textproduct: State College

WHAT HAS CHANGED

* Wind Advisory extended through 11 AM Monday across the northern tier along with Elk & Cameron Counties. * Slight uptick in snow totals across far NW PA including potential lake effect snow showers into the middle of the week.

KEY MESSAGES

1) Gusty winds continue through Monday morning ahead of a cold frontal passage.

2) A strong cold front will bring an Enhanced Risk of severe thunderstorms Monday afternoon and evening, with damaging winds and a few tornadoes as the primary threats.

3) Following Monday storminess, a period of sharply colder temperatures is expected through mid-week, with some snow possible Monday night, as the cold air first builds in.

DISCUSSION

KEY MESSAGE 1: Gusty winds continue through Monday morning ahead of a cold frontal passage.

Current observations across portions of central Pennsylvania outline highest wind gusts reaching between 30-40 mph range with the bulk of model guidance indicating wind gusts to increase this evening and into Monday morning with an ever tightening pressure gradient across Pennsylvania overnight. Have continued to increase wind gusts in the near-term as NBM wind gusts appear too low given current observational trend, pushing much of northern/western Pennsylvania above Wind Advisory criteria. The main changes to headlines this cycle was to tack on a couple of hours onto the Wind Advisory across the northern tier, where HREF model guidance continues to outline a favorable environment for continued wind gusts Monday morning with probabilities in exceeding Wind Advisory criteria over 50-60% in this timeframe. Some gusty winds will remain possible on Monday; however, looking at some potential for severe thunderstorms for areas south/east of the current advisory, thus have continued to keep wind headlines out of these areas given this severe threat.

KEY MESSAGE 2: A strong cold front will bring an Enhanced Risk of severe thunderstorms Monday afternoon and evening, with damaging winds and a few tornadoes as the primary threats.

Monday continues to be a period of high focus, as model consensus zeroes in on the potential for significant convective development. At this early juncture, confidence is quite high on the synoptic-scale evolution (the approach of a negatively tilted upper-trough, an associated sharp cold front, and impressively strong wind fields at all levels). More uncertainty still has to be dealt with, though, in terms of smaller-scale factors, such as: exact timing of the cold frontal passage, potential for a pre-frontal trough, whether any breaks develop in the overcast conditions that should prevail to start the day (which would enhance instability)?, and convective mode (will any discrete cells form ahead of a probable intense squall line?). Bottom line, though, ultimately we have increasing levels of confidence in high wind producing thunderstorms Monday afternoon, with the best potential for damaging gusts and tornadoes near and south of I-80, as well as east of I-99.

The Storm Prediction Center has continued with their Enhanced Risk (Level 3 out of 5) for Monday during the afternoon update, with very little changes across central Pennsylvania. The primary threat for Pennsylvania appears to be a line of storms along a cold front, progged to cross the region between 2PM and 8PM on Monday. Within the line, embedded supercells could support concentrated tornadic winds in the midst of a potentially expansive area of damaging wind gusts. A slight notable model difference that has been observed with more recent guidance is (potentially) a slightly less progressive pattern which could unlock more time for the forecast area to be unlocked in the warm sector. This could allow for slightly more convection ahead of the cold frontal passage; however, there still remains some uncertainty with regards to this solution so urging to continue to monitor trends for severe weather across much of the forecast area on Monday.

QPF will be 1 to 1.5" for locations east of the I-99/US-15 corridor, and isolated 2" amounts are possible at higher elevations in Schuylkill and Sullivan County. Antecedent dry conditions should preclude any major flooding concerns, but ponding on roadways is a real possibility. If you have outdoor plans on Monday afternoon and evening, be sure to monitor the weather and consider changing your plans.

KEY MESSAGE 3: Following Monday storminess, a period of sharply colder temperatures is expected through mid-week, with some snow possible Monday night, as the cold air first builds in.

On Monday evening, once the surface cold front pushes through, colder air should rush in. At issue is how much steadier precipitation will linger behind the boundary. Given favorable upper jet placement and the probability that some wrap-around moisture will get steered into the Commonwealth, there is the potential for a quick 1-3" of snow in some areas Monday night, particularly over the higher terrain and northern sections of the state. Again, this will come down to smaller details, so it is advised that you stay tuned for later forecast updates.

Tuesday looks windy and much colder, with temperatures a good 25-35 degrees lower than what will be experienced on Monday (highs no better than the upper 20s-mid 30s in many locales). Scattered snow showers/squalls with moisture sourcing from the Great Lakes will traverse the region and could lead to brief reductions in visibility and a quick accumulation of snowfall on untreated surfaces.

Temperatures will trend back to near average by Friday with occasional chances for rain or snow, but no significant accumulation is expected at this time.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Low clouds will continue to expand and lower through the rest of the evening, falling to IFR at most TAF sites by 06Z. Model soundings suggest that a few hours of LIFR ceilings will also be possible at some sites, but confidence is low in that occurring. Scattered showers will develop overnight and may lead to visibility restrictions developing as well. The main concern tonight into Monday morning is LLWS with a 50 to 75 knot low level jet moving into the region. Ceilings rise to MVFR during the morning and LLWS concerns decrease by early afternoon as surface winds begin to pick up.

Our focus then shifts to a line of thunderstorms that will develop over west-central PA in the early afternoon ahead of a very strong cold front. Showers will first approach JST and AOO around 15Z, though the threat for thunder at these sites is relatively low. They will strengthen as they move eastward into a more unstable environment, reaching UNV by 17Z. IPT, MDT, and LNS should see the line of thunderstorms move through in the 18 to 21Z timeframe. These thunderstorms will be capable of producing wind gusts in excess of 50 knots (possibly greater than 60 knots at MDT and LNS), as well as a few tornadoes.

Winds will be out of the southeast ahead of the cold front, sustained around 15 knots with gusts to 30 knots possible. A few gusts to 40 knots will remain possible at BFD through the rest of the evening. Winds quickly become westerly behind the front and will remain gusty, with gusts to 35 knots likely. Much colder air moves in behind the front and will change any precip to snow for Monday night.

Outlook...

Tue...Blustery. Sct SHSN northwest. IFR possible at BFD/JST.

Wed...Warm fropa. -SN or -RA possible northwest.

Thu...Sct showers over northern PA with restrictions possible.

Fri...PM Showers move in from W PA, restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

Wind Advisory until 11 AM EDT Monday for PAZ004>006-010-011- 037. Wind Advisory until 8 AM EDT Monday for PAZ012-017-018-024-033- 041-042.