textproduct: State College

WHAT HAS CHANGED

* Decrease in severe threat across much of southeastern Pennsylvania Thursday afternoon/evening. * Increasing potential for gradient winds on Thursday reaching close to Wind Advisory criteria across N/W PA.

KEY MESSAGES

1) Periods of showers and thunderstorms through Thursday evening bring some excessive rainfall tonight into Thursday morning and severe threat Thursday afternoon/evening.

2) Gusty winds, some of which approaching 40-50 mph, expected across the northwestern half of the Commonwealth on Thursday.

DISCUSSION

KEY MESSAGE 1: Periods of showers and thunderstorms through Thursday evening bring some excessive rainfall tonight into Thursday morning and severe threat Thursday afternoon/evening.

Current radar (as of 4:30AM EDT) outlines a line of gusty showers with an isolated thunderstorm generally along a line north/south of UNV. Recent observations at JST/AOO/FIG outline maximum wind gusts between 30-35kts and there remains not very much in the way of instability in place so generally expect these showers to gradually dwindle as they progress eastward and out of our forecast area around sunrise. In terms of appreciable rainfall, fast movement of these showers will likely make the ground wet but not expecting large-scale rainfall exceeding 0.05" this morning.

Focus will begin to shift towards the MCS and associated cold front that is expected to bring widespread severe weather the portions of central Illinois and the progression of this feature this evening as it tracks closer to the Commonwealth. Recent HRRR/RAP model guidance outlines that the bulk of convective activity will be west of the region before intensity begins to ramp down as it reaches the OH-PA border during the late evening hours. Thus, it looks that the severe threat for our western zones will likely be muted compared to previous concerns, but still should be monitored in the near-term as some model solutions could weaken this feature slightly more than what will be observed. The main concern with respect to this feature in out area; however, will be the prospect heavy rainfall threat across portions of NW PA. PWATs reaching potentially exceeding 1.50" across portions of NW PA with more recent HREF ensemble guidance in the one area of central PA that has relatively low FFGs. That being said, FFGs here do still remain over one inch and with forecast rainfall (even PMMs) not exceeding one inch, it looks as if the flooding concern is fairly low.

The period of most interest to many that have been following the forecast will likely be the Thursday afternoon and evening timeframe. In terms of the severe threat, SPC has downtrended much of the area from a Slight (Level 2/5) to a Marginal (Level 1/5) risk for severe weather. Initial look at guidance points towards timing of the cold front that has lowered the overall severe threat with a faster progression and much more cloud cover from remnant shower/storm activity mentioned in the previous paragraph. This line of showers and thunderstorms will likely be the main driver for the Marginal Risk across much of the region. Further southeast, a slightly more complicated forecast is in play, with the cold frontal passage moving quicker and potentially limiting the prospect of thunderstorms forming ahead of the cold front. In fact, model soundings across the region also look remarkably dry, thus we might have little coverage in thunderstorms in general Thursday afternoon/evening. That being said, there will still be ample shear in place across the region, thus if a thunderstorm does get going with sufficient instability, we cannot rule out that scattered strong-to-severe thunderstorm that will bring down some damaging wind potential across the area. This corridor in SE PA is also highlighted by the most recent HREF probability for severe winds; however, this guidance will also outline the threat has decreased across much of the region.

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KEY MESSAGE 2: Gusty winds, some of which approaching 40-50 mph, expected across the northwestern half of the Commonwealth on Thursday.

Anomalously strong surface low-pressure stationed across southern Ontario moving into Quebec will bring a fairly tight pressure gradient across much of central Pennsylvania Thursday morning and into the evening hours. Closest approach based on more recent deterministic model guidance brings increased winds/gusts ~03Z Thursday and likely continuing throughout the day across much of the region. Recent NBM QMD 24-hour maximum wind gusts brings the southeastern half of the Commonwealth in the 35-40 mph with much of the northwestern half of the Commonwealth closer to the 45-55 mph range. Given that this model guidance does tend to do slightly better than base NBM forecast gusts in this set-up have decided to increase gusts well above NBM model guidance in this timeframe. There remains some potential for Wind Advisory considerations for zones along the Allegheny Front with respect to potential wind gusts Thursday; however, lower confidence and marginal gusts approaching criteria will be make it to where mentions in the HWO suffice this cycle.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Current shower as of 11AM EDT is stationed along a IPT-SEG-MDT line, and this activity is expected to gradually wane as it moved eastward and out the forecast area by ~15Z Wednesday, with a SCT-BKN VFR cumulus deck expected to follow throughout the afternoon. A few diurnally driven showers/thundershowers may develop (PROB30) between 17Z-21Z over the eastern terminals (KIPT/KMDT/KLNS) as the shortwave continues exiting the region, though most areas will remain dry. The 12Z TAF package resolves these mentions with PROB30s with lowest confidence on timing and coverage. Otherwise, southwest winds will increase to around 10 kts, with gusts of 15 to 20 kts likely during the day.

High clouds will filter overhead Wednesday evening ahead of a potent low pressure system moving across the Great Lakes. A band of -SHRA/-RA will overspread much of the Commonwealth from west to east overnight, followed by the passage of a decaying MCS. While the MCS will be weakening as it encounters more stable air across central Pennsylvania, it will be coincident with an unseasonably strong nocturnal LLJ (~65 kts at 850 mb, above the 99th percentile for this time of year). As such, a period of LLWS concerns + southerly ridgetop wind gusts are likely overnight into Thursday morning for virtually all terminals, along with the potential for gusty winds to be transported within the weakening MCS. The main change with regards to this was penciling in some PROB30s for TSRA at BFD where there is slightly higher confidence in some TS at the airfield and extending LLWS mentions further east towards IPT/UNV/JST/AOO towards the end of the TAF package.

Outlook...

Thu...Restrictions possible in -SHRA/-TSRA.

Fri...Showers ending.

Sat-Sun...Chance of afternoon showers and thunderstorms over the north and west.

CTP WATCHES/WARNINGS/ADVISORIES

None.