textproduct: State College

WHAT HAS CHANGED

* Tweaks in near term for ongoing and future weak convection across the nrn tier

KEY MESSAGES

0) Made minor adjustments to near term to better reflect convection.

1) Frost concerns return tonight with lower chances for widespread frost formation across southeastern Pennsylvania.

2) Fire weather concerns continue through Saturday with lower relative humidities and breezy winds. Little precipitation will exacerbate drought across southern Pennsylvania.

DISCUSSION

KEY MESSAGE 0: Made minor adjustments to near term to better reflect convection.

Gust front moved through the NW counties and produced a 38KT gust at BFD, despite having only shallow convection and no CG in our CWA thus far this evening. Cooling, more-stable air is now in place there, but dewpoints are into the U40s after the rain. We expect there to be additional convection move across the nrn tier overnight. Current batches of TSRAs over LE are headed this way. It also should diminish in intensity and carry only light rain as it moves through.

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KEY MESSAGE 1: Frost concerns return tonight with lower chances for widespread frost formation across southeastern Pennsylvania.

A sneaky set-up for frost exists across portions of southern and southeastern Pennsylvania overnight tonight, mainly for areas along and east of the I-81 corridor. The exact coverage of frost will remain extremely dependent on the presence of the low-level cloud deck that is expected to form late tonight into Friday morning. This low-level cloud deck would allow for temperatures to not radiate (temperatures get cooler) which could nix almost all mentions of frost in these regions. A secondary area of concern is that there could be a light breeze continuing throughout the hours where clear skies are most progged by model guidance with winds trending calm after the low-level deck clears the region.

Most of the zones in the aforementioned region have had their growing season activated due to the recent warm temperatures and looking at current growing degree days (GDDs) compared to climatology, so we'll need to continue watching near-term trends in these zones over the next couple of hours for any hazard mentions. Dewpoints are expected to rise late this afternoon/evening and are forecast to rise into the mid-to-upper 30s based on a combination of HREF/NBM model guidance. Recent hi-res model guidance also supports this idea with model guidance also keeping onto a light breeze throughout much of the overnight period. RH cross-sections at MDT/LNS do indicate increasing low-level moisture, with HREF model guidance (which typically performs fairly well with respect to cloud cover) indicating the potential for low-level cloud deck formation after midnight and into the early morning hours on Friday. Joint probabilities from HREF indicate that the potential for a favorable environment for frost formation (T <= 36, Gusts <= 5) as achievable; however, only a 10-20% chance for this to occur.

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KEY MESSAGE 2: Fire weather concerns continue through Saturday with lower relative humidities and breezy winds. Little precipitation will exacerbate drought across southern Pennsylvania.

Relative humidities this afternoon have dipped into the upper 20% to lower 30% range, with southerly flow allowing for warming temperatures this afternoon. A similar set-up exists for Friday ahead of the cold frontal passage with recent model guidance indicating that portions of southwestern Pennsylvania could dip into the low-to-mid 20% range Friday afternoon. Low-level dry air above a weak inversion has been noted on model soundings on Saturday as well; however, the best chances for this low-level air to mix down across portions of southeastern Pennsylvania with some model guidance outlining 850mb Tds pushing closer to the -20C mark at LNS on Saturday afternoon. This area will also be compounded by lack of rainfall with the upcoming system, as recent NBM model guidance has dropped nil in terms of measurable rainfall south of the Turnpike through Saturday evening. This lack of rainfall, in areas that are currently under D1-D2 (moderate to severe) drought will continue to be dry in the mid-range with no precipitation mentions coming until the middle of next week.

The main concern with respect to the forecast will be minimum relative humidities over Friday and Saturday where recent HREF model guidance indicated a fairly wide spread in the 25th and 75th percentiles for RHs. The highest spread in these RHs on Friday will be across the central third of the area, where spreads pushing 30% area possible. Current forecast trends closer to the 25th (mixed in with NBM model guidance) given dry air in place across the region; however, there is a situation where these might still be slightly too high. Reasonable floor for Friday ranges between 25%-30% across the central third of the Commonwealth with slightly higher east and much higher west closer to the cold front. Much drier air in the lower levels behind the cold frontal passage will also allow for undercutting of NBM Tds on Saturday; however, the cold front is not notably strong so winds might not be as robust and could limit fire weather concerns; however, this will need to be monitored in future forecast packages.

AVIATION /01Z FRIDAY THROUGH TUESDAY/

A gusty line of showers moved across Bradford just before 00Z and proceeded to fall apart as it moved east. A few more rounds of showers are expected there late tonight, but VFR conditions should persist. The main area of concern in the first 12-15 hours of this TAF package will be at KLNS and KMDT as southeast winds usher in a marine layer of low clouds after midnight. Similar to last night, a low-end MVFR deck is favored but its extent and duration will likely exceed that of Thursday morning. IFR is favored at LNS, MVFR is favored at MDT, and MVFR is possible (~30% prob) at IPT. If a low deck of clouds develops at those airfields, it will likely be slowest to erode at IPT, perhaps lingering until 15 or 16Z.

Gusty winds will develop again on Friday afternoon across the entire area, perhaps gusting as high as 25 or 30 knots at KBFD, and KJST. By Friday afternoon, another round of rain will begin to encroach on northwest PA, with lowering clouds ahead of the rain. Restrictions should arrive at KBFD between 20-22Z with subsequent lowering to IFR ceilings. Additional restrictions are expected at KJST, KAOO, KUNV, and KIPT along and behind the front, but any impacts will occur after 00Z (beyond the current TAF period).

Outlook...

Friday Night...Lingering restrictions northwest

Sat...Restrictions possible early, then trending VFR.

Sun...Mainly dry. Maybe a shower near the NY border.

Mon-Tue...Chance of showers, mainly N/W later in the day.

CTP WATCHES/WARNINGS/ADVISORIES

None.