textproduct: State College

WHAT HAS CHANGED

Refined rainfall timing on Friday, especially across the south- central mountains where low-level dry air could slow down the eastern push of precipitation. Newest forecast also trends down with respect to elevated instability (and thus thunderstorm potential) on Friday/Saturday across the southern tier of Pennsylvania. Increased wind gusts on Sunday and Monday under the presence of a 850mb LLJ that could bring wind gusts into Advisory range (still too far out and too low confidence for headlines). Also, knocked down temperatures slightly on Saturday with cold air damming potential across S/E PA.

KEY MESSAGES

1) Mild conditions continue through Saturday evening with periods of rain across much of central Pennsylvania.

2) Monitoring potential for ice jams across northern Pennsylvania, primarily north of the Interstate 80 corridor.

3) Breezy to gusty winds begin on Sunday and continue through Monday with lake effect showers (and potentially squalls) increasingly likely to bring snow accumulations to favored locations in western Pennsylvania.

4) Dry and near-average temperatures on Monday and Tuesday before a warming trend and unsettled pattern begins for the middle of the week.

DISCUSSION

KEY MESSAGE 1...Mild conditions continue through Saturday evening, periods of rain across much of central Pennsylvania.

Influence of high pressure continues in the near-term with high clouds filtering in across the region. Recent HREF model guidance continues to depict some breaks in cloud cover over the next couple of hours before increasing overnight ahead of an approaching low pressure system with increasing low-level moisture suggesting an arrival time of rainfall near 12Z/7am across the western periphery of the forecast area. Recent HREF/RAP model guidance does indicate low-level dry air winning out across portions of the south-central mountains, promoting a slightly delayed arrival time closer to the late morning hours which is in good agreement with recent NBM model guidance. These slight changes have been outlined in the newest forecast which does have some slight deviations from the previous forecast cycle. After 18Z/1pm EST Friday, increasing PWATs outlined in HREF/RAP model guidance indicate PWATs in the 1.00-1.25" range across portions of southeastern Pennsylvania, where moderate rain mentions have been added to incorporate a heavier band of rainfall across the Lower Susquehanna Valley along the cold frontal passage.

Some uncertainty begins to creep into the forecast in the Friday night to Saturday night timeframe, with recent model guidance beginning to introduce a slight decrease in rainfall mentions. This decrease appears to be well-founded with the presence of subsidence inversion and dry air aloft on recent RAP/NAM model soundings. The main concern for keeping some precipitation mentions will be the potential of a stationary boundary setting up just south of the PA-MD border that would allow for precipitation mentions to be retained. For this forecast cycle, have decided to keep with NBM PoPs in this timeframe, but could realistically see an increase/decrease in mentions Friday night depending on where the frontal boundary sets up before increasing moisture Saturday morning allows for the second period of rainfall to overspread south-to-north across the region and continuing throughout the day on Saturday.

Another aspect of the forecast (not entirely a part of this key message but still warrants mention) to keep an eye on is the potential for cold air damming across eastern/southeastern Pennsylvania. Recent NBM model guidance indicates a fairly expansive spread with regards to high temperatures on Saturday, with portions of southeastern Pennsylvania showing a 10-15F range between the 25th and 75th percentiles. Given the event is approaching and there is quite an expansive spread, have nudged temperatures down slightly to be more in line with a slight decreasing trend in model guidance.

KEY MESSAGE 2...Monitoring potential for ice jams across northern Pennsylvania, primarily north of the I-80 corridor.

The aforementioned mild temperatures across the region, coupled with rainfall will bring about a non-zero chance for ice jams across portions of central Pennsylvania on Friday with the threat continuing into Saturday. Ice jam threat will be highest north of Interstate 80, where any remaining snow on the ground and river ice will continue to break up over the next couple of mild days. While we're not expecting any widespread hydrologic concerns at this time and thus no thoughts on issuing any sort of headline/mentions in products, it is still worth highlighting that there is a non-zero chance for ice jams on the smaller streams of northern Pennsylvania.

For areas south of Interstate 80, there remains little in the way of snow cover and temperatures are expected to stay above freezing for the next ~36 hours which will likely allow for remaining river ice to continue melting. The aforementioned rainfall also does not begin entering these areas until later in the day Friday, which will lead to lower ice jam concerns.

KEY MESSAGE 3...Breezy to gusty winds begin on Sunday and continue through Monday with lake effect showers (and potentially squalls) increasingly likely to bring snow accumulations to favored locations in western Pennsylvania.

As the cold front passage moves east of the area, northwesterly flow kicks up the typical lake effect snow machine across northwestern and northern Pennsylvania late Saturday night into Sunday morning. Recent GFS/NAM model guidance continues to indicate some snow squall potential Sunday morning and into the afternoon hours across mainly northern and western Pennsylvania. Squall potential might slightly decreased based on recent model guidance; however, potential still exists and will continue to be monitored ahead of the event. Current storm total snowfall outlines a corridor of 1-2" across Warren County; however, this does not encompass the entire event and additional snowfall into Sunday morning will need to be assessed for Winter Weather Advisory concerns.

Winds behind the cold frontal passage are expected to increase behind the cold frontal passage with a fairly tight pressure gradient. Given this tight pressure gradient, NBM is likely underestimating the strength of the wind field, thus have blended in with NBM 90pct in order to push wind/gusts higher and pushing portions of the Laurel Highlands closer to Wind Advisory criteria. Time horizon on lack of confidence warrants HWO mentions at this time while continuing to monitor this threat as it comes into focus.

KEY MESSAGE 4...Dry and near-average temperatures on Monday and Tuesday before a warming trend and unsettled pattern begins for the middle of the week.

High pressure south of the area builds in for the beginning of the weak, leading to tranquil weather conditions with near- average temperatures on Monday. A slight warming trend with tranquil weather is expected for the bulk of Tuesday before the next approaching surface low southern Canada tracks a cold front across central Pennsylvania late Tuesday into Wednesday morning.

Temperature profiles near freezing across the northern tier and western Pennsylvania will support a wintry mix at onset with rainfall the predominant precipitation type ahead of the cold frontal passage. Cold air filtering in behind the front will likely allow for a transition to snow across the northwestern Alleghenies and the Laurel Highlands with warming temperatures allowing for rain across the Lower Susquehanna Valley with some mix in between these two locations. There remains a fair amount of uncertainty in this timeframe; thus, have stayed close to NBM model guidance this forecast cycle.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

Came in around 11 PM, was 37 degrees outside the office here, then it jumped to 45 degrees as of Midnight. Narrow band of showers moving across the area, but CIGS still around 10,000 ft.

Not a lot of change from the earlier 00Z TAF package. Main thing was to adjust winds a bit, gusty across the far west, while calm and cold across the east.

Earlier discussion below.

VFR conditions continue tonight as high clouds overspread the area. An area of low pressure that will track to our northwest tomorrow, bringing widespread rainfall to the region. Rain will approach BFD and JST in the 14-16Z timeframe, gradually spreading eastward through the morning and into the early afternoon.

A 40 to 55 knot southerly low-level jet will move into Pennsylvania overnight, resulting in a prolonged period of LLWS. Surface wind gusts increase through the night, with gusts of 20 to 25 knots possible at all TAF sites.

A narrow cold frontal rainband along a cold front will move through on Friday evening into the overnight hours and should weaken/fizzle out with eastward extent. Winds will shift from southwesterly ahead of the line to northwesterly behind the line. A period of stronger winds is also possible.

Outlook...

Fri Night-Sat...Restrictions with a batch of steady rain and cold front Sat night.

Sun...Snow showers possible mainly west of UNV.

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

Tue...Mainly VFR.

CTP WATCHES/WARNINGS/ADVISORIES

None.