Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 201736 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 136 PM EDT Tue Sep 20 2016 .SYNOPSIS... An upper low combined with the remnants of former Tropical Storm Julia will meander about eastern North Carolina and South Carolina through late week before finally shifting out into the Atlantic ocean. This will keep fairly unsettled conditions east of the Blue Ridge over the next several days, less so in western areas. High pressure will gradually build in from the west during the later half of the week before a cold front approaches late in the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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As of 130 PM EDT Tuesday... Slowed the arrival of showers into the piedmont. With more sun and less rain this afternoon, increase temperatures a few degrees especially across the foothills and mountains. As of 330 AM EDT Tuesday... A fairly complicated forecast through this period as a weak upper low cuts off across the Carolinas. This upper low is combined from the remnants of former T.S. Julia and an upper trough that cutoff from the westerlies yesterday. Models continue to offer several solutions as to how far west and north tropical moisture from these combined systems will track back into the RNK CWA. The GFS tends to be fairly dry keeping the associated rainfall closer to the coast, while the NAM/Canadian/ECMWF spread the moisture much further back west and north, all the way to the Blue Ridge. ECMWF QPF is excessive, while the GFS is minimal at best. The concern, however, is that the ECMWF, NAM, and Canadian are more similar in QPF amounts, namely several inches as compared to the GFS. Having said this, all models are in good agreement that more appreciable rain will not spread back into the CWA until later today or this evening. The HRRR and NCEP WRF models show little to no precipitation making it any further west than the eastern parts of the Piedmont today/this afternoon. Mid-level air spreading east from the TN/OH valley is quite dry with mean RH less than 15%. Therefore, in agreement with other offices, have decreased pops for today/this afternoon, then increased into the high chance range along RAH border after 00Z when a weak spiral band is evident moving west into the region. As always with this type of pattern of high tropical PWAT air yielding an efficient warm rain process and enhanced upslope easterly flow developing, there are concerns of enhanced, heavy rainfall along the east slopes of the Blue Ridge. This will certainly need to be watched over the next few days. While we are almost a year to the date of a very heavy rain/flooding event that evolved along the Blue Ridge in 2015, the synoptic pattern this week is not similar. Several features in place during the event last year are lacking at this time. We lack a T.D. moving north out of the Gulf with PWATS in excess of 2.0 inches, an upper trough to the west, a stationary front over the region, and strong easterly flow to the east of the front thanks to a strong surface high over New England. So, at this point, concerns are far less than at this time last year for such type of event. Nonetheless, as noted above, we will need to keep a close eye as the ECMWF/NAM/Canadian depict several inches of rainfall across eastern NC very close to the RNK border and we are dealing with remnants of a tropical storm, always problematic. The key take away here is that confidence is low that the deep tropical moisture from remnant Julia will ever make it this far west. It is more likely to stay across the coastal plain in association with the coastal trough. As has been the case for weeks, no appreciable change in 850mb temps noted through the forecast period. Temperatures will largely be dependent on cloud cover today, less in the west, more in the east. This will most likely yield max temps in the upper 70s to lower 80s with lows in the upper 50s to mid 60s. These temperatures are 5-10 degrees above normal, especially min temperatures.
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&& .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 315 AM EDT Tuesday... During this portion of the forecast, a frontal boundary will remain stalled from near Jacksonville, FL, northeast along the east coast to the Outer Banks of North Carolina. A closed upper low will meander north and south along this boundary, and be centered somewhere near the coast of South Carolina. Model guidance varies as to the degree to which moisture on the northern side of this system gets advected into parts of our region. The GFS is the driest with little if any precipitation impacting our area. The ECMWF, NAM, and Canadian solutions all to some degree offer solutions that have precipitation impacting roughly the southern third of the region, with the greatest concentration across the southeast parts of the area. WPC also has been leaning towards this wetter scenario as noted in their QPF forecasts. Our ongoing forecast has been been mirroring these wetter solutions, and the one from this morning will continue likewise. The biggest alteration will be a slight shift southeast of the northern most extent of the slight chance probabilities. Other than that, the general orientation of the precipitation in the forecast has been altered very little. By Friday into Friday night, the guidance is in fairly good agreement that the closed low will transition into an open wave and start to progress more out to sea. This process will allow for a decrease in coverage across the southern sections of the area. By late Friday night, all the precipitation is expected to be out of the area. Thanks to decreasing wind speeds, less cloud cover, and a moist boundary layer, late night and early morning river and mountain valley fog is expected, especially by Thursday night and Friday night. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 330 PM EDT Monday... The weak closed 500mb low opens to become absorbed in the general west to east flow which gradually takes the clouds and rain east and off the Atlantic coast Friday night. Upper ridge strengthens by Saturday morning but remains undercut by low level southeast flow Saturday ahead of a shallow backdoor front that may slide south into the area by Sunday. This supports isolated diurnal pops Saturday before deeper upstream moisture ahead of a stronger Midwest cold front works east along the backdoor boundary resulting in perhaps more widespread showers by early next week. Leaned temperatures towards Superblend with cooler readings especially for Sunday and Monday with moderating high temperatures for Tuesday. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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As of 140 PM EDT Tuesday... Weather through the TAF valid period will be driven largely by remnants of T.S. Julia and a stalled frontal boundary along the coast. This will result in a strengthening northeast flow and even low end gusts across the Piedmont this afternoon. This will also keep low-end VFR clouds across the piedmont, moreso towards KDAN. Areas across the foothills and west of the Blue Ridge will see scattered fair weather cumulus through the rest of today. Outer bands of light rain may creep toward Danville tonight with the best chance toward sunrise based on 12z model data. Western valleys will likely see a repeat of the dense ground fog after 06Z Wed, although confidence in this occurring is not as high as it was this morning. Extended aviation discussion... Wednesday through Friday, outside of late night/early morning fog/low clouds, expect mainly VFR conditions with the caveat to this being any lingering slight chance of showers from Julia per GFS/ECMWF, mainly affecting areas east of the Blue Ridge.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RAB NEAR TERM...AMS/RAB/RCS SHORT TERM...KK LONG TERM...KK AVIATION...RCS is the U.S. government's official web portal to all federal, state and local government web resources and services.