Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 250820 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 420 AM EDT Sun Sep 25 2016 .SYNOPSIS... A backdoor front slide south into the Carolinas today, before stalling. High pressure following the front will wedge south along the eastern slopes of the mountains Sunday 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 TONIGHT/...
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As of 342 AM EDT Sunday... Backdoor front situated from eastern Kentucky, southeast into the mountains of NC early this morning will continue to drift slowly south to the Smokys and toward the coastal region of NC/SC. High pressure will move across the Northeast setting up a wedge over us. Satellite showed the stratus layer had advanced as far southwest as Pulaski, VA to Danbury NC at 330am. Will see these clouds make it to most of the forecast area by mid morning but confidence not too high that it will encompass the entire forecast area given shallowness of the moisture. As far as precip goes, models generating light qpf across the Blue Ridge but vary on exact location. Given depth of moisture threat is very low but could see patchy light rain or drizzle from near the Blue Ridge and Alleghanys east to the piedmont this morning. As head through the day should see edge of cloud cover erode somewhat in the west, but looks to hold firm roughly along and east of the WV/VA border south to the NC mountains. Days getting shorter with lower sun angle will be a factor on how much erosion occurs. Will hold onto slight chance pops most of the day across the Blue Ridge into the piedmont. Previous forecast of lower temperatures today looks on target with readings in the upper 60s to mid 70s east to lower 80s far southwest. Tonight, the flow will turn more southeast as previous backdoor front shifts back to the north across the Southern Appalachians. Should see better lift enter across the southern forecast area overnight so kept pops in the chance range but still no higher than 30, with slight chance pops across most of the rest of the region. Lows are going to be held up by the clouds with upper 50s to lower 60s common throughout southern WV and SW VA into NW NC.
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&& .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/...
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As of 330 AM EDT Sunday... The backdoor front stalled across western NC will begin to return north fairly quickly by Monday afternoon as a strong cold, front, basically our first fall cold front of significance, arrives Monday night into Tuesday. Early morning low clouds and drizzle along the Blue Ridge should give way to partly cloudy and continued fairly warm and humid conditions in advance of the front. Synoptic scale models are in good agreement on the arrival and timing of the front, reaching the western part of the CWA during peak heating, then tracking east across the Piedmont during the overnight hours. With the wedge gone and a warm humid air mass, expect some thunderstorms. However, best instability remains west of the region, diminishing considerably as the front tracks across the Alleghany front. Have included chance thunder, but do not expect much in the way of severe. Due to the broad nature of the parent upper low/trough sinking southward through the Great Lakes, dynamic forcing is not impressive. Have opted to drop mention of thunder after 06Z Tue. The front will slow forward speed Tuesday, but should still track far enough east to allow most of the precipitation to exist the CWA. However, weak surface low pressure developing along the front across the southeast states and the southward sinking upper low will result in potential retrogression of precipitation across eastern NC Tuesday. Consequently, kept a slight chance pop along the far southeastern border to account for this possibility. Beyond Tuesday, considerable discrepancies are noted among the various models regarding the evolution of the Great Lakes/Central Appalachians upper low. Initially the upper low will sag southward into northern WV/VA, then lift out by Thursday into the northeast states, per GFS. However, the ECMWF, for the second run, progs an anomalous situation where the upper low sags southward into the Carolinas, then retrogrades northwestward back into the Ohio Valley during the later half of the week. This solution would bring considerably more rain to the region than the GFS (and most other model solutions), as well as much colder temperatures. The pattern appears fairly unrealistic and is not accepted at this time, although the relative consistency from run-to-run of the ECMWF regarding this solution is somewhat disconcerting. For now, have just advertised slight chance pops, focusing mainly across the western mountains and northern areas closer to the more likely path of the upper low. Temperatures will finally trend toward more normal fall readings after Tuesday. 850mb temps will drop back toward +10C by midweek, bringing lows in the 40s and 50s and highs in the 60s and 70s. Something we have not seen since last spring, yet only near normal for late September.
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&& .LONG TERM /THURSDAY THROUGH SATURDAY/...
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As of 400 AM EDT Sunday... As noted above, the extended periods hinge largely on the progression of a deep closed low progged to be somewhere in the Appalachians or Mid-Atlantic region. ECMWF solution has been thrown out at this juncture, opting for the more progressive solution of the GFS and other models. Thus, expect a drying trend with increasing amounts of sunshine during the later half of the week. Giving some small credence to the ECMWF solution, however, given its run-to-run consistency, have opted to increase pops into the slight chance category in the Wed-Thu time frame, especially north and west, until we can obtain a better handle on the eventual pattern that will evolve. Temperatures will remain near seasonal normals during the period, but could be considerably cooler if the ECMWF solution verifies and the area remains in a cloudy, wet pattern for several days under the upper low. Really just to many uncertainties at this point to make any wholesale changes.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/... As of 130 AM EDT Sunday... Backdoor front situated along the NC/VA border will slide south today. Lower ceilings already working as far southwest as ROA/MTV/SIF and points northeast. Question will be how far southwest this canopy of MVFR to IFR ceilings makes it. Think bCB will soon drop to MVFR as will LWB. BLF should stay out of the wedge clouds but could see some patchy IFR ceilings or vsbys after 09z. Models show this wedge slow to erode Sunday with MVFR or worse conditions prevailing most of the morning, and do not see LYH/DAN/BCB getting above 3kft at all this taf period while ROA may briefly rise above 3kft this afternoon only to fall back after 00z. Drizzle may impact LYH/DAN but overall should keep vsbys VFR, while ROA could see enhancement of drizzle this morning given some upslope component which will drop vsbys to MVFR. BCB/LWB vsbys will vary depending on how thick the low clouds get but do not foresee LIFR so keeping it more IFR at times. Needless to say not good flying conditions east of the Alleghany Front. Extended aviation discussion... The backdoor front will drift to just south of the area by Sunday night and stall. Overrunning will begin in earnest Sunday night into Monday in advance of a strong, but slow moving, cold front that will approach the area from the west late Monday. A healthy line of showers and a few thunderstorms can be expected in advance of the front as it moves through the region during this time frame. Widespread sub-VFR cigs/vsbys are expected in the wedge air mass Monday before the front arrives, which will also be accompanied by a period of sub-VFR cigs/vsbys. Conditions will improve by midweek, although as an upper low sinks southward over the Appalachians, conditions could become unsettled enough to continue periods of sub-VFR cigs and scattered showers. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...RAB LONG TERM...RAB AVIATION...RAB/WP

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