Forecast Discussion
Summary
SYNOPSIS
... Low pressure over upstate South Carolina will continue to push northward through Wednesday bringing increased shower chances, cloud cover, and below normal temperatures to the region. Rain chances begin to decrease while temperatures increase later this week as high pressure builds back into the region.
NEAR TERM /UNTIL 7 AM MONDAY MORNING/... Relative to the past 24 hours, the lower atmosphere has significantly moistened up per ACARS soundings. There still remains some dry air in the lowest 150-mb, but radar echoes approaching from the southeast should further saturate the sub- cloud layer. Current surface observations show dew point depressions in the 10 to 15 degree range. While most approaching showers are relatively weak in nature, some 35 to 40 dBZ echoes have begun to emerge out of the Virginia Tidewater region. As these showers lift toward the northwest, a brief moderate downpour is possible over portions of central Virginia.
Aside from any enhancements along frontogenetic bands, a lot of the activity will be loosely defined. There will likely be a defined cut off where precipitation ceases as dry air sits to the north. Some drizzle may squeeze out for areas north of I-70, but these locations will largely remain dry overnight. In terms of temperatures, enhanced cloud cover and persistent easterly flow will hold temperatures in the 50s, locally into the low 60s for the metro regions and along the Cheseapeake Bay.
SHORT TERM /7 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... A lull in activity is likely Monday morning. The highest coverage of showers/storms is expected during peak heating each afternoon Monday and Tuesday. Most of the model guidance now keep rain chances low north of US-50, though that will ultimately depend on how far north the frontal boundary makes it.
The 12Z high res guidance today indicates a signal for possibly some heavy rainfall along the eastern slopes of the Blue Ridge and in parts of central VA. This area could see repeated rounds of rain through Tuesday, and will be aided by orographic lift. Widespread rainfall amounts of 0.25-0.75" are forecast south of I-66, with isolated higher amounts of 1.00-1.25". Along the Blue Ridge, rainfall amounts of 2-3" are forecast with isolated amounts up to 4" possible. This could lead to isolated instance of flooding in poor drainage and low-lying areas.
Afternoon temperatures over the next couple of days will reach the upper 60s to lower 70s. Muggy overnight conditions with lows in the 50s to low 60s.
LONG TERM /WEDNESDAY THROUGH SUNDAY/... Shower and thunderstorm chances will continue with moderating temperatures later this week. Overall not much change in the blocky pattern with residual troughing moving across Canada and a gradually flattening of the upper level ridge to the east. At the surface, closed low pressure will continue to linger across the region while gradually weakening and drifting offshore Wednesday into Thursday. This will allow the chance for diurnal showers and thunderstorms on both days with the higher coverage on Wednesday compared to Thursday as an area of diffuse high pressure builds aloft. Although ensemble and deterministic guidance show a bit of variance the tendency remains for higher coverage of showers and perhaps a few thunderstorms during the daytime hours Wednesday compared to Thursday as the high builds in aloft. Precipitation chances do not go to zero completely Thursday nor Friday although Friday appears to be drier as high pressure continues build into the region. Convection once again will be confined to the daylight hours, but should be mainly terrain induced on both days with the potential of one or two isolated showers/thunderstorms flaring up further east along the bay/river breeze.
A backdoor cold front looks to drop into the region from the northeast for the upcoming weekend ahead. 12z deterministic/ensemble guidance continues to show some differences on the timing of front along with the potential for showers and thunderstorms compared to earlier runs from this morning. Latest guidance suggest a frontal passage on Saturday afternoon and evening leading to increased shower and thunderstorm chances across the region. The front will quickly pass through Saturday night with high pressure building in from the north Sunday into early next week.
Near normal temperatures are expected Wednesday with excess cloud cover and showers. Highs by Thursday and Friday will warm back above average with highs potentially around 90 degrees in most locations. This trend will likely continue into Saturday ahead of the backdoor cold front as 850 mb temperatures approach +12 to +16 degrees C. Humidity values will also be on the increase as well with relief not expected until the end of the weekend as cooler and drier air filter in.
Forecast confidence in the long term periods remains moderate due to large model spread for both precipitation chances and temperatures within the mid and late week period.