Area Forecast Discussion
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793 FXUS61 KPBZ 212209 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 509 PM EST Sat Jan 21 2017 .SYNOPSIS... Mild weather will continue through the weekend. Rain will move back over the area on Sunday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Mid and high clouds have overspread the region as of 5 PM, with model indications of a fair amount of lift entering with a mid- level wave generally at or above 20 kft. However, a chasm in the RH field below this level has yielded little but virga as of this hour, and as the wave ejects northeastward this evening, all signs are pointing at this being the most likely continuing scenario. Thus, showers were replaced with sprinkles in the forecast. Behind this early evening wave, satellite imagery indicates a brief but rather abrupt period of clearing is translating northeastward. Hi-res guidance has some handle on this with sky cover backing off considerably centered around 03z. Skies were thus cut for all locations for a period of 2-4 hours from SW to NE as this area of clearing moves through. With still high surface dewpoints, this may yield a decent period of radiational cooling and thus some patchy fog development as it does so likewise. Behind the clearing, mid and high clouds look to increase again, however decent lift really does not enter the picture from the south again until well into the daytime on Sunday, so the remainder of the overnight will be dry. Fries
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/... Model depictions of the cutoff low pressure set to dig toward the Upper Ohio Region have deviated from previous days solutions with a more westward track of the center. Increasing ascent via deformed flow and warm advection with increased moisture through the mid levels all support escalating precip probabilities to categorical numbers for the period. With the northern branch of split flow holding colder air north and west of the region, and with warm advection into the mid levels of the cutoff, this looks to remain a rain event although the latest NAM has developed sub-zero air over the region at the 850mb level despite moist advection and an otherwise warm column. QPF was progged using a RFC/WPC blend for the period. An inch to inch and a half is anticipated for the early week with limitations imposed by low pressure progression and mid level dry slotting. Relative warmth will continue, although the diurnal temperature spread will be restricted by clouds and precip. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... The trend of the a slow low-pressure-exit on Tuesday continued in todays deterministic and ensemble depictions. Colder air wrapping into the western flank of the departing low may thus turn rain to snow, before ridging ensues in response to plains low pressure. That system is forecast to interrupt the resulting temperature moderation via a cold front across the Upper Ohio, which will drive temperature back to seasonal readings for the latter half of the week. && .AVIATION /22Z SATURDAY THROUGH THURSDAY/... VFR will prevail as high clouds overspread the region ahead of the next low pressure system, progged to approach Sunday. While the latest sounding data would suggest otherwise, it appears that high clouds should limit the development of fog across the region. That being said, the newest run of the hrrr generates a bit more precipitation with an overnight advancing shortwave than previous runs. Will have to continue to monitor trends to see if boundary layer and sky conditions begin to support more widespread fog. Otherwise, widespread restrictions will return by late tomorrow as the next system moves toward the region. Wind will remain light, out of the s-se through the period. .OUTLOOK... More restrictions are likely through Monday with slow moving low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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