Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 131405 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1005 AM EDT Sun Aug 13 2017 .SYNOPSIS... A weak frontal boundary will remain stalled just to our south for the next several days. A broad area of low pressure aloft, located across the Great Lakes, will continue to send a series of upper-level disturbances toward the region from the west. These disturbances will interact with the front helping to enhance periods of showers and thunderstorms at times through the week. A new frontal system will move into the region from the west Friday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1000 AM EDT Sunday... Have not made a lot of changes to the general trend of the forecast for today. However, have made some minor adjustments as to how we trend to the over expected pattern of the day. For example, have reduced the rate by which isolated showers return to the southwestern sections of the area, and then spread northward, by a few hours based upon the latest trends of the HRRR guidance. Although the final coverage of precipitation by the late afternoon is not to be all that much different than our earlier prediction. Have also made minor adjustments to the hourly temperature, dew point, wind/gusts, and sky cover based upon the latest observations and expected trends through the early afternoon. As of 415 AM EDT Sunday... What was appearing to be relatively dry day compared to the past few and the upcoming several, does not appear so dry now. Model trends continue to stall the frontal boundary, which moved through the area slowly late Friday and yesterday, further north. This will leave the associated moisture further north as well. Lingering showers and isolated thunderstorms early this morning across the far southeastern portions of the CWA, associated with an upper-level disturbance, are moving east and will move out of the region before daybreak. By afternoon, moisture will return northward through especially the southwest and western parts of the CWA as surface high pressure drifts from the eastern Great Lakes toward New England. This will help to set up easterly flow and enhance moisture and lift along the Blue Ridge going into Monday. Would look for rain showers, and possibly even a thunderstorm in the far southwestern areas, during the afternoon/evening hours. Moisture will continue to closely creep northward overnight as the east-southeast flow further enhances lift along the Blue Ridge. Rain chances will increase from the south overnight. There may also be areas of drizzle and fog, especially along and near the Blue Ridge from northwest NC into southwest VA overnight. So, this may leave a period this morning where much of the CWA, especially the northern sections, will have a rain free period, but still lots of clouds. Throughout this section of the forecast, the best rain chances will be located east and south of the Blue Ridge, with the lowest chances toward the I-64 corridor. 850mb temps drop back into the +12C north to +14C south range today. This combined with the developing northeast to east flow, hence weak wedge, should result in similar or cooler daytime max times than what was observed Saturday. The air mass will be slightly drier with lower dewpoints northern areas today. A few glimpses of sun may help warm northern areas a bit more than southern areas where clouds will be more solid. MET MOS seemed to have the best handle on the cooler temperatures as it was generally a full category lower on max temps than the GFS or ECMWF MOS. Thus, given the synoptic situation in place, leaned heavily toward the MET MOS. This will yield highs near 80 across the Piedmont, with mostly 70s elsewhere. Lows remain muggy at night with increasing humidity again, mainly in the 60s, but near 70 across the eastern and southeastern parts of the Piedmont.
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&& .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 245 AM EDT Sunday... High pressure centered over the northeastern states will wedge south into the Carolinas Monday. The parent high will move off the New England coast Monday night with a bubble high remaining over southern Virginia and western North Carolina. A series of mid level short waves, being ejected out of an upper level trough over the Midwest, will move over the wedge keeping the chance for showers along and east of the Blue Ridge Monday. The highest probability for showers through the day will be along the NC/VA border. Northeasterly flow may keep the morning dry for areas north of hwy 460. As the parent high shifts towards the New England coast, low level flow will become more east to southeast, increasing the chances for showers to the north Monday afternoon and evening. Downsloping southeast flow should help deter rain from falling across extreme southwestern Virgina and southeastern West Virginia. However, the wedge will partially erode across the Mountain Empire to the Bluefield area. This area may see some late day thunderstorms that will fade during the evening. All in all, Monday will be a damp cool day with highs generally in the 70s. Areas outside of the wedge may touch 80F. Rain chances and areal coverage decrease Monday night. Areas not seeing rain overnight will see fog developing and hanging around into Tuesday morning. The wedge is expected to erode Tuesday as the upper level trough approaches from the west. Areas across south-central Virginia may not see the wedge erode until mid to late afternoon. Forecast soundings still showing a moist environment when the wedge breaks. So, as the sun peeks through the clouds, afternoon heating will lead to showers and thunderstorms Tuesday afternoon and evening. Afternoon temperatures should warm towards normal levels with near 80F west of the Blue Ridge to the low to mid 80s east. Fog is again possible Tuesday night, especially in mountain valleys. && .LONG TERM /THURSDAY THROUGH SATURDAY/... As of 130 PM EDT Saturday... Overall not a lot of change in the pattern from the past couple of weeks as any mid week ridging looks to evolve back into a weak upper trough pattern by next weekend. This will again force weak surface ridging Tuesday night into Wednesday to give way to return flow by the end of the week with a residual front to the south lifting back north as a warm front into Thursday. Upper flow then briefly amplifies with a surface low near the Great Lakes pushing another weak cold front southeast toward the region by Saturday. This should result in less shower activity to start the period, per lower PWATs off extended guidance, followed by increasing coverage with the warm front espcly southern/western sections Thursday, and then ahead of the cold front Friday into Saturday. Appears best coverage likely Friday as deeper moisture returns behind the passing warm front and ahead of the pre-frontal axis in advance of the cold front. Temps to rebound to more normal levels through the week under increasing 850 mb temps as a more predominate west to southwest flow aloft takes shape ahead of the next weak upstream cold front. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... As of 745 AM EDT Sunday... Poor aviation conditions expected through much of the TAF valid period. A weak front has stalled just to the south across central NC. Widespread MVFR ceilings in place this morning, with localized IFR-LIFR in BR/FG and low clouds. Conditions are expected to improve to mainly MVFR or low end VFR throughout the day as weak high pressure moves toward New England and induces a light northeast flow across the region. This flow will enhance lift along the Blue Ridge and Alleghany Front overnight into Monday morning resulting in ceilings lowering into the MVFR-IFR category east of the Blue Ridge and IFR-LIFR category west of the Blue Ridge, with local exceptions. Fog development Monday morning is less certain because of expected cloud cover, but some upslope enhanced fog or low clouds ceilings possible west of the Blue Ridge. Not much in the way of shower activity expected until later today, when light rain showers may develop mainly along the Blue Ridge of northwest NC into southwest VA. Instability may support an isolated thunderstorm in far southwest VA or near the NC/TN border, but certainly not at the TAF sites. Light showers will expand further north overnight, but likely not make it too far north of U.S. 460. However, drizzle will likely develop in enhanced upslope areas along the Blue Ridge and Alleghany front. Once early morning dissipates this morning, visibilities should improve to mainly VFR throughout the day today. But as noted above, MVFR to locally IFR visibilities will be possible in the morning. LIFR-VLIFR visibilities not expected Monday morning given cloud cover, with the possible exception of LWB. Winds will generally be light northeast to east throughout the TAF valid period, with some locations calm at night. Medium confidence in ceilings through the TAF valid period. Medium confidence in visibilities throughout the TAF valid period, except medium to high during the 15Z-23Z time frame. Medium confidence in wind direction, high confidence in wind speed through the TAF valid period. Extended Aviation Discussion... The wedge may be slow to erode Monday with the potential for widespread low-mid clouds and a few showers Monday resulting in periods of sub-VFR ceilings and visibilities. Late in the day Monday, a cold front will move into the region from the west resulting in a better chance for showers and thunderstorms and accompanying sub-VFR conditions. The front will stall, as the past several have done so, and linger just to our south throughout much of the Wed-Fri period until another frontal system arrives and potentially finally dries out the region. All-in- all look for unsettled week with scattered to numerous showers, with thunderstorms possible during the afternoon and evening hours. && .EQUIPMENT... As of 755 PM EDT Saturday Aug 12th... KFCX doppler radar will continue to be down for the rest of the month due to a failing bull gear. It will only be operational during this time frame for brief periods if an imminent significant widespread severe weather and/or hydrologic event occurs. Operating the system for any amount of time until the bull gear is replaced risks a catastrophic failure that would further extend system downtime. From the 17th through the 27th the radar will most likely be down completely as the repairs are expected to be made during this period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RAB NEAR TERM...DS/RAB SHORT TERM...RCS LONG TERM...JH AVIATION...RAB EQUIPMENT...RAB/WERT

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