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FXUS61 KPBZ 042322 AAA AFDPBZ

Area Forecast Discussion...UPDATED National Weather Service Pittsburgh PA 722 PM EDT Wed Oct 4 2023

SYNOPSIS
Dry and seasonably warm weather through most of the day Thursday will give way to much cooler temperature and periods of rain through the weekend as a potent upper low crosses north of the region. Seasonably cool weather is favored to continue thereafter under persistent troughing.

NEAR TERM /UNTIL 6 AM THURSDAY MORNING/
No changes needed for the evening update.

Previous discussion...
The initiation of southerly flow aloft ahead of an incoming potent upper trough will promote weak moist advection. The result will be increasing cirrus through the overnight period.
This, along with a more turbulent boundary layer, should mitigate fog formation for most locations. However, low probabilities remain for pre-dawn fog within valleys near the Laurel Highlands where surface moisture will remain more intact and less cirrus coverage will develop.

SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/
An upper level trough will begin to drop out of Canada Thursday towards the western Great Lakes, promoting strengthening southwesterly flow over the Ohio River Valley. The region will initially remain under the influence of high pressure off the Atlantic coast (with southerly flow promoting weak warm advection) while moist advection aloft increases area cloud cover. Expect temperature to be around 10 degrees above the daily average, with potential for strong jet dynamics and mid- level frontogenesis to foster late afternoon showers for the far eastern forecast zones.

Progression eastward of this jet and mid-level frontogenesis appears likely to be slow, though there is model variance in shower band movement. Under the low probability scenario (less than 10% chance likelihood) that weak instability develops along with strong shear, somewhat unidirectional flow to that boundary, and above average PWATs could create localized flood concerns. A more notable eastward push is likely through the day front as the upper trough axis deepens over the western Great Lakes and pushes the initial cold front boundary east.
Again, eastward timing progression varies as well as positioning of dry slot behind the cold front, but the expectation is for a downward PoP trend for Friday night after FROPA.

Beyond the rain, temperature will more closely mirror seasonal averages Friday given area rain and cloud cover; locations farther east are likely to see greater temperature rises though given later cold front arrival. Strengthening low-level jets and surface pressure gradients may allow for breezier conditions through Friday afternoon, but the lack of deeper mixing ahead of the front should limit areawide gustiness.

LONG TERM /SATURDAY THROUGH WEDNESDAY/
There remains high confidence in a prolonged shift in the pattern as the upper level trough positions itself over the Great Lakes region and experiences shortwave movement through its base. This will promote seasonably cool temperature (averaging 10-15 degrees below the average), occasionally gusty afternoon wind (generally remain less than 40 mph), and periodic shower activity Saturday through early next week.
Depending on the degree of cloud cover and shower activity, frost concerns may develop for both the Sunday and Monday morning periods when the cold airmass is more in place.

Regarding the showers, their timing and coverage is the least certain of the forecast through this period as models diverge more notably in upper low shape, shortwave timing, and even additional surface low development/movement. Assuming northwest flow dominates after the axis passes late Saturday, strong cold advection should properly tap warm lake waters to get lake enhancement that may not be fully conveyed in the current forecast. Also of note, depending on whether showers develop far enough south over the higher terrain during overnight hours, it remains possible (less than 30% probability though) that snow mixes in with rain for those elevated locations.

Ensembles favor upper troughing to persist through the middle of next week, though that flow likely will become drier and thin out precipitation chances (save for any lake influence).
Temperature will remain below the seasonal average as well.

AVIATION /00Z THURSDAY THROUGH MONDAY/
High pressure will maintain VFR through Thursday. High clouds will continue to build from the west overnight, making near-dawn fog much less likely than previous mornings.

Widespread IFR cig/vis restrictions are likely to return late Thursday night as a cold front and associated showers cross the area. Restrictions could develop as early as 00Z Friday for ZZV.

OUTLOOK

Residual moisture associated with a lingering upper-lvl low and cold advection under northwest flow will likely keep restrictions ranging from MVFR to IFR through Saturday across the region.

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

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