Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170920 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 420 AM EST Tue Jan 17 2017 .SYNOPSIS... Warm front lifts north across our area today. A cold front will track into the mountains this evening then southeast to the coast by Wednesday morning. Weak high pressure builds over by late Wednesday, then another storm system moves over the southern Appalachians by Friday. && .NEAR TERM /THROUGH TONIGHT/... AS OF 330 AM EST Tuesday... Surface high situated over Eastern North Carolina shifts east today with warm front lifting north across us as approach of cold front with showers erodes the shallow cold air east of the WV/VA border. Expect the fog this morning to erode after 12z/7am, maybe later near Fancy Gap, VA and the higher ridges of NC before showers move in scour it out. Coverage of dense fog is not enough for an advisory but will keep the Special Wx Statement going. Cold front early this morning from Northwest Indiana, south to the lower Mississippi Valley should be close to the mountains by late afternoon. Best lift should stay over WV into the Alleghanys today with less threat of showers further south. Nonetheless, should still see high enough coverage with the warm front/cold front over most of the area to warrant high pops, from categorical in the Greenbrier County area toward Bath County, VA to high chance in southside VA and NW NC mountains. The best threat will be late this morning into early afternoon. Some hint of instability showing up in the high-res models later this afternoon, so thunderstorms not out of the question over the Mountain Empire, but do not see a good enough chance to have mention in the forecast, given extent of clouds/showers, and main energy heading north. Temperatures should stay mild as warm front moves north of us today, despite the showers and cloud cover. Most locations should reach the mid to upper 50s, with some lower 60s not out of the question. Front moves southeast through the forecast area tonight. Some difference in model solutions, and could see area of deeper convection form east or southeast of our forecast area. Appears another area of lift with shortwave energy moves toward Southeast WV during this time so could see a lull in precip in between. Went with this trend but slower. Running likely pops over WV to NW NC mountains to chance pops east. Temperatures staying mild with lows in the mid 40s west to mid 50 southeast. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 415 AM EST Tuesday.... After an initial northern-stream trough exits Wednesday, relatively quiet weather conditions are anticipated for Wednesday night into the first part of Thursday. Late in the forecast period, a complicated mid-level pattern begins to evolve with southern stream becomes rather active/unsettled. The first of what may prove to be several southern-stream disturbances (continuing into the medium- range period) begins to affect the Appalachians and Piedmont Thursday night into Friday. For Wednesday: Relatively sharp positively-tilted trough treks across northern West Virginia into the mid-Atlantic region through the day, while an associated 850 mb front sags southward into North Carolina. Despite otherwise favorable broader lift associated with the trough, moisture becomes rather shallow through the column of atmosphere. Maintained chance-level PoPs across southeast West Virginia and into the Virginia Mountain Empire, give I`ve included mention of patchy drizzle along with chance showers. Thus, I really don`t think upslope QPF Wednesday will be all that substantial, maybe amounting to a couple hundreths at most. Despite variable cloud cover (greatest west), it will turn rather breezy Wednesday due to sharp cold advection occuring during the day. Per NAM/GFS BUFKIT soundings, northwest winds at the top of the mixed layer during the aftn are around 35 kts at Roanoke and Bluefield. Feel that northwesterly gusts in the 30-35 mph would be possible in western areas into the NC foothills, more in the 20-25 mph range east of the Blue Ridge into the Virginia and NC piedmont areas. Highs in the lower 60s furthest east aided by downsloping and greater sun, but may only top out in the 40s to mid 50s west of the Blue Ridge. For Wednesday night: High pressure builds in and essentially anchors itself over the CWA overnight. Result should be slackening winds and generally clear skies allowing for good radiational cooling. I wasn`t confident enough to place in the grids given the time of year, but I did think of the prospect of radiational fog across parts of the area given a few days of damp conditions, light winds and crossover temps in the upper 30s/low 40s being exceeded. Something for later shifts to evaluate. Temps should cool into the 30s, coolest in the Greenbrier Valley/Alleghany Highlands. For Thursday: Ridging sfc and aloft begins to shift east. It is replaced late in the day by isentropic lift in advance of a potent closed low over the Central Plains and surface low near northeast Arkansas. 00z suite of guidance shows significant differences in timing eastward increase in PoPs. The 00z NAM and the past couple runs of the SREF mean are the most aggressive in introducing PoPs into our western zones later Thursday, while the GFS is later Thursday evening and the ECMWF is more into Friday. Given the amplified nature of the pattern in mid-levels which can lend itself to slower eastward progression, I`ve sided toward the slower solutions, keeping Thursday dry. I would however expect an increase in clouds, capping high temperatures only in the lower to middle 50s. For Thursday Night and Friday: Upper closed low deamplifies as it advances into the Ohio Valley Thurs night and Friday. Surface warm front looks to advance into our western zones early Thursday night and continuing to progress northeast overnight and Friday. Result should be PoPs increasing into the Likely/low Categorical range. Given PWAT values that increase to near 1" and strong mid-level diffluent flow, appears that a window of steady widespread rain may be possible during the overnight hours and into Friday associated with the warm front`s advance. PoPs then decrease into Friday evening as system moves off the coast. Highs on Friday may be tempered significantly by abundant clouds and leftover CAD and I`ve leaned on the colder end of guidance, in the low 50s. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 330 PM EST Monday... The upper pattern will be in a state of flux for the end of the week and through the weekend, as one closed low which weakened/moved east Friday is replaced by a very large closed low which dominates the eastern US by Sunday night. This will keep a progression of weather system s affecting our region through Monday. In response to the changing upper pattern, another low will develop over the mid Mississippi valley and push what appears to be a dynamic occluded front into the area on Sunday. By this time deep moist southerly flow will be well established above the boundary layer, and when coupled with the dynamics of the front has the potential to produce a significant amount of rainfall. Guidance also indicates the presence of some instability which would support a chance for some thunder in an environment of robust low level winds. This is still quite a ways off but well worth keeping an eye on in the coming days. We eventually get into the dry slot, but moisture and colder air wrapping well around the vertically stacked system will keep a good chance of rain around through Monday. Temperatures will remain well above normal through the weekend, but colder air wrapping around the vigorous low on Monday will bring temperatures down a bit west of the Blue Ridge. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... As of 1145 PM EST Monday... The coverage of the IFR/MVFR ceilings remains a void from roughly KLYH-KDAN. Indications are that this area will return to sub-VFR conditions as the night progresses, obviously later than earlier anticipated. The latest TAF package delays the return for a few more hours. Most of the rest of the region already is experiencing sub-VFR conditions, and this trend will continue through the night. About the only region that may see more VFR conditions than sub-VFR will be areas near and west of KMKJ. Rain showers will move into the region late tonight in advance of an approaching cold front. Coverage will continue to increase during Tuesday morning, especially across the western and northern parts of the region. Increasing southwest flow in advance of the cold front will advect warm air over the cold air damming, helping to slowly erode it from the top to bottom. However, this feature that will terminate the wedge will be the passage of the cold front in the afternoon. VFR conditions are expected at all terminals by the mid afternoon. The initial round of precipitation will likely be pre-frontal. The 850mb front may bring an additional round of showers to the region Tuesday evening. A brief period of low level wind shear is expected just in advance of the cold front near a KBLF-KLWB line Tuesday morning. Extended Aviation Discussion... Clearing may take place behind the front on Wednesday with potential period of VFR Wednesday into early Thursday. However, this is expected to be short-lived per return of unsettled weather by weeks end as moisture returns from the southwest. No precipitation type issues are expected through the end of this week...just plain ole rain. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...AL LONG TERM...MBS AVIATION...DS/PM

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