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Forecast Discussion

Summary

SYNOPSIS

... A weak cold front over the Mid-Atlantic region today will trigger scattered showers and thunderstorms today. These daily afternoon and evening showers and storms will linger through at least the middle of the week thanks to a stalled front across the Carolinas.

NEAR TERM /THROUGH TONIGHT/... As of 330 AM EDT Sunday...

Stubborn isolated showers continue to develop during the pre-dawn hours along a weak sfc boundary and in a region of MLCAPES of 500- 1000 J/KG. Introduced a slight chance of showers/isolated storms for spots east of the Blue Ridge through 10Z (6 AM EDT) to account for addition development, but most high res models fade these showers earlier.

The trend of increasing coverage of showers and thunderstorms continues today, as a high CAPE/low shear environment with a diffuse boundary in the region along with weak upslope southeast low level flow will likely lead to scattered convection across the region. With the lack of shear, storms will be fairly disorganized, so was reluctant to push POPs into the likely category. However, with an unstable air mass in place, blanketed the entire region under a high chc of POPS, but the areas most likely to see storms will be the southern Blue Ridge (upslope enhanced), and the Shenandoah Valley (closest to low level convergence boundary by this evening). SREF output shows the probability for DCAPE values greater than 1000 j/kg highest across the far eastern portion of our area, and would not be surprised if a couple of storms generate some damaging winds and locally heavy rain.

Today's high temperature forecast has a lower than average confidence level as model ensemble members show a higher than normal spread in forecast values. Additional clouds, slightly lower 1000- 850mb thicknesses should cut a few degrees off yesterday's values, but still leaned toward the warmer guidance values for highs today given a similar airmass to yesterday.

Would not be surprised to see a similar scenario unfold Sunday night with showers and isolated storms continuing past sunset as the deterministic medium range models indicate a weak upper level shortwave approaching from the northwest. Meanwhile at the surface, a subtle surge of low level moisture pushes into the region with a weak low level convergence zone lingering. Nudged up lows a degree or so east of the Blue Ridge with this nose of higher sfc dew point air arriving.

SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... As of 400 AM EDT Sunday...

This period will see widespread scattered diurnal showers and storms every afternoon and evening. A broad upper ridge on Monday will give way to weak troughing Tuesday and Wednesday. With deep moisture in the atmosphere and instability from afternoon heating, some storms could create strong wind gusts and heavy downpours. Localized heavy rainfall is possible with PWATS ~2 inches, and if training were to setup, some locations could get a lot of rain.

A little bit of cooling off will occur into the midweek as showers and cloud cover become more widespread. Highs in the 80s east of the Blue Ridge, upper 70s/low 80s west. Nights will stay in the upper 60s/lower 70s for most. Early morning fog is a distinct possibility, especially in those locales that are on the receiving end of showers/storms in the previous 24 hours.

LONG TERM /THURSDAY THROUGH SATURDAY/... As of 400 AM EDT Sunday...

An upper trough and surface low in the southeast CONUS will form up in the Carolinas and intensify easterly flow and continue to feed moisture into the Appalachians. Guidance has begun to edge westward with the track of the surface low with each successive run. With the center of the low passing over the tidewater region of VA early in the weekend, Friday will see widespread storm coverage. Late in the weekend the low will make its exit towards New England and a trough axis over the Ohio Valley will return us to large scale westerly/northwesterly flow. This will decrease the overall moisture in the environment and downsloping over the mountains will also serve to suppress shower coverage.

Temps for this period are generally cooler than normal, with the coastal low pulling air from the northern Atlantic and cloud coverage preventing some of the afternoon insolation.

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