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FXUS61 KPBZ 091139 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 639 AM EST Sat Dec 9 2023

SYNOPSIS
Scattered showers are possible today, mainly west of Pittsburgh, with southerly flow providing above-normal temperature. More widespread rain is forecast tonight into Sunday as a cold front approaches and crosses. Rain will change over to snow Sunday night. A few inches may accumulate on the ridges into Monday, with less expected elsewhere.

NEAR TERM /UNTIL 6 PM THIS EVENING/
The main challenge during the overnight hours has been the temperature and dewpoint forecast, as low-level mixing of dry air and ongoing warm advection have played havoc with the values. Many locations remain in the 50s at this hour. Several record high min temps would be in jeopardy if not for the expected cooler temperatures this evening.

The first push of 1+ inch precipitable water air will arrive in in the western CWA over the next few hours, and this is shown by the bands of rain showers moving into the middle Ohio Valley with lift supported by convergence as a result of a low-level jet. Expect these to arrive in the eastern Ohio counties in the hours leading up to and around sunrise, providing some light QPF. This first round of showers will likely slowly fade over western PA through midday as they run into very dry low-level air in place, and as influence of the low-level jet wanes.
Clouds will abound otherwise.

As a potent shortwave digs across the Upper Midwest and western Great Lakes today, more moisture will get drawn northward ahead of an advancing cold front, which will arrive in western Ohio by sunset. Showers will once again increase in coverage in our western counties during the late afternoon hours. The strong southerly flow will help high temperatures remain well above normal, reaching into the lower 60s for areas south of Pittsburgh.

SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/
The main period of rainfall will still occur during the Saturday night/Sunday period, associated with the main cold frontal passage. There are still some ingredients present for periods of heavy rain. Anomalously high moisture for December will be present, with precipitable water climbing to above an inch just ahead of the front. Lift will be supported by a 130-150 knot upper jet passing to our west, along with isentropic upglide ahead of the boundary. A line of forced convection will likely occur ahead of the frontal boundary during the evening and early overnight hours, in a tongue of less than 500 J/kg of elevated instability. Severe chances remain quite low due to the expected stable boundary layer, but the convection could locally enhance rainfall rates, and relatively rare December lightning may be experienced in some areas.

While the potential period of heavy rain will be relatively brief for western portions of the CWA, a surface wave riding north along the front on Sunday may slow frontal progress as it approaches the ridges, extending the period of steadier rain.
This is reflected in NBM 48-hour rainfall probabilities of one inch or more exceeding 50 percent mainly south and east of Pittsburgh. Probabilities of 2+ inches remain low, in the range of 10 to 25 percent in the same region. The deterministic QPF forecast follows this trend, and was lowered slightly. Totals of 1 inch or more through 00Z Monday are now limited to some pockets south of Pittsburgh. Also continued is the de-emphasis on any sort of excessive rainfall/flood threat outside of very isolated issues.

Additional shortwaves/vorticity maxima arrive behind the boundary Sunday night, with one taking on a negative tilt as it lifts across the Central Appalachians to the Mid-Atlantic coast. Wind gusts of at least 25 to 30 knots will be possible for a time late Sunday night into Monday, perhaps a bit higher on the ridges. Still, no Wind Advisory is anticipated at this time, as model soundings show that mixing should be shallow enough to keep stronger wind locked up aloft. As colder air spills in behind the boundary, rain will change to snow starting late Sunday and continuing into early Monday morning. Behind the cold front, moisture appears to only barely penetrate into the dendritic growth layer in many cases Sunday night into Monday, keeping many areas below an inch total accumulation. The ridges will be an exception, as stronger forcing and better moisture may linger through the changeover period. NBM probabilities of two inches of snow are 50 percent or greater in the Laurel Highlands down into Preston and Tucker Counties. It is this area that may eventually require a Winter Weather Advisory. Lake enhancement may provide a small accumulation boost to locations north of I-80 as well perhaps into the 1 to 2 inch range, although moisture still looks shallow overall.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/
Surface ridging from Central Appalachians high pressure should help to clear the sky Monday night. Low-level backing wind will also signal rising 850mb temperature. Minimums will still dip into the 20s, and gusty wind from a lingering surface pressure gradient will also provide a wind chill, with apparent temperatures dropping into the single digits in the higher terrain.

Dry weather will then take hold for the remainder of the extended period, with quasi-zonal flow Tuesday and Wednesday eventually giving way to strong ridging over the Eastern CONUS. Temperatures will moderate close to normal in this pattern for Tuesday. A weak and likely dry frontal passage Wednesday may drop temps slightly below normal again for Wednesday. The rising heights will create a slow warming trend Thursday and Friday, perhaps to above-normal levels.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
A mid-level cloud deck with cigs around 7-9kft AGL has overspread the area to kick off the TAF period, with scattered lower cigs around 3-4kft also being observed at FKL/DUJ. A band of showers over central Ohio will move east into the area this morning, bringing some scattered light rain to areas mainly west of PIT. Not expecting these to make it too far into the area given the very dry sub-cloud air over western PA right now.
Additionally, current upstream obs being impacted by these showers are showing no signs if notable reduction in visibility, so left any impacted terminals in our area at 6SM or better.

However, these showers will be accompanied by the onset of MVFR cigs, which should reach ZZV within the first couple hours of the TAF period and then spread eastward throughout the day, reaching western PA (PIT/AGC/BVI) by 18Z and the rest of the area after 00Z. South to southwest winds will increase through the day as well, with gusts of 20-25 knots likely by this afternoon and lasting into the overnight hours tonight.

OUTLOOK
Restrictions with rain are expected on Sunday, followed by a transition to snow and continued restrictions Sunday night into Monday. A return to VFR is expected Monday night and should persist through much of next week with building high pressure.

HYDROLOGY
Widespread rain of 0.5 to 1.25 inches is expected for most of the Upper Ohio River Valley Saturday night into Sunday. Rapid rises are expected Sunday night due to runoff, especially along the Monongahela Basin. With rivers running 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.

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

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