Area Forecast Discussion
Issued by NWS Wakefield, VA
Issued by NWS Wakefield, VA
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298 FXUS61 KAKQ 261936 AFDAKQ AREA FORECAST DISCUSSION National Weather Service Wakefield VA 336 PM EDT Thu Sep 26 2024 .SYNOPSIS... Hurricane Helene intensifies today, making landfall on the Florida Gulf Coast early tonight. The remnants of Helene will then pass well west of the area on Friday. Drier conditions briefly return on Saturday, followed by unsettled weather Sunday through next Tuesday. && .NEAR TERM /THROUGH FRIDAY NIGHT/... As of 330 PM EDT Thursday... Key Message: - Isolated to scattered showers redevelop this afternoon, mainly W of I-95, with only a slight chance to the east. - Widespread showers and isolated thunderstorms expected Friday, associated with the remnants of Tropical Cyclone Helene. Heavy rain and brief tornadoes are the main threats. The main wx story regionally (and nationally) remains Hurricane Helene in the eastern Gulf of Mexico. As of a 230 PM special NHC advisory, max sustained winds have increased to near 120 mph. Ahead of Helene, high pressure is gradually shifting further offshore from Atlantic Canada, leading to rather light SE/ESE flow over the region. An expansive shield of precip extends northward from Helene, with a predecessor (PRE) rainfall event ongoing in the higher terrain of SC/NC/VA. Aloft, a sprawling upper low is situated over the mid MS Valley. Temps are generally around 80 F this afternoon and highs will average 80-82, except upper 70s along the immediate coast. Light radar echoes are noted over our far western counties, but this does not appear to be reaching the ground as of this writing. For the rest of this afternoon into this evening, showers currently over central NC are forecast to spread northward into our SW and W counties as rich tropical moisture advances ahead of Helene. The highest coverage remains W of our area, but have 30-40% PoPs W of I-95, with 10-20% PoPs further E. Helene will make landfall in the FL Big Bend region tonight, moving inland along the southern spine of the Appalachians Friday morning. The remnant sfc circulation will then interact with the upper low over the MS Valley, eventually pivoting NW into the OH Valley. Isolated-scattered showers or light rain are expected through around sunrise, with the bulk of the rain forecast to hold off until the late morning and afternoon hours Friday. Rain chances ramp up quickly heading into this timeframe and PoPs of 70-90% have been continued areawide. Per the latest consensus across the CAMs, the bulk of the precip will be associated with a northward/northeastward pivoting band of heavier showers and embedded thunderstorms along a warm front-type feature. Given the tropical moisture source, PWATs of at least 2-2.2" will overspread the region and very heavy rain can be expected in any shower or storm. The duration of heavy rain is expected to remain on the shorter side, so aerial rain totals will range from 0.25-1.0" across the area (highest across the Piedmont). However, the very heavy rain rates could quickly drop an inch or two of rain in localized areas. There could be isolated instances of flash flooding in urban areas, but the overall flooding risk remains marginal per the latest WPC excessive rainfall outlook. The other side to the forecast, and perhaps the more concerning aspect, is the tornado potential. With sfc dew points surging into the mid 70s and temps in the upper 70s or lower 80s, there could be a narrow axis of higher sfc-based instability along and just ahead of the line of showers/storms. Forecast sounding show large, looping hodographs with plentiful 0-1 and 0-3 km SRH. If enough instability can be realized, a tornado threat could materialize. The highest threat is over central and southern VA and NE NC, where SPC has expanded the slight risk. A marginal risk borders the slight risk to the N and it is questionable how much destabilization there will be in these areas. The lowest tornado threat appears to be on the Eastern Shore. With strong winds aloft (850 and 925 mb winds of ~60 and ~40 kt, respectively), any convective core could also mix down some of these winds to the sfc, especially if low-level lapse rates can be sufficiently steepened in areas that see sfc heating and/or cloud breaks. Highs Friday range from the lower 80s across the E and S, with mid-upper 70s across the W and N. Skies average mostly cloudy to overcast. Lows Friday night in the mid-upper 60s, with slowly clearing skies and PoPs tapering off (though a few showers may remain). && .SHORT TERM /SATURDAY THROUGH SUNDAY/... As of 405 AM EDT Thursday... Key Messages: - Helene weakens and tracks well west of the area Friday, bringing the potential for moderate to locally heavy rain and a tornado threat (mainly to southern VA and NE NC zones). - Mainly rain-free and warm/humid Saturday. Rain chances rapidly increase after daybreak Friday, with PoPs rising to likely to categorical over the W/SW 1/2 of the area, overspreading the entire CWA by late morning or early aftn. The overall trends with respect to total QPF are similar or slightly lower compared to the previous model cycle. The ensemble means are less than 0.50" over the NE portion of of the CWA to ~1.00" over the western piedmont, but as usual much higher local amounts will be possible. Perhaps the greater concern will be for a tornado threat sometime from later Fri morning through Fri aftn as instability develops across NC and into at least southern portions of VA. Ample low level shear will be in place with strong SSW flow aloft and SE low level winds, yielding large, curving hodographs. Farther north, the amount of instability will be less so the threat is lower. Uncertainty in the exact track of Helene makes the severe threat somewhat uncertain, but there is enough consensus for SPC to have NE NC into a Day Slight Risk (primarily for the 5% TOR threat), with a marginal (2% TOR threat) into southern and central VA (roughly to the I-64 corridor). A dry slot moves in from the south late Fri aftn/Friday evening into Friday night, bringing an end to the precip. On Saturday, the remnants of (weakening) Helene will be located well off to our west over western KY. Drier weather returns outside of some low- end rain chances across the NW late. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 330 PM EDT Thursday... Key Messages: - Multiple low-end chances of rain Sunday through Tuesday early from the post tropical remnant of Hurricane Helene. - Drier and cooler air arrives mid to late next week. The latest model guidance indicates the remnant low of Hurricane Helene will gradually shift to the east through Tue after stalling near the TN valley on Sunday. This will keep conditions unsettled, with chcs for showers and a few tstms. High temperatures Mon-Wed will be in the mid-to-upper 70s with lows in the low 60s Mon-Tue night. A dry cold front is expected around Tuesday night. There is some model differences on the timing, but cooler overnight lows in the low-to-mid 50s across the piedmont and inland and upper 50s near the coast for Wed-Thur night are possible. After the cold front, significantly lower humidities are expected. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... As of 230 PM EDT Thursday... Scattered showers remain to the NW of RIC. High clouds from Hurricane Helene with VFR conditions are present across the area. Some lower clouds (around 2,000ft) are present across most of the area, but they are mainly scattered. Winds are primarily SE at 5- 10kt. MVFR CIGs will begin overnight with the piedmont reaching lower heights first. Patchy fog near SBY and PHF is possible before sunrise. Cloud heights will decrease throughout the day to IFR and locally LIFR as rain chcs increase. There is a chc for scattered storms/tstms starting tomorrow afternoon but timing and coverage are uncertain. Locally reduced visibility and higher winds are expected in any shower or storm. After sunrise, winds will pick up and gusts around 20kt are possible across the area. Outlook: The direct impacts of Hurricane Helene and its remnant low are somewhat uncertain, but Friday afternoon and into overnight has a continuing chc of storms. While Friday is likely wet, Saturday will be drier. A chc of showers is possible with unsettled wx Sun- Mon. && .MARINE... As of 405 AM EDT Thursday... Key Messages: - Small Craft Advisories have been extended into late Friday for all Atlantic zones due to persistent seas above 5 ft. - Southeast winds increase on Friday bringing the potential for Small Craft conditions in the Chesapeake Bay, lower James River, and Currituck Sound. High pressure has moved off the eastern Canadian coast with Hurricane Helene making northward progress into the eastern Gulf of Mexico. Winds are generally E or SE at 5-10 kt across the local waters. Waves are mainly 1-2 ft in the Chesapeake with 2-3 ft near the mouth. Seas range from 3-5 ft, highest away from the immediate coast. Southeast flow has overspread the waters as high pressure well to the north continues to translate eastward/offshore. This has allowed near-shore wave heights to finally fall below 5 ft but have maintained SCA headlines for all coastal waters as heights remain 4- 5 ft farther offshore. Hurricane Helene continues to intensify over the extremely warm waters of the eastern Gulf. The storm is forecast to make landfall along the Florida Big Bend early this evening. Helene continues quickly north tonight into Georgia. SE winds 5-10 kt continue today and tonight before the gradient strengthens locally on Friday. Winds increase to 15-20 kt with gusts to 25 kt on Friday into Friday evening and SCA headlines are likely for the Chesapeake Bay, lower James, and Currituck Sound. The gradient relaxes early Saturday as the remnant circulation from Helene gets shunted NW on the northern periphery of an upper level low. Waves in the bay increase to 2-3 ft with 3-4 ft likely near the mouth Friday afternoon and evening. Calmer conditions are expected on Saturday and even the seemingly never-ending SCA headlines for the coastal waters may even be allowed to expire. For now, have only extended the ongoing coastal waters SCAs for an additional six hours into Friday night with less confidence that 5 ft seas will persist toward sunrise Saturday. Uncertainty increases for Sunday into next week with the 00z GFS the most bullish on some of Helene`s energy combining with the lingering upper low to result in the development of another area of (non-tropical) low pressure along the NC coast. Will show ENE winds increasing to around 15 kt for now pending better agreement in the models. High rip current risk continues for all beaches today and Friday given 3-5 ft nearshore waves and swell periods 10-12 seconds. && .TIDES/COASTAL FLOODING... As of 405 AM EDT Thursday... Key Messages: -Tidal departures continue to fall across the region. - Coastal flood advisories have been issued for the next two high tide cycles for the tidal Potomac and Rappahannock as well as bay side portions of the MD Eastern Shore. Tidal departures continue to fall this morning, but remain highest (1.5-1.75 ft) in the upper Chesapeake Bay, along the tidal Potomac and Rappahannock, and on the bay side of the MD Eastern Shore. Coastal flood advisories have been issued through late this evening. Water levels fall further to end the week and any nuisance-minor flooding will likely be constrained to tidal Potomac and bay side of the MD Eastern Shore. Latest guidance does show the potential for increasing anomalies late Friday across the middle and upper bay with the potential for minor to locally moderate coastal flooding at Bishop`s Head and Lewisetta. Lastly, will continue to watch areas adjacent to the Currituck Sound (including Back Bay) and the Albemarle Sound today into Friday as SE winds could push already elevated water levels up further into minor flooding thresholds. As of 405 AM EDT Thursday: Consecutive high tides reaching at least Minor Flood with this event: - Lewisetta (P.O.R. since 1979): 18 (with several more to come), previous record was 8 in Oct 2021, Oct 2019, Oct 2015. - Cambridge (P.O.R. since 1980): 13 (with this morning`s high tide unlikely to reach minor flood), previous record was 5 in Oct 2021, Oct 2019, Sep 2018, Oct likely 2011. Consecutive high tides reaching at least Moderate Flood with this event: - Lewisetta (P.O.R. since 1979): 7 (with additional moderate flood unlikely), ***tying the record of 7 in Oct 2015*** && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...Coastal Flood Advisory until 8 AM EDT Saturday for MDZ021-022. NC...None. VA...None. MARINE...Small Craft Advisory from 7 AM Friday to midnight EDT Friday night for ANZ630>638. Small Craft Advisory until 1 AM EDT Saturday for ANZ650-652- 654-656-658. && $$ SYNOPSIS...LKB/SW NEAR TERM...SW SHORT TERM...LKB/SW LONG TERM...KMC AVIATION...KMC/LKB MARINE...RHR TIDES/COASTAL FLOODING...