Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
236 FXUS61 KLWX 121404 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1004 AM EDT Fri Jul 12 2024 .SYNOPSIS... A stalled frontal boundary will continue to drift in the vicinity of the Interstate 95 corridor today through Saturday bringing widespread showers and thunderstorms to areas east of the Blue Ridge. Weak high pressure briefly builds back into the area Sunday then gradually slides offshore Monday into Tuesday allowing the excessive heat, humidity, and daily thunderstorm chances to return. Another more substantial cold front approaches for the middle part of next week. && .NEAR TERM /THROUGH TONIGHT/... MORNING UPDATE: Widespread light to moderate showers are building in from the southeast for areas east of the Blue Ridge at this time. At this point, nothing has changed in our thinking with the forecast, so refer to the previous discussion. PREVIOUS DISCUSSION: A cooler day in store today with thickening clouds from southeast to northwest as a stationary front meanders back toward the west from the Delmarva Peninsula. Accompanying this stationary front and increasing moisture will be numerous showers and thunderstorms through the day today and continuing through tonight. Showers and storms are expected to develop as far west as the I-81 corridor by later this afternoon. The heaviest rainfall should be across far Northeast Maryland and much of southern Maryland. There is currently a Flood Watch for the southern Maryland counties of Charles, Calvert, and St. Mary`s from 2am this morning until midnight tonight. There is also a Flood Watch for Cecil County from 6am this morning until 2am early Saturday morning. High temperatures will be mainly in the lower 80s for most with a few northwestern zones of western Maryland and eastern West Virginia reaching the middle to upper 80s as these locations will be the last to receive thickening clouds and showers and thunderstorms. While showers and scattered thunderstorms are expected to continue tonight over Northeast Maryland, the I-95 corridor, the DC metro, and southern Maryland, showers and thunderstorms will weaken, diminish and end from west to east with the stationary front wiggling its way to the east overnight and slowly during the day on Saturday. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... During Saturday morning until around midday, showers and thunderstorms will be predominantly located along and east of the I- 95 corridor. As for farther to the west of I-95 and the remainder of the day Saturday, there will be a slight chance of a shower or popup thunderstorm with daytime heating. High temperatures will have a chance to rebound back up into the lower 90s on Saturday with clouds breaking for sunshine and a light westerly wind. Total rainfall for this event today through Saturday night will be 2 to 3 inches over a 30 hour period in Northeast Maryland and southern Maryland. Amounts of 1 to 2 inches on both sides of I-95 from Harford County area in Northeast Maryland to the Stafford County area of eastern Virginia. Rain amounts of one-third up to 1 inch expected between the Blue Ridge Mountains and the I-95 corridor. Drier and hotter conditions expected Sunday and Sunday night with highs Sunday in the middle to upper 90s possible. Increasing humidity, combined with this heat, could result in heat headlines Sunday and perhaps into early next week. && .LONG TERM /MONDAY THROUGH THURSDAY/... A prolonged stretch of excessive heat and humidity are expected for the beginning of the new workweek. Diurnally driven shower and thunderstorms are expected each afternoon with more widespread activity coming with a front mid to late week. Synoptically, we`ll sit sandwiched between the broad upper level ridge over the southeastern U.S and an incumbent neutral to positively tilted upper level trough ejecting from the eastern Great Lakes/upper Midwest region. Some uncertainty remains in regards to the timing and strength of this trough along with it`s associated cold front which is progged by the latest 00z guidance to cross the area Wednesday into early Thursday. A notable change in airmass is expected in the wake of this front with broad high pressure expected to return for the upcoming weekend. The biggest hazard during the majority of the extended period will be the excessive heat and humidity. High temperatures Monday through Wednesday will push into the mid to upper 90s (mountains mid to upper 80s and low 90s). A few locations may even hit 100 degrees with 850 mb temps running +21 to +24 degrees C which is right at the 97.5 to 99 percentile. Dewpoints remain in question through the period as model guidance suggest values in the upper 60s and low to mid 70s. Wind direction aloft appears to be more westerly/southwest Monday into Tuesday before turning southerly mid next week. With that said, we could mix down some lower dew points (mid 60s) during the afternoons. This would be the one thing keeping us from another lengthy period of heat headlines, but will continue to monitor model trends as we get closer in time. The excessive heat and humidity will bring the return of diurnally driven showers and thunderstorms. Overall forcing and steering flow will be lacking Monday and Tuesday leading to the bulk of any convective activity tied to the mountains. Coverage should be fairly isolated Monday into Tuesday due to the lack of any forcing mechanisms to work with. Should a storm get locked over one location for an hour or two, could see some isolated instances of flooding in the higher terrain. However, the recent drought has flash flood guidance very high area-wide. The greatest coverage or uptick of thunderstorms looks to arrive Wednesday and Thursday as the upper level trough and cold front approach from the Ohio River Valley. Some these thunderstorms could be strong to severe based upon increasing wind shear and the antecedent airmass in place. Both SPC and the CSU Learning Machine Probabilities also indicate this threat Tuesday and especially Wednesday as the front pushes through. Long range model soundings indicate plenty of SBCAPE and DCAPE to work with along with subtle shear. One thing that could be lacking is our mid-level lapse rates which seem to be an issue with convection last week. Something that will have to continue to monitor over the next few days ahead. Primary threats with storms during the extended period appears to be damaging winds and isolated instances of flooding (mainly urban and poor drainage areas). Once again this will not be a drought busting front, but at this point anything helps. The front clears the area Thursday into Friday with seasonable temps expected next weekend. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... MVFR to IFR conditions are expected at terminals east of MRB and SHD today as a stalled front wavers in the vicinity of the I-95 corridor. Cigs will deteriorate from southeast to northeast between 9- 12z/5am-8am this morning. The worst conditions and highest probability for IFR will likely come later this afternoon and into the overnight hours as heavier showers pivot through. A few thunderstorms are also possible at KCHO, KIAD, KDCA and KBWI although confidence is low given limited instability with the increased cloud cover and persistent precipitation chances throughout the day. Areas west of MRB will likely remain VFR throughout the day although a few light to moderate showers cannot be ruled out late morning and into the afternoon hours. East winds will remain light around 5 to 10 kts changing to the southeast later this afternoon and evening. Terminals east of the Blue Ridge and especially along the I-95 corridor will see shower activity continuing through the overnight hours. Cigs will remain low along with some vsby reductions due to the showers and areas of fog/low stratus overhead. Conditions will be slow to improve in these same locations Saturday as the front washes out across the region. Additional periods of sub-VFR can be expected for the corridor terminals Saturday morning and afternoon. Areas further west will see the influence of drier air with weak high pressure building into the region. VFR conditions return for all terminals Saturday night into Sunday morning. Some fog is possible leading to reductions in areas that see additional rainfall heading into late Saturday night and early Sunday morning. Prevailing VFR conditions are expected Sunday through Tuesday as weak high pressure builds back into the region. Winds will turn toward the west late Saturday before switching back to the southwest Sunday into early next week. A few thunderstorms cannot be ruled out at terminals along and west of the Blue Ridge (MRB, CHO, HGR, and SHD) although coverage should be isolated. However, cannot completely rule out an afternoon storm making it as far east as IAD Monday , and even all the way to I-95 on Tuesday. If these were to make it over the terminals, could see some gusty winds, VSBY reductions, and of course lightning. High probabilities for thunderstorm activity return to the forecast Wednesday and Thursday next week as another front pushes into the area. && .MARINE... A stalled front over southeast VA and the Delmarva will continue to waver over the local waters through Saturday. The front will dissipate over the waters Saturday night with calmer conditions under the weak influence of high pressure Sunday. SCA conditions are likely over the open waters of the Chesapeake Bay around sunrise this morning through this evening with southerly wind gusts up to 20 kts. Elsewhere, across the waters winds will remain variable out of the east and southeast at 5-10 kts with intermittent gusts up to 15 kts. Widespread showers and scattered thunderstorms are expected all day Friday into Friday night. A few Special Marine Warnings may be needed for wind gusts of 35+ knots. Additional showers and thunderstorms are expected over the waters Saturday although coverage will gradually decrease throughout the day as the front sweeps back to the east. Weak high pressure brings sub-SCA level winds to the waters Saturday night into Sunday. This trend continues into much of the day Monday although winds will switch back to the south west and eventually south Monday night. Some southerly channeling could occur Monday night into Tuesday morning over the open waters although confidence is low. A shower or thunderstorm also cannot be ruled out, but the coverage will remain isolated. A greater potential of showers and thunderstorms is possible Wednesday and THursday as a strong cold front crosses the waters. && .TIDES/COASTAL FLOODING... Tides will remain below action stage through the next 24 hours. Winds will be light out of the north and perhaps northeast through tonight before becoming more southeasterly Saturday into early Sunday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Flood Watch through late tonight for MDZ008. Flood Watch through this evening for MDZ016>018. VA...Flood Watch through this evening for VAZ057. WV...None. MARINE...Flood Watch through late tonight for ANZ530. Small Craft Advisory until 6 AM EDT Saturday for ANZ533-534- 543. && $$ SYNOPSIS...EST NEAR TERM...KLW/CJL SHORT TERM...KLW LONG TERM...EST AVIATION...EST MARINE...EST TIDES/COASTAL FLOODING...LWX