textproduct: State College

WHAT HAS CHANGED

*Minimal changes since the last discussion

KEY MESSAGES

1) Multiple chances for showers and thunderstorms through the end of the week. Severe thunderstorms possible today, mainly across the northwest.

2) Very warm temperatures continue through the weekend, although a backdoor cold front will likely bring cooler temps across eastern zones Thursday.

DISCUSSION

KEY MESSAGE 1: Multiple chances for showers and thunderstorms through the end of the week. Severe thunderstorms possible today, mainly across the northwest.

As of 0330 AM Tue, central PA is mainly dry with earlier batch of showers and thundershowers moving out of Sullivan Co. Looking upstream, there are a few clusters of convection over the Great Lakes region. The first is a decaying MCV across southern Michigan. Over the past couple hours, it lost all of its lightning and heavier precip. If it maintains steady state motion and intensity, it will arrive in Warren County by 0630 AM with nothing more than light rain. The second convective cluster is to the north of the aforementioned MCV. This cluster is much more electrically active, but it is tracking to the east and will likely stay north of the PA NY border for the time being. However if this cluster develops more of a flanking line of convection on its south side, it would probably move into our NW zones between 0730 and 0800 AM. A third cluster of convection is located just west of Chicago. This convection is also showing signs of weakening lately, but its remnants would most likely be over our NW zones by mid afternoon.

Model soundings show a capping inversion and just 200-500 J/kg of MLCAPE through this afternoon, so currently not expecting much intensification of convection as it moves in from upstream. However there is a noticeable uptick in instability progged for the evening hours (5-11 PM). The HREF shows temperatures in the mid 60s-70s, dewpoints in the mid to upper 50s, and surface- based CAPE in the 500-1000 J/kg range during this timeframe. 0-6km bulk shear of 30 to 45 knots will be supportive of organized convection. Model hodographs show decent curvature in the low levels, with 0-3 KM SRH in excess of 200 m2/s2. Damaging winds appear to be the primary threat with these storms, but a conditional hail and tornado threat is not off the table especially with any discrete cells. SPC continues the slight risk over the northern half of Central PA, with the highest probabilities of thunder in the NW.

As a slow moving cold front approaches NW PA Tuesday night, additional rounds of showers and thunderstorms are likely to ride along it. With instability decreasing by midnight, these storms will gradually weaken. Main concern may transition to a heavy rainfall threat for any training heavy precip. Isolated flash flooding may become possible, but as of right now the threat is too low to include any of our counties in a Flood Watch (there is one up for much of western New York State).

The cold front will inch southward across Central PA on Wednesday, bringing additional showers and thunderstorms to the region. With the front dividing the area, high temps will range from the mid 50s in the northern tier to the mid 70s across the south. SPC draws a MRGL risk for severe weather south of Interstate 80.

KEY MESSAGE 2: Very warm temperatures continue through the weekend, although a backdoor cold front will likely bring cooler temps across eastern zones Thursday.

There is fairly large spread in model guidance for temperatures on Thursday. For Harrisburg as an example, the interquartile range for high temps from the NBM is 59-74F. Some guidance (NAM) even keeps highs in the 40s. Much of the uncertainty traces back to limited predictability with respect to the placement of a backdoor cold front. Don't be surprised if we end up with temps in the 40s and 50s with low clouds, drizzle, and patchy fog across our eastern zones Thursday. The cool weather does look to be limited just to Thursday. By Friday the front lifts north as a warm front and model guidance is more tightly clustered showing temps in the 70s areawide. Warm weather sticks around for the weekend before a cold front moves through later Sunday knocking temps back down for Monday.

AVIATION /07Z TUESDAY THROUGH SATURDAY/

Thunder at BFD earlier, now over Wellsboro, soon to exit our area.

Most of the showers and storms today will be north of the TAF sites. Main issue early on will be LLWS.

As cold front sags southward late this evening, BFD will be the most likely TAF site for showers and storms late.

Some potential for strong storms, but most of the area remains capped off today. Thus did not hit the activity very hard in the TAFS at this point, mainly at BFD.

For Wednesday, cold front slows down, so one could see more widespread showers and storms.

Wet weather at times into the upcoming Easter weekend.

Earlier discussion below.

Rain showers have developed across portions of northern PA this evening as west-southwesterly flow ahead of an approaching low pressure system brings mild, moist air across the region.

Prevailing VFR is expected across most of central PA through 00z Wednesday, though flight restrictions are possible across the Northern Tier (KBFD) and the Allegheny Front (KJST). Moreover, LLWS is expected regionwide through 12z-16z due to a 40-50 kt LLJ developing overnight. LLWS chances diminish by midday Tuesday as daytime heating mixes out the boundary layer, with 10-15 kt winds gusting to 20-25 kts expected regionwide in the afternoon.

Across the Alleghenies, a combination of moisture advection and orographic lift may contribute to lower ceilings overnight, with the latest LAMP bringing 30-50% probabilities for MVFR ceilings at KJST from 10z to 16z. While the HREF suggests MVFR ceilings might linger after 16z, it seems more probable that daytime heating and southwesterly winds of around 15 kts would raise ceilings to VFR through mixing, as suggested by the HRRR.

Elsewhere, VFR is expected through 00z Wednesday, along with lower rain shower coverage as the best forcing for ascent is positioned farther north. With that said, mean SBCAPE values on the HREF rise to around 1000 J/kg Tuesday afternoon, which suggests at least a conditional risk for showers and thunderstorms. The HREF has 1-hr PoPs fairly low (10-20%) for much of central PA Tuesday afternoon/evening, owing to a lack of organized forcing during that timeframe, with higher PoPs (30%+) primarily after 00z Wednesday.

Outlook...

Wed-Sat...Numerous SHRA with restrictions possible, especially on the cool side of a frontal boundary that oscillates across central PA during this timeframe.

CTP WATCHES/WARNINGS/ADVISORIES

None.