Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
679 FXUS61 KLWX 261502 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1102 AM EDT Wed Jun 26 2024 .SYNOPSIS... Excessive heat and humidity are expected across the region today ahead of a strong cold front set to cross the area early Thursday morning. The cold front will bring the return of strong to severe thunderstorms across portions of the area later this afternoon and into the late evening hours. High pressure briefly returns from the north late Thursday into Friday bringing with it slightly cooler temperatures. Heat and humidity return this weekend along with increased thunderstorm chances as a series of fronts push through the region. && .NEAR TERM /THROUGH TONIGHT/... Overall not much change to the current forecast. Still monitoring the influx of moisture pushing north this afternoon and winds aloft to determine the extent of our severe weather threat later today. The current 12z KIAD this morning shows plenty of dry air in the mid to upper levels and even at the surface. PWATS this morning are around 1.14 inches with low level RH values ta 50 percent and mid level RH values around 55 percent. As a result, expect an excessively hot and humid afternoon ahead of a strong shortwave disturbance/cold front that is set to cross tonight into early Thursday morning. Convective activity looks to be a bit more organized although some uncertainty remains with the better forcing forecast north of the area up across PA/NJ. Severe THREATS:This convective activity is due largely in part to a lee trough ahead of the main boundary leading to increased height falls and a quickly moistening thermodynamic profile which should initiate convection mid to late afternoon (between 3-7pm). The Storm Prediction Center continues to highlight the majority of the forecast area north of Interstate 64 in a Slight Risk (Level 2 out of 5) Wednesday afternoon and evening. The primary threats with these thunderstorms will be damaging winds and large hail. The threat for damaging winds (70 mph or more) continues to increase per the latest 12z guidance although some uncertainty remains to how widespread this threat will be resolved with two rounds of convection set to take place. Additionally, there is a secondary threat of an isolated tornado mainly along the PA/MD line and back into portions of western MD given the back flow. This threat has lowered somewhat in the last few model runs, but something that we are still monitoring at this time. Isolated instances of urban and poor drainage flooding are also possible given increased PWAT values above 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. Some urban locations especially from DC points south and east may push up and over 100 degrees. Of course this will be determined by how much low level moisture influx there is over the region and how early convective initiation takes place. Models have dewpoints quickly getting back into the low 70s this afternoon with precipitable water values on the order of 1.8-2.2 inches. The confidence for Heat Advisories still remains low although a few areas may touch criteria for a brief period of time. Overall heat index values this afternoon will run between 98 to 104 degrees during the peak of the afternoon. Will continue to monitor the need for any such heat-related products. SEVERE TIMING:Expect convective initiation to kick off along the lee side trough later this afternoon before propagating east of the metros tonight (between 3-7pm). Storms will feed off of CAPE values between 1500-2500 j/kg. This is especially true east of the Alleghenies where the bulk of the instability looks to remain untapped from limited convective debris. 0-6 km shear values will be on the order of 30-40 kts with lapse rate 6-7 degrees C/km favoring well organized storms. This is backed up in several hi-res CAMS and model soundings. 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 may be on it`s way down due in part to the initial round of thunderstorm activity during the afternoon and evening hours. We`ll have to continue to monitor this timeframe though as some of the latest guidance has trended toward isolated activity mainly east of the Blue Ridge this afternoon with the next round blowing up west of the Alleghenies this evening and propagating east overnight. Lows tonight 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 /15Z 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- southeast 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 1 AM EDT Thursday for ANZ530>534- 537>543. && $$ SYNOPSIS...EST NEAR TERM...EST SHORT TERM...CAS LONG TERM...BRO AVIATION...BRO/CAS MARINE...BRO/CAS TIDES/COASTAL FLOODING...LWX CLIMATE...LWX