Bryce Resort / Basye

Summary

SYNOPSIS

... High pressure will move offshore through Wednesday. A cold front will approach from the Midwest late Wednesday, then cross the northern Mid-Atlantic Thursday. The front will stall to the north across Pennsylvania keeping the local area in a warm and humid air mass through early next week.

NEAR TERM /THROUGH TONIGHT/... 945 AM Update: Early morning fog has lifted across much of the area, with abundant sunshine building in. Temperatures have already risen into the 60s to low 70s this morning. High cirrus clouds will continue to pass through early today.

Previous Discussion Follows: Slightly lower heights associated with a weak lead shortwave, coupled with heating and orographic lift could result in a few showers and thunderstorms generally west of the Blue Ridge and south of US-50 mid to late afternoon into this evening. Flow aloft is relatively weak (W 15-20 kts 700-300 hPa, SE 5-10 kts 850 hPa). Although light, the veering nature of the flow could help prop or lock cells to the terrain, with subsequent outflow collisions resulting in only a slow east- southeastward drift. There is a stable layer around 600 hPa in forecast soundings, and this plus a lack of a large scale trigger may inhibit more widespread deeper convection for much of the day. Although storm motion may be slow, any excessive rainfall threat should be mitigated by lower PWs of around 1 inch. Still, isolated totals of 1-2 inches are possible under any slow-moving/repeat storms. CAPE around 1500 J/kg and steepening low-level lapse rates could result in some spotty small hail or gusty winds and frequent lightning.

Any shower and thunderstorm activity should dwindle this evening with additional mid and high level clouds beginning to move in from the west. These clouds may help reduce the threat of more widespread and/or dense fog tonight, but some patchy fog or perhaps marine stratus are possible.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... High pressure at the surface will shift offshore Wednesday into Wednesday night. Meanwhile, a broad area of troughing and associated surface low pressure will trek gradually eastward from the Midwest. This will lead to a period of unsettled weather starting as early as Wednesday afternoon, and lingering through at least Thursday evening.

Deep layer shear is expected to remain weak Wednesday (AOB 20 kts sfc-500 hPa). However, flow will increase subtly later in the day, and heights will begin to come down slightly as well. This, coupled with above normal temperatures and increasing humidity and instability will lend to a threat of at least widely scattered showers and thunderstorms come afternoon. The threat appears greatest west of I-95 further away from the departing ridge and any associated subsidence. A couple storms could produce a little hail or gusty winds given the increased instability, but the lack of shear will probably keep a more organized or widespread severe weather threat marginal, and relegated to north of US-33/west of US-15 for the most part.

Heights fall more appreciably on Thursday as a surface cold front impinges on the area. Latest guidance times the front favorably with peak heating. The background airmass will be very warm and humid, but cloud debris from previous day's convection over the middle of the country could inhibit more significant CAPE. If higher instability is realized, a more organized threat for strong to severe thunderstorm clusters could develop given the strong surface front and increasing flow/shear aloft in the right entrance region of a seasonably strong upper jet. The threat for strong to severe thunderstorms may be greater south of US-50/I-66 and east of US-15 where the potential for more heating is highest.

Activity should gradually dissipate heading into Thursday night as daytime instability wanes and the front sags south. But, the front's progress will be slow, so the potential of showers may linger well into the night south of US-50/I-66.

LONG TERM /FRIDAY THROUGH MONDAY/... A front will remain generally north of the Pennsylvania Turnpike through early next week keeping the local area in a warm and humid air mass with the risk of almost daily afternoon and early evening showers and thunderstorms. A well defined 700 mb shortwave-trough is fcst to approach the area Fri night with its axis crossing the area around 18Z Saturday. With shear vectors on the weaker side and slow storm motions, storms Fri afternoon and evening may pose a risk of heavy rainfall and flash flooding. Saturday's threat really depends on the timing of that shortwave as it appears the trough axis will be too far east to enhance t-storm activity.

On Sunday, the models show significant 925-850 mb drying under low- mid level ridging, which is likely to keep t-storm activity isolated at best.

On Monday, a potent shortwave-trough and attendant sfc cyclone will move across Ohio and northern Mid-Atlantic. This system could pose a more substantial severe wx threat given strong winds aloft especially if the timing of the frontal passage coincides with peak heating. The latest timing suggests a severe threat during the morning hours, but things could definitely still change between now and next Monday.