Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
708 FXUS61 KLWX 032342 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 742 PM EDT Wed Jul 3 2024 .SYNOPSIS... High pressure will remain offshore through the first half of the weekend. A cold front will move through the area Saturday night. High pressure will re-establish itself offshore early next week. && .NEAR TERM /THROUGH THURSDAY/... High pressure remains off the Mid-Atlantic/New England coastline this evening. Meanwhile, much further north, low pressure has tracked northeastward into Hudson Bay. Southwesterly flow aloft/southerly flow at the surface between these two features continues to advect a warmer airmass into the area. High clouds will start to increase across the area tonight, but dry conditions are expected to persist for most. The exception will along the Allegheny Front into the Potomac Highlands, where a few showers may be possible especially during the second half of the night. Temperatures will remain warm overnight, with lows in the upper 60s and lower 70s. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... High pressure will remain in place offshore tomorrow, maintaining southerly flow at the surface. Aloft, flow will be west to southwesterly, which will draw a very warm and humid airmass into the area from the center of the country. High temperatures should reach into the low-mid 90s for most, which when combined with dewpoints in the lower-middle 70s will result in peak heat indices of 95-105. After a dry start to the day for most, thunderstorms are expected to form during the afternoon hours in response to daytime heating within a very moist, uncapped environment with gradually falling heights aloft. The storms will likely form in the vicinity of the higher terrain to the west of the Blue Ridge by the early afternoon hours, and then progress eastward through the remainder of the afternoon and into the evening. Flood Threat: Tomorrow`s environment will be characterized by highly anomalous levels of moisture, with precipitable water values of 2.0-2.5 inches across the area. Warm cloud layers will be very deep, and MBE velocities are also very low. As a result, any storms that form will be capable of producing very heavy rainfall. There isn`t an obvious surface boundary present to focus low-level convergence (except perhaps along the bay breeze in NE MD), which should serve as a limiting factor (in terms of how long storms can last in a given location). Recent CAM runs show storms being somewhat progressive in nature, which makes sense given the lack of a focusing boundary, and with around 20-30 knots of flow present in the mid-levels. Antecedent conditions have also been very dry. 1-hour flash flood guidance is around 3 inches in rural areas, which will make flooding highly unlikely in those areas. Urban areas have lower flash flood guidance, and may be a bit more susceptible to flooding. WPC currently has the area outlooked in a Marginal Risk, which seems appropriate given the potential for very heavy rainfall producing storms, but also the limiting factors listed above. Severe Thunderstorm Threat: Model soundings show around 1000-2000 J/kg of MLCAPE and 20-30 knots of effective bulk shear, with well-mixed boundary layers and steep low-level lapse rates, but saturated profiles aloft. The saturated profiles aloft limit DCAPE and resultant evaporatively driven downdraft accelerations, but wet microbursts may still be possible with any stronger storms. SPC currently has much of the area outlooked in a Marginal Risk for severe thunderstorms, with damaging winds being the primary threat. Any thunderstorms should gradually wind down through the evening hours, giving way to predominantly dry conditions during the second half of the night. Overnight lows will be in the low to mid 70s for most. Friday looks like a similar day to Thursday, with continued hot and humid conditions and additional chances for afternoon/evening thunderstorms. Dewpoints may be a couple degrees higher, which may make heat indices a few degrees higher as well. An isolated downburst or instance of flash flooding can`t be ruled out with any of the stronger storms, but the overall coverage of storms may be slightly lower than Thursday. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... On Saturday, an upper shortwave and associated surface low pressure system are expected over the Great Lakes. By early Saturday morning, a weak cold front is expected to approach from our west and then gradually move through the area through Sunday morning. Guidance currently quickly fills in the upper trough and takes it northeast, resulting in a continuing drier trend for Saturday as the best forcing leaves with it. Scattered showers and thunderstorms remain possible Saturday afternoon and evening with PoPs increasing west to east, though with only slight PoPs along and west of the I-81 corridor. We`re continuing to monitor the potential for severe thunderstorms and isolated flooding, but with the drier trend confidence is low. Additionally, Temperatures on Saturday are expected in the 80s to low 90s for most of the non-mountainous areas and heat indices near 100-105 east of the Blue Ridge. The front is expected to be mostly through the area by Sunday morning, but since the upper low is expected to cut and run, the weak boundary likely stalls and hangs around. Brief ridging looks to keep us dry on Sunday, with highs a little cooler than Saturday, but not as humid with dewpoints in the 60s. Heading into the work week, continuing moisture advection will allow afternoon showers and thunderstorms during the afternoon on Monday and Tuesday, focused along the weak stalled boundary over the area. This wet pattern likely sticks around as long as the boundary does, which might not take its leave until another trough comes to pick it up late in the week. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... Prevailing VFR conditions are expected through Friday. However, afternoon and evening thunderstorms may lead to temporary restrictions on Thursday and Friday. VCTS has been introduced to the TAFs tomorrow afternoon, but some timing uncertainty still remains. Winds will generally be light out of the south over the next few days. Prevailing VFR conditions are expected Saturday, but sub-VFR conditions are likely with any scattered showers and thunderstorms passing over the terminals in the afternoon associated with a weak frontal boundary. Sunday looks drier with high pressure building in behind the boundary. && .MARINE... SCAs are in effect within channeled southerly flow through tonight. Sub-SCA southerly winds are expected through much of the day tomorrow, but the lower tidal Potomac and central portions of the Bay likely reach SCA levels in channeled southerly flow again especially tomorrow afternoon. Elsewhere, SMWs may be needed in association with any thunderstorms that move over the waters tomorrow afternoon into tomorrow evening. Sub-SCA southerly winds are expected again on Friday, and SMWs may be needed once again during the afternoon and evening hours. Winds increase out of the south on Saturday and could approach SCA criteria with any channeling through the Bay in the afternoon. Winds then diminish overnight and Sunday as a weak boundary stalls over the waters. && .TIDES/COASTAL FLOODING... Tidal anomalies are expected to continue their steady climb through the rest of this week, into the weekend, and possibly next week as south to southeast winds prevail for most of the period. Sensitive locations are expected to reach Action Stage during the higher of the two daily high tides. A Coastal Flood Advisory is in effect in Annapolis for tomorrow morning`s tide cycle. Beyond that, will need to continue monitoring trends to see if, and when, any Coastal Flood Advisories are needed through the weekend. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Heat Advisory from noon to 8 PM EDT Thursday for DCZ001. MD...Small Craft Advisory until 9 AM EDT Thursday for MDZ008. Heat Advisory from noon to 8 PM EDT Thursday for MDZ011-013- 014-016-504-506. Coastal Flood Advisory from 1 AM to 8 AM EDT Thursday for MDZ014. VA...Heat Advisory from noon to 8 PM EDT Thursday for VAZ053>056- 527. WV...None. MARINE...Small Craft Advisory until 9 AM EDT Thursday for ANZ530>533- 537-539>541. Small Craft Advisory from 3 PM to 8 PM EDT Thursday for ANZ533- 537-541. Small Craft Advisory until 1 AM EDT Friday for ANZ534-543. Small Craft Advisory until 10 PM EDT this evening for ANZ535. Small Craft Advisory until 6 AM EDT Thursday for ANZ536-538- 542. && $$ SYNOPSIS...DHOF/KJP NEAR TERM...DHOF/KJP SHORT TERM...DHOF/KJP LONG TERM...CAS AVIATION...DHOF/KJP/CAS MARINE...DHOF/KJP/CAS TIDES/COASTAL FLOODING...LWX