... A series of upper-level disturbances will pass through the region today through Sunday while high pressure builds over the Atlantic. Low pressure will pass to the south Monday and a strong cold front will pass through Tuesday. High pressure will build overhead for the middle portion of next week.
NEAR TERM /THROUGH TONIGHT/... ...Update to include Small Craft Advisory for the upper Tidal Potomac River and the northern Bay today. Previous discussion follows...
A warm front over western PA into the Potomac Highlands early this morning will push northeast today. A few showers and even a thunderstorm are possible early this morning, mainly across the Allegheny Highlands and near the Mason-Dixon Line, but most of the coverage should be to our north where the forcing is a little stronger on the warm front.
A deep westerly flow behind the warm front will allow for warmer and more humid conditions as the flow rotates around high pressure over the Atlantic and a subtropical ridge over southern Florida. At the same time, the jetstream will remain over our area, so upper-level disturbances will pass through during this time. Timing of these systems will be difficult, which does have an impact on the forecast and it does lower confidence. That is because these disturbances will have the potential to trigger showers and thunderstorms due to increasing instability associated with the warmer and more humid airmass. Therefore, there can be a shower or thunderstorm at anytime today and tonight. As of now, it does appear that one of the disturbances (perhaps an MCS) will pass through the area from west to east, first across the Allegheny Highlands late this morning/early afternoon, then across the DC metro area around mid-afternoon, and through Baltimore by late this afternoon/early evening. This is when the best chance for showers and a few thunderstorms will be. There is an isolated threat for severe storms due to stronger shear profiles associated with the jetstream and increasing instability. Also, with heavy rain from thunderstorms there is an isolated flood threat especially if an organized bowing segment develops where the shear vector will be parallel to the northern and southern portions of such a feature (oriented west to east). Confidence is too low for a flash flood watch at this time since the storms will be moving fast and there is uncertainty as to how organized this system will be when moving through.
Left pops in for tonight as well since there will be more shortwave energy passing through, but coverage should be isolated to widely scattered for most of the area. Tonight will feel noticeably more humid with lows ranging from the 60s west of the Blue Ridge to the 70s for most places east of the Blue Ridge.
SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure will remain over the Atlantic while low pressure passes through the southeastern CONUS. Some subsidence and riding north of the surface low may keep most of the shortwave energy to the north for Sunday, and this may cause convection to be more isolated or scattered. However, more hot and humid conditions are expected and this will increase instability to the point that if something does develop it may contain gusty winds and locally heavy rainfall.
The low will pass by to our southeast Sunday night through Monday and a stronger cold front will approach from the northwest Monday night as a potent upper-level trough digs over the Great Lakes and Midwest. Timing of these systems will have a significant impact on our forecast. As of now, we are leaning toward the less progressive solution that has the cold front holding off to our west through Monday night. If this solution plays out, then convection will be isolated or widely scattered Monday for most areas with perhaps more widespread convection near the Allgheny Highland late Monday/Monday night near the approaching cold front. If the front is faster, then convection will be more widespread and the threat for severe storms will increase. Will continue to monitor.
LONG TERM /TUESDAY THROUGH FRIDAY/... Long term will start out active as a strong trough and frontal boundary passes through the region Tuesday. Timing is still uncertain, but showers and thunderstorms, some with heavy rain, are likely, especially in the morning. Flooding is a risk given high PW's and potential slow movement of the front. Could be something gusty as well, but given timing, uncertainty on any severe risk is a little higher.
After that, Canadian high pressure will build across the region, with cooler and drier conditions and plenty of sunshine. Highs will be notably below normal for late June. Warming trend starts thereafter, but slowly, with low to mid 80s Thursday/Friday. Dry weather is most likely with the high pressure most likely to remain in control, but increasing humidity could allow something convective to pop, especially along the higher terrain, by Friday afternoon.