... Eroding high pressure will support the return of warm and humid conditions with thunderstorms possible on Thursday and Friday.
NEAR TERM /THROUGH TONIGHT/... Morning Update: No major changes were needed to this period of the forecast.
Previous Discussion: A generally pleasant day across the Upper Ohio Valley is about to get underway as we remain under the influence of a high pressure that's centered across the far northeast US and Canadian Maritimes. Looking aloft, GOES Water Vapor Imagery and 500mb isobaric analysis shows a deep trough digging across the Great Plains, with downstream ridging in the process of amplifying from the Mid- Atlantic northwestward into the Great Lakes region.
Over the course of the day, the deep Great Plains trough will just about become stacked and "cut off" from the main synoptic northern stream flow, resulting in a very slow progression of this system over the next few days as it traverses east across the US. For the Upper Ohio Valley, this will ultimately result in a few days of southerly lower tropospheric flow that will bring warmer, moister air into the area. Each day through the passage of the low should be a bit more warm and humid than the previous day.
For today specifically, 850mb southerly flow on the eastern periphery of the Plains low will bring about 15C air (at that level) into the Upper Ohio Valley. Forecast model soundings suggest we'll remain a bit decoupled from this air today as ESE flow in the lowest 100mb will bring in only modest warm air advection thanks to the retreating sfc high, allowing for temps to generally remain in the 70s, though portions of SE Ohio and WV may reach around 80.
Wouldn't rule out a rogue shower or two west of the I-77 corridor as the environment is a bit more moist and in closer proximity to ripples moving northward in the deep southerly flow.
Overnight lows will range from the mid 50s towards the Allegheny Mtns to the mid 60s along the I-77 corridor, which is about 5-10 degrees above average.
SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... By Thursday, models project the closed, stacked low to make it into the mid Mississippi Valley area. Deep meridional poleward flow on the east flank of this system will continue to bring in warm, moist air into the Upper Ohio Valley and Allegheny Mountains. The aforementioned retreating high will shift far enough east such that sfc flow will also become more southerly and in align with the flow/air in the mid-levels. Temps Thursday should get into the 80s for most areas outside of the Allegheny Mountains, with dewpoints rising into the 60s.
As for the Thursday thunderstorm risk... 850mb ramps up to 35 to 45kts by the afternoon, allowing effective and deep-layer shear values to increase to the 35-40kt. Surface heating in the warm, moist low-level air will result in instability increasing to between 500 to 1000 J/kg by peak heating, coincident with the onset of higher shear. This should suffice for some thunderstorm organization across Ohio and potentially into WV and far western PA as well. Convection allowing models (CAMs) are suggesting quick-moving convective segments. Damaging wind gusts should be the most-likely severe threat with the stronger segments, with a lesser (but not zero) threat for large hail or even a tornado. Additionally, high rain rates coupled with any training convective segments could result in some isolated instances of flash flooding.
Thursday night into Friday... severe thunderstorm chances should decrease with the loss of heating and potentially a stabilized surface layer if there was any antecedent convection. Models continue to advertise a slug of precipitation and likely thunderstorm activity given some very modest CAPE present. The low is projected to weaken by the time the center moves into the Upper Ohio Valley late Friday. Should we get enough heating, wouldn't rule out a few storms on Friday. Mid-level lapse rates within the low are 6-6.5 C/km. Should any storms form as the crosses, small hail should be the primary threat.
Showers/thunderstorms will decrease and shift southeast late Friday night along with the low pressure.
LONG TERM /SATURDAY THROUGH TUESDAY/... Looking ahead at the holiday weekend-- generally nice weather is anticipated! The departing low pressure system may yield a few shorts early on Saturday south and east of Pittsburgh along an axis of stretched/deformed flow on the northside of the low.
From the second half of Saturday through the weekend into early next week, both primary global models (GFS and ECMWF) are showing high pressure firmly in place across the eastern half of the US, with overall ridging growing in intensity each day, with potentially 594- 595dam heights reached by Tuesday or Wednesday! What does this mean? We're going to generally get warmer each day as we sunset Meteorological spring and head into summer. Look for highs in the 80s by Sunday and then potentially 90s by early or mid next week with little to no rain.