Area Forecast Discussion
Issued by NWS Pittsburgh, PA
Issued by NWS Pittsburgh, PA
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985 FXUS61 KPBZ 281228 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 828 AM EDT Fri Jun 28 2024 .SYNOPSIS... Quiet weather expected today under high pressure. A warm front brings showers and thunderstorms to region early Saturday. The severe weather increases Saturday and eveing ahead of a cold front. Tornadoes and damaging winds will be the main threat. && .NEAR TERM /THROUGH TONIGHT/... KEY MESSAGES: - Dry weather and light wind expected today. - Rain chances increase tonight with approaching low pressure. ------------------------------------------------------------------ 8:30am Update: No major chances have been made to this forecast period. Surface high pressure shifts eastward off the northeast Atlantic coast today as low pressure upstream advances from the northern Plains into the Great Lakes region. We remain warm and dry through much of today under the departing high pressure, with high temperatures running around 5 degrees above normal. Southerly flow tonight will prompt a warm, moist airmass to return to the region ahead of an approaching low pressure system. Overnight low temperatures will likely range in the upper-60s to low-70s; 10 degrees above the climatological average. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... KEY MESSAGES: - Showers and thunderstorms expected Saturday and Saturday night as a cold front approaches the area. Some storms could become strong to severe. - The primary threats will be damaging winds, hail, and possibly a few tornadoes. - Flash flooding will also be possible due to torrential downpours in the heavier thunderstorms. ------------------------------------------------------------------- Saturday is shaping up to be another potential severe weather day across the region. Low pressure advancing eastward from the upper Great Lakes into southern Ontario will result in southerly flow developing across the Ohio Valley and lower Great Lakes tonight, bringing moisture and showers back into the area. Ongoing convection is anticipated Saturday morning, most likely over eastern Ohio and northwest PA where a weak warm frontal zone will reside along with deeper moisture. Despite the negative impacts this morning round may have on diurnal heating, convective-allowing models continue to suggest moderate instability developing by mid to late afternoon, with HREF probabilities for 1000+ J/kg SBCAPE exceeding 60% areawide. Meanwhile, deep-layer bulk shear profiles will be supportive of organized storm modes (sfc-6km around 40kts), including bowing segments and possibly supercells. What catches my eye when interrogating model-derived soundings for Saturday afternoon is how they compare to Wednesday`s setup. There are striking similarities between the two, most notably the fact that instability and wind shear are packed in the lowest ~4km of the atmosphere, the former being driven by steep 850-700mb lapse rates and the latter by jets at those two respective levels overspreading the region ahead of the advancing cold front. These two factors combined could support strong low-level updrafts where your best streamwise vorticity will also reside, leading to low-level mesocyclones that can rapidly spin up and locally enhance tornado potential. We saw this manifest in the QLCS that moved through southwest PA on Wednesday, which produced 4 confirmed tornadoes. This will be something to watch closely as this system evolves. The Storm Prediction Center has highlighted this local enhanced tornado threat with a 5% tornado probability contour extending across portions of western PA and eastern OH (the rest of the region remains in a 2% probability area). The final threat to discuss for Saturday`s convection is flash flooding. The setup may be such that there is training, or at least multiple rounds, of thunderstorms in some areas (particularly north and west of Pittsburgh) at least initially. Eventually, the cold front will sweep through and storm motion will become more progressive before ending altogether behind the front. However, ahead of the front, corfidi vectors decrease to 5-10 knots in a moisture-rich environment characterized by PWATS exceeding 2 inches, nearly an inch above normal levels for this time of year. These factors, along with abundant buoyancy and warm cloud depths >13kft supporting warm rain processes, are an indicator for slow-moving or backbuilding thunderstorms capable of producing torrential downpours. The Weather Prediction Center has highlighted much of the area in a Marginal Risk (Level 1 of 5) for excessive rainfall leading to flooding, with the low-end risk reflecting the isolated nature of flash flooding and its dependence on thunderstorms. If the pattern evolves such that a more widespread coverage is expected, an upgrade to a Slight Risk (Level 2 of 4) is certainly not out of the question. && .LONG TERM /SUNDAY THROUGH THURSDAY/... KEY MESSAGES: - Dry weather expected Sunday through mid week under building high pressure. - Showers and thunderstorms return Thursday with another low pressure system. ------------------------------------------------------------------- A cold front sweeps through the area late Saturday night or early Sunday morning, bringing an end to showers and thunderstorms and ushering in a drier airmass. This will bring dry weather and a slight cool down Sunday and Monday with temperatures returning to normal or just below normal levels for this time of year. Dry weather persists through Wednesday under high pressure. The upper ridge then pushes off to the east Wednesday night and low pressure moves back into the Great Lakes region, bringing showers and thunderstorms back to the area on Thursday. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... River valley fog has dissipated early this morning, winds will veer south- southeasterly with the development of VFR diurnal cu. Upper clouds increase overnight with some small chances of rain west of the mountains, primarily after 04Z. .Outlook... Restrictions are likely early Sat into late Sat night with showers and thunderstorms ahead of and along a crossing cold front. Some storms may be severe. VFR returns Sun under high pressure and is likely to persist Mon and Tue. && .PBZ WATCHES/WARNINGS/ADVISORIES... PA...None. OH...None. WV...None. && $$ SYNOPSIS...Hefferan NEAR TERM...Cermak/Hefferan SHORT TERM...Cermak/Craven LONG TERM...Cermak/Craven AVIATION...Milcarek/Hefferan