Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
953 FXUS61 KLWX 260800 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 400 AM EDT Wed Jun 26 2024 .SYNOPSIS... Excessive heat and humidity return today ahead of a strong cold front approaching from the Ohio River Valley. The cold front will cross the region late this afternoon into early Thursday morning bringing the next risk for strong to severe thunderstorms. High pressure briefly returns from the north and west late Thursday into Friday. Unsettled conditions are possible this weekend which comes with increasing heat and humidity. && .NEAR TERM /THROUGH TONIGHT/... Early this morning some high clouds are streaming over the area with spotty showers ongoing to our southwest over Kentucky, and remnant cloud cover from previous activity moving into SW VA and WV. This cloud cover will be one variable in today`s equation for potential severe weather. It looks to arrive in our SW around the 10-12z time frame if it holds together as it progresses east, in which case it could put a damper on the amount of heating and instability we have available for convection this afternoon and evening. Latest hi-res guidance has trended a few hours later with initiation from the lee trough this afternoon, looking closer to 20z than 18z. Timing remains uncertain though, with continuing run-to-run inconsistencies. Guidance has been consistent indicating a lull in convection between the lee trough and the arrival of the front around 00z, but how far ahead of the front the lee trough will be remains a question. How much instability we have to start and the gap between rounds (and thus ability to recover) will be key to severe potential. The primary threat remains damaging winds and large hail. Guidance has backed off somewhat on tornado potential, and SPC 6z outlook removed the 2% prob from our northwest, but still won`t rule out the possibility. SPC`s Slight risk has also expanded to encompass essentially the entire CWA. Isolated instances of urban and poor drainage flooding are also possible given increased PWAT values around 1.5 inches. With the antecedent dry conditions expect a bit more runoff as water will have a bit of a harder time percolating through. Ahead of the front, south to southwesterly flow will help boost afternoon temperatures into the mid to upper 90s. Of course this will be determined by how much convective debris funnels into the area this morning. One uncertainty is the influx of low level moisture into the area. Dewpoints in the low/mid 60s have already returned this morning, though model soundings have trended lower with PWATs. Confidence has continued to decrease for Heat Advisory criteria being reached today. HREF 90th percentile AptT just scratches 105 in a few spots. Will continue to monitor the need for any such heat- related products. As for timing, expect convective initiation to kick off along the lee side trough mid to late afternoon before propagating east of the metros Wednesday night. Storms will feed off of CAPE values between 1500-2000 j/kg, based on the recent downward trend in guidance. This is especially true east of the Alleghenies where the bulk of the instability looks to remain untapped from convective debris and the first round of convection. 0-6 km shear values have trended down as well, but 25-35 kts with lapse rate 6-7 degrees C/km still favor organized storms. This is backed up in several hi- res CAMS and model soundings. Stay tuned and weather aware as we continue to monitor the evolving conditions leading up to this afternoon/evening. Convection is slow to wane as we get into tonight and Thursday morning as the cold front slowly pushes through. The severe threat at this point will be on it`s way down due in part to the initial round of thunderstorm activity during the afternoon and evening hours. Lows tonight night will fall into the mid to upper 60s with low to mid 70s along and east of I-95. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As a cold front exits early Thursday, some shower activity could linger into Thursday morning/afternoon. Temperatures lower behind the cold front Thursday into the 80s to low 90s, but dewpoints look to remain in the low-mid 60s during the day, dropping into the upper 50s overnight with lows temps in the 60s to near 70 east of the mountains. A quick-moving area of high pressure passes to our north on Friday. Mostly dry conditions are expected during the day, but some showers could develop in the evening/overnight around the Shenandoah Valley and Virginia Piedmont as the center of high pressure swiftly moves offshore by the evening and a shortwaves passes to the northwest. Temperatures look relatively cooler, topping out in the upper 80s. Lows hover in the 60s to mid 70s. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As high pressure further retreats offshore into the western Atlantic, a return to a warm advection dominant regime ensues. Above average mid-level heights persist over the weekend as forecast heights range from 590 to 594 dm. A net warming of the troposphere will yield another period of hot and humid conditions across the area. A tongue of moisture is set to transport northward in response to the prevailing southerly flow. Multi-ensemble solutions favor dew points into the mid 70s which appears to be a tad aggressive. With forecast high temperatures in the low 90s over the weekend, the addition of high dew points would yield heat indices into the 100 to 105 degree range. If this degree of moisture transport comes to fruition, the region could teeter near Heat Advisory thresholds. This all comes with increasing convective threats as the atmosphere becomes more unstable due to the upticks in heat and moisture. Looking more closely at the threat for thunderstorm activity, Saturday`s setup looks a bit more unpredictable as it will be dependent on low-amplitude impulses in the west-southwesterly flow regime. Heading to the second half of the weekend, storm-scale organization should improve with the approach of a cold front. This boundary is currently slated to track through the local area sometime on Sunday. The added lift from the front coupled with modest height falls aloft should help enhance the coverage of showers and thunderstorms. Any severe aspect of this activity would be too difficult to predict this far out in time. In the wake of the frontal passage, winds shift to northwesterly by late Sunday before turning more northerly on Monday. This ultimately ushers in a cooler and drier air mass for the start of the work week. This shift to below average temperatures also coincides with the first day of July. On Monday, forecast highs are in the upper 70s to low 80s, accompanied by dew points in the 50s. This pattern holds for the start of the work week before the heat and humidity return by mid-week. The Climate Prediction Center Week 2 outlook does suggest some return of excessive heat during the mid/late week period next week. && .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/... VFR conditions are expected to start today with winds out of the W/SW. A cold front arrives tonight, preceded by a lee trough this afternoon to kick off convection. Guidance has trended a few hours slower with initiation of showers and thunderstorms this afternoon, around the 19-21z time frame for the terminals. A lull in convection is likely between the lee trough afternoon convection and the arrival of the front tonight, but the timing isn`t certain. Approximated about a 4 hr duration for the decrease in coverage around the terminals, around the 23z-4z time frame. Don`t currently explicitly mention TS in the second round arriving 3-4z due to low confidence on how much instability exists after the first round and loss of daytime heating. Latest guidance has showers exit around 9z for most of the area. Some showers and thunderstorms could linger into Thursday afternoon, with the greatest chance for CHO. Winds turn NW behind the cold front by late morning. Dry conditions follow and continue into Friday as high pressure passes to the north. Some shower activity is possible again Friday afternoon/evening but should be isolated, and again most likely near CHO if so. As a warm front pushes through the area on Saturday morning, this brings a return of the heat and humidity. Daily showers and thunderstorms can be expected, particularly on Sunday as a cold front approaches from the west. While some restrictions are possible at times on Saturday, the better shot for any impacts to the terminals would be the frontal convection on Sunday. Initial south to southwesterly winds give way to northwesterly flow by Sunday evening into the night. && .MARINE... Small Craft Advisories continue for the Chesapeake Bay and tidal Potomac early this morning as marginal criteria winds continue to be observed. SCA my be canceled early if winds continue to diminish. Additional Small Craft Advisories are in effect this afternoon and evening as a slow moving cold front pushes through the region. The front will yield strong to severe thunderstorms which may initiate the need for Special Marine Warnings over the waters. Initial activity looks to push toward the waters around 21z before crossing the waters 00z. Additional showers and perhaps a few thunderstorms will follow late tonight into early Thursday morning. This frontal system exits by Thursday morning with sub-advisory level winds expected through Thursday night. Winds over the waters may approach SCA criteria Friday morning, diminishing in the afternoon. Behind a warm front that tracks through Saturday morning, southerly flow returns across the area. This leads to some southerly channeling effects over the Chesapeake Bay. With forecast wind gusts up to around 20 to 25 knots, Small Craft Advisories may be needed from Saturday morning through midday Sunday. Behind a cold front, a shift to northwesterlies may bring near advisory caliber winds to the waters late Sunday. Besides the background wind fields, convection may lead to some Special Marine Warnings. This is especially the case on Sunday as an approaching cold front increases thunderstorm activity. && .TIDES/COASTAL FLOODING... Prolonged southerly flow has led to a large ramp up in tidal anomalies which currently average around 0.75 to 1.25 feet. These elevated water levels will bring Annapolis and Fort McHenry into Action stage this morning. However, do not expect any location to see a rise toward minor flooding. A cold frontal passage tonight should help lower the tides into Thursday. A shift to east- southeasterlies on Friday could lead to another spike in water levels. && .CLIMATE... Hot temperatures are expected on Wednesday. Several records could be in jeopardy. Below is a list of record high temperatures for June 26th, the year the record was set, and the current forecast high temperatures for that day. A plus sign after the date signifies the record was set multiple times, with the most recent year indicated below. RERs are only issued for DCA, IAD, BWI, and MRB, but other sites are shown for reference. Wednesday Jun 26th Washington-National (DCA) 101F (1952) 99F Washington-Dulles (IAD) 95F (1998) 98F Baltimore (BWI) 99F (1954+) 99F Martinsburg (MRB) 102F (1943) 92F Charlottesville (CHO) 99F (1998) 99F Annapolis (NAK) 98F (1952) 94F Hagerstown (HGR) 98F (1954+) 94F && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 AM EDT early this morning for ANZ530>543. Small Craft Advisory from noon today to 1 AM EDT Thursday for ANZ530>534-537>543. && $$ SYNOPSIS...EST/CAS NEAR TERM...CAS/EST SHORT TERM...CAS LONG TERM...BRO AVIATION...BRO/CAS MARINE...BRO/CAS TIDES/COASTAL FLOODING...LWX CLIMATE...LWX