Area Forecast Discussion
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000 FXUS61 KPBZ 111746 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1246 PM EST Thu Jan 11 2018 .SYNOPSIS... A very warm Thursday will transition into an unsettled end of the work week. Much colder temperatures will arrive Friday afternoon with a quick changeover to wintry precipitation. && .NEAR TERM /THROUGH TONIGHT/... Warm advection is under way with values already into the 50`s and 60s. Expect this to continue through the afternoon and evening as 850hpa temperatures approach +10C. In addition to the warm temperature, rain will make it`s approach as a shortwave rides northeastward along the departing ridge over the Atlantic. A Gulf moisture feed and mid-level forcing will support periods of heavier rain through the night, with the axis over the upper Ohio Valley. PWAT values will be on the high side climatologically, and the combination of QPF and snowmelt does lend to some concerns for localized flooding, especially on the tributaries that remain ice covered. As such, a flood watch is being considered, and will be likely issued this afternoon. Rain will persist through the night - which warrants categorical PoPs. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... All eyes then turn toward the storm system later Friday through Saturday. Model solutions have trended together a bit more with a decent large scale consensus centered around the ECMWF and GEM. As the Mississippi Valley trough ejects northeast through Saturday, most models track a series of weak surface lows up the Appalachians. Trends that have favored more rapid evolution of the cold air below 850 mb have become fairly uniform, however a favored 850 mb low track through the eastern half of the CWA will mean that cold air will have difficulty becoming very deep until after 06z Saturday or so. This will mean that the transitional phase precipitation may become problematic in a SW- NE corridor that will likely include the Zanesville-Wheeling- Pittsburgh-Punxsutawney corridor. While rain will envelop the region Friday morning, an earlier arrival of low level cold air will mean changeovers to freezing rain and sleet will already be evident in Ohio at least by 18z Friday. And the consensus favors a rapid surge eastward toward Pittsburgh by 00z Saturday. All areas look to finally get into deeper cold air by 12Z Saturday, which would mean a changeover to all snow everywhere by that juncture. However, this leaves a very long interlude over eastern Ohio toward western PA, especially north of Wheeling and Pittsburgh, during which freezing and frozen precipitation are likely to be the dominant p-type. Further dissection of thermal fields yields some concern for a long duration sleet event over a portion of the area immediately southeast of where cold air should become deep enough to support snow upon the arrival of impressive deformation in the 850-700 mb layer in the 00z-06z Saturday time frame. Further, some banding of snowfall appears likely, and a backbuilding of the right entrance region of an anti- cyclonically curved upper jet streak would not favor an exceedingly rapid departure of the system to the NE. With all of this in mind, winter storm watches have been issued. While only one segment was included in the watch, generally from Columbiana to Forest County and NW, it would seem snow accumulation will be the largest impact. Farther to the SE from there, while at least moderate snow accumulation is likely after the changeover, thermal fields yield enough concern to issue a winter storm watch for a combination of sleet and snow combined with the fact that the vast majority of the heaviest winter precipitation is likely to fall in a window late Friday evening. Across the rest of the area, widespread advisories still look like a good bet, however considerable uncertainty remains as to how lift and cold advection will overlay as to what the most problematic p-types will be. We still have more than 36 hours to sort it out, hence no additional headlines were issued at this time. As the system rolls out of town on Saturday, precipitation chances rapidly edge down from the SW. Much drier Arctic air quickly makes inroads. Due northerly flow will allow for this to take residence overnight Saturday night into Sunday, allowing for lows to challenge 0F and highs to be slow to recover on Sunday. Fries && .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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Upper troughing and below normal temperatures will persist across the eastern CONUS through the middle of next week. Sunday figures to be dry, but a shortwave may drop through the trough to bring snow chances by Monday, with snow showers lingering thereafter in northwest flow.
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&& .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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VFR conditions will continue through the afternoon. Approaching low pressure will result in deterioration through MVFR to IFR during the evening hours, with periods of moderate to heavy rain possible. Low level wind shear concerns will continue into the evening as well, until a low level jet weakens towards morning. IFR/low MVFR conditions will linger through midday Friday as precipitation temporarily eases a bit. Colder air, and the resulting changeover to mixed precipitation and snow, will mainly occur after 18Z Friday. .OUTLOOK... Restrictions can be expected through Saturday as a cold front changes rain to sleet and freezing rain, and then snow.
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&& .PBZ WATCHES/WARNINGS/ADVISORIES...
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MD...None. OH...Flood Watch from 7 PM EST this evening through Saturday afternoon for OHZ039>041-048>050-057>059-068-069. Winter Storm Watch from Friday afternoon through Saturday afternoon for OHZ039>041-048>050-057>059. PA...Flood Watch from 7 PM EST this evening through Saturday afternoon for PAZ007>009-013>016-020>023-029-031-073>076. Winter Storm Watch from Friday afternoon through Saturday afternoon for PAZ007>009-013>016-020>022-029. WV...Flood Watch from 7 PM EST this evening through Saturday afternoon for WVZ001>004-012-021-509>512. Winter Storm Watch from Friday afternoon through Saturday afternoon for WVZ001>003.
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&& $$

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