Area Forecast Discussion
Issued by NWS Blacksburg, VA

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498 FXUS61 KRNK 210829 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 429 AM EDT Fri Apr 21 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 320 AM EDT Friday... 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.
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&& .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
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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.
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&& .LONG TERM /TUESDAY THROUGH THURSDAY/...
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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.
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&& .AVIATION /09Z FRIDAY THROUGH TUESDAY/... As of 120 AM EDT Friday... Going to see clouds increasing and lowering toward dawn but mainly VFR. Showers will move into the BLF/LWB area by mid-late morning with thunder in vicinity in the afternoon. Cannot rule out a few showers earlier but majority of the threat will be late this morning into the afternoon. VFR to high end MVFR should be prevalent, though some vsbys could drop to IFR in heavier downpours. Further east the showers will make it to Roanoke/Blacksburg midday-2pm, the LYH/DAN by 19-22z. Thunderstorms will occur though will not have it predominant in the tafs keeping it VCTS for now. As front moves southeast over the NC piedmont/mountains, some drying takes place in the north along and north of a BLF-ROA-LYH line, but should still see showers or storms in vicinity through the end of the taf period with better chance of showers across the BLF-DAN and south corridor. Expect MVFR cigs BLF/BCB Friday evening with VFR elsewhere, but close to 3-4kft. Winds will be light this morning, then increase from the southwest ahead of the front between 09z-15z in the west and midday in the east. Once front moves across, winds shift to the west and weaken later this evening. Extended Aviation Discussion... VFR to MVFR cigs into Friday night with this front. As front moves south Saturday morning, will see more coverage and this is especially true Sunday. During this time, expect cigs/vsbys to be highly variable but most of the time should be sub-VFR. Could see lingering rain threat Monday with sub-VFR before VFR returns Tuesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...MBS LONG TERM...DS/PM AVIATION...AL/KK/WP

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