Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
479 FXUS61 KLWX 181406 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1006 AM EDT Wed Sep 18 2024 .SYNOPSIS... Low pressure over the southern Appalachians will weaken today. Meanwhile, another low will develop off the coast by tonight and only slowly drift eastward through the weekend. A backdoor cold front may push into the area Saturday. Strong high pressure will build northeast of the area early next week while a low pressure system approaches the Great Lakes. && .NEAR TERM /THROUGH TONIGHT/... Synoptically, not too much of a change in the near term period with vertically stacked low pressure continuing to weaken while drifting northeast this morning and into the afternoon hours. Meanwhile, another area of low pressure looks to form along the VA/NC coast as the remnant energy from the stacked upper level low transfers east tonight. As a result, expect a continuation of scattered to widespread light to moderate showers pivoting through along with a few isolated thunderstorms. The highest coverage for rain will be in areas along and east of I- 81 through mid-afternoon where an easterly low level jet combined with a piece of shortwave energy rotating around the decaying upper level low remain most pronounced. This is evident on both satellite and radar this morning (as of 10am) with a few thunderstorms traversing the Shenandaoh Valley. Additional isolated thunderstorm activity is possible in this area this afternoon given the cooler air aloft and leftover surface convergence from the occluded boundary accompanying the stacked low. A couple hundred j/kg of CAPE are possible in these locations through at least 6pm today. Confidence though is medium given the fact that the jet will gradually weaken this afternoon and the deeper moisture will likely contract into the developing low off the VA/NC coast. However, current 12z CAM guidance continues to show a wide array of solutions in regards to rain coverage, intensity, and amounts over the next 12-24 hours. Rain rates will generally sit between 0.25 to 0.50"/hr although heavier amounts are possible in any elevated convective elements that can form. We`ll have to continue to monitor areas southwest of Charlottesville and directly along the Blue Ridge given some localized heavy/multi- inch rain totals. Current drought conditions and the transient/progressive nature of the precipitation any flood threat will be fairly isolated/localized at most. High temperatures will struggle to get out of the mid to upper 60s and low to mid 70s today given the increased cloud cover and showers. Rain chances gradually decrease tonight as northerly winds bring dry air back into the region. This is due largely in part to low pressure taking over along the VA/NC coast and the upper level low overhead. Any shower activity will be isolated to scattered in areas mainly east of the Blue Ridge. Areas further west including the mountains may see a few breaks in the clouds which will warrant fog development. Lows will be in the mid 50s to mid 60s. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... The upper level and surface lows are forecast to gradually drift east or northeast Thursday and Friday, but there is still some notable spread amongst guidance in their positioning. While the upper low will still be close by Thursday and a few showers or even a thunderstorm could pop up almost anywhere, the highest chances appear to be closer to the Chesapeake (closer to the surface low) and across the Appalachians (as a disturbance rotates through the west side of the gyre). Some clearing could occur if the low pulls away fast enough. Temperatures will respond into the mid 70s to near 80. The current forecast consensus has surface high pressure settling over the area Friday and Friday night as the upper trough/closed low moves to the east. This would result in dry conditions and more sunshine, with temperatures rising into the upper 70s to lower 80s. However, there is still some chance (such as in the 00Z NAM) that they low remains closer to the coast, which could result in more clouds and a chance of showers. Have slight chance PoPs and more cloud cover along the immediate Chesapeake for this forecast package. && .LONG TERM /SATURDAY THROUGH TUESDAY/... A backdoor cold front will move through the region Saturday. Cooler and drier air will move in from the northwest and north Saturday and Sunday. High temperatures will be below average both days. High pressure building southeast behind the backdoor cold front will keep conditions dry for the majority of our western and central zones on Monday as well. There will be a low pressure system off of the eastern seaboard that will have some energy filling around the backside of it on Monday. Therefore, there is a slight chance for showers in our eastern zones and could primarily be over the Maryland eastern shore. High temperatures on Monday will be below average too with highs in the lower to middle 70s. Higher elevations stay cooler. Later on Monday and into the day on Tuesday, a low pressure system will approach our region from the west. This low could bring rain chances to our western zones Monday evening through Tuesday. Highs Tuesday could be milder with highs in the upper 70s as a return flow develops ahead of the low pressure arriving from the west. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... Periods of IFR conditions look to continue through midday, with LIFT possible at CHO. Showers, some moderate to briefly heavy, may also reduce visibility at times, but the heaviest elements seem to be transient. Showers should become more spotty in nature this afternoon as developing off the coast starts to turn winds more northerly and bring in some drying air. This also seems like it will raise ceilings to VFR by late afternoon except at CHO. Those northeast winds may gust close to 20 kt at times today. MVFR to IFR conditions may redevelop late tonight, although there is some uncertainty in whether this would be in the form of low clouds or fog and the location of the restrictions. A few showers may continue Thursday but overall there should be a drying trend into Friday. There is some potential of low clouds and/or fog developing both Thursday night and Friday night however. VFR conditions Saturday through Sunday night. Winds northerly 5 to 10 knots Saturday, becoming predominantly northeast 5 to 10 knots Saturday night through Sunday night. && .MARINE... Small Craft Advisory conditions will continue in northeast flow today as low pressure develops offshore. An isolated thunderstorm can`t be ruled out amongst showers today. Winds should gradually decrease tonight as the low starts to drift away from the area. Advisory conditions may linger along the bay into Thursday though depending on the position and strength of the low. It does appear winds should be relatively lighter Thursday night into Friday, but may strengthen again Friday night amongst a tightening pressure gradient. Small craft advisories possible Saturday and again Sunday afternoon and evening as northeasterly winds kick up a little over the open waters of the Tidal Potomac and central and southern Chesapeake Bay. Winds northeast 10 to 15 knots gusts up to 20 knots both periods. && .TIDES/COASTAL FLOODING... Anomalies remain highest this morning along the Potomac River. Several advisories have been issued through this afternoon`s high tide cycle. As low pressure turns winds more northerly tonight, there may be a brief decrease in tidal levels. However, as the low strengthens offshore, surge guidance shows increasing water levels by Thursday evening into the weekend. Combined with high astronomical tide levels, additional coastal flooding can be expected. Some guidance suggests Annapolis and DC SW Waterfront could even approach moderate flood levels. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 11 PM EDT this evening for DCZ001. MD...Small Craft Advisory until 9 PM EDT this evening for MDZ008. Coastal Flood Advisory until 6 PM EDT this evening for MDZ016. VA...Coastal Flood Advisory until 11 PM EDT this evening for VAZ054. Coastal Flood Advisory until 6 PM EDT this evening for VAZ057. WV...None. MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ530- 538-539-542. Small Craft Advisory until 6 AM EDT Thursday for ANZ531>534- 537-540-541-543. Small Craft Advisory until noon EDT today for ANZ535. Small Craft Advisory until 6 PM EDT this evening for ANZ536. && $$ SYNOPSIS...ADS NEAR TERM...EST SHORT TERM...ADS LONG TERM...KLW AVIATION...ADS/KLW/EST MARINE...ADS/KLW TIDES/COASTAL FLOODING...ADS