Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
034 FXUS61 KLWX 170805 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 405 AM EDT Wed Jul 17 2024 .SYNOPSIS... A cold front will approach the area today and slowly push through the region by Thursday. This front will then stall to the south Friday before gradually moving back to the north this weekend into early next week. && .NEAR TERM /THROUGH TONIGHT/... Some leftover mid and high level clouds to start the day as high pressure remains situated off the coast. A cold front stretching from the lower Great Lakes through the Ohio Valley is poised to approach later today. One last hot day in this stretch, although the ridge being suppressed and increasing cloud cover will likely result in temperatures a few degrees cooler than Tuesday (highs generally mid to upper 90s). Dew points will be a bit higher though, so heat index values will still range from 100-109 near and east of Interstate 81, where a Heat Advisory is in effect. Height falls, and possibly perturbations originating from ongoing convection upstream, will serve to initiate convection this afternoon. Instability forecasts have trended down slightly but should still be sufficient when coupled with up to 30 knots of bulk shear to result in strong to severe thunderstorms. Damaging winds will be the main threat, especially should any linear segments form. Greatest risk for severe storms will be east of the Blue Ridge Mountains, where a surface trough will focus convergence and greater moisture/instability. Multiple models indicate there could be multiple rounds or foci for thunderstorm development. This could include a earlier round across northern areas, with redevelopment or mergers along its southern flank. It`s in this type of scenario where models have been suggesting localized multi-inch rain totals. Precipitable water values exceeding 2 inches and deep warm cloud layers could result in intense rainfall rates, so any prolonged duration could result in flash flooding. Due to the ongoing drought, flood-prone urban/suburban areas will be most susceptible to runoff issues and flash flooding. For this reason, have issued only a targeted watch at this time. Unfortunately for areas with the worst drought conditions west of the Blue Ridge, guidance suggests lesser convective coverage in this area. The cold front may not reach the area until later tonight, so can`t rule out some showers lingering. However, convection looks to be widespread enough that the atmosphere will likely be overturned and most of the heavier activity exiting around or just after midnight. Some patchy fog could develop if there are breaks in the clouds. Lows will drop into the 60s west of the Blue Ridge and remain in the 70s to the east. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... The surface cold front will likely be south of the area by Thursday morning. However, some guidance is still slow enough with the frontal zone progression, combined with shortwaves aloft, that showers and a few thunderstorms could still develop through Thursday, mainly across the southern half of the area. It will be noticeably cooler with highs in the 80s for most, although some breaks of sun could allow some northern areas to reach 90. Lower dew points will gradually advect in and will be most noticeable Thursday night when lows drop into the 60s for most of the area, with 50s in the mountains. High pressure will build north of the area and the front will reach its most southern extent Friday before stalling against the Bermuda high. Therefore dry and seasonable conditions should prevail, although some increased cloud cover may persist in southern areas. Some guidance has moisture advancing back northward as quickly as Friday night, so have some slight chance PoPs across the southern half of the area. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Increasing moisture will approach the area from the south by the weekend as a result of a warm front spreading northward from the Carolinas. Expect daily chances for showers and thunderstorms for the weekend and into early next week. Highs are expected to be more tame compared to the previous few days with upper 80s to low 90s for the lower elevations and upper 70s for the mountains. WE start to get closer to heat advisory criteria by midweek into late next week but still a lot of uncertainty with the heat intensity. Overnight lows will drop down into the 60s to low 70s most of the period with winds remaining light. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... VFR conditions and light winds are expected through the morning. Thunderstorms will become likely this afternoon into the evening, with the least confidence at MRB. Hope to be able to narrow down the timing some, but guidance indicates the potential for multiple rounds, or at least some kind of redevelopment along the cluster`s southern flank which could affect the metro hubs into the evening. Strong to severe downbursts, frequent lightning, and intense rain rates are possible with the strongest storms. While storm chances decrease during the evening, a few showers could linger into the night as the surface cold front approaches. There is also some potential of MVFR ceilings behind the storms in the metro areas, but confidence is low. Patchy fog could also develop if there are breaks in the clouds. The front will be south of the area Thursday, but showers and thunderstorms may still develop in its vicinity, which could affect CHO. VFR conditions appear probable Thursday night into Friday as high pressure builds to the north. Mostly VFR conditions are expected Saturday into Sunday with some isolated instances of sub-VFR ceilings as a result of any showers or thunderstorms affect the terminals. Winds will remain light out of the south Saturday and continue into the remainder of the weekend. && .MARINE... Southwest winds continue to reach marginal advisory levels across the wide waters of the middle bay early this morning. There could be a brief break in this area during the mid to late morning (with lighter winds continuing elsewhere). Southwesterly flow will then increase again this afternoon across all waters. While marginal and perhaps sporadic, thought it was best to cover this with an advisory. Thunderstorms with strong winds will become also become more likely this afternoon and evening as a cold front approaches with Special Marine Warnings likely. Winds become northerly Thursday behind the front. A few thunderstorms are possible near southern Maryland through Thursday but seem unlikely to be strong. There`s still some potential for northerly channeling down the bay Thursday night, but it would be marginal for an advisory. Lighter winds are expected Friday and Friday night as high pressure builds to the north and the front stalls to the south. Sub-SCA winds are expected for the waters for the weekend. Cannot completely rule out an SMW for any potential strong thunderstorms that cross the waters either afternoon/evening. && .CLIMATE... Here are daily high temperature records for July 17. 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. ***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...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for DCZ001. Flood Watch from 2 PM EDT this afternoon through this evening for DCZ001. MD...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for MDZ003>006-008-011-013-014-016>018-503>508. Small Craft Advisory from 1 PM this afternoon to 11 PM EDT this evening for MDZ008. Flood Watch from 2 PM EDT this afternoon through this evening for MDZ011-013-014-504-506. VA...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for VAZ026>031-037>040-050-051-053>057-501-502-505-506- 526-527. Flood Watch from 2 PM EDT this afternoon through this evening for VAZ053-054-527. WV...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for WVZ051>053. MARINE...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for ANZ530. Small Craft Advisory from 1 PM this afternoon to 11 PM EDT this evening for ANZ530>533-535-536-538>542. Small Craft Advisory until 5 AM EDT Thursday for ANZ534-537- 543. && $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...ADM AVIATION...ADS/ADM MARINE...ADS/ADM CLIMATE...LWX