Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
090 FXUS61 KLWX 180018 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 818 PM EDT Tue Sep 17 2024 .SYNOPSIS... Low pressure over the Carolinas will gradually weaken through Wednesday. Another low may develop off the coast during the second half of the week. A backdoor cold front may push into the area Saturday. Strong high pressure will build northeast of the area in its wake. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Low pressure over west-central North Carolina will continue to move toward the northwest tonight. Low clouds will continue to cover the forecast area, especially south of I-70. Waves of light to at times moderate rain will continue to inch further north through the night. PWATs were in excess of 1.75 inches south of I-66/US-50 and east of the Blue Ridge Mountains as of early this evening, with PWATs of 1.90-2.00 inches advecting in from the southeast. Guidance remains split on the strength of a low-level jet tonight, and thus how far northwest these higher PWAT values get. Waves of rain will continue for much of the area regardless, but the intensity varies among the latest model runs. The NAM12/NAMNest and ECMWF, for example, have an area of heavier rainfall totals from far northern VA into central and northeast MD (including the greater Baltimore/Washington metro areas - generally 1-3"). However, recent RAP/HRRR runs are much less enthused. Given the antecedent dry conditions, no Flood Watch has been issued as any flooding will be conditional and isolated wherever any heavier rain bands set up over vulnerable (i.e. urban) basins. MUCAPE of less than 500 J/kg should tend to keep rainfall rates modest (less than a half inch per hour). Lows tonight in the middle 50s in the west and the lower to middle 60s in the eastern zones. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/... The aforementioned low pressure will continue to move across western NC and further west on Wednesday. Rain will continue through most of the day for a large portion of the area, with the VA Piedmont likely receiving the most precipitation. Additional totals of 2-3" are certainly possible through the day on Wednesday as the low begins to break down and a coastal low forms off the Delmarva. Cannot rule out a rumble of thunder in the afternoon hours but overall convective activity should be fairly suppressed. Highs top out in the upper 60s to low 70s along and west of the Blue Ridge where the more cooler side of the low resides and a few degrees warmer further east towards the waters. Overnight lows Wednesday night drop down into the mid to upper 50s along the Allegheny Front and low 60s further east. As the coastal low begins to shift further northeast, expect shower chances to decrease from west to east during the day on Thursday. There may be some general clearing later in the day. Highs will top out in the upper 70s to near 80 for most areas. Northwesterly flow will allow temperatures to drop down into the upper 40s to low 50s along portions of the Allegheny Front Thursday night, with mid to upper 50s more common further east with low 60s near the waters. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Long term period continues to trend drier as model guidance keeps the trough axis offshore. A backdoor cold front is set to drop through the forecast area Friday afternoon and throughout the day on Saturday as an upper level trough deepens over Canada into New England. Cooler and drier air will move in with temperatures expected to stay below normal. High pressure will keep conditions primarily dry with a low pressure system off the coast of the Carolinas leading to slight chance of PoPs in the southeastern portions of the area through Monday. Precipitation chances increase elsewhere on Monday as a low pressure system approaches from the west. High temperatures will be in the upper 70s to low 80s on Friday with higher elevations staying in the mid to low 70s. Temperatures cool slightly each day with high temperatures on Monday expected to be in the low to mid 70s for most. High elevations will stay in the upper 50s to 60s. Overnight low temperatures will be in the 50s for most each night with metro areas staying in the low 60s. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... IFR CIGs were expanding out of the central VA Piedmont as of 00Z. These CIGs should expand to the DC area terminals through 06Z, then the Baltimore terminals and MRB by 09Z. Intermittent IFR CIGs (or VSBY in any heavier rain) is possible prior to that, with patchy LIFR possible heading into Wednesday morning. Winds will remain out of the east 5-10 kts with occasional gusts to around 15 kts. Rain showers will likely continue off and on Wednesday with low pressure nearby. Winds will be northeast to east 5 to 15 kts through Wednesday. Gusts of 20 to 25 kts are possible, mainly each afternoon and evening, especially at terminals close to the waters and along the ridges. With the residual low pressure nearby, expect periods of MVFR to IFR conditions. Underneath a mid-level low pressure system on Thursday, scattered showers may develop across the area. Clouds should be abundant which may afford periods of sub-VFR conditions. Prevailing VFR conditions are expected Friday and Saturday with dry conditions at all terminals. Northeasterly winds on Friday shift to easterly on Saturday. && .MARINE... SCA conditions will persist through the night, especially south of the Chesapeake Bay Bridge on the Chesapeake Bay and below Dahlgren on the tidal Potomac River. This is in association with a tightened pressure gradient from blocking high pressure to the north and low pressure moving inland across the coastal Carolinas. As a weakening area of low pressure tracks through the area, winds will be out of the east on Wednesday while likely remaining at SCA levels for at least portions of the waters. Winds will become more northeast Wednesday, then more northerly while diminishing on Thursday. Northeasterly winds are expected to stay below SCA criteria on Friday with winds gusting 10 to 15 knots. Winds shift more easterly on Saturday with Small Craft Advisories likely as winds gust 15 to 20 knots. && .TIDES/COASTAL FLOODING... With low pressure inland over the Carolinas and winds from a northeasterly direction, there have not been significant rises in anomalies, with the highest values greater than one foot along the tidal Potomac River. Advisories remain in effect, but most flooding will be marginal. Between the full moon and continued onshore flow, tide levels will likely remain elevated and near minor flood thresholds much of the week. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 10 PM EDT this evening for DCZ001. MD...Small Craft Advisory until 9 PM EDT Wednesday for MDZ008. VA...Coastal Flood Advisory until 10 PM EDT this evening for VAZ054. WV...None. MARINE...Small Craft Advisory until 9 PM EDT Wednesday for ANZ530>534- 537>543. Small Craft Advisory until noon EDT Wednesday for ANZ535. Small Craft Advisory until 6 PM EDT Wednesday for ANZ536. && $$ SYNOPSIS...ADS/DHOF/ADM NEAR TERM...KLW/DHOF/ADM SHORT TERM...KLW/DHOF/ADM LONG TERM...AVS AVIATION...AVS/DHOF/ADM MARINE...AVS/DHOF/ADM TIDES/COASTAL FLOODING...LWX