Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
868 FXUS61 KLWX 161832 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 232 PM EDT Tue Jul 16 2024 .SYNOPSIS... High pressure will persist offshore through Wednesday while a surface trough will reside east of the Blue Ridge Mountains. The surface trough will be overtaken by a cold front late Wednesday into Thursday. This front will then stall to the south through the end of the week and into the weekend. && .NEAR TERM /THROUGH TONIGHT/... Key Messages: - Another day of dangerously high excessive heat and humidity continues across the region. - Any thunderstorm that does form today has the potential to produce very intense, damaging winds in the form of downbursts. _____________________________________________________________ High pressure is situated offshore with a surface trough east of the Blue Ridge Mountains. A CU field is building in the vicinity of the lee sfc trough around 18Z. Recent radar imagery shows an area of storms developing just west of the CWA near Morgantown, WV. Guidance has this moving east through the afternoon, but split on if any storms persist. Better chance of development on the lee sfc trough. There is notable dry air in the low and mid levels evident in ACARS data and various satellite products. This may inhibit storm coverage today, but anything that does go will be volatile with damaging winds. The convective threat does appear to wane quickly by late evening, although there is an outside chance some activity moves into the mountains during the second half of the night. Overnight relief will be lacking again, with much of the area between 70 and 80. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Key Messages: - One last day of dangerous high heat and humidity is likely. - More widespread thunderstorms are likely Wednesday. - Damaging winds are the main threat. - The potential for Flash Flooding exists, especially around the vicinity of the DC and Baltimore metros if multiple rounds come to fruition. ____________________________________________________________ Better synoptic forcing arrives Wednesday with H5 height falls and convergence along a cold front. Widespread showers and thunderstorms are increasingly likely during the afternoon and evenings hours. Initiation could be early in the day (~16-18Z) along the lee sfc trough based on soundings and CAMs. There remains uncertainty surrounding how the evolution of storms plays out tomorrow afternoon. The most intense storms are likely in the form of linear clusters, and are most probable east of the Blue Ridge. Even though mid level lapse rates will be weak and bulk shear around 20-30 kt, the surface trough will enhance convergence and pool much higher moisture/instability to its east. PWs will be at or above two inches, so water loaded downbursts are likely as well. Storms do look to be mostly progressive, but there are some indications for multiple rounds. This could pose a risk of localized flooding, especially over the urban areas where precipitable water values over 2 inches could support intense rain rates. Have collaborated with WPC to introduce a Level 2 out of 5 (Slight Risk) for Flash Flooding across the Greater DC and Baltimore Metros into Northeast MD. A chance of showers and thunderstorms may linger into the night as the surface cold front gradually moves into the area. While the weakening ridge and increasing clouds will result in temperatures a little lower Wednesday compared to Tuesday, dew points will be higher in the lower to perhaps mid 70s. This will still result in heat index values between 100 and 109, especially along and east of I-81. Have issued a Heat Advisory for most of the area along/east of I-81 Wednesday. There remains uncertainty with Thursday as the front continues to press south of the area. Slower solutions, or ones that involve any sort of low pressure wave, would result in a lingering chance of showers and some thunderstorms, although severe weather would not be expected. However, there is still a reasonable chance most of the rain stays south of the area Thursday, which is what most guidance has trended towards today. It will be much cooler however, with highs topping out in the mid 80s for most of the area. More comfortable air will be working in by Thursday night, with lows generally in the 60s. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Shortwave troughing will depart off to our east on Friday as high pressure builds into the Ohio/Mississippi Valleys at the surface. Northerly flow ahead of the high will advect a somewhat drier airmass (precipitable water values below one inch) into the region. The combination of drier air and weak subsidence behind the departing shortwave should lead to dry conditions locally. Temperatures should be seasonable, with highs in the mid to upper 80s. Flow at the surface turns southerly by next weekend, which will allow moisture to work back into the area from the south. As deeper moisture works in from the south, daily chances for showers and thunderstorms will return, although overall coverage shouldn`t be overly high in the absence of any well-defined upper level features. Seasonable temperatures will continue through the weekend into early next week, with highs generally in the mid 80s to low 90s. Dewpoints will climb into the 60s and lower 70s, making it feel a bit more humid compared to Friday. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Widespread gusts to 18-24 kts have been observed across the terminals, which is notable for mid-July. Scattered thunderstorms are possible this afternoon and evening. Have maintained timing of VCTS for CHO and the metros with the idea that storms will form off the surface trough during the mid afternoon and press east. However, timing could be in the back half of that window, or even later, should the solutions verify that have storms developing farther west and arriving during the early evening. That second option, should it occur, would be most likely to affect MRB (with them likely too far west for the first option). Strong downbursts are possible with some storms. Lots of uncertainty wrt thunderstorm coverage, but any thunderstorm that does develop could result in microbursts near the terminals. A higher likelihood of thunderstorms is expected Wednesday afternoon and evening as a cold front approaches. Multiple rounds of showers and thunderstorms are possible. Some showers and storms, along with localized sub- VFR ceilings/visibilities may linger into Wednesday night as the front pushes to the south. Some uncertainty with how quickly the front pushes south of the area Thursday. The highest convective chances will be near CHO and to the south. Prevailing VFR conditions are expected on both Friday and Saturday, although a passing thunderstorm can`t be ruled out Saturday afternoon. Winds will be light out of the north on Friday, and then light out of the south on Saturday. && .MARINE... Small Craft Advisories have been issued for all waters and continuing for the bay into tonight. Scattered thunderstorms with gusty winds may warrant Special Marine Warnings this afternoon and evening. Thunderstorms with strong winds will become more likely Wednesday afternoon and evening as a cold front approaches with Special Marine Warnings likely. Marginal advisory conditions are possible in southwesterly flow ahead of the front. Sub-SCA winds are expected on both Friday and Saturday. Winds will be out of the north to northeast on Friday, and out of the south to southeast on Saturday. && .CLIMATE... Here are some daily high temperature records during the July 16-17 timeframe this week. Records (RERs) are only issued for DCA, IAD, BWI, and MRB, but other sites are shown for reference. A `+` sign indicates multiple years currently hold that record. ***TUESDAY, JULY 16TH, 2024*** LOCATION RECORD (YEAR) FORECAST Washington-National (DCA) 104F (1988) 102F Washington-Dulles (IAD) 104F (1988) 102F Baltimore (BWI) 104F (1988) 102F Martinsburg (MRB) 107F (1988) 99F Charlottesville (CHO) 102F (1988) 100F Annapolis (NAK) 98F (1997)+ 98F Hagerstown (HGR) 104F (1988) 100F ***WEDNESDAY, JULY 17TH, 2024*** LOCATION RECORD (YEAR) FORECAST Washington-National (DCA) 102F (1980) 97F Washington-Dulles (IAD) 100F (1997)+ 96F Baltimore (BWI) 101F (1988) 97F Martinsburg (MRB) 102F (1988)+ 93F Charlottesville (CHO) 100F (1988)+ 96F Annapolis (NAK) 100F (1900) 94F Hagerstown (HGR) 100F (1953)+ 93F && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Excessive Heat Warning until 8 PM EDT this evening for DCZ001. Heat Advisory from 11 AM to 8 PM EDT Wednesday for DCZ001. MD...Excessive Heat Warning until 8 PM EDT this evening for MDZ003- 006-008-011-013-014-016>018-504-506>508. Small Craft Advisory until 2 AM EDT Wednesday for MDZ008. Heat Advisory from 11 AM to 8 PM EDT Wednesday for MDZ003>006- 008-011-013-014-016>018-503>508. Heat Advisory until 8 PM EDT this evening for MDZ004-005-502- 503-505. VA...Excessive Heat Warning until 8 PM EDT this evening for VAZ027>031-053>055-057-527. Heat Advisory from 11 AM to 8 PM EDT Wednesday for VAZ026>031- 037>040-050-051-053>057-501-502-505-506-526-527. Heat Advisory until 8 PM EDT this evening for VAZ025-026- 036>040-050-051-056-501-502-504>506-526. WV...Excessive Heat Warning until 8 PM EDT this evening for WVZ051>053. Heat Advisory from 11 AM to 8 PM EDT Wednesday for WVZ051>053. Heat Advisory until 8 PM EDT this evening for WVZ050-055-502- 504-506. MARINE... Small Craft Advisory until 6 AM EDT Wednesday for ANZ534-543. Small Craft Advisory until 8 PM EDT this evening for ANZ535- 536-538-542. && $$ SYNOPSIS...CPB NEAR TERM...CPB SHORT TERM...CPB LONG TERM...KJP AVIATION...KJP/CPB MARINE...KJP/CPB CLIMATE...LWX