textproduct: State College

WHAT HAS CHANGED

* Increased odds for rain across the far northwest Alleghenies this evening into tonight

KEY MESSAGES

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 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.

-------------------------------------------

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 /18Z THURSDAY THROUGH TUESDAY/

Current satellite indicates much of the area with clear skies with the main area of clouds stationed along Lake Erie and the spine of the Alleghenies. Current guidance continues to indicate widespread VFR conditions across the entire region through 18Z Friday with very high (over 90%) confidence at all airfields outside of BFD which still looks to have high confidence (~60-70%) in VFR conditions continuing through 18Z Friday. The main reason for slightly lower confidence at BFD is due to some potential for light rain showers between 00Z and 10Z Friday with a slow-moving cold front northwest of the region continuing to track closer to the region. Low-level dry air will limit precipitation mentions for much of the area throughout the rest of the TAF package, with precipitation chances becoming more plausible after 18Z Friday with BFD being the main candidate for a return to rain showers towards the end of the TAF package. Shower activity (thunderstorm potential looks low with not a lot of instability available) becomes more of an issue for JST/IPT/UNV after 18Z with lower chances further south/east across central Pennsylvania.

Outlook...

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.