textproduct: State College

WHAT HAS CHANGED

* Added higher fog probabilities in for early Friday morning * Adjusted pops for scattered afternoon rain

KEY MESSAGES

1) Very humid with locally heavy rainfall possible this afternoon

2) Cooler this weekend with rain returning by Sunday

DISCUSSION

KEY MESSAGE 1: Very humid with locally heavy rainfall possible this afternoon

Morning rounds of rain have slid off to the southeast with only isolated showers in the northern tier of counties across our CWA. Afternoon coverage of showers and thunderstorms remains uncertain, but with additional convection beginning to develop to our west over Pittsburg's area we can expect at least a few heavy downpours later this afternoon before the stratiform rain shield over Ohio arrives later tonight. By midnight the cold front should be south of the PA/MD border, and with some scattering of clouds anticipated and light winds leading into Friday morning fog development seems probable over areas that saw rain today.

Previous Discussion... Highly anomalous moisture for this time of year (NAEFS mean pwat values near climatological maximum percentiles) pooling around the periphery of subtropical high over the southern Plains and lower MS Valley will interact with a surface cold front moving slowly south/southeastward from the lower Great Lakes to produce periods of showers and a few locally heavy thunderstorms across Pennsylvania later today. 17/00Z HREF indicates the potential for spot rain amounts of 2-3 inches. It's very likely that most of central PA picks up some rainfall by tonight. However, we do not anticipate an all-day rain/washout. Cloud trends remain a tough call with early morning convection over north central PA, but fcst instability this afternoon across south central PA will be more than supportive for thunderstorm development. SPC continues to maintain a general (non-severe) convective outlook for today which is likely due to the lack of deep layer shear (displaced farther to the north) and poor mid level lapse rates (very moist, tall/skinny soundings with warm cloud depths greater than 10kft).

KEY MESSAGE 2: Cooler this weekend with rain returning by Sunday

Another taste of Fall is on the way for the end of the week with noticeably cooler and much drier/less humid air poised to return behind the front (Td crashing into the 40s/50s). Expect a crisp and cool overnight Friday into Saturday morning with some upper 30s possible across the northern tier. Dry/fair wx conditions are forecast to start the weekend and last through the day on Saturday followed by periods of potentially heavy rainfall Saturday night through early next week. Both NAEFS and ECMWF SATs show at least 90th percentile PWs returning the the region which is supportive for at least a marginal excessive rain signal. A surface low expected to track over the Great Lakes will usher in this moisture and more than enough forcing for widespread precipitation. NBM temps have also cooled/moderated quite a bit Mon- Wed with most locations struggling to exceed 70F for daytime highs.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Highly variable conditions ranging from MVFR to brief periods of LIFR/VLIFR across Central and Western PA airfields, while mainly VFR dominates conditions across the Susquehanna Valley terminals early this morning.

During the late morning hours, conditions at most terminals should improve to higher end MVFR or VFR as temps warm, improving vsbys and raising ceiling heights.

For this afternoon, the very slow moving cold frontal boundary, draped along the Interstate 90 corridor in Northwestern PA and Northern Ohio this early this morning, will drift southeast bringing a trend toward a few brief periods of MVFR cigs and vsbys as a result of widely scattered showers and a few thunderstorms.

As the front pushes through the NW Mtns (this afternoon) and Central and Southern PA (tonight), sfc winds will shift to the northwest and precipitation will end.

Lowering cigs and vsbys (into mainly the MVFR range) will occur in the wake of the front tonight/early Friday associated with a layer of stratocu clouds.

Outlook...

Fri-Sat...Valley fog and the likelihood of a MVFR stratocu cloud layer in the morning (mainly across the Central and NW Mtns); otherwise VFR.

Sat night-Monday...Periods of rain and sub-VFR likely.

CTP WATCHES/WARNINGS/ADVISORIES

None.