Area Forecast Discussion
Issued by NWS

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KPBZ 121233 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 733 AM EST Tue Dec 12 2017 .SYNOPSIS... Much colder weather arrives today along with snow. Heavy lake effect snow showers will be possible through mid-week. Another system early Thursday will bring additional accumulation. A brief warm up follows this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --
As of 7:30am, bands of heavier snow showers are tracking across Allegheny County from west to east. Colder air is moving in behind these bands, which will allow for accumulation to begin picking up. Previous discussion follows. Surface low pressure is currently tracking through the area with quite impressive warm advection ahead of it in a well-mixed and remarkably dry column. This changes quickly with the approach and passage of the low as evaporational cooling and cold advection start quickly with its passage. Thus, most all areas will be seeing their highs for the day in the next hour or two before summarily slowly falling this morning then falling remarkably quicker this afternoon as a secondary frontal boundary surges through the area. Large scale deformation around the surface low tracking through is spreading snow across areas north of its track this morning largely in line with previous expectations, even if the snow got started a bit later than previously projected. Temperatures in the deformation zone quickly fall toward and below freezing with precipitation onset, and given weak snow-to-liquid ratios through the start of the snowfall that have always been expected, snow accumulations from the system itself this morning should largely be relegated to areas north of Pittsburgh. Things rapidly change as the Arctic boundary pushes into the area as the last vestiges of deformational ascent depart to the east. Rapid cold advection will work to quickly destabilize the column and saturate the dendritic growth zone. This is evidenced by the linear band of snow activity pushing southeast of the Michigan/Ohio border, and it will continue southeastward through our area today. As it passes, a burst of snow in a more unstable dendritic growth regime will be likely and rapidly cooling temperatures along the much stronger wind gusts will follow in its wake. This boundary would seem likely to drop up to an inch in just about every location in the lower elevations it passes, but the calculus looks to change dramatically as it intercepts the terrain. As boundary layer moisture gets progressively sandwiched into the terrain with the incoming boundary, rapid cooling aloft and upslope WNW flow will mean a drastically increasing snowfall intensity late this morning. A period of heavy upslope snow showers is likely as the -18C isotherm falls toward the ridgetops and wind gusts increase toward 40 mph. It is possible that local blizzard conditions will result, particularly in Garrett/Preston/Tucker Counties, however the duration of heavy snowfall should only continue through the afternoon as once the immediate post-frontal instability effects are exhausted, flow looks to be too westerly for a decent moisture feed toward the terrain. Thus, the winter weather advisory has been maintained, even with high snow rates expected through the daytime today, as they should fall off tonight. Fries
-- End Changed Discussion --
&& .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/... Right as the moisture into the ridges starts to become exhausted, yet another mid-level short wave tracks southward across Lake Erie, which will turn our eyes farther north for the overnight hours. An increase in inversion heights and reorientation of the flow over basically all of the Great Lakes starts to occur tonight. A multi-lake fetch from Lake Huron over eastern Lake Erie is probably the moist poignant feature to follow. In this area, with inversion heights running up over 11 kft and lake-induced troughing downstream from the lake increasing boundary layer friction effects, a single dominant band is suggested by both large scale and meso-models that tracks into the northern CWA overnight into Wednesday morning. The models are showing good consistency with this feature, and previous discussions and headlines have captured the threat quite realistically with the meteorology being rehashed ad nauseum. Timing on tongiht`s runs is similar, and thus tonight still looks like the highest accumulation period for Venango/Forest/northern Clarion. Elsewhere tonight, again two other bands seem to be showing up in large scale model guidances with remarkable persistence such that incorporating them into the forecast became a bit more necessary. The first and probably most impactful of these is a Superior-northern Michigan-western Erie band with a trajectory over northern Ohio toward Columbiana, Beaver, Allegheny, and Westmoreland Counties. Given the long overland trajectory from the lake, it would seem substantial accumulations would be unlikely, however the persistence in this band in guidance coupled with an impressive instability profile have resulted in PoPs being increased along this corridor with general snow expectations kicked up about an inch from previous forecasts. This includes the Pittsburgh area. Given the first dominant banding position, while upslope snows should be fairly minimized tonight into Wednesday for most of the ridge locations, this many not end up being the case for the ridges in Westmoreland County. Here again, the evolution of this band will need to be evaluated, as any persistence of it combined with upslope flow will necessitate an upgrade of the current advisory to a warning. The second band is the shore-parallel band that is currently extending down the eastern shore of Lake Michigan that models across suggest will traverse all the way into central Ohio and WV as the Arctic front surges southeast by tonight. While the consensus suggests this band will lie just to the south of our CWA, it seemed worth mentioning here as it may play a role in the forecast depending upon low-level wind evolution. While flow starts to turn more southwesterly by Wednesday afternoon, lake effect bands will quickly transition to the northeast and out of the area. However, as warm advection begins in earnest ahead of the next system dropping toward the area for Wednesday night, isentropic ascent in the 290-300K layer and strong southwesterly moisture transport will rapidly increase a field of snowfall from west to east across the area. This will be in advance of a surface low that the model consensus tracks southeastward from the western Great Lakes toward the PA/WV border during Wednesday night. Some disagreement exists on the strength of the system, with the NAM persisting a closed low- level center and thus stronger deformation than the GFS/EC/Canadian, however indications are that at least modest deformation/frontogenetical light will follow the isentropic ascent such that a maximized swath of QPF will be likely from northern Ohio through western PA toward the Alleghenies. Lifting and moisture profiles suggest a much deeper moist layer and a far higher dendritic growth region. Thus, snow-to-liquid ratios will not be as impressive as earlier in the week, however a gradation from 15 to 10 from N to S is suggested, and as such a swath of winter weather advisories may be necessary as this system dives southeastward. These will need to be covered in future forecast updates, however, as ongoing headlines preclude issuing them at the moment for the sake of clarity. Fries && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Brief cold advection follows behind the departure of Wednesday night/Thursday`s system. This will be short-lived, however, as 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. However, all signs continue to point toward the reorganization of the upper trough over our region sometime early next work week with the return of below normal conditions shortly after the end of the current forecast. Fries && .AVIATION /13Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
General MVFR conditions today, with IFR expected over FKL and DUJ. Frontal snow will move through this morning, which will provide general MVFR restrictions, with IFR possible at times, again to the north. Expect some improvement later today outside of the terminals impacted by lake-enhanced snow but general MVFR should prevail. Flow will swing to the wnw with gusts to near 30kts. .Outlook... Periodic restrictions in snow showers and northwest flow will continue through Wednesday. Another system will bring more snow and restrictions to many of the terminals Thursday.
-- End Changed Discussion --
&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...Winter Weather Advisory until 7 AM EST Wednesday for MDZ001. Wind Chill Advisory from 7 PM this evening to 1 PM EST Wednesday for MDZ001. OH...None. PA...Winter Weather Advisory until 1 PM EST Wednesday for PAZ007- 016. Lake Effect Snow Warning until 1 PM EST Wednesday for PAZ008- 009-015. Winter Weather Advisory until 7 AM EST Wednesday for PAZ023- 074-076. WV...Winter Weather Advisory until 7 AM EST Wednesday for WVZ510>514. Wind Chill Advisory from 7 PM this evening to 1 PM EST Wednesday for WVZ512>514. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.