textproduct: State College

WHAT HAS CHANGED

* Frost Advisory issued for late tonight through early Tuesday morning across the Laurel Highlands and portions of the Central and Northern Mtns of PA. A Freeze Watch remains in effect or McKean, Potter, Elk, and Cameron Counties during this same period.

KEY MESSAGES

1) Frost/freeze conditions are probable late tonight and early Tuesday morning over the western and central Alleghenies

2) Extended stretch of dry and seasonal Fall weather this week

DISCUSSION

KEY MESSAGE 1: Frost/freeze conditions are probable late tonight and early Tuesday morning over the western and central Alleghenies

As of 0730Z, a light southwesterly breeze developing just ahead of a SEWD-pushing cold front was helping to erode the northern and western edge of the widespread stratus deck covering much of Central and Eastern PA. Patchy dense valley fog was occurring over and just SE of the Allegheny Front in Scent PA - in the narrow transition zone between the initial breaks and slight increase in SW wind. The relatively minor fog problems will develop a bit more to the east (toward the Lower Susq Valley) through 11Z before improvement begins.

Timing of the cold front goes like this - the boundary will push through the Northwest Mountains during the early morning hours today, crossing the Laurel Highlands and Central Mountains shortly after sunrise and the Lower Susquehanna Valley late this morning.

A rather deep upper level trough over northern New York state will deepen overnight, with a tight 500 mb height gradient by morning. This will lead to bands of lower SC clouds and gusty winds early Monday, but 500 mb heights come up fast later in the day. The rising 500 mb heights will aid in a cold night Monday Night, as winds slacken, and clouds diminish with the warming aloft and much drier air being advected into central PA.

The coldest air of the early Autumn season will spread southeast across Central PA early this week with frost/freeze conditions expected tonight into early Tuesday morning over much of the western and central Alleghenies (northwest of Interstate-81).

We maintained the Freeze Watch for 4 NW PA counties for late tonight/early Tuesday (as there is some questions whether there will be widespread near to slightly below freezing temps or lows 33-35F INVOF KBFD and other perennial cold spots across the NW Mtns).

A Frost Advisory was issued for the Laurel Highlands, and portions of the Central and North-Central Mtns. Continued to lean a toward slightly colder guidance in the form of a NBM/MOS blend. Placement of sfc high becomes somewhat less favorable for frost/freeze Tuesday night.

------------------------------------------------------------- KEY MESSAGE 2: Extended stretch of dry and seasonable Fall weather this week

Low clouds and patchy dense valley fog will cover much the Central Ridge and Valley Region of the state and Susquehanna Valley early today.

Conditions will improve sharply around and shortly after sunrise as the mean low level flow shifts from the east/southeast to the west/northwest behind a secondary cold front. The colder WNW flow could bring a couple of rain showers downwind of Lake Erie tonight into early Monday.

Otherwise, there is high confidence in a prolonged stretch of dry and seasonable Fall wx this week (Mon- Fri) as a large area of Canadian high pressure dominates the pattern.

The next chance of showers will likely arrive Sunday.

AVIATION /07Z MONDAY THROUGH FRIDAY/

Poor conditions currently in Central and Eastern PA that will be in the IFR/LIFR ceiling category in the back half of the night/early morning. However, conditions begin to improve as high pressure moves in from the west and dry air filters in with more northwesterly flow. Another brief wave may knock some western terminals down to MVFR/IFR around dawn but this isn't expected to persist more than two hours or so.

VFR prevails during daytime tomorrow with a gusty northwest flow in place with gusts in the 15-30mph range as a drier, high pressure airmass sets in.

Outlook...

Tue-Fri. VFR prevailing. Brief restrictions possible in late night and early morning fog.

CTP WATCHES/WARNINGS/ADVISORIES

Frost Advisory from 1 AM to 9 AM EDT Tuesday for PAZ004-012- 017>019-024-033-037-041-042-045. Freeze Watch from late tonight through Tuesday morning for PAZ005-006-010-011.