Area Forecast Discussion
Issued by NWS State College, PA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
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
-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KCTP 072222 AFDCTP Area Forecast Discussion National Weather Service State College PA 522 PM EST Thu Dec 7 2017 .SYNOPSIS... A prolonged period of mainly dry and cold weather will continue into next week. && .NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
-- Changed Discussion --
The single, intense, Lakeshore parallel LES snowband will remain near, or just a few miles to the north of the I-90 corridor in far NW PA until late today when a sfc (and weak upper level) trough will slide east across the region. Slight veering of the mean boundary layer flow in the wake of these features will help to shift the single snowband south and into NW Warren county for a few to svrl hour period late today/early tonight. Painted in a tight gradient of snow probs and accums across the NW half of Warren county. Much of Warren county will see less than an inch of snow, while the highly rural NW part of the county could see between 2-3 inches. There is some uncertainty with respect to the southern extent of this snowband between 23Z Thu and 05Z Friday, but the last several runs of the HRRR and coarser RAP have been fairly consistent in brushing just far NW Warren county with any briefly heavier snow showers. No Winter Weather Advisory planned ATTM for this brief, borderline episode of LES that will impact just a small portion of NW Warren county. Prior to, and after this particular 6 hour window, the single LES band should be over the I-90/NYS Route 17 corridor, and of little (or no) consequence to our NW snowbelt. In the meantime, weak low-mid level ascent upglide ahead of the sfc trough will slowly moisten-up the very dry lowest several KFT AGL, and support a rather compact area of very light snow/flurries spreading ENE across the Laurel Highlands and Central Mtns, where vsbys briefly dipping into the 2-4 SM range could result in a light coating of snow (up to 2-3 tents of an inch) across the Laurels and along the Allegheny front just NW of the I-99 corridor. Throughout the Central Valleys, weak low- level downsloping/drying (and sfc temps in the mid-upper 30s will preclude the chc for any light accumulation. Little or no precip (aside from a brief flurry) is expected across the Lower Susq Valley and much of the NW mtns through 23Z Today. Clouds will be clearing out late in the evening near the I-80 corridor, while skies stay mostly cloudy across the southern Tier counties of PA as layered mid/high clouds thicken up from the SW following the departure of the weak shortwave. Wind will still be gusty from the WSW late this afternoon and evening - sustained between 10-15 mph, with gusts of 20-25 mph.
-- End Changed Discussion --
&& .SHORT TERM /MIDNIGHT THROUGH 6 PM FRIDAY/... The mean boundary layer flow backs to the SW again by shortly after 05Z Friday, causing the persistent band of LES to shift north again into SWRN New York. As previously noted, tonight will be the best chance for snow accums across the far NW zones. Will let it top out at 2 or 3 inches in far NW Warren Co, less than inch elsewhere in the NW mtns. Elk county could even stay flurry-free as the fetch is rather poor to get any bands there. Any flurries and patchy 2-4SM light snow in the southwest/central counties during the evening will dissipate as weak forcing aloft moves off to the east. No additional accums are expected there. Clouds should break up. Mins should be into the teens west and L-M20s elsewhere, but will not drop too drastically with some wind still going through the night. It does seem like the gradient relaxes some on Friday (especially across southern PA) as a ridge of high pressure at the sfc extends from the Ohio River Valley to the Southern New England Coast, so expect there to be a little less westerly wind. Max temps will be 4-7F colder than Thursday`s. The old front off to the SE may continue to push cirrus over the SErn cos, but few other clouds will be found across the central and northern 2/3rds of the CWA. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Central PA will contend with two winter weather systems this weekend which could bring snow to different parts of the area. Temperatures will trend progressively colder through the weekend, with the coldest air of the season gradually taking hold by the middle of next week. The first system is a low pressure wave lifting north along a stalled frontal boundary draped from the Gulf Coast into the western Atlantic. 07/12z models have trended further northwest with the low track in response to a sharper upstream trough and more pronounced ridging downstream. This trend also resulted in a northwest shift in the precip shield which introduces low probs for snow as far west at the I-81 corridor. The 12z ECMWF is the farthest NW with its precip shield vs. GFS/WPC/NBM. The middle ground "edge" is probably somewhere between I-81 and I-95 at this time. Subtle shifts in the low track will make the difference in a potential period of light snow Saturday across the Lower Susquehanna Valley. If models continue to trend NW, snow risk will increase for places such as Harrisburg and Lancaster and may extend to the west of the Susq. River into interior parts of central PA. Continue to monitor the forecast. The second system is a clipper tracking eastward from the Great Lakes and eventually merging with the strengthening storm off the Northeast coast by the second half of the weekend. Snow showers with light accumulations are most likely over the higher terrain of the Alleghenies but may coat the ground quickly into parts of east-central PA. A period of lake effect and orographic snow showers should continue into Sunday with gusty winds and wind chills in the single digits and teens. Lake effect trajectories back to the west-southwest into Monday as the next shortwave digs southeast through the MS/OH Valley. There is a decent model spread in the low track and evolution of this system, but regardless of the details this feature should provide a good opportunity for most of the area to see some snow early next week. The coldest air of the season is forecast to arrive behind this system with a shot of arctic air and more lake effect snow lasting through midweek. && .AVIATION /22Z THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --
Bands of snow showers from the gusty westerly flow will continue through central PA through the first half of the evening. JST is seeing VLIFR cigs and vsbys. The snow bands are wavering though periods of IFR and lower will continue until lifting between 00Z to 03Z. Otherwise, widespread VFR conditions are anticipated for the rest of the night and into tomorrow as another ridge of high pressure will build over the PA airfields behind the cold front tonight. This will bring a diminishing westerly wind, and resulting VFR conditions across most, if not all, of the forecast area. Will have to watch KBFD and KJST, where upslope flow could potentially yield tempo MVFR to IFR cigs tonight into Friday morning. Outlook... Fri...No sig wx expected. Sat...Thickening clouds with late afternoon and nighttime snow showers/vis reductions poss Allegheny Plateau. Sun...AM snow showers/vis reductions poss Allegheny Plateau. Mon...PM light snow possible, mainly northwest Pa. Min night and Tuesday...Periods of snow and snow showers. Widespread MVFR likely, with periods of IFR and brief LIFR cigs and vsbys.
-- End Changed Discussion --
&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Steinbugl AVIATION...Lambert/Ceru

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.