Area Forecast Discussion
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000 FXUS61 KPBZ 181759 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1259 PM EST Sat Nov 18 2017 .SYNOPSIS... Periods of rain along with a few isolated thunderstorms expected today as low pressure moves towards the region. A strong cold front will cross tonight bringing strong winds and a changeover to snow showers by early Sunday morning. && .NEAR TERM /THROUGH TONIGHT/... Elected to issue a Flood Watch into tonight for the northern portions of the CWA. Areas of moderate to heavy rain have lined up along and north of a CAPE gradient in place across OH/IN/IL. Last few runs of the HRRR have been fairly consistent in bringing 1 to 2 inches of additional rainfall from roughly Pittsburgh on north, consistent with some other hi-res guidance. Much of the watch area has already received 0.75 to 1.25 inches of rain, and fear that the additional QPF could cause flooding. Went with the generic Flood Watch given the overall long-term nature of the rain, but cannot rule out Flash Flood Warning issuances in any heavier convective elements. Ran the watch through and an hour or two past the cold frontal passage tonight. Made some adjustments to temps/PoPs in the grids. Previous discussion... A brief lull in the precipitation late this afternoon into early evening before the associated cold front begins to sweep across the forecast area. Strong winds are expected to accompany the front as it crosses and behind with steepening lapse rates in cold advection. Based off of analogs and models guidance max gusts look to largely remain below advisory criteria outside of the higher elevations. That being said...the majority of the area WILL see at least a brief window of winds approaching criteria...up to 45mph...higher in a few isolated locations. Have issued an advisory for the ridges overnight but opted to not go blanket the entire forecast area with an advisory based analogs, guidance, and known biases of momentum transfer during high wind events for the forecast area. Total precipitation amounts have not changed much from the previous forecast, ranging from just under an inch south of Pittsburgh...increasing as you head to the north...to just under 2 inches north of I-80. Strong warm air advection ahead of the front will result in temperatures climbing to between 55 and 60F on average across the region...holding steady into the evening until frontal passage. && .SHORT TERM /SUNDAY THROUGH MONDAY/... The upper trough will pivot across the region Sunday with a changeover to snow at most locations by daybreak. Northwesterly flow will continue scattered snow showers and keep temperatures in the 30s through the day. Highest PoPs remain the snow belt north of I-80 and in the mountains of PA/WV/MD where lake enhancement/upslope is anticipated. Still thinking 2-3 is reasonable, though some higher amounts may be possible. Depth of saturation will quickly fall below the prime snow growth zone in building subsidence in the terrain, which should keep rates down. Snow showers will continue through Sunday night as another reinforcing shortwave digs across the region. Conditions should begin to improve on Monday, as a surface high builds eastward and winds back toward the west. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... Minimal change was needed in the extended. The majority of the week will be dry with seasonal temperatures outside of a weak wave progged to pass to the north midweek. Have kept slight chances in for locations north of I80 on Wednesday. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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Widespread MVFR at the start of the TAF period, save for near MGW in the warm sector, and IFR near/north of I-80. Expecting general MVFR to continue through the frontal passage tonight. Still a brief period of MVFR could creep as far north as PIT/AGC later this afternoon, and likewise brief IFR conditions will be possible in any heavier shower. Low level wind shear remains a concern this afternoon as strong winds remain trapped above a surface-based inversion. Stronger gusts are expected to reach the surface later this evening and overnight with the front. Peaks of 30-40 knots are expected in general, with some gusts up to around 45 knots expected on the ridges. Behind the front, MVFR conditions will linger. Rain showers will mix with and change to snow showers with time, with coverage tapering. Gusty winds will continue out of the west and northwest behind the front. .Outlook... Restrictions are likely with Sun/Sun night upper troughing and cold NW flow.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES...
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MD...Wind Advisory from 7 PM this evening to 10 AM EST Sunday for MDZ001. OH...Flood Watch until midnight EST tonight for OHZ039>041-048>050. PA...Flood Watch through late tonight for PAZ007>009-013>016- 020>023-029-073-074. Wind Advisory from 7 PM this evening to 10 AM EST Sunday for PAZ074-076. WV...Flood Watch until midnight EST tonight for WVZ001-002. Wind Advisory from 7 PM this evening to 10 AM EST Sunday for WVZ510>514.
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