FXUS61 KLWX 030756 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 356 AM EDT Sat May 3 2025 .SYNOPSIS... A slow-moving cold front will approach from the Midwest through tonight, then gradually cross the Mid-Atlantic from west to east through Tuesday. High pressure may follow briefly for Wednesday. Another frontal system and area of low pressure may gradually approach the region from the Tennessee Valley late next week. && .NEAR TERM /THROUGH TONIGHT/... Early morning GOES-16 IR/nighttime microphysics RGB imagery shows rather large expanses of mainly clear skies across much of the Mid-Atlantic east of I-81 and roughly north of I-64. Mid/high level clouds leftover from prior convection are streaming NNE up the Appalachians, with another area of clouds over southern VA working north but dissipating. These mainly clear skies will play an important role in heating and the evolution of convection later today. As a large and deep upper-level low cuts off over the Ohio Valley and slowly approaches from the west, a surface cold front will encroach upon the Appalachians by tonight. Substantial height falls ahead of these features amid a seasonably warm and humid airmass will set the stage for an active 24 to 48 hours. Heating may allow storms to begin initiating as early as midday or shortly thereafter, though some lingering subsidence or capping aloft may keep stronger updrafts at bay until mid afternoon. Storms are most likely to initiate in the vicinity of a surface trough oriented roughly along the Blue Ridge as of early this morning. Cooling mid-level temps and strong PVA/height falls as well as RER dynamics of the attendant upper jet will overlay isentropic advection in the low levels. Modest mid-level lapse rates of 6-6.5 C/km overtop surface temps rising well into the 80s with dew points into the lower 60s is likely to result in a broad 750-1250 J/kg MLCAPE by mid afternoon between the foothills of the Allegheny and Potomac Highlands west of I-81 to near and just west of I-95. Within this broad zone, pockets of locally greater heating and/or subtly higher low-level moisture pooling in some valleys or in the lee of terrain features could result in local maxima of 1500-2000 J/kg MLCAPE, as noted in some hi- res guidance. The flow in the 850-500 hPa layer will increase to 30-45 kts through the afternoon into this evening, offering the potential of storm organization into lines/clusters and perhaps transient supercell structures. Given moderate DCAPE and steep low-level lapse rates as well as pockets of mid-level dry air (favorable for evaporational cooling and acceleration of downdrafts) amid the modest background flow, strong to damaging straight line wind gusts will likely be the primary threat with the more intense updrafts/line segments. Favorable shear and modest CAPE in the -10 to -30 deg C (hail growth) layer could result in some severe hail with any persistent semi-discrete updrafts. There is some curvature to the low-level hodograph in most forecast soundings, but modest wind speeds in the lowest 2 km AGL may keep any tornado threat limited/conditional on any favorable storm-scale boundary interactions, or perhaps interaction with enhanced environmental vorticity near the surface trough or just east of higher terrain features (where surface flow could be slightly enhanced parallel to the ridges). Height falls and mid-level flow strengthen tonight, with continued moisture advection in the low levels. This very well may keep widespread showers and at least a few thunderstorms going well into the night, though the severe threat should gradually wane with the loss of daytime heating and an increasingly stable boundary layer. PWATs increasing to 1.25-1.50 inches with storm motions parallel to the approaching cold front could afford some training and heavy rain potential tonight. Ongoing drought conditions should limit a more significant flood threat, keeping the risk marginal and most likely relegated to hydrophobic (urban) basins, or basins with flashy complex terrain. Guidance differs on exactly where any heavier rainfall totals may focus, but in a broad sense this looks most likely west of I-95 and east of the Appalachians. If more widespread organized convection develops this afternoon and early evening, then it could result in a conglomerate outflow and subsequent renewed convection shifting further east closer to the more vulnerable urban basins tonight, however. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Unsettled weather will continue Sunday and Monday as the upper low and surface cold front only slowly drift west to east across the Mid-Atlantic. Mid-level lapse rates will steepen a bit, perhaps exceeding 7 C/km by Monday afternoon. Enhanced flow east of the upper low will also linger. This could set the stage for multiple rounds of at least isolated strong to severe storms, with the potential for some locally heavier rainfall totals given repeat/training potential. Cooler mid-level temps and lowering freezing levels could result in an uptick in hail potential should deeper updrafts evolve in such an environment Sunday into Monday, as well. Low-level flow appears to back on Monday, which could also increase SRH. && .LONG TERM /TUESDAY THROUGH FRIDAY/... A nearby upper-level low will bring showers and thunderstorms to the forecast area through Tuesday evening. While there continues to be model discrepancy regarding the location of the upper low, global models indicate the low tracking northeast between the Ohio River Valley and Great Lakes Region. At the surface, a cold front associated with a low pressure system will move through the region on Tuesday. As the surface low tracks to the NE towards New England, surface high pressure builds in from the northwest. While conditions dry out overnight Tuesday and into Wednesday, precipitation chances linger throughout the long term as the upper low remains nearby and the aforementioned cold front stalls to the south. High temperatures each day will be in the 60s to 70s with Tuesday being the warmest day of the period. Overnight low temperatures will be in the 40s to 50s each night, with the exception being Wednesday night when the metro regions stay in the 60s. Another upper-level low may approach the region bringing a rejuvenation of unsettled weather late next week. && .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/... Bouts of sub-VFR conditions are expected beginning this afternoon and lasting right through early next week. This is due to waves of showers and thunderstorms ahead of a slowly approaching upper low. Timing of the lowest conditions from convection is a bit difficult, though should be focused broadly on the afternoon and evening hours each day. During the overnight hours, a slight decrease in surface winds with enhanced flow at a few thousand feet could result in spotty/marginal LLWS. Lower CIGs are also possible in the increasingly moist low-level environment late at night into the morning hours each day. Flow will be mainly southerly with daytime gusts to around 20 kts possible outside of thunderstorms, becoming southeasterly Monday. Showers and thunderstorms on Tuesday will likely lead to periods of sub-VFR conditions. Sub-VFR conditions are possible again on Wednesday with a chance of precipitation. Southwest winds on Tuesday shift to northwest on Wednesday. && .MARINE... Southerly gradient winds will increase today, with a very favorable setup for southerly channeling expected this afternoon into this evening. Winds of 25 to 35 knots out of the south a few thousand feet above the surface will likely be drawn toward the surface by subsidence over the cooler waterways due to developing bay/river breezes since the land/water temp difference will be 20+ deg F this afternoon. This downward transport and enhancement from the channeling and bay/river breeze should overcome shallow stability and could even result in a few gusts near gale force early this evening over the main channel of the Chesapeake Bay. Enhanced southerly flow will continue through the day Sunday for at least part of the waters, with southeast flow possibly nearing SCA levels Monday. Otherwise, multiple rounds of showers and thunderstorms are anticipated over the next few days, resulting in the potential for hazardous winds, waves, lightning, and reduced visibility in heavy rain at times. Winds remain below Small Craft Advisory criteria both Tuesday and Wednesday. Southwest winds on Tuesday shift to northwest on Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 AM this morning to 8 PM EDT this evening for ANZ530>532-535-536-538>540-542. Small Craft Advisory from 6 AM this morning to 6 PM EDT Sunday for ANZ533-537-541. Small Craft Advisory until 6 PM EDT Sunday for ANZ534-543. && $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...AVS AVIATION...AVS/DHOF MARINE...AVS/DHOF