Area Forecast Discussion
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002 FXUS61 KPBZ 131514 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1014 AM EST Wed Dec 13 2017 .SYNOPSIS... A widespread snow event is expected to bring accumulating snow to the region through tonight. Below normal temperatures will continue until we return to near normal for the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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Lake effect activity is slowly diminishing as northwest flow is cut off by increasing southwest flow ahead of the next system. Snow is continuing at FKL and DUJ currently, but expect this activity to mostly shut off by noon, just as isentropic lift starts to develop snow across Ohio. Have made some minor adjustments to PoPs and temperatures, but have left forecast largely intact for now. Still seeing a few wind chill values below -10F on the ridges, so will allow advisory to continue for now. Quick peek at 12Z NAM shows surface low track similar to previous runs, so max snow placement generally north of Pittsburgh still looks good at first glance. Previous discussion... All eyes then turn toward the northwest as a fast-moving clipper system approaches the area. Warm advection ahead of it should start to develop snow in isentropic ascent. However, dry air will be encroaching rapidly from the southwest in a large scale dry slot associated with the system. What is interesting is that it remains saturated below 700 mb in this dry slot, however the warm advection into the dry slot brings the temperatures at 700 mb above -12C, thus precluding efficient crystal development. Thus, it seems as the system dives southeast through the area, there will be a fairly stark diving line between accumulating snow and essentially dry conditions. With deeper moisture farther north and better deformation lift to the north of the surface low track, and a more optimal overlay of the dendritic growth layer with the moisture and lift profile, a very efficient snow seems to be in the cards. Models have really trended quite closely together with regard to the surface track of the system from around Fort Wayne, IN, at 7 PM to just west of Pittsburgh at 1 AM tonight with reorganization offshore quickly thereafter. That said, optimal forcing for ascent being north of this track and consensus favoring roughly 0.2-0.25 inches of QPF with roughly 20:1 ratios means a swath of 4-5 inches of snow centered on the U.S. 422 corridor. Amounts to the south of there will taper quickly, while to the north, they will taper far more slowly. That said, winter weather advisories were issued from Columbiana County eastward to Indiana County to focus the attention on the heaviest snowfall. This does not mean it will not snow in metropolitan Pittsburgh, however. But a less efficient profile, substantial dry air aloft, and less consistent forcing will make widespread 3 inches amounts tough to muster. As the surface low tracks eastward, moisture and backside deformation wrap in a period of snow showers for all locations by tonight. This may be the only measurable snowfall that the southernmost locations in the CWA see during the event. That said, large scale deformation combined with a period of upslope into the ridges could accumulate a quick 3 inches there, so winter weather advisories have been hoisted for the terrain. Fries
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&& .SHORT TERM /THURSDAY THROUGH SATURDAY/... With the formation of the coastal system, isallobaric flow across the terrain again increases through Thursday morning. This will cause wind gusts to extend from Wednesday night into Thursday for the ridges. However, because of the location of the formation of the coastal low, geostrophic flow will not favor much more than a short window of lake effect snow showers in the wake of this system. As a result, things will trend dry pretty quickly on Thursday in continental cold advection. Dry weather continues until at least Friday when a weak system moves through the central Great Lakes. Warm advection ahead of it is rather weak, and forcing is rather weak as well. Thus, just a few snow showers are expected. Behind that system, dry weather returns on Saturday in nearly neutral thermal advection. Fries && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... A general retrogression of the upper trough toward the high plains seems in store for next weekend. This will result in more normal temperatures returning to the region, but with the threat of a southern jet branch storm that should bring primarily rain across the area on Sunday. && .AVIATION /15Z WEDNESDAY THROUGH SUNDAY/... Restrictions this morning will be confined to lingering snow bands, have decided on a general VFR forecast. Included the mention of restrictions at FKL and DUJ. Winds will pickup again this morning, and swing toward the south. Gusts to 20kts are possible. MVFR cigs will begin to overspread the area late in the day and into the evening hours, as the next system rushes toward the area. Snow showers will move in as well, and provide restrictions to visibility, but these could be confined to ports from PIT northward. .Outlook... Next chance for restrictions is Friday into Saturday. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...Winter Weather Advisory from 7 PM this evening to 10 AM EST Thursday for MDZ001. Wind Chill Advisory until 1 PM EST this afternoon for MDZ001. OH...Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Thursday for OHZ041. PA...Winter Weather Advisory from 4 PM this afternoon to 7 AM EST Thursday for PAZ007>009-013>016-020-022-023-074. Winter Weather Advisory from 7 PM this evening to 10 AM EST Thursday for PAZ076. WV...Winter Weather Advisory from 7 PM this evening to 10 AM EST Thursday for WVZ512-514. Wind Chill Advisory until 1 PM EST this afternoon for WVZ512>514. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.