textproduct: State College

WHAT HAS CHANGED

* Frost Advisory and Freeze Warning issued for parts of central PA for tonight.

KEY MESSAGES

1) Cool and unsettled pattern to start May with shower chances and a frost/freeze risk each night through the weekend, mainly across Central and Northern PA.

DISCUSSION

KEY MESSAGE 1: Cool and unsettled pattern to start May with shower chances and frost/freeze risk each night through the weekend, mainly across Central and Northern PA.

Several disturbances rotating around a broad, persistent upper level closed low centered over Ontario and Quebec will help to reinforce broadly deep troughing across the eastern U.S. through much of next week. This pattern will keep conditions cool and unsettled at times as multiple frontal systems move through. In addition to shower chances, the fronts will bring reinforcing surges of cool air, keeping temperatures near to below seasonal averages.

The first of these disturbances aloft and weak low pressure at the surface will drift east across the Commonwealth today, bringing periods of rain through the rest of the afternoon. Rainfall amounts will range from around 3 tenths of an inch over the northern tier, to less than a tenth of an inch in the Lower Susquehanna Valley. Temperatures will be cold enough just 1 to 2 thousand feet above ground to allow wet snowflakes to mix in at times, especially alter this evening, at elevations above 2000 feet MSL.

Another cold night is on tap with partial clearing and light winds allowing for frost and freeze conditions to develop. We have issued Freeze Warnings and Frost Advisories for roughly the northwestern half of the forecast area that are in effect from midnight through 9 am Sunday.

The lowest temperatures of this current cool spell will be Saturday night and early Sunday with frost or freeze conditions across all of the CWA except for perhaps the Lower Susquehanna Valley and adjacent South-Central Counties of PA where low temps will be in the mid-upper 30s. There is still some uncertainty regarding the southeastward extent of frost as there will be a rather tight pressure gradient across the region due to a developing area of low pressure off of the coast.

Temperatures trend warmer for the beginning of next week, but all guidance indicates that unsettled weather will continue with multiple chances for rain.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

An upper level wave and attendant weak surface frontal boundary are beginning to move through PA. This is leading to widespread SHRA across the northern and western tiers of the state. Have introduced MVFR into prevailing and early 18Z TAF windows to account for this. -RASN may be mixed in around 00Z at BFD where the best overlap of DGZ saturation and surface cooling will be. BFD and JST are the most likely areas for IFR conditions to prevail for a few hours this evening and nighttime due to lower ceilings and the right conditions for visibility drops near JST. Impacts from rain will be minimized in the SE airfields MDT & LNS with confidence conditions will remain VFR throughout the 18Z TAF period.

Winds will shift from a prevailing southwesterly flow to a more northwesterly one across the state by daybreak tomorrow. Prevailing winds will mostly be in the single digits (5-10 kts) with some scattered gusts in the teens possible (10-15 kts). After daybreak tomorrow across most TAF sites, VFR conditions are expected to prevail. JST may have the most trouble removing low level clouds into the morning.

Outlook...

Sat PM...Restrictions possible in SCT PM -SHRA.

Sun...SHRA moves in late; otherwise, VFR expected.

Mon...SHRA potential increases during PM hours, some TSRA possible across N/W PA. Restrictions possible.

Tue...SHRA becomes more widespread, TSRA chances expand across much of central PA. Restrictions possible.

Wed...Scattered SHRA remain. Restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

Freeze Warning from midnight tonight to 9 AM EDT Saturday for PAZ004>006-010>012-017-018-024-033-037. Frost Advisory from midnight tonight to 9 AM EDT Saturday for PAZ019-025>027-034-041-042-045-046.