Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
474 FXUS61 KLWX 251857 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 257 PM EDT Wed Sep 25 2024 .SYNOPSIS... Light rain and cloudy conditions persist through today and into tonight. Weak front arrives Thursday with mostly dry conditions expected. Hurricane Helene moves further north into the Gulf of Mexico and into the southern Appalachians and Tennessee River Valley over the next several days. Widespread rainfall possible Friday, especially across parts of Central Virginia. High pressure builds in from the north this weekend, but rain chances remain into next week. && .NEAR TERM /THROUGH TONIGHT/... Off and on showers will persist through the remainder of the day. Moderate to heavy rain may be possible at times for areas along and west of the Blue Ridge. Cannot rule out a rumble of thunder but confidence is low in this coming to fruition locally. Rainfall amounts of 1 to 2 inches will be possible for the aforementioned areas through tonight. Given the ongoing drought concerns and the more prolonged periods of rain, the flood threat should be fairly limited and localized heading into early Thursday. Afternoon highs will top out in the upper 60s to low 70s for most areas aside from the Allegheny Front where low 60s will be more common. For tonight, lows will only drop a few degrees given the increased cloud coverage across the area. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... A brief relief from the precipitation will take place on Thursday as southerly flow builds in and a quick break between the departing frontal boundary and the impacts associated with Hurricane Helene. Cannot rule out a few isolated showers Thursday afternoon but most areas should remain fairly dry for the majority of the day. Per the NHC forecast, Hurricane Helene makes landfall along the northeast Gulf Thursday evening, then moves north toward the TN Valley / Southern Appalachians Thursday night. Deep tropical moisture moves into our area by early Friday morning, bringing widespread rain/showers to central/northwest VA and surrounding highlands. Models have trended weaker with a surface low moving south from Canada, meaning most of the area is likely to see high rain chances Friday into Friday night. The heaviest is going to be along the eastern slopes of ridges south of I-66/US-48, and in Central VA where 2-4" of rain is possible with localized higher amounts. Localized flooding is possible but confidence is low at this time given the track uncertainty. The main circulation of Helene retrogrades west towards the Mid- South as it interacts with the cutoff low, keeping the better tornado environment to the west of the forecast area. Continue to monitor our local forecast and the National Hurricane Center as Helene continues to move further north. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... The remnants of Helene will continue to drift over the Tennessee and lower Ohio River Valleys while being absorbed into a large cutoff low this weekend. The cutoff upper level low will gradually fill and weaken as over the Mid-South through early next week. With the low nearby, and pieces of embedded energy pivoting around it, expect increased chances for showers and perhaps a few thunderstorms throughout the weekend into early next week. Some uncertainty remains in regards to the placement of the cutoff low and timing/placement of embedded pieces of energy pivoting around it. The influence of surface high pressure over New England wedging south along the Appalachians will play into the overall coverage of precipitation as well. Overall not expecting any washouts this weekend although abundant cloud cover and intermittent shower chances will continue as remnant moisture from Helene overrides the wedge high overhead. The upper level low continues to fill and push to the east Monday into Tuesday. Moist flow will likely remain with incoming troughing and an associated cold front pushing out of the middle part of the country. This deepening trough continues to show up on 12z deterministic/ensemble model suites today and over past couple of days. It`s exact amplitude, timing, and whether or not it cuts off remains in question. Regardless, there is the potential for a strong frontal passage during the Tuesday and Wednesday timeframe providing additional shower and thunderstorm chances. Drier conditions look to return wit high pressure by the latter half of next week. Due to the added cloud cover and cool air wedge, temperatures are expected to remain at or below normal through Tuesday . Highs will be around 80 Saturday with 60s and 70s Sunday into early next week. Lows will fall into the 50s and 60s. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... Sub-VFR conditions look to continue through Thursday as a series of slow moving fronts cross the region. Still dealing with a pseudo CAD wedge and a weak warm frontal boundary gradually lifting north from central and southern VA. MVFR to IFR (brief periods LIFR) ceilings will prevail through the remainder of the day especially at terminals west of a line from KBWI-KRIC. Light showers/drizzle will continue to work through the corridor over the next 2-4 hours between now-21z/5pm. Further west of KMRB and KSHD will continue to monitor the progression of a line of moderate to heavy rain and embedded thunderstorms working north along the incoming cold front from the Ohio River Valley. This activity will gradually push east later this evening and into the overnight hours reinforcing reductions in vsbys and cigs across the area. IFR to possibly LIFR CIGs are likely tonight into Thursday morning. Conditions slowly improve Thursday as weak southerly flow ensue with mid-level ridging overhead. Conditions should dry out for the corridor terminals with perhaps a few showers and thunderstorms, mainly at terminals along and west of a line from KMRB to KCHO/KSHD. Ceilings should remain MVFR in these locations(IFR possible at CHO for most of the day) with VFR further east. Winds will remain light out of the east and southeast at 5-10 kts. Sub-VFR ceilings are likely Thursday night into Friday as the remnants of Helene work into the southern Appalachians. Widespread rainfall is expected from this system with the heaviest at terminals south of the corridor and down into southwest portions of VA. Sub-VFR conditions are likely at times over the weekend into early next week as remnant moisture from Helene continues to push over the region. Wedging high pressure will keep abundant cloud cover and shower chances in place through at least Monday. A few thunderstorms are possible during this time. && .MARINE... A Small Craft Advisory goes into effect through this evening for the Chesapeake Bay and surrounding rivers/tributaries, and the Lower Tidal Potomac. Expect winds to gust up to 20 knots into tonight. Winds will fall back below SCA levels tonight and remain at sub-SCA levels through Thursday afternoon. The remnants of Helene approach the area from the south late Friday into Saturday, with a surge of east to southeast winds likely ahead of the tropical system. SCA conditions are likely to develop Friday afternoon, and continue into the weekend. Depending on the evolution of low pressure approaching from the OH/TN Valleys this weekend, onshore flow could be strong enough to prompt the issuance of Small Craft Advisories at times. Uncertainty is moderate to high at this time, and ranges from marginal SCA to near gale-force. && .TIDES/COASTAL FLOODING... Decreasing astronomical tides will lead to slightly lower (though still elevated) tides, with bouts of minor flooding likely around times of high tide through Friday morning. The higher of the two daily tides is during the overnight/early morning hours, and would be susceptible to the most widespread minor to perhaps locally near moderate tidal flooding. By Friday, Helene will orbit around an upper low to the southwest as high pressure holds firmly to the north. This pattern results in enhanced onshore east to southeast flow at times through the weekend, and has the potential to result in multiple rounds of widespread minor to moderate tidal flooding through at least Sunday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 6 AM EDT Thursday for DCZ001. MD...Coastal Flood Advisory until 5 AM EDT Thursday for MDZ011. Coastal Flood Warning until 6 AM EDT Thursday for MDZ014. Coastal Flood Advisory until 1 AM EDT Thursday for MDZ016-017. Coastal Flood Advisory until 2 AM EDT Thursday for MDZ018. Coastal Flood Advisory until 9 AM EDT Thursday for MDZ508. VA...Coastal Flood Advisory until 6 AM EDT Thursday for VAZ054. Coastal Flood Advisory until 1 AM EDT Thursday for VAZ057. WV...None. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ531>534-537-538-542-543. && $$ SYNOPSIS...ADM/KRR/EST NEAR TERM...ADM/KRR SHORT TERM...ADM/KRR LONG TERM...EST AVIATION...DHOF/ADM/EST MARINE...DHOF/ADM/EST TIDES/COASTAL FLOODING...LWX