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WHAT HAS CHANGED

* Extreme Cold Watch issued for the Allegheny and Laurel Mountains for Saturday morning. * Considering additional cold weather hazards for Friday night through Sunday morning * The already-light snow totals for Friday and Friday night have decreased slightly.

KEY MESSAGES

1) Accumulating snow mid-day Friday into Friday night

2) Dangerous cold Saturday and Sunday

3) Moderating temperatures during the second week of February

DISCUSSION

KEY MESSAGE 1: Accumulating snow mid-day Friday into Friday night

Shortwave trough drops SE into the OH Valley during the daytime on Friday. Very cold air (temps already 10+F below normals) in place will briefly be overrun by a spot of relatively warmer air. The advection of the poorly-named warmer air will lift the meager moisture available to it. This will produce a widespread but (very) light snowfall. The Gulf is effectively cut off and the northern stream moisture won't have much residence time over PA due to the quick progression of the system. Cloud temps and near-sfc temps should be good for 18-20:1 SLRs through the time frame we are talking about. Most places will only see light accumulations with probability of 2+" of snow less than 20% for 60% of the CWA land-mass. The mountains, especially the Laurels will have the highest probs of getting a plowable snow to stick. However, almost everyone will get at least a dusting. The intensity of the snow will likely peak with the passage of one or two main mesoscale bands of snow during the mid-day (W) to evening hours (central). No risk of 1"+/hr rates. The forecast QPF has decreased over the Susq Valley, and is likely to be a good/solid forecast judging by the lacking moisture content of the air. The lone place to worry for anything more than 3" is the Laurels. We have (for the time being) stuck with the Winter Storm Watch there. But, it's looking like a stretch with the downward trend on QPF/SF. There are two things that could allow 6"+ to accumulate - Upslope/orographic influence during the warm advection and after the passage of the sharp cold front (Fri evening), and the potential for a fluffy snow if the SLRs bump up to more like 25-30:1. There could be a couple of stronger SHSN along and immediately in the wake of the FROPA. But, timing is not favorable for CAPE/instability. So, the possibility of any SNSQs is not high (maybe not even medium). Gusty wind along and after the front will create minor problems for visby as it blows the fluff around.

KEY MESSAGE 2: Dangerous cold Saturday and Sunday

This threat looks more serious/certain than the problems that the light snow Friday/night presents. 8H temps drop hard after the front passes Fri evening, with -25C or lower values across the northern tier. Confidence has increased greatly in the sharp drop in temps aligned with the gusty wind as the much colder air brute-forces it's way in from the NW. This airmass and the tight pressure gradient will drop the wind chills well below zero. Minus 20F to even minus 30F wind chills are almost a certainty for Saturday morning over the NW. Some rise in the chills is expected during the daytime, but they will drop off again for Sunday morning. But, they will not be as low as the prior morning. So, we've posted an Extreme Cold Watch for the NW half of the CWA for later Fri night and Sat AM. The 12 hrs around sunrise will be about the coldest of what we've experienced all winter long. Most of the area will certainly need a Cold Weather Advisory - perhaps for much of the time from Sat morning into Sunday morning. But, we have tried to take the first step with this watch as confidence has increased in warning criteria being met. Warning criteria is -25F and below across the northern mountains (northern two layers of zones in the CWA), and -20 or lower for the rest of the CWA.

KEY MESSAGE 3: Moderating temperatures during the second week of February

For those looking for a break in the cold pattern, here you go. Confidence continues to increase that temperatures will moderate during the second week of February and climb back to seasonal levels or near the historical average for early to mid February. This "warmup" puts mixed precipitation on the table with the next weather system that could arrive by the middle of next week. The guidance is highly variable concerning the precip pattern, so we feel most confident in signal for relatively milder temps next week.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Restrictions remain at BFD/JST, with freezing fog potential realized at JST. Restrictions between 1/4SM and 1SM are expected to continue through 05/14Z based on more recent RAP model guidance with NBM/HREF model guidance also indicating lower conditions compared to more recent GLAMP model guidance. MVFR conditions become more likely after 05/14Z based on HREF/RAP model guidance, before the bulk of model guidance outlines VFR conditions prevailing closer to the 06/00Z. Low-level cloud cover at JST is expected to increase after 06/06Z with a brief period of MVFR conditions shifting towards IFR towards the end of the TAF period based on a consensus of HREF/NBM/GLAMP model guidance. At BFD, model guidance continues to hold onto low-end MVFR to high-end IFR over the next couple of hours (through 05/14Z) before IFR conditions become more likely. Brief periods of IFR will remain possible through 06/06Z at BFD with increasing potential for IFR conditions in the 06/06Z to 06/12Z timeframe with moderate-to- high (~60-80% confidence).

Elsewhere, VFR conditions are expected (~80-90% confidence at all airfields) through 12/12Z. Clear skies are currently being observed, which tracks fairly well with HREF/NBM model guidance. Recent model guidance indicates a scattered low-level deck with a more expansive mid-to-high cloud deck this afternoon (after 05/15Z) and into the evening hours. Low-level cloud deck is expected to thicken based on all model guidance after 06/00Z; however, model guidance indicate all ceilings staying above 3000ft AGL through 06/12Z.

Outlook...

Fri...SN spreading W-E through the day. N & W: IFR (100% chc), LIFR likely (70% chc). SE: MVFR (100% chc), IFR (70% chc).

Fri PM...CFROPA. Gusty NW wind. W: SHSN with IFR (90%), LIFR poss (50%), SNSQ also poss (30%). Cent: SHSN. IFR likely (70%). SE: -SN ending then VFR.

Sat...Sct SHSN/MVFR AM (IFR poss) BFD/JST. Gusty NW wind.

Sun...-SN/MVFR poss W. Mainly VFR E.

Mon...No sig wx.

EQUIPMENT

KCCX will be down for intermittently through Thursday afternoon for an important software upgrade.

CTP WATCHES/WARNINGS/ADVISORIES

Extreme Cold Watch from late Friday night through Saturday morning for PAZ004>006-010>012-017-018-024-025-033-034-037-041- 042. Winter Storm Watch from Friday morning through Saturday morning for PAZ024-033.