Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 260145 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 945 PM EDT Sun Sep 25 2016 .SYNOPSIS... High pressure following the backdoor front will wedge south along the eastern slopes of the mountains tonight into early Monday. Another cold front will shift in from the west Monday night into Tuesday bringing better chances of showers and isolated thunderstorms for early next week. && .NEAR TERM /THROUGH MONDAY/... As of 945 PM EDT Sunday... The only changes required were to slightly adjust up low temps in the lower lying areas as they do not seem to be falling as fast as previously advertised, nor does short term guidance suggest they will get as low. Otherwise, sky, POPs, and WX appear to be on track. As of 800 PM EDT Sunday... Made some changes to pops overnight as advertised precipitation has failed to materialize anywhere around the region at this time. Nothing showing on any neighboring radar for several hundred miles and spotty showers that were earlier ongoing in southwest NC have dissipated. Do expect drizzle to develop toward midnight near and along the Blue Ridge in moist maritime air mass and easterly surface flow. As upper forcing increases overnight from upstream trough and overrunning of shallow wedge air mass, expect areas of light rain to develop, again mainly along and near the Blue Ridge. Right now, pops range from just under 15% at the current hour to around 25% toward daybreak along the Blue Ridge from southwest to northeast. Minor modifications were made to temperatures and dewpoints to account for air mass currently being a tad cooler than expected, but cloud cover and warm advection aloft will limit cooling overnight. As of 330 PM EDT Sunday... A frontal boundary will drop south this afternoon into tonight in the South Carolina and then lift northward as a warm front on Monday. High pressure located over eastern Ontario will move east tonight into Monday and push off the New England coast Monday. Low level moisture is trapped under subsidence inversion in combination with light upslope flow into the higher terrain. This is resulting in plenty of clouds especially across eastern portions of the forecast area. The shallow nature of the moisture this afternoon is allowing cloud cover to erode and thin. convective cu fields are forming in the west and parts of the south this afternoon. Tonight, the flow turns more southeast across the Southern Appalachians. The increased lift along with upslope flow will produce areas of light rain,drizzle and fog overnight into Monday morning. Low temperatures tonight will range from the mid 50s across the north to the mid 60s along the southern Blue ridge mountains. The warm frontal boundary will return north fairly quickly Monday as a strong cold front approaches from the west. Considerable cloudiness, fairly warm and humid conditions with scattered showers and thunderstorms are expected in advance of the front. Swody2 has placed a marginal risk of severe thunderstorms to our west across western Pennsylvania and West Virginia. The main concern to our west is for organized line segments which could produce strong winds. The cold front will move across our area Monday night. High temperatures on Monday will the mid 60s along the northern Blue ridge to the upper 70s in the far west mountains. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/... As of 300 PM EDT Sunday... Cold front will slowly cross the region from west to east Monday night and early Tuesday in advance of a closing off upper system rotating through the Great Lakes into midweek. Models have again trended slightly slower with deeper pre-frontal convection over the mountains during the evening, with this swath of lift gradually shifting east, reaching the Piedmont during the overnight hours. Best support will also reside west of the Blue Ridge during the evening with somewhat of a residual low level wedge out east until it mixes out overnight. Thus still appears best thunder chances western third early Monday night, with a possible decrease in coverage upon crossing the Blue Ridge late evening, before some redevelopment possibly occurs out east as the axis of deeper moisture slows into midday Tuesday. This supports keeping likely to categorical pops west through midnight with highest qpf across the northwest and perhaps southwest. Then high chances to low likelys east overnight where could still see some isolated thunder far southeast per latest weak progged instability. Still appears a half to locally one inch of rain possible pending just how fast showers move east before loss of heating takes place. Boundary likely to stall just southeast of the area Tuesday afternoon/evening before finally getting a bit more of a push as another faint wave rides along and northeast of the region overnight. This should finally let a strong push of 850 mb cool advection take place with a nose of surface high pressure sneaking in between the departing front and moisture with the developing Ohio Valley cutoff low. Will still be rather mild with lagging cool advection Tuesday allowing highs to reach well into the 70s Piedmont and 69-75 elsewhere given some sun. Appears enough progged 850 mb cooling out west under a period of clearing and light winds to result in some of the coolest lows of the season with low/mid 50s east, and 40s west including perhaps a few 30s deepest valleys outside of any fog by early Wednesday. Otherwise kept in some low pops southeast third Tuesday afternoon/evening, then dry with clearing from northwest to southeast Tuesday night. Any drying however to be short lived as the flow quickly backs aloft Wednesday in response to the upper low dropping south and deeper moisture to the east shifting back inland per latest ECMWF. This to bring a combination of increasing shower chances Blue Ridge east by afternoon with weak surface low pressure to the east, and showers along the western slopes along the leading edge of the deeper moisture axis on the front of the upper low. Kept pops mostly low end chance most sections with less southern Blue Ridge for now where expecting a gap between the two. Otherwise becoming partly/mostly cloudy with highs mostly 60s mountains to low/mid 70s east. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 200 PM EDT Sunday... Trend toward a cloudier/wetter and much cooler scenario continues at least to start the period with models swinging a strong upper low from the Ohio Valley Wednesday night southeast into/and around the region into early next weekend. This would allow for some degree of deeper moisture with the residual front along the coast to get swung back inland around this feature espcly by weeks end per latest GFS. However model spread remains rather large with earlier ECMWF farther to the southwest with this feature, while the 12z GFS much stronger now with the cold pool about directly overhead Thursday into Friday night before the upper low lifts out. Latest ensembles were closer to the earlier weaker 00z runs excluding the Euro with a much weaker upper system keeping most showers to the north and east. Since great uncertainty plan to just continue trend of including more chance pops Wednesday night into Friday night, at least for diurnally driven showers under the cold air aloft. Should finally see this slow moving system weaken and lift out during the weekend with drier air aloft spilling in. This will allow for a return to dry weather for later Saturday into Sunday although clouds may linger given possible redevelopment of another low level wedge under leftover weakness aloft at this point. Temps likely to be below normal under very cool air aloft with highs 60s to low 70s through Friday with potential 50s at elevation pending clouds. Some rebound likely during the weekend as highs slide back into the 70s overall. Lows also chilly, mostly 40s to low/mid 50s, with possibly even a few 30s again deeper valleys early Thursday if a period of more clearing develops early on. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... As of 800 PM EDT Sunday... Generally poor aviation conditions expected through much of this TAF valid period. Wedge air mass in place with stalled front across western NC toward the SC/NC border. This front will remain in place through Monday morning, then begin to lift back to the north as a warm front in advance of a stronger cold front expected to arrive from the west Monday night into early Tuesday. Low clouds already in place, as they have been much of the day, in the wedge air mass with widespread MVFR ceilings. Tendency will be for ceilings to lower into the high end of the IFR range overnight, especially east of the Alleghany front. It may take a little longer for the MVFR-IFR cigs to reach the WV TAF sites, but they will arrive before daybreak. Fog development in question because of dense cloud cover. However haze was already prevalent at sunset, denoting a moist low-level air mass and with easterly flow expect fairly widespread development of MVFR BR overnight. No radiational cooling expected and with dense cloud cover, IFR-LIFR FG conditions appear less likely, but such could develop near the Blue Ridge, e.g., KBCB in -DZ FG. Wedge expected to erode from the southwest Monday afternoon. Cigs will improve to high end MVFR or low end VFR by 18Z. Strong cold front will push into WV/KY by late afternoon with a line of showers and a few thunderstorms expected to accompany the front, diminishing as it reaches the KBLF and KLWB TAFs. Have included VCSH in the western TAF sites after 18Z Monday, but did not mention any TS yet. Winds are expected to remain ESE-SE overnight at speeds of 4-7kts becoming SSE-SSW 6-8kts after 14Z Monday. Medium confidence in cigs and vsbys through the TAF valid period. Medium to high confidence in wind speed/direction through the TAF valid period. Low confidence in TSRA potential late Monday afternoon. Extended aviation discussion... The front will slow in its forward progression across the Piedmont Tuesday, so showers and a few thunderstorms may linger across that region as a result. Initially to the west conditions will improve with mostly VFR to MVFR conditions within this time frame. During midweek, a rather unsettled pattern is expected to evolve as a deep upper low sags southward from the Great Lakes into the Central Appalachians. This will keep variable clouds, MVFR cigs, and periods of -SHRA across the region through much of the period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/WP NEAR TERM...JR/KK/RAB SHORT TERM...JH LONG TERM...JH AVIATION...RAB/WP

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