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FXUS61 KPBZ 080026 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 726 PM EST Thu Dec 7 2023

SYNOPSIS
There is high confidence in above average temperatures through this weekend, followed by below average temperatures to start next week. One to two inches of rain with low-end hydrologic impacts are expected Sunday, before a transition to snow with some impacts through Monday morning, particularly for higher elevations.

NEAR TERM /THROUGH FRIDAY/
An Ohio Valley ridge will build in throughout the night, allowing warmer air to filter in aloft. The warm nose is expected to progressively lower, limiting radiative cooling potential and keeping lows just above average for most of the region. Lingering mid- level clouds north of the Pittsburgh metro are expected to decay with upper subsidence, but upper- level clouds will progressively advect in from the west through dawn before quickly departing in the late morning.

As the diurnal inversion breaks and mixing taps into the elevated warm layer, temperatures will readily and rapidly rise beyond 8am. Daytime highs are expect to peak 10 to 15 degrees above average with mostly sunny skies.

SHORT TERM /FRIDAY NIGHT/
Temperatures will be 10 to 15 degrees above average under strong southerly flow across the Ohio River Valley Friday night.
Clouds will increase from west to east as upper-lvl moisture associated with a low over the Midwest overspreads the region.

LONG TERM /SATURDAY THROUGH THURSDAY/
Saturday may be the warmest day in the long term forecast period if precipitation holds off until late afternoon due to low-lvl dry air. Probabilities remain elevated (above 70%) that areas near and south of Pittsburgh will climb into the upper-50/low-60s before rain advances into the region with our next disturbance. Breezy conditions with diurnal heating and boundary layer mixing of strong low-lvl winds is expected after 12pm Saturday for areas west of Pittsburgh. Probabilities of 20mph to 30mph gusts are above 70%.

Significant rainfall is expected to occur Sunday as moisture from the Gulf of Mexico advects into the region as a deep trough digs into the Mississippi River Valley and slowly advances across the Ohio River Valley. Probability of 1.0 inch of rainfall in a 24hr time period is high across a large portion of region (above 70%) through late Sunday night. The probability of exceeding 2.0 inches of precipitation is low (less than 20%).
However, it isn't zero and a few isolated areas may see localized higher amounts with stronger convection. In fact, thunderstorms may develop early Sunday morning with noted elevated instability within model soundings.

The probability of heavy rainfall decreases after 10pm Sunday as cold air aloft advances and rain quickly chances into a wintry mix followed by snow. With the ridges expected to see the snow change over sooner than the lower elevations, noted accumulation will likely occur over the Laurel Highlands (for elevations above 2kft). The probability of 2 inches in eastern Tucker is high, above 80%. However, the probability of 6 inches of more is low (below 10%). A Winter Weather Advisory may need to be issued if confidence continues to remain high for this projected outcome. Elsewhere, low accumulations (less than 0.5 inches) is expected given snow character leaning more on the wet side opposed to dry and fluffy.

High pressure builds into the Ohio Valley on Monday with northwest flow and some upslope snows continuing over the ridges. 500 mb height rises on the order of 150 meters occur, setting the stage for a return to near normal temperatures for Tuesday and Wednesday as surface high settles over the eastern states.

AVIATION /00Z FRIDAY THROUGH TUESDAY/
VFR is expected through the TAF period as ridging continues to build over the eastern CONUS. Wind will remain southwesterly through Friday, increasing again Friday afternoon with mixing.
Gusts of 10 to 20 kts are expected areawide.

OUTLOOK
VFR conditions will prevail through Saturday morning. The next impactful system with associated restrictions and rain is expected late Saturday through Sunday.

HYDROLOGY
Widespread rain of 1 to 2 inches is expected for most of the Upper Ohio River Valley Saturday night into Sunday. Rapid rises are expected Sunday night due to runoff. With rivers run below average across the region, the probability of river forecast points approaching flood stage is low. Local tributaries may be impacted in areas with localized heavy amounts.

With the latest GEFS run, probability of Stillwater Creek at Uhrichvilles approaching flood stage is 20% while the grand ensemble of Hi-Res guidance suggest only a 39% chance of reach "Action" stage.

PBZ WATCHES/WARNINGS/ADVISORIES
PA...None.
OH...None.
WV...None.

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