Area Forecast Discussion
Issued by NWS Blacksburg, VA
Issued by NWS Blacksburg, VA
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555 FXUS61 KRNK 240118 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 918 PM EDT Mon Sep 23 2024 .SYNOPSIS... High pressure is situated across the Mid-Atlantic today, wedged against the eastern face of the Appalachians. A slow-moving cold front will meander along the Ohio River Valley over the next several days, triggering rounds of showers and thunderstorm activity that will persist through much of the workweek. PTC 9 lifting northward from the Gulf of Mexico toward the end of the work week may bring more widespread rainfall to the region. && .NEAR TERM /THROUGH TUESDAY/... As of 915 PM EDT Monday... Key Messages: 1) Additional rounds of showers and thunderstorms are expected again on Tuesday, with a few bringing locally heavy rain and damaging winds. 2) Cloudy conditions again tonight are expected with patchy fog and additional showers and a few storms. The only changes made to the forecast for tonight were adjustments to the POPs. At 8 pm EDT, low level moisture was pooling along a warm front located roughly along the eastern slopes of the Blue Ridge. The 00Z/24 RNK sounding measured a PWAT value of 1.56 inches which is above the 90th percentile for this date. This anomalously high moisture combined with the low level boundary and an approaching upper level disturbance will likely spawn more showers and even a few storms with lingering instability during the overnight hours, especially near the low level boundary. Many of the CAMs, including the HRW NSSL which has a fairly good grasp of the location of precipitation at 8 PM EDT, reflect this as well. As a result, modified POPs to account for showers/storms continuing through the overnight hours. No changes were made to the temperature or wind forecasts for tonight. A mid-level shortwave vorticity perturbation is expected to push across the region late Tuesday afternoon and evening. This coupled with a stalling cold front west of the Appalachian mountains, and an upper level trough digging into the mid- Mississippi valley will provide sufficient upper level support/forcing for ascent for widespread shower and thunderstorm developing Tuesday afternoon. A surface wedge situated across the coastal plain will lead to southeasterly flow across the VA/NC border during the afternoon and evening hours on Tuesday. Current CAM guidance suggests that a warm front will remain situated along or just north of the VA/NC border tomorrow, which may provide a focus for severe weather, and training thunderstorms. The biggest caveat will be the placement of this boundary Tuesday afternoon, and the amount of clearing south and west of this boundary to allow for destabilization. WPC currently has an areawide marginal risk for excessive rainfall on Tuesday as widespread rainfall totals of 1-2 inches are forecast. SPC similarly has a marginal risk for severe thunderstorms for much of the western portion of the area where destabilization is expected to be greatest. Overall, expect highs in the upper 70s where clearing occurs across SW Virginia and the VA/NC border, and low 70s to upper 60s where low clouds hang around across interior Piedmont locations. Confidence in the near term is moderate. && .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 215 PM EDT Monday... Key Messages: 1: Daily showers and storms 2: Potential for outer bands of PTC9 to bring further rain Thursday A glacially slow cold front attendant to a low in the southern Great Lakes will continue its march to the east in our direction. The southerly flow will fetch plenty of Gulf moisture to interact with a stationary front over the Mid-Atlantic, leading to showers and storms for most of the day through Wednesday evening. Wednesday there is a marginal risk of severe storms west of the Blue Ridge, where orographic effects could enhance the freely available moisture and moderate instability into stronger convection. Main threat would be damaging thunderstorm winds. Localized flooding may also be a concern, as PWATs will be in the 1.5-1.75" range for most of the CWA, and storms in this environment have been observed to be extremely efficient rain producers. This problem could be exacerbated by freshly fallen leaves as we are getting into the fall season, which could be piled up from the aforementioned thunderstorm winds. This can clog drains and sewer openings, preventing runoff in streets and sidewalks. By Thursday evening, we may be close enough to feel the impacts of Potential Tropical Cyclone 9`s outer rain bands. PTC 9 is progged to be quite a wide storm, so even though the center of the storm (according to the current NHC track) will be near the panhandle of Florida, the outer rain bands (and tropical moisture to interact with the pre-frontal environment) could impact our NC counties as early as Thursday evening. Temperatures will be around normal, with little diurnal sway due to high dewpoints and widespread thick cloud cover. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 200 PM EDT Monday... Key Messages: 1) Confidence is increasing that Potential Tropical Cyclone Nine will affect the area Friday into the weekend, though exact impacts remain uncertain. 2) Rainy, cloudy, and breezy conditions will persist and keep below average high temperatures in place through the weekend. High pressure over New England and the Great Lakes region will begin to back out of the area on Friday, as PTC9 moves northward towards our area. This high pressure, along with a cutoff upper-level low over the Great Plains will steer PTC9 towards the northwest. Where this turn occurs is still uncertain and will determine the magnitude of impacts seen across the RNK CWA. Most model ensembles continue to indicate the center of the low somewhere in the southern Appalachian Mountains on Friday, but the past few model runs have not been consistent with the track post-landfall, varying east to west. Should the current forecast track continue with the storm moving into far western North Carolina, heavy rainfall will be expected across the CWA on Friday into Saturday as models are consistently showing a band of rain spreading over the area. Regardless of the exact track of the system, moisture is expected to increase with showers possible each day through the period. PoPs have been raised to reflect recent model runs and taking into account that a tight gradient between the low pressure system to the south and the high pressure system to the north will allow strong easterly upslope flow to further enhance rainfall across the area. This setup will continue into Saturday, though PTC9 will weaken as dry air begins to overtake the system. The aforementioned high pressure over New England will wedge back in on Saturday, keeping cloudy and cool conditions in place, but decreasing rain chances. Showers remain possible Sunday into Monday, though Monday is trending drier. Highs throughout the period will generally be in the 60s and low 70s, with breezy conditions persisting. Low temperatures will remain mild, around 60 degrees this weekend before lowering into the 50s next week. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... As of 736 PM EDT Monday... Early this evening, MVFR to IFR CIGs were observed along and east of the Blue Ridge and confidence is high that these ceilings will fall tonight keeping poor flying conditions in place well into Tuesday. For the mountain sites of KLWB, KBCB and KBLF,while VFR conditions were observed this evening, a moist airmass will allow CIGs to drop to IFR levels quickly tonight. These low cigs will continue well into Tuesday morning and likely through Tuesday afternoon, with KDAN having the best chance of seeing improving conditions. Confidence is low regarding the timing of when/if CIGs improve to VFR category. A nearly stationary front bisecting our area will allow periodic showers to re-develop during the overnight hours, so kept VCSH in for most locations tonight. A better chance for showers and storms arrives tomorrow afternoon after 17-18Z. EXTENDED AVIATION OUTLOOK... MVFR/IFR stratus and fog are expected to occur in rounds through at least the middle of the week, and increasingly likely to persist through the end of the workweek. Rounds of showers and thunderstorms will also continue to pass across the lower Mid- Atlantic. Low pressure lifting northward from the Gulf of Mexico toward the end of the week may bring more widespread rainfall and sub- VFR conditions to the region. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...EB/NF NEAR TERM...PH SHORT TERM...VFJ LONG TERM...JCB AVIATION...PH