Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
064 FXUS61 KLWX 050757 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 357 AM EDT Wed Jun 5 2024 .SYNOPSIS... Unsettled weather today through Thursday as a series of fronts cross the region. Mostly dry conditions on Friday in the wake of the cold front. A slow moving upper trough approaches the area from the Great Lakes this weekend leading to more unsettled conditions. && .NEAR TERM /THROUGH TONIGHT/... Muggy conditions early this morning as temps hover in the mid 60s to lower 70s. The noticeable increase in moisture and lift has is already produce showers along the US-15 corridor early this morning. Expect these showers to continue developing through the morning, bringing periods of moderate rainfall during the morning commute. An active weather day is likely for most of the area today as multiple rounds of showers and thunderstorms (possibly 3-4) move across the area. Severe thunderstorms capable of producing damaging wind gusts, and locally heavy rainfall leading to flash flooding are the main threats. A weak cold front moves through the area tonight, though isolated showers storms will linger through the overnight. Synoptic/Mesoscale Setup: A mid-level shortwave through will move across the area today, bringing around 20-30 knots of deep-layer shear. Some of the high res models indicate higher shear values are possible during the peak convective window this afternoon/evening. Instability will be limited for the first half of the day as widespread cloud cover is slow to dissipate this morning. Increasing southerly winds within the boundary layer should be strong enough to break the inversion, leading to sufficient heating/instability this afternoon. Given dew points in the lower 70s east of the Blue Ridge, though mid-level lapse rates are meager, there should be at least 1000-1500 J/kg of SBCAPE. Afternoon highs are forecast to reach the upper 70s to lower 80s, though mid 80s are possible along/south of I-66 if cloud cover can break earlier in the afternoon. Flash Flood Threat: This is likely to be the biggest threat of the day as multiple rounds of showers and thunderstorms move across the area. WPC has expanded the Slight Risk of flooding to encompass most areas along/north of I-66, the northern Shenandoah Valley, and Alleghenies. Widespread rainfall amounts of 1-2 inches is likely, with isolated amounts up to 3-4 inches is possible. A very important feature to watch is the subtle surface boundary located from Central VA to around Norfolk early this morning. The increasing southerly flow will push this boundary north into our area this afternoon, likely somewhere along or north of the Potomac River. Storms today are likely to be efficient rain producers given the deep warm cloud layer, a moist airmass, sufficient shear, and low-level convergence into the previously mentioned boundary. While mean storm motion is likely to be fast, around 15-20 knots, the multiple rounds of heavy rain could cause issues in urban areas. The 00Z guidance shows the potential for locally heavy rainfall, but exactly where that occurs remains to be seen. The heavy rain footprint various still varies quite a bit from model to model, which is expected in this type of disorganized/cluster storm mode. Given the sensitivities of the DC and Baltimore metros, a targeted Flood Watch has been issued from 12PM today through tonight to account for possible flash flooding due to multiple rounds of storms. Elsewhere, FFG values are high enough and convection spaced out enough that a Flood Watch is not warranted at this time. However, the current watch may be expanded later today depending on future model data. Severe Threat: SPC has added a Marginal Risk (level 1 of 5) of severe storms for most areas along/east of I-81 and north of I-64. Damaging wind gusts will be the main threat, and a tornado or two is also possible. Saturated soundings, with a deep warm cloud layer and modestly steep low-level lapse rates (7-8 C/km) favor wet microbursts in any organized line segments/clusters of storms. The tornado threat is evident in the favorably curved low-level hodographs in the model soundings, though it is limited by weaker shear and lower instability. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... The leading mid-level shortwave moves east of the area Thursday morning, as a weak cold front continues its slow trek through the area. This boundary is likely to produce a broken line of showers and thunderstorms during the late morning to afternoon, mostly along and east of the Blue Ridge. Additional showers could develop in the Alleghenies. Very warm afternoon temperatures in the low to mid 80s, with some spots possibly reaching the upper 80s. Not much cooling in wake of the front for Thursday night as lows settle in the upper 50s to mid 60s. High pressure starts to build in from the west Friday as westerly winds around 5-10 mph continue. Aside from a few showers in the Alleghenies, mostly dry conditions are forecast Friday. Afternoon highs will be closer to normal, in the upper 70s to low 80s. Seasonal lows Friday night in the 50s, with low 60s along the immediate Western Shore and Potomac River. && .LONG TERM /SATURDAY THROUGH TUESDAY/... An upper level trough is expected to remain over the area for the duration of the extended with the main feature of interest being a strong cut-off low descending into the Northeast. Guidance has struggled getting a handle on the cut-off low with significant variance between models and run-to-run. While there is general agreement on taking the low into SE Canada/NE US, some guidance brings it a bit further south, and there is a range of possible shortwaves rounding the base of the low through the weekend. At the very least, the general expected location of the upper low will yield unsettled conditions for our area over the weekend and into early next week, with the focus of diurnal shower and t-storm activity being the northwestern portions of the CWA. Sunday has the greatest potential for precip as a cold front associated with the low moves through Sunday into Monday. High temperatures will be in the mid 70s to low 80s for most, a bit cooler in the 60s for the mountains. Lows will generally be in the mid 50s to mid 60s, though a few degrees cooler Sunday night with the passage of the cold front. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... Low stratus is likely to develop across much of the area early this morning, resulting in MVFR to IFR conditions at most terminals. These clouds lift to MVFR mid to late in the morning, then VFR by noon (or sooner). Expect multiple rounds of showers and thunderstorms to move across the area throughout the day and into this evening. The strongest storms will produce periods of sub-VFR conditions and strong wind gusts in excess of 35 knots. A 4-5 hour window of most likely VCTS was kept in the 06Z TAFs. Storms are expected to be widely scattered, but still difficult to time exactly when the worst conditions will occur (and how many times any terminal could see impacts). Showers and storms decrease late this evening, though showers remain in vicinity through most of tonight. Low clouds are likely to build in again at BWI and MTN tonight, and could reach MRB, IAD, and DCA though confidence is low at this time. A weak cold front crosses the waters Thursday morning, causing winds to turn southwest, then west by Thursday afternoon. Another round of showers and thunderstorms is likely, though mainly along and east of I-95. High pressure builds into the region Thursday night into Friday, bringing dry conditions and continued west winds. Predominantly VFR conditions are expected over the weekend, but showers and thunderstorms in the afternoon/evening could bring restrictions to the terminals. MRB is the most likely to see precip on Saturday. A cold front moves through Sunday into Monday with greater coverage of convection, so restrictions could be more widespread. && .MARINE... Multiple rounds of showers and thunderstorms will move across the local waters throughout the day, and into this evening. Special Marine Warnings may be needed for any strong storm that could produce wind gusts in excess of 35 knots. A waterspout or two is also possible. Increasing southerly winds tonight into Thursday morning will likely result in SCA conditions in the open waters of the central and southern Chesapeake Bay, along with the Lower Tidal Potomac. A weak cold front crosses the area Thursday morning, causing winds to veer southwest, then west Thursday afternoon. At this time, winds are forecast to remain below SCA level. However, occasional gusts to around 15-18 knots is possible Thursday afternoon. Additional showers and thunderstorms are likely to cross the waters Thursday afternoon/evening. Westerly winds around 5-10 knots Friday, with dry conditions. Westerly winds over the weekend are expected to remain below SCA criteria. Showers and thunderstorms on Sunday could bring stronger gusts to the waters. && .TIDES/COASTAL FLOODING... Increasing southerly flow has resulted in rising tidal anomalies, with a few sensitive sites already reaching minor flood. Annapolis and Straits Point likely reach minor again during Wednesday night`s high tide, and other sites may approach or exceed minor as well. This will be even further enhanced by the presence of the New Moon. A weak cold front crosses the area early Thursday, with the resulting west winds causing tidal anomalies to drop. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flood Watch from noon EDT today through late tonight for DCZ001. MD...Flood Watch from noon EDT today through late tonight for MDZ011-013-014-503>506. Coastal Flood Advisory until 9 AM EDT this morning for MDZ014. Coastal Flood Advisory until 4 AM EDT early this morning for MDZ017. VA...Flood Watch from noon EDT today through late tonight for VAZ053-054-506-526-527. WV...None. MARINE...Small Craft Advisory from 2 AM to noon EDT Thursday for ANZ532- 533-537-540-541. Small Craft Advisory from 10 PM this evening to 2 PM EDT Thursday for ANZ534-543. && $$ SYNOPSIS...KRR NEAR TERM...KRR SHORT TERM...KRR LONG TERM...CAS AVIATION...KRR/CAS MARINE...KRR/CAS TIDES/COASTAL FLOODING...CAS