445 FXUS61 KRNK 101405 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1005 AM EDT Wed Oct 10 2018 .SYNOPSIS... Wet weather for the next two days as Hurricane Michael moves through the southeast U.S. A cold front will cross the area by Thursday evening and shift Michael well offshore. Cooler high pressure builds in to start the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 1004 AM EDT Wednesday...Increased PoPs across the Alleghany Highlands over the next couple hrs - essentially an earlier start of rains as current radar shows showers already that far north. Though it isn't surprising given the radar beam blockage, observed rainfall totals have been higher in the NC mountains than radar estimates would otherwise indicate, with totals nearing three-quarters an inch across Watauga and Wilkes Counties as of the last 3 hours. With heavy rains affecting parts of southern NC well east of Charlotte likely to set eyes on the Blue Ridge, southern Shenandoah and the VA Piedmont toward mid to late afternoon per general high-res guidance consensus, raised QPF a little more for the afternoon in those described areas as well. Still looks like the steadiest rains move in late this morning and into the afternoon/early evening areawide. Otherwise, see no compelling reason to make further changes at this point. Will be digesting incoming 12z guidance as it pertains to rainfall associated with Michael's approach to determine if northward changes to the FFA are necessary. No changes imminent as yet, however. Previous discussion from 320 AM Wednesday... Wet weather in store today as southeast flow combined increasing pwats and upper level energy slides into the mountains this morning and spreads north thru the day. High-res and synoptic models painting in more efficient rainfall this morning over the NC mountains, therefore, with collaboration with WFO GSP, will be starting the Flash Flood Watch at 8am for the NC mountains/Wilkes. Will see best upslope today, then low level flow turns more south tonight ahead of Hurricane Michael, which should over southwest Georgia by midnight. Updated the storm total rainfall, which still reflects locally 3-4 inches along the Blue Ridge southwest of Meadows of Dan, but appears a lull this evening/overnight may limit amounts to a degree. Nonetheless, forecast still appears on track for likely to categorical pops for most of the forecast area today, with maybe just scattered chances east of Lynchburg. As far as thunder chance, instability for now looks very limited given tropical like sounding and skinny CAPE. Am not seeing a threat of thunder, except perhaps isolated rumbles south of the VA/NC border this afternoon. Any convection will also likely bring heavy downpours. Current flash flood watch locations still look good, so no areal extension at this point. Tonight, expect the flow to turn south at 8h and will see a break in the overall coverage of rain, but still not enough break to drop pops to below 30-40 percent. At the moment will leave likely to cat pops across the Blue Ridge as majority of solutions keep best low level convergence in this area. Some subsidence ahead of Michael however may limit coverage further east in the piedmont. As far as temps clouds/rain will keep most if not all of the forecast area in the 70s, with some 60s in the NC mountains. May see 80 across southside east of Danville to Buckingham. Muggy/tropical tonight with lows in the mid 60s west to around 70 east, as Augtober continues. Forecast confidence average on rainfall amounts, and high on everything else. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 330 AM EDT Wednesday... Primary focus is the track and anticipated rainfall footprint from Hurricane Michael. In general, model guidance is in reasonable agreement, featuring a highly amplified upper air pattern with an anomalous subtropical ridge of high pressure off the Mid-Atlantic Coast, and a deep longwave trough over the southern Plains. Caught in- between is Hurricane Michael over the eastern Gulf of Mexico which is forecast to gain latitude in a proverbial sling shot between the two upper air features. Latest track guidance suggests the center of Michael will bisect the Florida Panhandle Wednesday afternoon/evening then trek northeast across south central GA and into the Carolinas Wed-night and Thursday. A generous amount of moisture is forecast into our forecast area Thursday as the center of the system passes to our southeast, an axis of heavy rainfall occurring within the northwest quadrant of the storm where the low level winds converge with the increasing westerly winds aloft. At then moment, we are forecasting a general 1-4 inches of rain across the forecast area with the higher amounts along eastern slopes of the Blue Ridge. There are two areas which may see locally higher amounts, one being the Blue Ridge Parkway southwest of Roanoke into the NC High Country where the southeast upslope wind component will be a premium before the storm arrives. The second area to watch will be over the NC/VA Piedmont on the northwest side of the storm track where convergent flow will be maximized (CLT-GSO-DAN corridor). If all comes to fruition, watershed within our CWA which will have the most impact from runoff would be the Dan River Basin. Wind impacts for our County Warning Area (CWA) from this system may be felt more so on the back side of the storm as it exits the region Thursday night and Friday. Direct impact from core tropical storm strength winds will occur along the central track of the storm (southeast of our CWA). However, as the storm moves rapidly to the northeast and away from the area Thursday night, pressure rises in the wake of the system across our mountains, in addition to the passage of a surface front will bring strong cross-barrier flow with 85h winds of near 40 kts. This will result in gusty surface winds, similar to a strong frontal passage, across our CWA Thursday night and Friday with gusts of 20 to 30 mph. A noticeable airmass change will take place Thursday night into Friday. A surface front associated with the upper trough to our west will finally makes its entrance, northwest winds quickly introducing an airmass change that will reflect much lower dewpoints and cooler temperatures. These temperatures/dewpoints will be a seasonal eyeopening change, more of a reflection of what mid October should actually feel like weatherwise. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 400 PM EDT Tuesday... Upper level pattern will deflate over the weekend and into the following week as the Bermuda high becomes a summer ghost. One last surge of higher dewpoint air will try to work back into the area Sunday into Monday ahead of what will be the remnants of Hurricane Sergio, which is expected to get drawn into the southwestern CONUS from the Pacific Saturday, then into the base of the central CONUS long wave trough. Attm it appears the energy associated with this feature will pass through the OH valley Sunday with showers primarily to our northwest. However, with the passage of this feature it will open the door for a pattern change with the northern jet stream making a dive into the central and eastern CONUS, a feature that will favor below normal temperatures across much of the central and eastern U.S. by the middle of next week. As for the details, the upcoming weekend as a whole does not look that bad...featuring dry conditions and near seasonal temperatures. Near seasonal temperatures however, means temperatures at night will be in the 40s, so it will feel noticeably cooler compared to our extended summer-like nights. By Monday, showers associated with the deepening upper level trough to our northwest are expected to move across the area. After that...it dries out for the middle of next week, but expect much cooler nights with the potential for frost in some areas toward 18th of the month. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... As of 720 AM EDT Wednesday... Well, we actually had VFR conditions as low clouds either never materialized, or were scattered enough on the downslope side of the Blue Ridge. Worsening conditions expected this morning as influx of rain moves across from south to north. Am keeping more pessimistic on cigs and at times vsbys but confidence is low on this being the case constantly west of BCB as downslope flow may still keep cigs elevated at BLF/LWB. Main threat of moderate to heavy rain will be along/east of the Blue Ridge late this morning into the afternoon. Going to keep thunder out as not see enough instability to warrant mention. As we head to this evening, a lull of sorts but will still be seeing light rain/drizzle at times though flow turns more south overnight as Michael heads closer. Rain from Michael comes full force after 12z Thursday. Looking at increasing chances of rain later this morning at BCB/ROA then expanding to other sites by late morning. Rain will become moderate to heavy at times especially at ROA/BCB due to upslope enhancement this afternoon. Instability seems limited so no mention of thunder in tafs. Confidence is above average for sub-VFR most of the period but lower on timing of how low ceilings go and what times. Extended Discussion... Showers Thursday will be heavy at times especially along/east of BCB/ROA as tropical moisture shifts northward ahead of Hurricane Michael and a cold front moving in from the west. Conditions will improve back to VFR on Friday behind the front, then VFR over the weekend. Winds are expected to be gusty at times Thursday over the southeast near DAN and behind the front Friday at all sites, but mainly under 30kts. && .HYDROLOGY... As of 300 AM Wednesday... An inflow of deep tropical moisture will introduce the possibility of both flash flooding and, later, river flooding in the Wednesday afternoon through Friday time frame. A first round of heavy rain is expected to affect the northwestern North Carolina and southwestern Virginia Blue Ridge and foothills regions Wednesday afternoon/evening as winds shift southeasterly across the mid- Atlantic, carrying very deep moisture into the mountains. One to two inches, with locally higher amounts, are expected along the crest of the Blue Ridge and may result in flash flooding in spots by Wednesday evening. A flash flood watch has already been issued for this region. After a brief decrease in rainfall on Wednesday night, rainfall coverage and intensity is expected to increase again on Thursday late morning/early afternoon with the approach of what is expected to be (by that time) Tropical Storm Michael. Weather forecast models are still indicating a variability of rainfall amounts depending on the track and speed at which the storm pushes across the Carolinas, but the most likely scenario indicates 2 to 4 inches across the Southside of Virginia and the Piedmont of North Carolina, where a flash flood watch has been posted. Convective bands with high rates of rainfall within a tropical environment will obviously present an enhanced flash flood threat, with the Weather Prediction Center calling for a slight to moderate risk of excessive rainfall during the Thursday to early Friday timeframe, mainly across the southeast one-third of our forecast area. The Dan and lower Roanoke river basins will be most heavily affected by rainfall from Michael, and it appears reasonable that points these rivers will experience at least minor flooding...again: all dependent on where the heaviest rainfall occurs. Lesser amounts are expected to the northwest to include the Greenbrier and James River basins, which should limit the flood threat there. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flash Flood Watch from Thursday morning through late Thursday night for VAZ032-043-044-058. NC...Flash Flood Watch from Thursday morning through late Thursday night for NCZ003>006-020. Flash Flood Watch through Thursday evening for NCZ001-002-018- 019. WV...None. && $$ SYNOPSIS...WP NEAR TERM...AL/WP SHORT TERM...PM/RCS LONG TERM...PM AVIATION...AL/KK/WP HYDROLOGY...PC/RCS