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FXUS61 KCTP 110431 AFDCTP

Area Forecast Discussion National Weather Service State College PA 1131 PM EST Sun Dec 10 2023

SYNOPSIS
An early December storm will bring soaking rainfall of 1 to 2 inches followed by elevation driven wet snow accumulation and gusty winds into Monday, before tapering off later Monday.
As the main storm starts to pull away on Monday, seasonably colder air and locally heavy snow showers forming downwind of Lake Erie will impact parts of the western and northern Alleghenies through Monday afternoon before shifting into southwest NY Monday night. The weather pattern from Tuesday through Friday will feature seasonal temperatures and high confidence for mainly dry weather across much of the Commonwealth.

NEAR TERM /UNTIL 6 AM MONDAY MORNING/
Late evening update.

Bands of snow now over the western areas of the central PA, will continue to spread eastward overnight. Much of the area still above freezing, issued a SPS for areas near or just below freezing not in the advisory area.

Left the flood watch up, as still a lot of moderate rain over the east.

Early evening update.

Patchy light snow has pushed into the far western areas to include BFD and JST. Adjusted temperatures for the next few hours.

Will continue the flood watch for the far east for now, given returns on the radar.

Heaviest convection will stay just southeast of our area by a few miles.

Earlier information below.

Late afternoon update.

Current fcst in good shape, wind shift into the west with temperatures starting to fall. Still above freezing back there. Will continue to monitor trends.

Earlier discussion below.

Southern stream trough situated over the lower MS Valley at midday will take on a negative tilt over the mid Atlantic by tomorrow morning as it digs through the central Gulf Coast and zips north/east over the Eastern Seaboard tonight into Monday.
Favorable coupled upper jet dynamics/strong diffluence aloft and 100-150m 500mb height falls will spawn an elongate wave of low pressure along slowing sfc front located just east of State College at 17Z. As the low deepens tracking to the northeast, strong CAA behind the front amid a sufficiently moist column will transition rain to snow from west to east this evening and overnight.

Rainfall totals have continued a lower/downward trend (<2 inches) with higher values shifting slight to the east over the last few model runs. Rate of ptype changeover to snow has also increased with colder air arriving sooner with each successive model run. Flood watch continues only for Schuylkill County at this time.

Colder air will pour into the area behind the front tonight accompanying an increasingly gusty NW wind. This will result in a rain to snow transition from west to east across the entire CWA between 00-12Z Monday. There remains some uncertainty with how the interplay between moisture and colder air materializes.
Recent high-resolution guidance favors little to no snow west of the I-99 corridor and more widespread accumulation to the east.

Elevation will be a significant delineating factor for snow accums. Official expected forecast shows at least few inches (borderline advisory) the from the Laurel Highlands into the Endless mtns/western Poconos. The most consistent signal for advisory level snow has been over Sullivan County, where 4"+ appear possible, esp AOA 1800 ft MSL.

We issued a winter wx advisory late this morning for Sullivan, Columbia, and Schuylkill Counties with 1-2" forecast in the valleys and 3-5" in the surrounding higher elevations AOA 1500ft.

It's possible some of the highest terrain over east-central PA picks up a couple inches with a large spread or gradient between ridgetop and valley locations. Snowfall amounts have trended lower over the NW mtns (with weaker ensemble QPF signal for the synoptic snow), but we raised amounts slightly across the eastern part of the forecast area.

Peak wind gusts late tonight into Monday (especially later Monday morning and Monday afternoon as the colder air dramatically deepens and taps stronger winds aloft) are forecast in the 35 to 40 mph range (just under Wind Advisory Criteria).

SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/
Late afternoon update.

Did end snow showers across the northwest Monday evening a few hours earlier, to fit better with others and guidance.

Earlier discussion below.

As winds become northwesterly behind the exiting system, lake effect snow showers are likely on Monday before fading into Monday night. High-resolution guidance indicates the potential for lake effect streams to reach as far southeast as the I-80/I-99 corridor. Sufficient instability could also lead to an increase risk of snow squalls. The Snow Squall Parameter lights up northwest and Central PA downwind of Lake Erie during the day on Monday. Additional accumulation of a dusting to an inch or two is possible with lake effect snow during the day on Monday.

LONG TERM /TUESDAY THROUGH SUNDAY/
Late evening update.

Some chance of some light rain or snow later next weekend, but models diverge on the large scale pattern as one gets into early next week. Question is does an upper level trough interact with a warm core low across the far south.

Otherwise the fcst is dry after later Monday.

Earlier discussion below.

High pressure will provide an extended stretch of dry weather through the end of the work week. There does look to be one shot of reinforcement to the cold air Tues PM which could make some SHSN in the nrn mtns. So, while temps will moderate a few degs on Tuesday to maxes of 35-45F, we'll drop back 3-5F on Wed.
Then, temps crawl back up into the 40s for Fri.

The next significant chance for precip is Sunday (Day7). GFS and GEFS mean of solutions brew up a Miller Type-A coastal/storm later in the week over the nrn GOMEX and then roll it across FL and up the East Coast. The temps locally will be 5-10F milder than normal for mid-Dec on Sat. That's not conducive to snow except for the higher elevs. ECMWF is not on board with this idea, keeping development in the GOMEX slower/suppressed to the south and east. That results in only a weak nrn stream/Clipper rolling across NY on Sun. CFS also makes development in the GOMEX farther S than the GFS and very similar to the EC in many respects. National Blend brings some increase in PoPs for Sunday, but does not jump on either bandwagon. Obviously, considerable uncertainty remains regarding the precipitation type and track of the low, but the system bears watching over the next week.

AVIATION /04Z MONDAY THROUGH FRIDAY/
Rain continues to transition to snow from west to east across the area. Airfields from UNV westward have transitioned to snow prior to 04z, with a transition likely at IPT by 06z, and MDT and LNS by 09-10z. Thus far, snow has generally been light, but there is some potential for a brief period of moderate snow between 06Z-12Z, mainly from IPT east and south through MDT and LNS shortly after changing over. Snow will be short lived, however, only lasting up to a couple hours at any location before the low level moisture gets cutoff by dry NW flow.

Cigs are largely MVFR outside of the steadier snow bands, except for JST and BFD where cigs are lower in cold advection upslope flow.

Gusty northwest (290-310 degree) winds to 25 or 30kts will develop later tonight and linger into Monday, resulting in lake effect/upslope snow showers and persistent MVFR ceilings at BFD and JST through all or much of the day. Farther south and east, VFR conditions should prevail by midday.

Outlook...

Tue-Fri...No sig wx expected.

HYDROLOGY
Late evening update.

RFC new fcst is similar to earlier, but with a large area of rain across the east, some moderate, will hold onto the watch.

Earlier discussion below.

The Flood Watch was trimmed back overnight with the majority of counties removed with only Schuylkill County remaining at this point.

Given the generally dry soil conditions and near to below average streamflows over east-central Pennsylvania, the 1-2" rainfall will be largely beneficial in nature. Total rainfall continues to trend lower (<2 inches) with higher amounts shifting to the east.

We still can't rule out some isolated minor flooding issues given cold season hydrologic conditions. The threat of flash flooding remains very low given modest rainfall rates less than 1 inch per hour owing to lack of instability. Minor urban, poor drainage, and small stream flooding remain a possibility and would be the most likely outcome.

Within-bank rises of a few to several feet will occur on rivers and streams across east central Pennsylvania. Response time will be quicker on small streams (tonight-Monday) and longer on larger main stem points (Tuesday-Wednesday). The latest MARFC guidance now keeps Swatara Creek at Harper Tavern below action or caution stage. No river flooding is forecast at this time.

CLIMATE
It has been 318 days since the last measurable snowfall (>=0.1") at Harrisburg. This ranks as the 3rd longest run on record.

1. 330 days ending 12/13/2020 2. 328 days ending 01/21/2007 3. 318 days ending 12/09/2023

CTP WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory until 1 PM EST Monday for PAZ042-053- 058.
Flood Watch through Monday afternoon for PAZ058.

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