of the latest TAF forecast period. South to southeast winds to also increase along the western ridges with gusts topping 25 kts possible at KBLF this afternoon/evening. Confidence in the general trend through the forecast period is moderate to high. Confidence specific boundaries between VFR and MVFR ceilings low to moderate. A complex storm system approaches Thursday night with a warm front lifting into the area by early Friday ahead of a passing cold front Friday afternoon. Widespread showers and low cigs likely to result in a period of sub-VFR continuing from Thursday night into much of Friday before some improvement possibly occurs with the cold frontal passage later Friday. Best rain chances appear later Thursday night into Friday morning when could see embedded thunderstorms as well as MVFR to IFR vsbys. With the frontal passage by Friday evening, MVFR and IFR conditions look to persist over the western mountains along with gusty northwest winds. Extended Aviation Discussion... High pressure should bring improving conditions during Saturday although appears sub-VFR cigs along with a few residual showers may linger at KBLF/KLWB into Saturday afternoon. Otherwise will see VFR return later Saturday through Sunday under diminishing winds as high pressure slides across. Next similar low pressure system to that seen at the end of this week will approach by Monday with sub-VFR conditions likely returning Sunday night into Monday as rain again transitions to showers and storms to start the week. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...AL LONG TERM...AL/JH AVIATION...DS/JH/JR/PH

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