textproduct: State College

WHAT HAS CHANGED

* Confidence continues to increase for the development of strong to severe thunderstorms across parts of central PA Monday afternoon or early evening.

KEY MESSAGES

1) After a lull tonight and early Sunday, gusty winds return Sunday afternoon through Monday, with the potential for strong to severe thunderstorms Monday afternoon or early evening.

2) 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: After a lull tonight and early Sunday, gusty winds return Sunday afternoon through Monday, with the potential for strong to severe thunderstorms Monday afternoon or early evening.

As of late Saturday afternoon, skies continue to clear from south to north across the Commonwealth, heralding the approach of a narrow surface ridge axis. Fair and seasonably chilly conditions are foreseen overnight and early Sunday.

By mid to late afternoon Sunday, as the above mentioned ridge axis shifts offshore and a significant storm tracks from the Plains towards the Upper Midwest, a return S-SE flow will start to increase across PA. As this occurs, we also expect lower stratus clouds to begin to overspread much of the region, along with periods of light rain and drizzle Sunday night.

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, can 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 near and south of I-80, as well as east of I-99. Please stay tuned for later updates over the next couple of days.

KEY MESSAGE 2: 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).

Wednesday should be less harsh, but still quite chilly for mid- March, with highs in the 30s-lower 40s for much of central PA.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Gusty wind is noticeably less-so (in the obs) over the wrn terminals (JST/BFD), and will be reducing gradually from west to east late this afternoon. Around sunset, decoupling will stop most gusts and quickly reduce the sustained speeds, too. The sfc high will slide in overhead for the night - so the wind should go light/calm for most. Low moisture/RH should preclude fog. Plenty of high/mid clouds around tonight as warm air advection also brings a moisture increase. The clouds may get thick enough tonight (right around 06Z) at BFD to allow a few snowflakes to hit the ground. A dusting of snow is possible there. Just have the briefest of mentions of MVFR -SN/cig in a PROB30 group there from 05-07Z. The WAA will be driven by a decent low level jet of 40-50KT in the west, but lighter (30-35KT) in the east. Have left LLWS out of IPT for the time being.

After a VFR (but high-cloud-covered) start to Sunday, low clouds creep in from the S. They will likely arrive around 15Z in between MDT and AOO, then spread into all the central PA between then and 21Z terminals (with JST and BFD last). IFR cigs probably (50%) wait until 00Z Mon or later - and are most likely on the higher elevation locations - but they will be lowering during Sun aftn. Most locations will (90%) be IFR for a time Sun night, and some may (40%) hit LIFR before sunrise Mon.

A 55-65KT LLJet will (100%) return late Sun night, screaming/streaming over top the low clouds. With sfc sustained wind of 10-15KT, this makes very strong LLWS a certainty (100%).

Outlook...

Mon...Very strong LLWS will occur before a VERY strong cold fropa. Pre-frontal SHRA/TSRA with IFR possible. Then turning blustery and colder.

Mon night...SN/IFR possible (50%) post-front close to the front over eastern terminals (mainly IPT/MDT). MVFR cigs for most of the area. IFR likely N/W (60%) due to clouds and/or SHSN at BFD.

Tue...Blustery. Sct SHSN NW. IFR poss (50%) at BFD, (30%) JST.

Wed...Warm fropa. -SN or -RA poss (30%) NW. No sig wx elsewhere.

Thu...Sct showers (RA?SN?) N. MVFR likely (60%) BFD, IFR less likely (40%). Elsewhere, no sig wx.

CTP WATCHES/WARNINGS/ADVISORIES

None.