... Shower and thunderstorm chances will increase Thursday and Friday ahead of an approaching cold front, with the front likely crossing the region on Saturday. The front may stall near the southern part of the area for the latter half of the weekend into early next week, keeping isolated to scattered showers and thunderstorms in the forecast.
NEAR TERM /THROUGH THURSDAY/... As of 740 PM EDT: Main update for the 00Z TAF issuance along with minor adjustments to temperatures and PoPs to coincide with latest trends as the rest of the near term forecast remains on track. Aside from a few lingering showers in the mountains, conditions overall remain quiet across the area this evening with temperatures still in the low to mid 80s.
Previous discussion: Over the next 24 hours, the upper ridge off the southeast coast is expected to retreat as a baggy upper trof moves eastward over the TN Valley and srn Appalachians. Not a lot of mid/upper forcing is seen in the model guidance to encourage the deep convection, but with this setup, it is almost always conducive to greater-than-climatological coverage of showers and storms during peak heating. With little cloud cover tomorrow morning, we should be able to destabilize quickly, eventually realizing 1500-2500 J/kg of sbCAPE, but shear will be light. Convective initiation is expected much earlier in the day, by 15Z over the mtns, and coverage should be much better. A few pulse-severe-type storms can be expected in the afternoon. Temps were kept close to the blend of the guidance.
SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 215 pm EDT Wednesday: A 500 mb height weakness will persist over the southern Appalachians between the building Plains ridge and the offshore ridge axis Friday through Saturday. Any lingering Thursday evening convection under the developing baggy heights should steadily wane through the nighttime hours with the loss of heating. Then, anticipate convection returning a bit more quickly on Friday afternoon as dewpoints continue to recover in southwest flow and triggering increases under the shortwave trough. However, sbCAPE values should be a touch less, generally 1500 to 2000 J/kg, compared to previous days given the expected clouds. Will still feature solid chance east to likely west PoPs late day.
Lapse rates will steepen a bit from the north on Saturday under the persistent upper troughing. A weak boundary settling southward in the trough, and outflows from upstream convection to the north and west, could provide triggering during peak heating. Solid chance PoPs will be featured throughout with modest instability. Temperatures will run a couple of degrees above climo at most sites through the period.
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 230 pm EDT Wednesday: By Sunday, the central CONUS ridge axis will move east over the Mississippi and lower Ohio River Valleys, with deep northwest flow over our region. This ridge should prevent any eastward movement of Tropical System Cristobal from the central or western Gulf coast. Over our area, shallow return flow moisture may start developing over the weak, stalling boundary in the vicinity, and isolated shower PoPs will be featured for mainly the southern half.
The upper ridge will build eastward Monday through Tuesday. Meanwhile, the Cristobal remnants will get absorbed northward west of the Mississippi River as a prominent northern stream trough digs over the northern/central plains. At lower levels under the southeast ridge, however, south to southeast upslope flow moisture will likely blossom. With 1020+ mb surface high pressure developing from Great Lakes to the mid-Atlantic Coast, cold air damming could set up and keep temps below climo Monday and Tuesday. The resulting shortwave trough axis from the absorbed Cristobal system may move eastward to the Appalachian chain on Wednesday ahead of the main longwave trough axis. Likely PoPs will be featured with the passage of the narrow pre-frontal band of tropical moisture on Wednesday.