textproduct: State College

WHAT HAS CHANGED

* Little change in Sunday's severe thunderstorm potential, built out more details pertaining to timing and threats. * Added mentions of icing potential Monday morning with dropping temperatures and freezing rain/drizzle potential across the Poconos.

KEY MESSAGES

1) Severe thunderstorms possible this afternoon and evening across much of central Pennsylvania, highest chances south of the I-80 corridor.

2) Some icing potential Sunday night into Monday morning across portions of NE PA, slick spots possible for the Monday morning commute.

DISCUSSION

KEY MESSAGE 1: Severe thunderstorms possible this afternoon and evening across much of central Pennsylvania, highest chances south of the I-80 corridor.

Severe threat will be in play across much of central PA this afternoon and evening, with surface dewpoints across western Indiana surging towards the region. Recent HREF model guidance continues to indicate favorable probabilities (generally between 50-70%) for surface dewpoints reaching into the mid-to-upper 50s this afternoon for regions south of I-80. Bulk wind shear across the entire area by this time will also markedly increase which would support thunderstorm development to become organized enough to sustain themselves this afternoon/evening.

There does remain some uncertainty with respect to CAPE with some recent hi-res model guidance indicating some breaks in the clouds across south-central Pennsylvania during the afternoon hours. If these solutions begin to pan out, this will allow for an uptick in instability, which would increase the severe threat. For reference, model solutions on the lower end of CAPE values top out between 500-600 J/kg while higher-end solutions of CAPE values exceed 1000 J/kg and push closer to the 1500 J/kg mark which would be much more sufficient for severe weather potential this afternoon. The spread is most pronounced across western portions of the forecast area, which is where model guidance is the most uncertain with respect to cloud cover later today.

All of that being said, SPC has continued with the slight risk for severe thunderstorms this afternoon/evening for areas south of the I-80 corridor. Damaging wind threat continues to highlight areas mainly south of I-80 where there is there remains some higher potential for instability with ample shear in place. Fact cell movement will also give ample convective and kinematic energy to support the damaging wind threat this afternoon and into the evening. The hail threat across the southern half of Pennsylvania bears watching, with steep (7-7.5 C/km) 700-500mb lapse rates continuing to be signaled in recent hi-res model guidance. Large hail will be possible, as SPC outlines with their CIG1 hatching across the southern half of Pennsylvania. These hazards are expected to be focused in the 2PM to 9PM timeframe today, gradually spreading north-to-south throughout the afternoon and evening hours.

In terms of the tornado threat across the region, there remains a non-zero threat across portions south of I-80. This threat is not as robust as the wind/hail threat. One of the main negatives with respect to the tornado threat read out in looking at model guidance for LCLs this afternoon. LCLs look to be relatively high with recent RAP model guidance indicating LCLs between 2000-3000m across the southern half of the region. The higher cloud bases will make it slightly harder (not impossible) to form, especially with low-level dry air in place.

Lastly, flooding concerns given some locations have pushed slightly above one inch storm total rainfall amounts with more localized pockets of heavy rainfall. Highest rainfall signals appear across the southern tier, where recent Drought Monitor continues with D1-D2 drought. FFGs in this area would likely need to be exceeded in the one-hour timeframe, where we're looking at slightly below one inch to exceed guidance. Given this, not foreseeing a lot of hydrological issues with this system especially given quick movement of storms.

KEY MESSAGE 2: Some icing potential Sunday night into Monday morning across portions of NE PA, slick spots possible for the Monday morning commute.

A transition to rain/snow and potentially some freezing rain or drizzle across the Poconos is possible based on recent model soundings/guidance Sunday night into Monday morning. Most recent HREF model guidance has trended slightly up with respect to probabilities of measurable ice accumulation across portions of Tioga, northern Lycoming, and Sullivan counties with a steep drop off further south/east where precipitation looks to end slightly quicker as temperatures continue to fall behind the cold frontal passage. This threat seems relatively marginal at this time and a transition to more rain/snow seems like the most plausible solution, with some potential for a quick measurable snowfall (below one inch) possible across portions of the northern tier.

Comparing to recent events about a week ago, the drop in temperatures are not expected to be as rapid and winds will be slightly less gusty Sunday night into Monday compared to last week's event; however, cannot rule out with a fairly quick temperature drop that surfaces that remain wet with the cold frontal passage freeze and allow for isolated slick spots for the Monday morning commute. At this time, have introduced mentions in the HWO for the potential for a light glaze of ice across the Poconos with the freezing rain/drizzle; however, isolated slick spots could be possible across a larger area of central Pennsylvania Monday morning.

AVIATION /07Z SUNDAY THROUGH THURSDAY/

Some changes for the 06Z TAFS.

1. Took out mention of lower CIGS later on during the night here, airmass quite dry, and radar returns are mainly mid levels.

2. Added a few lines after 00Z Monday.

3. Adjusted LLWS direction, based on VAD winds showing up at this time.

Earlier discussion below.

VFR conditions are expected through the night and most or all of Sunday morning across Central and SW PA (KUNV, KAOO and KJST) with increasing mainly mid and high level cloud cover.

Low-level flow becomes southeasterly overnight as high pressure exits the region to the east, which will allow for low clouds to work their way into eastern PA after 06Z. This should bring a period of MVFR or IFR conditions to IPT, MDT, and LNS into Sunday morning. A SW to WSW, 40 to 55 knot low-level jet will lead to LLWS concerns late tonight through the mid morning hours Sunday for all TAF sites.

Clouds lower across northern PA through the morning, with ceilings at BFD likely becoming MVFR by 14-16Z as a few showers move into the area near and in advance of a cold front. VFR conditions will return across eastern PA by late Sun morning, as the low clouds dissipate.

Numerous thunderstorms should develop ahead of a cold front after 18Z on Sunday, particularly for areas south of Interstate 80 where the best combination of wind shear and instability will exist due to the warmest temps and sfc dewpoints. These storms will be capable of producing strong wind gusts (over 45 KTS) from the west to west-northwest and perhaps a few instances of large hail.

Outlook...

Mon...Rain gradually tapers off north to south, transition to snow possible mainly north of I-80 (BFD/IPT). Restrictions possible within rain/snow showers.

Tue...Mainly dry with VFR conditions. Slight chance of snow shower across northern PA late which could bring some restrictions.

Wed...A few rain showers across northern PA.

Thu...Rain with restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.