Area Forecast Discussion
Issued by NWS Sterling, VA
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Issued by NWS Sterling, VA
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-- Discussion containing changed information from previous version are highlighted. --403 FXUS61 KLWX 261904 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 304 PM 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 /UNTIL 6 AM THURSDAY MORNING/...-- Changed Discussion --Hot and not too terribly humid conditions continue this afternoon. Many locations east of the Alleghenies continue to see temperatures in the 95 to 99 degree range with heat indices around 100 degrees. One thing to note is the dewpoints which range from the low to mid 60s over the mountains to around 70 degrees in the Baltimore/DC metro area. These dewpoints combined with an incoming lee trough will determine the magnitude of coverage when it comes to our severe weather threat this afternoon. 12z soundings from both IAD and RNK show dry air in both the low and mid levels. RH values within the 700-925mb level ranged from 40-50 percent with precipitable water values of 1.1 to 1.4 inches. Further west towards PBZ these ingredients were a bit better as an elevated mixed layer plume gradually advects east- northeast into the area.This should help erode the cap a bit into the remainder of the afternoon hours. Current ACAR sounds at IAD, DCA, and BWI also signal some of the dry air with deep westerly flow aloft. With that said, coverage looks to be fairly isolated across the region this afternoon with initiation mainly along and east of I-81 as the lee trough swings through. Also watching a bay breeze along the western shore of the bay and how it interacts with the pending trough arcing through the area. The better focus this afternoon will be up toward the western PA and western New York as an MCV associated with the main cold front swings through these areas. Hi-res CAMS and ensemble runs continue to paint this area along with locations along and north of the PA/MD line and into NJ for the highest probabilities for severe weather development. TIMING:The initial round of convective development will run between now through 6pm before a lull. The secondary round of convection will be a bit more scattered to widespread as it pushes through the area after 7pm and continues through midnight.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) this afternoon and evening. Storms will feed off of CAPE values between 1000-2000 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 mid level lapse rates 6-7 degrees C/km favoring well organized storms. This is backed up in several hi-res CAMS and model soundings. THREATS:The primary threats with these thunderstorms will be damaging winds and large hail. The threat for damaging winds (70 mph or more) also continues based upon the current environment. 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 continues lower 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 antecent dry conditions expect a bit more runoff as water will have a bit of a harder time percolating through. Convection is slow to wane as we get into early Thursday morning as the cold front slowly pushes through. Some additional showers will linger through sunrise Thursday as the front continues south and east of the region. Lows tonight will fall into the mid to upper 60s with low to mid 70s along and east of I-95.-- End Changed Discussion --&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...-- Changed Discussion --Calmer conditions are expected Thursday as a cold front exits south and east with high pressure building in from the Great Lakes region. A few showers and perhaps a few thunderstorms may linger along and east of the boundary (i.e east of the Blue Ridge) and south of I-64 Thursday morning into early Thursday afternoon. As for temperatures Thursday, expect highs to drop back into the mid to upper 80s with dewpoints in the low to mid 60s. Additional relief arrives Thursday night as dewpoints fall into the 50s under light north to northwest flow. Lows Thursday night will range from the 50s and low to mid 60s west of US-15 to upper 60s and low 70s in the metros. High pressure continues to push eastward into the New England states Friday before pushing offshore Friday night into Saturday. This will allow for a light onshore flow component to converge against the Blue Ridge promoting a few showers and perhaps an isolated thunderstorms over the central/southern Blue Ridge (Shenandoah Valley/central VA Piedmont). Most locations will remain dry with temperatures once again in the mid to upper 80s and low 90s. Overnight lows Friday night will fall back into the upper 60s and low 70s areawide with increasing moisture as the flow turns toward the south.-- End Changed Discussion --&& .LONG TERM /SATURDAY THROUGH WEDNESDAY/...-- Changed Discussion --A strong Bermuda High will be in place for this coming weekend, which will usher in hot and humid air into the region. While temperatures may not get into the upper 90s like we have seen in the past week or two, the humidity is what will be noticeably worse. Forecast dew points at this point look to be in the mid to upper 70s, which will make those highs in the low 90s feel like it is into the low 100s. Several areas could come close to Heat Advisory criteria once again, but it will be another borderline event. Either way, it will be hot both days, and anyone with outdoor plans this weekend should plan to have a way to stay cool and hydrated throughout. Beyond just the heat threat, there will be an increasing threat for severe thunderstorms each afternoon ahead of a cold front that will push through Sunday evening. As low pressure crosses through the Great Lakes on Saturday, the associated warm front lifts through the region and ushers in those hot and humid conditions mentioned previously. Likely to see some showers and storms during the morning hours as the front lifts through, but not expecting severe weather at that point in time as we should still be stable at the surface. However, as we get into Saturday afternoon and most of the regions is within the warm sector, expect showers and storms to develop once again along the lee trough before drifting east towards the metros that evening. I would expect we will have ample instability in place within this air mass, but poor mid-level lapse rates and only modest shear could make it a bit difficult to get quite as much organized convection. There is the possibility we get some help aloft with a weak shortwave sliding by near/north of the area. This level of detail is just too difficult to nail down at this time, as it would likely be a smaller scale feature we have to pin down the evening before or the day of. The key is though, that the potential is there, but confidence is just low in level of coverage/severity at this time. Saturday night will be one of those nights that barely drops below 80 for those areas near the metro centers. Lows area-wide will be generally in the mid 70s. Depending on the progression of things earlier in the evening, we may have a decent amount of ongoing convection even into the nighttime hours. The severe threat would diminish of course with the developing stable layer, but could still be some elevated thunderstorms. By Sunday however, things get a bit more interesting. As low pressure moves further east out of the Great Lakes, the attendant cold front will swing through the region later in the day. Southerly flow between this and the aforementioned high pressure offshore will continue to usher in hot and humid air. Sunday could see temperatures into the mid 90s with dew points well into the mid and upper 70s! Not only will this yield some potentially dangerous heat, mentioned previously, but also this will be a volatile environment for the approaching cold front. This looks to be a rather significant cold front with some decent upper-level support as well. Guidance is still a bit split on how far south the associated trough sags into our area, but we could see anywhere from 30-40+ knots of deep layer shear with this fropa. This will be paired with around 1500-2500 J/kg of CAPE (perhaps even higher, especially along/east of I-95. The timing of this front will be one of 2 key factors to consider, with the other being if there are any lingering clouds/convection from Saturday night ongoing Sunday morning. Those are details that will get ironed out as we get closer to the event. With the thermodynamic and kinematic fields discussed above, both hail and damaging winds would be in play Sunday. Looking towards the surface, even think there is a marginal tornadic environment in place depending on bay/river breeze interactions. This would pin this threat to areas along/east of the I-95 corridor. Lots of uncertainty still and time for things to change, but something to think about and prepare for in the coming days should these trends continue. 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.-- End Changed Discussion --&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...-- Changed Discussion --VFR conditions look to prevail this afternoon with intermittent periods of sub-VFR as multiple rounds of showers and thunderstorms push through the region. This activity is in association with a strong cold front which looks to cross the region late Wednesday evening into early Thursday morning. Coverage of storms will be isolated to scattered mainly across the corridor terminals between 20-23z. Went ahead and included TEMPOs for this window at KIAD, KBWI, and KDCA. Confidence was a bit lower to go with straight -TSRA given the westerly flow aloft and residual dry air with both the low and mid-levels. More organized convection arrives at all terminals beyond 23-01z/7-9pm. This convection will be a bit more widespread and organized as the cold front kicks into the region. Made an inclusion of this in the form of VCTS/-TSRA at all terminals within the 00-05z/8pm-1am window. The primary threats with storms will be strong wind gusts and hail. West/southwest winds are expected this afternoon and evening with gusts up to 20kts. Winds will shift to west-northwesterly late tonight into Thursday morning behind a well defined cold front. Prevailing VFR conditions are expected Thursday through Saturday. Showers and thunderstorms may lead to brief restrictions on Thursday morning and early afternoon at terminals south of the corridor toward KCHO. Post frontal northwesterly winds will gusts up to 15 kts Thursday. Additional spotty showers and thunderstorms may lead to brief restrictions at terminals along and south of a line from KSHD to KCHO Friday. Unsettled conditions this weekend could lead to afternoon thunderstorms each day. Winds will be out of the south/south- southwest through Sunday afternoon before turning abruptly out of the west behind a departing cold front Sunday evening. Winds could gust up to 15-20 knots at times.-- End Changed Discussion --&& .MARINE...-- Changed Discussion --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 20z before crossing the waters 22z. Additional showers and perhaps a few thunderstorms will follow between (00-04z)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. Channeled southerly flow will likely necessitate SCAs on both Saturday and Sunday, with gusts up to 25 knots possible over the wide portions of the Chesapeake Bay. Additionally, daily chances for thunderstorms that could produce strong winds could necessitate some SMWs each afternoon/evening.-- End Changed Discussion --&& .TIDES/COASTAL FLOODING...-- Changed Discussion --Prolonged southerly flow has led to a large ramp up in tidal anomalies which currently average around 0.75 to 1.25 feet. However, do not expect any location to see a rise toward minor flooding during the next high tide cycle. 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.-- End Changed Discussion --&& .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...EST LONG TERM...CJL AVIATION...CJL/EST MARINE...CJL/EST TIDES/COASTAL FLOODING... CLIMATE...