Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
453 FXUS61 KLWX 210133 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 933 PM EDT Thu Jun 20 2024 .SYNOPSIS... High pressure will remain in place offshore through this weekend. A cold front will progress through on Monday, with high pressure building back in by Tuesday. && .NEAR TERM /THROUGH FRIDAY/... High pressure system will remain close to the region, resulting in hot and dry conditions with mostly sunny skies for the day. During the evening, however, as the upper-level ridge starts to break down, some convection west of I-81 could cause some scattered showers and possibly a few thunderstorms in that area, but they are expected to quickly dissipate. High temperatures are expected to be in the mid 90s for most areas and upper 80s in the mountains. However, dew points being in the mid to upper 60s indicate that the heat indices show the temperature feeling like upper 90s and even reach 100 in some spots. Low temperatures will range within the upper 60s and low 70s in most areas and potentially the mid 70s in the DC and Baltimore areas. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... Friday night`s low temperatures will reach the lower to middle 70s in the eastern two-thirds of the region. Dry conditions expected once Friday afternoon and early evening`s thunderstorms dissipate. The upper level ridge of high pressure will break down even further Saturday. A light to modest westerly flow will develop within and just above the boundary layer. High temperatures Saturday afternoon will reach the middle to upper 90s to 100 degrees mainly east of the Blue Ridge and interior valleys between the Appalachians and the Shenandoah Valley. Humidity will also increase with this heat having dewpoint temperatures reach the lower 70s. Heat indices are expected to easily reach the 100-105 degree mark over much of the forecast region. Areas within the metro areas could exceed 105. There is currently an Excessive Heat Watch in effect Saturday afternoon and Saturday evening for an area from eastern West Virginia to the Chesapeake Bay, excluding far southern Maryland as a Chesapeake Bay breeze expected in the afternoon could keep values below criteria. With the high pressure ridge breaking down, showers and thunderstorms could find it easier to develop during the heat of the afternoon and early evening. Wind shear will be relatively weak, but instability will not be scarce. Thunderstorm activity should be disorganized. However, thunderstorms could be capable of producing localized downbursts given high levels of CAPE and DCAPE in place. It isn`t entirely impossible for storms to get all the way to the Chesapeake Bay if development continues along outflow boundaries from previous storms. Saturday night will be even warmer than previous nights with lows dropping only into the mid-upper 70s to the east of the Blue Ridge, with lower 80s even possible in DC and Baltimore. This level of heat overnight, when the daytime temperatures are near 100, is especially dangerous because there will be very little relief. && .LONG TERM /SUNDAY THROUGH THURSDAY/... The conclusion of this weekend will mark a temporary end to the lengthy period of above average temperatures. On Sunday, 850-mb temperatures rise to around 21-23C which dry adiabatically mixed to the surface would support some century degree readings. Consequently, the forecast does call for some spots reaching 100 degrees. This is accompanied increasing humidity levels as dew points rise into the upper 60s to low 70s. The resultant heat indices range from 100 to 107 degrees across the forecast area. If this were to be maintained, Heat Advisories may be needed for much of the region. An additional threat will be some strong to severe thunderstorms, particularly during the afternoon to evening hours. After over a week of continued heating of the surface, instability levels should be maximized as mixed-layer CAPE values reach 2,500 to 3,000 J/kg. The guidance vary in convective coverage which suggests there could be some capping issues. After around 9 to 10 days without a cold frontal passage, the guidance does bring a boundary through during the second half of Monday. This would bring an additional threat for some severe convection, especially for those east of the Blue Ridge Mountains. The added clouds and thunderstorm activity does carry Monday`s high temperatures back down into the upper 80s to mid 90s. Through Tuesday, this boundary eventually settles off to the south across the coastal Carolinas. Another round of hot temperatures ensue toward mid-week as highs rise into the mid/upper 90s again. Heat indices begin to approach advisory levels again. Another cold front races across the local area by Thursday which helps usher temperatures down. This would again come with another risk of severe thunderstorms. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... VFR conditions overnight through Saturday morning. Brief MVFR conditions possible is strong thunderstorms develop with gusty winds on Saturday afternoon and evening. Downpours could reduce visibility to 2 miles or so while brief wind gusts from thunderstorms could gust 35 to 40 knots. Given Sunday and Monday will likely bring a risk of showers and strong to severe thunderstorms, restrictions are possible at times, particularly during the afternoon to evening hours. By Monday, the threat for such severe weather ends up focusing mainly from the Blue Ridge eastward. Regarding winds, Sunday`s southwesterly winds may gust up to 15 knots before shifting over to west-northwesterly by Monday evening behind the cold front. VFR conditions are expected for Tuesday with winds shifting from westerly to southerly by the second half of the day. && .MARINE... No marine hazards overnight through Saturday morning. Small craft advisories possible Saturday afternoon and more likely Saturday night. As a matter of fact, special marine warnings or marine weather statements may be warranted in thunderstorms Saturday afternoon and evening. A period of southerly channeling may unfold from Sunday afternoon through Monday afternoon. These channeling effects may warrant Small Craft Advisories, especially for the wider waters of the Chesapeake Bay. A cold front races through the waters by late Monday evening. Given the threat for convection each day, Special Marine Warnings may be needed each afternoon and evening. && .CLIMATE... Very hot temperatures are expected Friday through Sunday. Several records could be in jeopardy, as seen below. Below is a list of record high temperatures for Jun 21st, 22nd, and the 23rd and the year the record was set, and the current forecast high temperatures for those days. RERs are only issued for DCA, IAD, BWI, and MRB, but other sites are shown for reference. Friday Jun 21st Climate Site Record High Forecast High Washington-National (DCA) 99F (2012) 96F Washington-Dulles (IAD) 98F (1988) 95F Baltimore (BWI) 100F (2012+) 93F Martinsburg (MRB) 102F (1931) 94F Charlottesville (CHO) 99F (1933) 94F Annapolis (NAK) 100F (1988) 89F Hagerstown (HGR) 97F (1923) 94F Saturday Jun 22nd Climate Site Record High Forecast High Washington-National (DCA) 101F (1988) 100F Washington-Dulles (IAD) 99F (1988) 99F Baltimore (BWI) 100F (1988) 97F Martinsburg (MRB) 102F (1933) 95F Charlottesville (CHO) 101F (1933) 97F Annapolis (NAK) 101F (1988) 94F Hagerstown (HGR) 100F (1988) 97F Sunday Jun 23nd Climate Site Record High Forecast High Washington-National (DCA) 98F (1988) 101F Washington-Dulles (IAD) 96F (1988) 99F Baltimore (BWI) 97F (2010) 99F Martinsburg (MRB) 100F (1934) 96F Charlottesville (CHO) 100F (1894) 98F Annapolis (NAK) 98F (1988) 96F Hagerstown (HGR) 98F (1988) 97F && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Excessive Heat Watch from Saturday afternoon through Saturday evening for DCZ001. MD...Excessive Heat Watch from Saturday afternoon through Saturday evening for MDZ003>006-008-011-013-014-016-502>508. VA...Excessive Heat Watch from Saturday afternoon through Saturday evening for VAZ026>031-037>040-050-051-053>057-501-502- 505-506-526-527. WV...Excessive Heat Watch from Saturday afternoon through Saturday evening for WVZ050>053-055-502-504. MARINE...None. && $$ SYNOPSIS...CJL NEAR TERM...KLW/Guest SHORT TERM...KLW LONG TERM...BRO AVIATION...BRO/KLW MARINE...BRO/KLW CLIMATE...CJL