textproduct: State College

WHAT HAS CHANGED

* Slight adjustments to rainfall forecast on Wednesday, mainly across southeastern Pennsylvania. * Built out slightly more details on the severe threat for Wednesday afternoon/evening. * Temperatures trended slightly colder for late week and throughout the weekend, increasing freeze and freeze threat.

KEY MESSAGES

1) Beneficial rainfall Wednesday, a few strong to severe thunderstorms possible on Wednesday across the south.

2) Cool pattern late this week and into the weekend brings renewed frost/freeze concerns to central Pennsylvania.

DISCUSSION

1) Beneficial rainfall Wednesday, a few strong to severe thunderstorms possible on Wednesday across the south.

Looking at mainly cloudy skies overnight, with areas of drizzle and fog. Fog could shroud the ridgetops early Wednesday morning. This could lead to some hazardous travel overnight so any motorists expected to be on roadways that traverse higher elevations should be ready for rapidly changing conditions over short distances.

A widespread soaking rainfall is forecast across all of central Pennsylvania, with precipitations largely increasing late Wednesday morning (western half) into Wednesday afternoon (eastern half) and continue into Thursday morning. Current forecast continues to outline a widespread 0.75" to 1.00" across much of the area with far southeastern Pennsylvania potentially trending slightly lower in recent model guidance. Some thunderstorm development could lead to locally higher totals between 1.00" and 1.25" mainly across northwestern Pennsylvania where soils remain the most moist. Despite this, flooding risk across the entire forecast area remains low on Wednesday.

The Storm Prediction Center has retained the Day 2 marginal risk for severe thunderstorms across portions of SW PA, mainly along and south of the I-99/US-22 corridor. With regards to the severe threat, there remains considerable uncertainty mainly in the form of instability. Mean CAPE values from the most recent 12Z HREF run outline CAPE values ~200 J/kg which is generally not favorable for severe thunderstorm potential; however, spreads between the 10th-90th percentiles outline a spread of near ~500 J/kg. This spread is mainly driven by potential cloud cover, which looks to be fairly expansive across the region throughout much of the day on Wednesday. It is of note that most recent HREF guidance has had a slight uptick in CAPE values; however, will need to have some ability to destabilize in order for this severe potential to materialize. At this time, damaging winds will be the main concern; however, given ample low-level shear will also need to keep an eye on any rotating storms.

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KEY MESSAGE 2: Cool pattern late this week and into the weekend brings renewed frost/freeze concerns to central Pennsylvania.

Cool cyclonic flow pattern aloft sets up over the eastern United States for the late week period with guidance indicating a closed 500 mb low rotating over the northern Great Lakes into southeastern Canada. This allows for continued high confidence in temperatures 5-15 degrees below the historical average Thursday through Sunday. Breezy conditions due a slight pressure gradient set up over Pennsylvania on Thursday could allow for wind gusts between 25 and 35 mph, which will add to the relative chill factor.

Frost risk is non-zero given the colder nighttime temperatures especially later in the weekend, but it appears more likely to see an advective freeze or nothing scenario given the persistent cloud cover and sustained northwesterly wind. Guidance has trended slightly cooler for the weekend period, with sub- freezing temperatures expected across the northern tier beginning Thursday night into Friday morning, with multiple overnight periods of sub-freezing temperatures possible through Sunday morning. The coldest of these timeframes based on recent NBM guidance will be Saturday night into Sunday morning, where recent forecast brings sub- freezing temperatures to much of the area along and north/west of the I-80/I-99 corridor.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Scattered showers across the eastern airfields will taper off by dinner time and then some additional pockets of rain will redevelop overnight, bringing occasional reductions in visibility across the region. No thunder is expected with these showers.

Ceilings will gradually deteriorate areawide tonight into Wednesday, with IFR/LIFR likely everywhere across west and central PA (BFD, JST, AOO, UNV) and MVFR favored at IPT, MDT, and LNS. There is some uncertainty in the onset of ceiling restrictions across the region, and lower ceilings may work into the southeast earlier than currently conveyed in the TAF package. There is also some potential for reduced visibility in low clouds/fog, especially across the west. Whether this will manifest in simply low ceilings or in reduced visibility remains a bit unclear. But IFR/LIFR restrictions are expected regardless.

More widespread showers, and perhaps some thunderstorms, will overspread the area on Wednesday from southwest to northeast. Visibility restrictions are likely (MVFR/IFR) as ceilings remain low. The best chance for thunder will be in southern PA, first affecting JST and then potentially extending east toward MDT and LNS later in the afternoon/evening.

Outlook...

Wed...MVFR/IFR favored due to low cloud ceilings & rain overspreading the region. -TSRA possible south.

Thu...VFR/MVFR in the morning as rain ends west to east, becoming VFR in the afternoon.

Fri-Sat...Mainly VFR with brief restrictions possible in showers.

Sun...Restrictions possible northwest in -RA and possibly some -SN.

CTP WATCHES/WARNINGS/ADVISORIES

None.