030 FXUS61 KRNK 100541 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 141 AM EDT Wed Oct 10 2018 .SYNOPSIS... High pressure over the western Atlantic will begin to lose influence over the region late tonight into early Wednesday. A surge in subtropical moisture banked against the Blue Ridge will lead to a period of steady rains Wednesday. Hurricane Michael is expected to progress through the southeast US Wednesday into Thursday. Moderate to locally heavy rain in association with Michael's closest passage is expected to begin Thursday across the southern half of the region. A cold frontal passage late Thursday evening is expected to usher in a more seasonable air mass into the weekend. && .NEAR TERM /THROUGH TODAY/... As of 1000 PM EDT Tuesday... Latest updates remain minor, basically modest adjustments to rain chances through midnight based on the HRRR rapid update weather forecast model, as well as hourly temperatures. Overall, the afternoon forecast package remains on track. As of 205 PM EDT Tuesday... A persistent and stout anticyclone, both at surface and aloft is centered off the mid-Atlantic coast, which is extending a ridge back westward into the Appalachians. This has led to a persistent east to southeast fetch across a large part of the Appalachians and VA/NC Piedmont region. Partly to mostly cloudy skies exist across the region this afternoon and will continue to be the case at least for the first part of the evening. Widely scattered showers in this regime across northern/central NC will continue to progress westward and ascend upslope against mainly the southern Blue Ridge through mid-evening, as reflected in most finer-resolution guidance. While southeast upslope will continue, as we progress deeper into the evening, model guidance QPF becomes more nebulous and spotty but again is largely along or east of the Blue Ridge. Forecast BUFKIT RH profiles again show re-development of lower stratus with any cloud breaks filling in overnight, as was the case last night. As this moisture is quite shallow - a mostly unsaturated moisture profile above 850 mb - suspect that only drizzle or light rain would predominate again through the bulk of the overnight. Overall through overnight, rain amounts are pretty minimal and are generally a few hundreths of an inch or less, but amounts up to a tenth possible along the southern Blue Ridge from Carroll County southwest into Watauga County. Turbulent mixing from a continued southeasterly wind around 5-10 mph should keep mist from developing in most areas, though there could still be some patches at times into the Blue Ridge. Lows again on the whole similar to last night in the 60s, again quite unseasonable for this time of year. Wednesday is shaping up to be a pretty wet day, as we begin to feel the fringe effects of Hurricane Michael in the form of elevated tropical precipitable water values (GFS depicts values rising to on the order of 2"+). While still some minor timing differences exist between available 12z models, they each show a band of what is likely to be a period of moderate to locally at-times heavy rain showers progressing from south to north from around mid-morning across the NC Blue Ridge/foothills through early evening. This steady rain shield is the one currently affecting portions of northeast FL into coastal GA/SC, this rain enhanced by subtle vort energy in the weakness in the large high over the western Atlantic seaboard mentioned above. As a southeasterly low-level jet to the tune of 30-40 kts will be occurring, expect there to be some significant upslope enhancement to rainfall totals on the lee of the Blue Ridge and into the foothills with notable rain shadowing and a relative minimum in PoPs/QPF as one progresses further northwest across part of the New River Valley/Mtn Empire region into southeast WV. There's also a small amount of CAPE to the point where I did include a few possible rumbles of thunder for areas within or close to the Blue Ridge into the Piedmont. This will only serve to enhance rainfall totals, which through Wednesday afternoon should range from about a half to three-quarters of an inch from the NC foothills, Roanoke valley area into part of Southside, to locally near an inch along the escarpment of the Blue Ridge into Ashe and Watauga Counties. Otherwise, rain amounts taper to a quarter to a third of an inch in the New River Valley to less than a quarter inch into southeast WV and the Mtn Empire in the rain-shadowed area...with amounts of a quarter inch or so in the southern Shenandoah Valley area into central VA. This will set the stage for additional rainfall coming later Wednesday evening and particularly Thursday, in association with the approach of Michael. Coordinated with NWS Greer on a Flash Flood Watch which has been posted for the NC Mountains counties (Wilkes, Alleghany, Ashe and Watauga Counties) beginning at 2 pm Wednesday, ending early Thursday evening. Highs mainly in the low to mid 70s. Forecast confidence is moderate. && .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/... As of 400 AM EDT Tuesday... 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. Attm we are forecasting a general 1-3 inches of rain across the forecast area with the higher amounts along and east 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 /06Z WEDNESDAY THROUGH SUNDAY/... As of 130 AM EDT Wednesday... Worsening conditions expected this period for flying as ceilings and visibilities hang under VFR for most if not the entire next 24 hours and beyond. Increased southeast flow may allow for a period of VFR at BLF later this morning but for now confidence in mainly sub-VFR will stay in the forecast. 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. May be a period of less rain after 00z Thu before next round from Michael arrives later Thu morning. Since low levels remain moist think conditions will stay at or below MVFR after 00z Thu at all sites, except possibly BLF which may elevated above 1kft. 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 1100 PM Tuesday... 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 Blue Ridge and foothills region on Wednesday afternoon/evening as winds shift southeasterly across the mid-Atlantic, carrying very deep moisture into the mountains. Two to three 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 from this afternoon through Thursday evening for NCZ001-002-018-019. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL/KK/NF SHORT TERM...PM LONG TERM...PM AVIATION...AL/KK/WP HYDROLOGY...NF/PC