Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
034 FXUS61 KLWX 161416 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1016 AM 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/... Latest SFC observations show Tds running 2-5 degrees higher than this time yesterday. This is resulting in heat indices running higher than yesterday at this time as well and expected to persist into the afternoon. No other major changes to the forecast at this time. Another day of excessive heat and scattered afternoon thunderstorms is in store. High pressure is situated offshore with a surface trough east of the Blue Ridge Mountains. A few thunderstorms recently flared up in north central West Virginia and may spill into the Alleghenies but tend to dissipate as they move into weaker MUCAPE to the east. The southern end of the long-lived MCS has been weakening south of Lake Erie, while another MCS is moving across Illinois. The remnant perturbations from these features could loosely tie into convective chances today. While the best forcing will generally be north of the area today, the remnant MCVs may still serve to increase flow aloft (20-30 kt of bulk shear, especially northern half of the area) and steepen mid level lapse rates a bit. Some CAM solutions favor isolated to scattered storms developing along the Blue Ridge and surface trough. Other solutions suggest a more delayed threat, toward evening, and possibly farther west. That forcing may be tied to the second MCS. Regardless, ample CAPE and DCAPE in the hot and humid airmass will support a risk of locally damaging downbursts from the strongest storms, similar to the past two days. 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. Temperatures and dew points won`t show a lot of variance from Monday, with air temperatures on either side of 100 and heat index values from 100 to 110. Excessive Heat Warnings and Heat Advisories remain in effect for much of the area. Overnight relief will be lacking again, with much of the area between 70 and 80. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Stronger forcing arrives Wednesday in the form of height falls and increasing convergence along an approaching cold front. Showers and thunderstorms will become likely across much of the area during the afternoon and evening, although timing and placement remains somewhat uncertain. More intense storms, possibly in the form of linear clusters, 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. However, strong to severe downbursts will be possible over much of the area. 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. 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 east of the Blue Ridge. Additional Heat Advisories will likely be needed. There`s still a wide range of solutions for 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. 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 MONDAY/... Quasi-stationary front draped across the Carolinas on Friday coupled with a slight increase in heights locally will lead to a mostly dry finish to the workweek with highs only in the mid to upper 80s across the lower elevations. Overnight lows will fall into the 60s for most areas. Would not be shocked to see some mid to upper 50s in the valleys across the Allegheny Front. By the weekend, the stalled front to the south will begin nudging further north as a warm front. A slight uptick in heat and humidity will soon follow with the warm sector. Afternoon and evening showers and thunderstorms will pick back up each day into early next week. Highs will be in the upper 80s to low 90s heading into next week with a fairly stagnant airmass in place with fairly weak flow aloft. Increase in moisture locally may begin to help with the drought conditions at this time, little by little. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... South/southwest winds will increase toward midday, with some gusts as high as 20 kt possible this afternoon. Scattered thunderstorms are possible this afternoon and evening. Timed 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. A higher likelihood of thunderstorms is expected Wednesday afternoon and evening as a cold front approaches. 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. VFR conditions are expected Friday before some sub-VFR instances move in for the afternoon and evening hours for the weekend as a result of potential showers and thunderstorms. Winds out of the southeast will continue into next week with weak flow aloft. && .MARINE... Small Craft Advisories have been issued for this afternoon 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. Marginal advisory conditions are possible in southwesterly flow ahead of the front. A cold front will track across the waters Thursday morning which brings a shift to northerly winds. Will have to monitor Thursday night for possible marginal advisories due to northerly channeling on the bay. Winds look to be sub-SCA in nature heading into the weekend but cannot rule out an SMW for any strong to severe thunderstorms that cross the waters during the period. && .TIDES/COASTAL FLOODING... Anomalies will be slightly elevated over the next several days with some more sensitive tidal locations reaching action stage. No minor flooding is expected over the next several tide cycles locally. && .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. 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 from 2 PM this afternoon to 2 AM EDT Wednesday for MDZ008. 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 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 until 8 PM EDT this evening for WVZ050-055-502- 504-506. MARINE... Small Craft Advisory from 2 PM this afternoon to 2 AM EDT Wednesday for ANZ530>533-537-539>541. Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Wednesday for ANZ534-543. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this evening for ANZ535-536-538-542. && $$ SYNOPSIS...ADS NEAR TERM...ADS/CPB SHORT TERM...ADS LONG TERM...ADM AVIATION...ADS/ADM MARINE...ADS/ADM TIDES/COASTAL FLOODING...LWX CLIMATE...LWX