textproduct: Pittsburgh

SYNOPSIS

A frontal passage on Sunday brings a bit higher risk for severe storms and isolated flash flooding, with muted heat. Chance of rain with a post-frontal trough on Monday and relatively dry conditions on Tuesday. By midweek, heat, flooding and severe weather concerns return to the region.

NEAR TERM /THROUGH TONIGHT/

KEY MESSAGES:

- Slightly higher flash flooding threat Sunday; marginal severe risk to the east of Pittsburgh. - Temperatures continue to run above seasonal levels. - Patchy fog development possible Sunday night in areas that see rainfall during the day.

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The upper ridge that previously sat along the eastern seaboard pushes east over the northern Atlantic on Sunday in response to longwave troughing setting up to the west over the Great Lakes. This shift puts the local area under a region of more unsettled southwest flow, with height falls providing slightly more synoptic ascent/forcing than the last couple days which actually saw weak height rises and associated subsidence beneath the east coast ridge. This translates to higher confidence in scattered shower and thunderstorm activity across the region today, with the greatest chances coinciding with peak heating during afternoon and evening hours. Unlike yesterday when most of the thunderstorm activity occurred to our west over the lower Ohio Valley, on Sunday the best environment for convection will be across western PA and northern WV where the HREF suggests high probabilities (~90% chance) of at least 1000 J/kg SBCAPE. Convective-allowing models also suggest an area of roughly 600-800 J/kg DCAPE values across the same areas, which supports a low-end threat for damaging wind gusts in thunderstorm downbursts. The Storm Prediction Center has maintained its Marginal Risk (Level 1 out of 5) for severe weather generally east of I-79 for this reason.

Additionally, HREF probabilities for PWATs above 1.6" are high (>70%), which coupled with slow storm motions (<20 kts deep layer mean flow) will support a localized flood threat in slow- moving heavy downpours, especially for urban and poor-drainage areas. The Weather Prediction Center has maintained its Marginal Risk (level 1 out of 4) for excessive rainfall over most of the area east of I-77, with a Slight Risk (level 2 out of 4) even clipping eastern Indiana and Westmoreland counties where upslope flow could help focus/anchor thunderstorms over the ridges and lead to locally higher rainfall totals.

Convection will taper after sunset with the loss of daytime heating, followed by a quieter (but still warm and muggy) Sunday night period. Patchy fog will be possible in places that see accumulating rainfall during the day.

SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/

KEY MESSAGES:

- Lesser storm/shower chances Monday afternoon, mainly along the Appalachian ridges. - Mostly dry and warm Tuesday. - West Virginia could see some showers on Tuesday. ----------------------------------------------------------------

Tomorrow, a shortwave trough will move through the area with the axis expected to be east of Pittsburgh by the time of peak heating. The best chances for scattered showers/storms are along the ridges as well as southwestern PA and parts of norther WV. There appears to be a low probability for isolated severe or flooding threat. If the trough has a slower timing, this could increase these threats in our area. Temperatures remain in the mid to upper 80s (about 5F above normal) with a very modest air mass change behind the Sunday front.

Rain chances decrease into Tuesday with the arrival of surface high pressure. This will likely bring the region light winds with a slight decline in dew points and mostly clear skies. This should keep conditions pleasant despite general upper ridging and high temperatures above normal. Areas in WV are now looking to have a high probability of some rain due to a low level shortwave trough over western Virginia.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

KEY MESSAGES:

- Heat Wednesday and Thursday - Rain chances increasing Thursday into the weekend.

------------------------------------------------------------------- The area of high pressure will begin to gradually move eastward off the Atlantic coast, allowing the return of warm and moist southwest flow. This will likely result in Wednesday being the hottest day of the work-week with overnight lows staying elevated Thursday morning. Heat Risk tops out at "Major" levels with the most recent update. There is still about four degrees of temperature uncertainty Wednesday related to precipitation chances and cloud cover.

Depending on the speed of a trough passage and cloud cover trends, temperature spread increases Thursday. Thursday has the potential to be comparable to Wednesday's heat, although most ensembles have more cloud coverage and the best chance of rain this week due to a shortwave skirting south of the main surface front. Wednesday and Thursday also have the highest proportional chances of severe weather and flooding as indicated by machine learning, and CIPS analogs.

Rain chances linger through Friday as the probability for the main surface front passage increases, though at this point, there are varying solutions in clustered guidance with some solutions suggesting ridging in the upper Great Lakes which would maintain near 588dm heights and warmth, but some show trough development which might sweep downstream and return temperatures close to normal.

AVIATION /08Z SUNDAY THROUGH THURSDAY/

VFR conditions prevail at all area terminals to start the TAF period. Aside from passing mid and high clouds, conditions should remain quiet with only some patchy fog possible towards sunrise, particularly in areas that saw accumulating rainfall during the day Saturday.

A cold front brings higher precipitation chances to the area Sunday afternoon. The best coverage likely occurs after 18z, though some isolated morning showers could bring rain (and possibly a few rumbles of thunder) to terminals earlier than that. VFR will prevail outside of showers/storms, with brief restrictions possible in any precipitation. Some storms could become strong and will be capable of producing damaging wind gusts and frequent lightning. Convective activity will again generally follow the summertime diurnal cycle, tapering after sunset with the loss of daytime heating and giving way to quieter weather Sunday night.

Outlook... Ensemble models favor high pressure that drastically limits convective potential Monday/Tuesday (but localized morning fog can't be ruled out). Precipitation and restrictions become more likely on Wednesday and Thursday as southwest flow and moisture increase.

PBZ WATCHES/WARNINGS/ADVISORIES

PA...None. OH...None. WV...None.