textproduct: State College

WHAT HAS CHANGED

Line of low-topped convection has fallen apart and dissipated. Adding fog to all the area for later tonight.

KEY MESSAGES

1) Timing and location of any clearing will determine where fog develops. Cool and wet on Saturday.

2) Trending colder Saturday night and becoming windy on Sunday with max gusts 35-45 mph pushing advisory level thresholds.

3) Minor snow accumulation in the NW Alleghenies and Laurel Highlands could impact travel Sunday into Sunday night. A few localized heavy snow showers/squalls are also possible over the western half of PA.

DISCUSSION

KEY MESSAGE 1... Fog has been forming in the upslope/nil flow in the central mountains. The advent of wind in those places has started to improve visibility. Clearing of low clouds is a little better than progged, which is a plus for fog formation, especially later tonight when the wind should get lighter almost everywhere as they veer to the east. We will expand the mention of to all the CWA later tonight.

Prev... Warm conditions in place this afternoon with temperatures largely in the 40s while a few locales in the Laurels sit in the 50s. Light rain showers will come to an end by early this evening as a weak cool front pushes through central PA. Transient surface ridging will move in tonight and early Saturday, providing a lull/break in rain into Saturday morning while setting up a cold air damming wedge pattern. Patchy fog looks like a decent bet, but the amount of any clearing will determine longevity and placement. Hi-res models show overrunning precip returning northward by Saturday afternoon which will result in a rather unpleasant/chilly/dank start to the weekend. May have to monitor the northern tier into Sullivan County for a brief period of freezing rain/drizzle, but 850 mb thermal ridging along with diurnally warming temperatures should preclude much if any threat by the time precipitation begins falling. Rain totals on Saturday are expected to be higher than today (Friday) with widespread amounts in the 0.25-0.50 inch range. The expected rainfall combined with snowmelt from the ongoing/early January thaw will cause rises on rivers and streams; we will continue to monitor for ice jams.

KEY MESSAGE 2...

Winds increase on Sunday with strong low level CAA/steepening lapse rates promoting deep boundary layer mixing with max gusts likely reaching into the 35-45 mph range. Higher gusts >45 mph are possible and a wind advisory may be needed. Temperatures will drop below the freezing mark over the northwest half of central PA by 7AM Sunday behind the occluded front.

KEY MESSAGE 3...

Secondary cold front will deliver a colder/more seasonal west/northwest flow regime that will drive lake effect and upslope snow showers downwind of Lake Erie over the Allegheny Plateau Sunday into Sunday night. Minor accumulations are forecast from 0.5-2.0" in the favored northwest snowbelts and Laurel ridgetops. GFS/NAM fcst SNSQ parameter indicates a few heavier snow showers or squalls are possible over the western part of the state. Travel may be impacted particularly over the western mtns on Sunday due to a combination of blustery winds, slippery roads and reduced visibility.

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Look for improving conditions early next week with no precipitation and moderating temperatures Monday & Tuesday.

Precipitation chances will increase into the middle to later part of next week in response to an amplifying upper trough carving out over the Eastern US. Moderating/above normal temps into Wednesday are forecast to trend colder to more typical levels for mid January by Thursday and Friday.

AVIATION /02Z SATURDAY THROUGH WEDNESDAY/

Current observations outline MVFR ceilings across the western terminals where the cold front is tracking across central Pennsylvania with the bulk of model guidance continuing to outline IFR/LIFR ceilings overnight at BFD/JST as winds begin to slowly taper down as the frontal passage continues to truck east of the forecast area. Increasing confidence in LIFR conditions with fog formation close to BFD; however, question remains on if fog formation occurs low enough on the ridgetops surrounding the airfield. Rainfall tracking over the airfield recently is a tip towards the right direction; however, if winds remain gusty throughout much of the evening, a solution favoring high-end IFR is plausible to win out. Main timing of fog concerns would come in the 06-10Z timeframe; however, cannot rule out longer duration LIFR conditions if fog does manage to settle over the airfield. Further south, lower confidence of fog formation at JST as dew point depressions remain relatively higher compared to that of BFD.

Across southeastern Pennsylvania, rainfall has tracked east of the region with IFR ceilings currently observed at MDT/LNS. A combination of RAP/GLAMP model guidance continues to indicate favorable chances (> 80%) for IFR ceilings continuing through the 02Z-04Z Saturday timeframe. Progressing past 04Z, odds tip towards favoring VFR conditions (brief periods of MVFR possible between 04-05Z) at MDT/LNS with some erosion of the low level cloud deck outlined on HREF/RAP model guidance. VFR conditions are expected to continue through ~15-17Z Saturday (60-70% confidence) before the next system bring rainfall northwards across all of central Pennsylvania. As rainfall enters the forecast area south-to-north, expect widespread deterioration in flying conditions across central Pennsylvania with model consensus between recent RAP/GLAMP/HREF model guidance indicating widespread IFR conditions are likely (> 70% confidence) after a brief window of MVFR ceilings/visibilities within rainfall. LIFR remains most plausible based on RAP model guidance at MDT/LNS where ample low-level moisture will be in place. LLWS concerns also begin to be introduced after 20Z Saturday; however, based on recent NBM/NAM model guidance have decided to limit mentions to BFD due to slightly higher (30-40%) confidence in LLWS criteria being met at the airfield.

Outlook...

Sun...Scattered snow showers anticipated, mainly in the vicinity of KBFD and KJST.

Mon...A few lingering lake effect snow showers early, giving way to mainly VFR conditions.

Tue-Wed...Mainly VFR.

CTP WATCHES/WARNINGS/ADVISORIES

None.