235 FXUS61 KRNK 102027 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 427 PM EDT Wed Oct 10 2018 .SYNOPSIS... Wet weather is expected tonight and especially Thursday as Hurricane Michael moves through the southeast U.S. A cold front will cross the area by Thursday evening and shift Michael well offshore. A period of gusty northwest winds is expected in the mountains and foothills Thursday afternoon and evening. Cooler high pressure builds in to start the weekend. && .NEAR TERM /THROUGH THURSDAY NIGHT/... As of 330 PM EDT Wednesday... Most significant change in terms of headlines with the afternoon forecast package was with the expansion of the Flash Flood Watch north and west, to now encompass an area from the I-81 corridor eastward. Portion of the Watch which now includes the NW NC mountains, Wilkes County, Grayson, Smith and Wythe Counties is in effect until Thursday early evening. The portion of the Watch which previously included just the NC Piedmont, foothills and Southside now adds the the New River Valley and Roanoke County is in effect from 8 AM Thursday until late Thursday night. Finally, the final segment of the Watch includes the southern Shenandoah Valley eastward to include the central VA counties north of the Roanoke River. This part of the Watch begins Thursday morning and ends late Thursday night. This was done to better time when heavy rainfall is expected to begin. We still are contending with periods of moderate to at times heavy rain showers associated with tropical moisture infusion forced upslope in regime of southeast low-level flow. While not reaching overly excessive totals, nonetheless rainfall amounts to this point have proven to overproduce given highly efficient tropical warm rain processes, as regional radar shows this rain axis extending from central WV southeastward into northern SC, with greatest radar returns across northern SC. High resolution guidance continues to pivot this axis north to northeast over the next 3-5 hours, so the threat for heavy rain at times will still continue especially for areas along or east of the Blue Ridge. Couldn't rule out minor flood issues through mid-evening, but at the least this rain will only serve to prime the ground for the bigger slug of heavy rain which comes Thursday associated with Michael. Later tonight though, model guidance agrees that we'll begin to get into a relative minimum in rain chances and rain amounts, as we await the heavy rain associated with Michael's closest approach. For the latest on Michael's intensity and forecast track information, please refer to the latest advisories from the National Hurricane Center. Michael is forecast to eventually turn northeast across the central Carolinas into southeast Virginia and the Tidewater area on Thursday, its northeast turn aided by a strong cold front which is now working its way through the Mississippi River Valley. Michael's interaction with this cold front is expected to lead to an expected westward spatial expansion of Michael's heavy rain shield Thursday, and this has essentially played out in today's 12z NWP guidance. Expected local impacts for the region include the following: (1) Heavy rain, potential flash flooding and rises along smaller streams and creeks to mainstem rivers. Heaviest rains should occur through Thursday before clearing west-east Thursday late afternoon into Thursday evening in the east. (2) Post-Michael gusty to potentially damaging northwest winds Thursday afternoon into early morning Friday from the Blue Ridge, the immediate foothills counties and back westward to include the I- 81 corridor. Heavy rains should begin to progress northeastward starting earliest in the mountains of NW NC and the mountain empire around early Thursday morning, and then continue north and northeast the NC/VA Piedmont, Southside starting Thursday mid-morning and continuing into central VA by late morning. Area averaged rain forecast ranges from an inch to inch and a quarter in southeast WV and Tazewell County, 1.5-2 inches across the I-81 corridor into central VA, and then 2 to 5 inches across the southern Blue Ridge, the NC Piedmont/foothills and into Southside. These values closely resemble WPC's latest rain outlook. Localized higher amounts are possible, particularly the further south/east one goes. Overall confidence on rain totals is moderate to high on where the greatest rain totals will be observed, but is lower on exact magnitudes. See the Hydro section for more detail. As heavy rain begins to skirt eastward Thursday late afternoon into early Friday, focus will then shift toward strong and gusty northwest winds across the NC/VA mountains and into the immediate foothills areas. Of concern is that saturated ground in conjunction with this rain will lead to an environment which will make weak/dead trees easier to be felled. Very strong pressure rises and north- northwesterly 850 mb jet of 40-50 kts off the GFS will likely support a period of 45-50 mph wind gusts Thursday late afternoon into evening in these areas. To avoid any potential concern with confusing messaging with what really is more of a non-tropical wind hazard, haven't issued any wind headlines as yet in collaboration with NWS GSP, but it's quite likely that at least impact-based wind headlines will be needed at some point for the potential for tree/powerline damage and hazardous travel through mountain passes such as I-77/Fancy Gap. Have opted to message in the HWO for now. May also need to consider such headlines for the Piedmont as well but have highest confidence in the mountains. Forecast confidence is overall moderate on the heavy rain and flood threat, and is moderate to high on the post-Michael wind threat. && .SHORT TERM /FRIDAY THROUGH SUNDAY/... As of 330 PM EDT Wednesday... Focus Friday will be on post tropical storm conditions with rush of wind and cooler temperatures in the wake of Michael. Primary wind flow will be out of the northwest with dewpoints tanking into the 30s/40s as cool/dry October airmass finally makes an entrance. Temperatures Friday and into the weekend will resemble something closer to the climatological norm...a change that will stun folks into making an about-face for a jacket for outdoor activities. Weekend as a whole looks relatively dry with a couple of disturbances moving up through the Ohio Valley promising to cast a few clouds and possible showers to the WV mountains. In general, the upper air pattern will begin to shift into a Fall pattern, with broad troughing over the eastern 2/3rds of the country this weekend, trending toward a much deeper long wave trough by the middle of next week. During the transition, first in a series of short waves will pass through the base of the trough, the first wave being the remnants of the Pacific Hurricane Sergio which will pass through the Ohio Valley Sunday. The wave that follows Monday will be of Canadian origin. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... As of 400 PM EDT Wednesday... October finally feels like, October. A large scale upper trough is forecast to reside over the eastern 2/3rds of the country the entire week, amplifying by mid-week as series of short waves digging south from Canada resulting in a fairly extensive cold intrusion to much of the central and eastern CONUS. Model concensus is for some unseasonably cold air (yep summer to winter) to overtake areas which were basking in unseasonable warmth, and in some cases extreme enough to permit the frozen form of precipitation across the upper Mid-West and Great Lakes into parts of New England where 850 mb temps plunge low enough to support snow. For the central Mid-Atlantic it will certainly feel like October, if not favor some November sort of temperatures which would be permissible for frost formation for parts of the forecast area...especially the mountain valleys. Precipitation chances appear somewhat limited, but there will be opportunities for showers associated with each passing shortwave trough, Monday favoring a day for showers...and then again toward mid week. && .AVIATION /19Z WEDNESDAY THROUGH MONDAY/... As of 213 PM EDT Wednesday... Poor flying conditions generally will predominate the 18z TAF period, but especially for the Thursday portion of the TAF as the circulation associated with Michael makes its closest approach. Current MVFR/IFR cigs/visbys will remain relatively stable with periods of 2-6 SM SHRA through the afternoon and early evening as tropical moisture is lifted upslope. Toward the evening hours (after 02z), steady rain showers should taper off to intermittent unrestricted showers, but lingering MVFR-IFR ceilings will likely continue across the TAFs. Southeast winds 6-8 kts should predominate from the afternoon into the evening. Heavy rains associated with Michael's northern circulation then begin to progress north/northeast toward the pre-dawn hrs of Thursday morning in the NC mountains, and through the rest of the 18z TAF period across the remainder of the airspace. Look for initial VFR visbys to decrease to MVFR-IFR most everywhere as these rain bands move northeast, and may at times be LIFR vsby at Danville. Ceilings likely to be in the IFR range. In addition, while surface winds should be east to northeast around 4-8 kts, very late in the TAF period northwest winds along mountaintops will increase rather sharply as Michael passes, lead to a likely period of mechanical turbulence especially within and just downslope of the Blue Ridge. Confidence is above average for sub-VFR most of the period, but lower on timing heaviest rains Thursday and how low ceilings go tonight into Thursday. Extended Discussion... Heavy rains associated with Michael will begin to shift eastward late Thursday afternoon, but northwest winds will continue to increase Thursday night into midday Friday leading to gusty northwest surface wind conditions and areas of mountain turbulence. Conditions will improve back to VFR on Friday behind the front, then VFR over the weekend. && .HYDROLOGY... As of 300 PM 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 not directly associated with Hurricane Michael has been very impressive across northwestern North Carolina, especially parts of Watauga County today. Radar is grossly underestimating the rainfall in this area due to a combination of beam overshooting and tropical air mass characteristics. Some rain gages In Watauga are showing up to 5 inches during the day today, much of it falling in about 6 hours. The Watauga River and upper New River stream gages are rising very rapidly this afternoon and will be rising above flood stage shortly. The area remains under a Flood Advisory and a Flash Food Watch has already been issued for this region. Flood Warnings may be required After a lull in rainfall the tonight, the 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 some variability in rainfall amounts depending on the track and speed at which the storm pushes across the Carolinas, but the most likely scenario still indicates QPF of 2 to 5 inches across the Southside of Virginia and the Piedmont of North Carolina, where the flash flood watch continues. The highest end of this range falls in the Dan River basin where guidance already suggests moderate flooding is the most likely occurrence. But shifts in QPF will heavily affect forthcoming river forecasts and both minor or major are also possible depending on how much and where it falls. 1 to 2 inches of rain seems most likely across the upper Roanoke, James, Greenbrier and middle New river basins where river flooding will likely be minor if any. Again, modest shifts in the rainfall pattern could substantially affect these river forecasts, so remain attentive. 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. && .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...AL NEAR TERM...AL SHORT TERM...PM LONG TERM...PM AVIATION...AL HYDROLOGY...PC