textproduct: State College

WHAT HAS CHANGED

* Posted winter weather advisory for freezing rain for almost all of the forecast/warning area for later tonight and Tuesday morning.

KEY MESSAGES

1) A light wintry mix with light freezing rain accumulations likely late Monday night into Tuesday morning.

2) Periods of rain with temperatures very much above normal Wed onward.

DISCUSSION

KEY MESSAGE 1: A light wintry mix with light freezing rain accumulations expected later tonight into Tuesday morning.

Over the past 24 hours, the model solutions have come into better alignment w.r.t. timing. The QPF and temperatures have been very consistent from run to run. P-type will be a varied mix at first, but the gradual warming aloft will turn the precip to liquid. The consistency has raised our confidence in a light glaze of ice occurring late tonight and Tuesday morning.

We wondered whether it is a little early to post the advisory at this range (24-30hrs before start of the hazardous wx). But, the forecast has been highly consistent at painting measurable amounts of ZR over the entire CWA. WPC guidance, FRAM progs from HREF, and NBM forecasts all add together to make a high confidence forecast for a thin glaze of ice accretion in most places. We kept the goalposts wide on timing, that is, erring a little early on start of the ZR, and a little later than when the ZR should be done in each area. This should help our partners to be ready should it move in a little faster or end a little later. We didn't want to call for too tight of a window.

In all likelihood, the valley locations probably have more like a 3-6hr time period when ice may accrete before the temps warm enough to make it plain rain. But, the hills/ridgetops can stay sub-freezing longer into many events. For example, the 00Z NAM and GFS BUFKIT soundings both turn the precip type at UNV (1150ft MSL elevation) to plain rain at 17Z. At that time, though, the temp profile has the temps at 1000ft still sub- freezing. Our surrounding ridges are that high above the valley floor (1900-2400ft MSL). That may be due to a boundary layer of the cold air staying in place while the warmer air works down from above - and the valley sfc warms due to the sun. On the other hand, the fast (30-50KT) winds in the LLJet of the warm advection could mix/force the warm air down onto the ridgetops before or at the same time as the valleys gets warm. But, from personal observation, the scenario where the ridges stay colder-longer than the valley is just as common.

The arrival time of the precip lends itself to keeping the temps colder-longer. As for the NW: The northwestern zones may also need an advisory eventually, but confidence is just a little lower there for a glaze to occur. The temps may warm just enough before precip starts to keep it all rain. Collaboration among the WFOs resulted in holding off on issuing for them (Warren and McKean) on this shift.

NAM hints at the llvl moisture increasing before the higher clouds drop larger hydrometeors into it. Thus, there could be some freezing drizzle before the main push of mix arrives. This is another reason to start the advy a little earlier than most of the guidance brings the measurable precip in. The criteria for a winter weather advisory for freezing rain as the threat is: any. Even if it just the thinnest of glazes due to the hazard it presents to travel, even on foot.

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KEY MESSAGE 2: Periods of rain with temperatures very much above normal Wed onward

An amplifying mid/upper level ridge off the SE U.S. coast for Wed-Fri will support a big sfc high parked off the East Coast. These features will direct plenty of warm and moist air at the surface and aloft north and into the state for the rest of the week and weekend.

Temps are slated to be well above normal with sfc dewpoints climbing in to the 40s and possibly lower 50s in some areas later in the week. The W-E frontal boundary does become pretty stationary around our latitude, but will be wavy. There is a possibility of a small surge of cooler air sliding down from the NE (backdoor cold front) Friday-ish. But, by then, the temps will be so warm that no threat exists for any wintry precip. It should all be plain rain. Our wavy front will be the pathway for repeated shots of forcing and constant lift. So...it should be a wet time. But, that's not so bad, considering some of the area is still in drought.

AVIATION /07Z MONDAY THROUGH FRIDAY/

VFR conditions will continue into Monday afternoon under varying amounts of high cloud cover with a system passing to our south. A few flurries could sneak into southern PA during the morning, but they will remain south of all TAF sites. Winds will initially be out of the north-northeast, but will gradually become more southeasterly for the afternoon. This will usher in increasing low-level moisture and MVFR ceilings should develop at JST and AOO by 20Z. These low clouds will spread northward across the rest of Central PA through the evening and gradually lower through the night, likely falling to IFR for most of the region by 12Z Tuesday.

Light precipitation begins to enter Pennsylvania from the south after 06Z Tuesday ahead of a warm front lifting northward into the region. Most of this will likely fall as freezing rain and will reach most TAF sites by 12Z.

Outlook...

Tue...Restrictions likely in a light wintry mix which will change to rain Tuesday afternoon. A high probability for Low Level Wind Shear late Monday night through Tuesday morning. A second round of LLWS expected later Tuesday afternoon and Tuesday night.

Wed-Fri...Restrictions possible in periods of rain.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 8 AM to 4 PM EST Tuesday for PAZ006-037-041-042. Winter Weather Advisory from 7 AM to 1 PM EST Tuesday for PAZ010>012-045-046-049>053-058. Winter Weather Advisory from 4 AM to 1 PM EST Tuesday for PAZ017>019-025>028-056-057-059-063-065-066. Winter Weather Advisory from 1 AM to 1 PM EST Tuesday for PAZ024-033>036-064.