Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
623 FXUS61 KLWX 251901 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 301 PM EDT Tue Jun 25 2024 .SYNOPSIS... Dry conditions look to continue through tonight as high pressure slowly pushes offshore. Excessive heat and humidity return Wednesday ahead of a strong cold front approaching from the Ohio River Valley. The cold front will cross the region late Wednesday afternoon into early Thursday morning bringing the next risk for strong to severe thunderstorms. High pressure briefly returns from the north and west late Thursday into Friday. Unsettled conditions are possible this weekend which comes with increasing heat and humidity. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... Another hot and not too terribly humid afternoon ahead with high pressure nearby. The surface high over the region will gradually weaken while shifting south and east toward the VA/NC coast later this afternoon and evening. This will allow the winds to shift toward more of west to southwest flow. The downslope component to the wind will lead to compression off the mountains boosting temperatures 3 to 5 degrees above climatology for late June standards. Highs today will push into the upper 80s and low 90s for areas east of the Allegheny Front. Mountain locations will be a touch cooler in the upper 70s and low to mid 80s. Skies will remain mostly sunny with just a few instances of high cirrus passing through from time to time. Some additional high and perhaps mid-level cloud cover will sink into areas west of the Alleghenies and Blue Ridge Mountains later this evening and into the overnight hours. This is attributed to a complex of thunderstorms dropping south from the eastern Great Lakes and central Ohio River Valley. Some showers and perhaps a few spotty thunderstorms could develop in this location although confidence is low. For our region, mostly dry conditions will prevail through the evening and overnight period. With south to southwesterly flow in place expected a milder night ahead with lows in the upper 60s and low to mid 70s. Mountain locations will stay in the low to mid 60s. Some river valley fog is possible west of the Blue Ridge overnight. This will be fairly localized and dependent upon how much convective cloud debris is realized heading into early Wednesday morning. SEVERE THREAT: All eyes shift to Wednesday as the excessive heat and humidity return ahead of a strong shortwave disturbance/cold front that are set to cross the region Wednesday afternoon into late Wednesday night. Convective activity looks to be a bit more organized compared to what we saw this past weekend and earlier this week. This is due largely in part to a lee trough ahead of the main boundary leading to increased height falls and a quickly moistening thermodynamic profile which should initiate convection mid to late Wednesday afternoon. The Storm Prediction Center continues to highlight the majority of the forecast area north of Interstate 64 in a Slight Risk (Level 2 out of 5) Wednesday afternoon and evening. The primary threats with these thunderstorms will be damaging winds and large hail. The threat for damaging winds (70 mph or more) continue to increase per the latest 12z guidance although some uncertainty remains to how widespread this threat will be resolved. Additionally, there is a secondary threat of an isolated tornado mainly along the PA/MD line and back into portions of western MD given the back flow. CAM guidance hints at this threat as well as additional high supercell composite values down across the central VA Piedmont and central/southern MD. Isolated instances of urban and poor drainage flooding are also possible given increased PWAT values above 1.5 inches. With the antecedent dry conditions expect a bit more runoff as water will have a bit of a harder time percolating through. Ahead of the front, south to southwesterly flow will help boost afternoon temperatures into the mid to upper 90s. Of course this will be determined by how much convective debris funnels into the area overnight Tuesday into early Wednesday morning. One uncertainty is the influx of low level moisture into the area. Models have dewpoints quickly getting back into the mid to upper 60s Wednesday afternoon with precipitable water values on the order of 1.8-2.2 inches. With that said, the confidence for heat advisories across the area is a bit lower. Current forecast projections amongst the guidance suggest heat indices on the order of 98 to 104 degrees Wednesday afternoon. Will continue to monitor the need for any such heat-related products. As for timing, expect convective initiation to kick off along the lee side trough mid to late Wednesday afternoon before propagating east of the metros Wednesday night. Storms will feed off of CAPE values between 1500-2500 j/kg. This is especially true east of the Alleghenies where the bulk of the instability looks to remain untapped from early morning showers and convective debris. 0-6 km shear values will be on the order of 30-40 kts with lapse rate 6-7 degrees C/km favoring well organized storms. This is backed up in several hi-res CAMS and model soundings. Convection is slow to wane as we get into Wednesday night and Thursday morning as the cold front slowly pushes through. The severe threat at this point will be on it`s way down due in part to the initial round of thunderstorm activity during the afternoon and evening hours. Lows Wednesday night will fall into the mid to upper 60s with low to mid 70s along and east of I-95. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Calmer conditions are expected for the late week period as a cold front slowly exits south and east Thursday with high pressure building in briefly from the north and west Friday. A few showers and perhaps a few thunderstorms may linger along and east of the boundary (i.e east of the Blue Ridge) Thursday morning into early Thursday afternoon. As for temperatures Thursday, expect highs to drop back into the mid to upper 80s with dewpoints in the low to mid 60s. Additional relief arrives Thursday night as dewpoints fall into the 50s under light north to northwest flow. Lows Thursday night will range from the 50s and low to mid 60s west of US-15 to upper 60s and low 70s in the metros. High pressure continues to push eastward into the New England states Friday before pushing offshore Friday night into Saturday. This will allow for a light onshore flow component to converge against the Blue Ridge promoting a few showers and perhaps an isolated thunderstorms over the central/southern Blue Ridge (Shenandoah Valley/central VA Piedmont). Most locations will remain dry with temperatures once again in the mid to upper 80s and low 90s. Overnight lows Friday night will fall back into the upper 60s and low 70s areawide with increasing moisture as the flow turns toward the south. && .LONG TERM /FRIDAY THROUGH TUESDAY/... High pressure centered to the north will move further east early Thursday and into Friday. Most precipitation looks to stay to the south during the period with drier air locally. Cannot rule out a stray shower or thunderstorm late Friday night across central VA. Highs on Friday will be in the upper 80s to near 90 for most areas east of the Allegheny Front (low 80s). A warm front associated with a low pressure system will track further north for the weekend, increasing chances for showers and thunderstorms Saturday and Sunday. Some of these storms may be strong to severe but there continues to be variability in the timing and intensity of this particular event. Drier conditions build in post-cold front on Monday, leading to highs in the low to mid 80s across the lower elevations with low 70s across the mountains. && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... VFR conditions look to prevail through tonight as high pressure makes it`s way offshore. Winds will remain out of the west and southwest this afternoon with gusts up to 20 kts mainly across the ridges and at terminals close to the waters. Winds will drop off a bit overnight before coming back again Wednesday morning into Wednesday afternoon. Hi-res guidance notes a bit of LLWS across the mountains and over northern portions of the corridor before daybreak Wednesday morning. Just wanted to make a mention here in case consistency exists amongst the guidance to include in the next TAF cycle. Sub-VFR conditions return Wednesday afternoon into Wednesday night as strong cold front and shortwave disturbance traverse the region. This will lead to multiple rounds of showers and thunderstorms. The risk for severe weather has increased with a focus on the western terminals around 16-19z/12-3pm advancing into the corridor between 20-00z/4-8pm. This is for the initial round of storms with additional activity possible late Wednesday evening into Wednesday night. All convection should wrap up between 04-07z Thursday. The primary threats with storms will be strong wind gusts and hail. Winds shift to west- northwesterly late Wednesday into Thursday morning behind a well defined cold front. Prevailing VFR conditions are expected Thursday through Saturday. Showers and thunderstorms may lead to brief restrictions on Thursday morning and early afternoon at terminals along and east of the corridor and south of KCHO. Post frontal northwesterly winds will gusts up to 15 kts Thursday. Elevated southeasterly gusts up to 15 knots on Friday will turn more southwesterly heading into Saturday ahead of a cold front passage later in the weekend. VFR conditions are expected Friday while some sub-VFR conditions will be possible Saturday as more moisture returns across the terminals in the form of showers and isolated thunderstorms. && .MARINE... Small Craft Advisories continue for the Chesapeake Bay and tidal Potomac this afternoon and evening due to southerly channeling. Gusts up to 20 knots are possible for the most of the waters with 25 to 30 kts over the open waters of the bay and lower tidal Potomac. Additional Small Craft Advisories are likely Wednesday afternoon and evening as a slow moving cold front pushes through the region. The front will yield strong to severe thunderstorms which may initiate the need for Special Marine Warnings over the waters. Initial activity looks to push toward the waters around 20z before crossing the waters 00z. Additional showers and perhaps a few thunderstorms will follow late Wednesday evening into early Thursday morning. This frontal system exits by Thursday morning with sub-advisory level winds expected for the day. SCA winds are possible Friday and more likely on Saturday as a tighter pressure gradient settles over the waters, especially the lower waters of the Potomac River and open waters of the Chesapeake Bay. An SMW cannot be ruled out Saturday afternoon for any strong thunderstorms that cross the waters. && .TIDES/COASTAL FLOODING... A return of southwest winds later this afternoon into Wednesday will bring another increase in water levels, although no flooding is forecast at this time. && .CLIMATE... Very hot temperatures are expected on Wednesday. Several records could be in jeopardy. Below is a list of record high temperatures for June 26th, the year the record was set, and the current forecast high temperatures for that day. A plus sign after the date signifies the record was set multiple times, with the most recent year indicated below. RERs are only issued for DCA, IAD, BWI, and MRB, but other sites are shown for reference. Wednesday Jun 26th Washington-National (DCA) 101F (1952) 98F Washington-Dulles (IAD) 95F (1998) 98F Baltimore (BWI) 99F (1954+) 99F Martinsburg (MRB) 102F (1943) 93F Charlottesville (CHO) 99F (1998) 97F Annapolis (NAK) 98F (1952) 94F Hagerstown (HGR) 98F (1954+) 93F && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT Wednesday for ANZ530-538>543. Small Craft Advisory until 6 AM EDT Wednesday for ANZ531>537. && $$ SYNOPSIS...EST NEAR TERM...EST SHORT TERM...ADM/EST LONG TERM...ADM AVIATION...ADM/EST MARINE...ADM/EST TIDES/COASTAL FLOODING...LWX CLIMATE...LWX