Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 211749 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 149 PM EDT Fri Apr 21 2017 .SYNOPSIS... Going to see a cold front move to the mountains Friday morning, shifting southeast to the NC piedmont Friday evening. This front will stall near the NC/VA border Saturday, with a secondary low moving across the area Sunday. This will keep us in a wetter pattern through the weekend into Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 952 AM EDT Friday... Made some minor adjustments in temperatures shaping with latest surface obs and glamp for late morning into this afternoon. With a warm start, high temperatures this afternoon will climb into the lower 60s in the northwest mountains to the mid 80s in the Piedmont. Adjusted pops for this morning with latest radar images and trends, then shaped towards HRRR. As the cold front moves east this afternoon expected scattered showers and thunderstorms to develop along and ahead of this boundary. SWODY1 places forecast area in a marginal risk for severe storms with strong winds and hail the primary threat. The freezing level on rnk 12z sounding was around 9 kft, helping hail potential. Weak near-surface winds with a substantial westerly component will hinder low- level shear. More changes later this morning. As of 720 AM EDT Friday... A few showers shifting east across the NC/VA foothills picked up well by HRRR and RAP, and try to hang onto these showers all the way out to southside VA and the piedmont of NC, however, radar showing a diminishing trend. Otherwise, appears the morning will be mainly dry with high-res models holding off until after 18z for most of the showers and storms to move into the mountains and foothills. Made adjustments to account for this, otherwise no other major changes. Previous forecast discussion... Getting a few showers already into southwest VA/southern WV ahead of a cold front stretching from Erie PA southwest to Paducah KY. Mesoanalysis showed area of deep moisture convergence centered in southern WV as of 07z/3am EDT. High-res models are showing this well enough but as we head through dawn they weaken this area and move it east-northeast toward the Alleghanys/southern Shenandoah Valley. For now will keep scattered mention in the far west to isolated to the I-81 corridor then nudge pops to likely by 12z as better lift enters the area. With the front entering the mountains after 12z, the upper flow out of the west-southwest should move it further east toward 18z to the Blue Ridge. Low level flow stays westerly enough that models showing less coverage lee of the mountains but think enough convergence along the front with moisture pooling along front will bring likely pops to at least the western half of the forecast area with high chance in the east. The Storm Prediction Center continues to highlight marginal risk for the entire area. Svr threat will be isolated as cloud cover and instability is limited as is mid- level lapse rates. A modest mid-level flow of 30-40 kts may initiate some stronger winds if storms can get some altitude. So main threat will be isolated strong/damaging winds. The better instability will be in the NC piedmont while best wind energy appears to shift over northern VA. As for temperatures will be warm again with temps already starting out in the upper 50s to lower 60s. Piedmont will see highs in the mid 80s with lower to mid 70s west as clouds and showers move in this morning. This evening the cold front should be extending from southeast VA to Western NC. Models showing weak waves of low pressure riding along the front and in decent agreement in placing axis of better shower threat from far SW VA to southside VA. Since the front starts to turn more toward east-west orientation think better rain chances may stay somewhat further north but appears all of the forecast area will see a decent coverage of showers tonight, with thunderstorm chances subsiding with loss of heating. Lows should be in the 50s in the mountains, with lower 60s across the foothills/piedmont and Roanoke Valley. && .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... As of 430 AM EDT Friday... An upper level trof will dig into the southeast US and develop into a slow moving closed low as it drives a surface low through South Carolina, while a wedge of high pressure surges into the region east of the Appalachians. Dynamic forcing between these two features will be formidable while the low transports abundant moisture from the Gulf of Mexico and southeast Atlantic over the wedge. These features will all combine to bring significant rainfall to the region this weekend and into Monday. We start off Saturday in the warm sector with isentropic lift from the surface low over the lower Mississippi valley bringing widespread showers to the region, and high pressure poised to begin wedging down from the north. Most guidance brings the wedge in behind a backdoor cold front early enough on Saturday afternoon to shut down heating, but locations from the Mountain Empire of VA through the mountains and foothills of North Carolina will see some instability pool ahead of the wedge to fuel development of some thunderstorms, and shear along the backdoor front will be sufficient to help organize the convection. It currently appears that any threat for severe thunderstorms will be mainly across North Carolina where some breaks of sun will help enhance instability, but the axis of instability will have to be monitored closely to see if it can push further north from Interstate 77 westward. By Saturday night we will be on the cold side of the surface low sliding by slowly to our south with a strong wedge firmly in place blowing a stiff east to northeast breeze east of the Blue Ridge. The latest guidance has generally trended wetter and slower, so we can expect rainfall to persist from Saturday night into Sunday night. Precipitation will then slowly taper off in the west toward daybreak Monday, but locations east of the Ridge will see showers continue until Monday night. Ensemble precipitation plumes are exhibiting considerable spread, but a reasonable expectation will be for precipitation totals from Friday through Monday to be in the 2.5 to 4 inch range, and this amount of rainfall may be enough to push some streams, creeks, and rivers into minor flood. Temperatures will be trending much cooler as the wedge sets in this weekend. Highs on Sunday will only reach the upper 50s east of the Blue Ridge, with readings of 50 to 55 more common in the west. && .LONG TERM /TUESDAY THROUGH THURSDAY/... As of 850 PM EDT Thursday... Will hang onto some cloud cover for Tuesday. With an east or northeast wind, temperatures will be slow to recover until the sun returns for mid Week. A dry forecast will then be advertised for Wednesday and and Thursday. A surface front is forecast to move into the Ohio Valley from the Great Lakes, but think this front will stall north of the area as High pressure amplifies over the southeastern CONUS. Temperatures will be trending closer to normal Tuesday, and then above normal for the second half of the week. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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As of 149 PM EDT Friday... Scattered MVFR showers and thunderstorms will move east across the region this afternoon into tonight. Local IFR conditions are possible with heavier showers/thunderstorms. The cold front will continue moving southeast this afternoon into tonight and stall overnight into Saturday. A wave of low pressure will develop along the front and lift northward Saturday. Convection will develop along the boundary tonight into Saturday with rounds of showers and thunderstorms. Where it rains hard, patches of fog will develop. Areas of mvfr and local ifr conditions are expected tonight into Saturday. Southwest winds around 8 to 12 kts are expected this afternoon into tonight. However, winds can be gusty near convection and variable in direction. Winds will be come light tonight and shift to more northeast direction on Saturday. Moderate confidence in ceilings, visibilities and winds during the taf period. Extended Aviation Discussion... Weather pattern active this weekend into Monday with periods of rain/showers, heavy at times. Should expect mostly sub-VFR condition when raining, but some VFR possible, especially north of a LWB-LYH line. Rain lingers into Monday as does lower cigs, and finally seeing some VFR returning Tuesday. Drier weather continues Wednesday into Thursday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...KK/WP SHORT TERM...MBS LONG TERM...DS/PM AVIATION...KK/WP

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