textproduct: State College

WHAT HAS CHANGED

* Pushed back the primary threat for scattered strong to severe thunderstorms by a few to several hours later today and this evening and greatly reduced pops through the mid afternoon hours.

KEY MESSAGES

1) Numerous showers and thunderstorms will affect Central PA late today and tonight. Some may be severe, especially across the northwest third of CTP CWA.

2) Cold front moves through later tonight and early Easter Sunday. Much colder next week but warming up again by week's end. Light snow likely across the northern tier Monday/night.

DISCUSSION

KEY MESSAGE 1: Numerous showers and thunderstorms will affect Central PA late today and tonight. Some may be severe, especially across the northwest third of CTP CWA.

A slowly northward moving warm front extended from near Olean New York, southeast to near Selinsgrove and Allentown this midday hour.

A few showers were developing along this airmass boundary as warmer and moist air overrides it from the southwest.

A deg C or two of mid level WAA from the SW (with a several more hour period of weak mid-level lapse rates) and a neg tilt mid-upper level ridge (with slightly confluent flow aloft, beneath the right exit region of jet max approaching from the midwest) will result in a mainly dry and very warm afternoon across much of Central and Southwestern PA.

Temperatures will peaking in the 70s to low 80s with an increasing, moderately gusty southerly breeze of 15-20 mph. These temps will be close to record maxes in the south (see Climo section below).

Some showers will continue across the North-Central and Endless Mountains regions of the state this afternoon where temperatures will be a bit cooler in the 60s.

More showers and thunderstorms are expected tonight in the very warm and humid airmass along and ahead of a cold front.

The cold front and a band of rain and possible thunderstorms will push through late tonight and Sunday morning.

Dewpoints will also be rising as the wind becomes more- southerly through the day.

High res models continue to suggest that some of the first TSRA initiation may take place across the Scent Mtns (between 20-21Z), within an area of pooling moisture and PWATs AOA 1.4 inches. However, the storms in the south-central mountains will be displaced from the best shear, but should have some of the best instability/CAPE of the day.

These late afternoon/early evening storms expand and move northward through all of the central mountains. That should get them into some of the better shear, but also put them into the colder/more stable air.

As this convection moves over the warm frontal boundary, it could have some enhanced low level shear/helicity and have more potential to make a strong, rotating updraft and perhaps a risk for a tornado. Heavy rain is also possible, and may lead to isolated instances of minor flooding where several heavy SHRA/ TSRA northeast.

A second round of storms is expected to flare up in the NW where a strong cold front will be moving in during the late evening hours. Some storms in the warm sector/ahead of the cold front could have supercellular characteristics, but the main threat will be wind damage. The front will be the main focus for severe weather in the evening and early nighttime. Storms reach Warren Co just after 00Z and Coudersport/Clearfield by midnight. The showers/storms along the front continue eastward at a slow clip, and may not get to Harrisburg until 12Z. But, they'll only spend 2-3 hrs there before leaving the CWA around noon on Easter.

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KEY MESSAGE 2: Cold front moves through later tonight and early Easter Sunday. Much colder next week but warming up again by week's end. Light snow likely across the northern tier Monday/night.

The air behind the cold front is going to make temps fall throughout the day over all but the Lower Susq. Temps probably peak in the Lower Susq in the morning and fall after the rain starts and then continue falling through the day. Mid aftn temps will be in the L40s NW and ~60F in the SE. Sfc low will pass way to our N, but the center of the cold dome will slide to our S. Still, 8H temps drop to -8C along the NY border, and -4C in the S. Maxes on Monday will have some bust potential. Blended guidance mean maxes of 40F N and U50s S seems ripe for a bust with such cold temps aloft. Wild cards include: we may not mix to 8H that day, the April sunshine will try to do it's best, and downsloping in the Lower Susq.

A Clipper-like system dives across the Upper Great Lakes and into NY/PA later Mon/MOn night. After that, the flow favors lake-effect showers. The temps will be cold enough across the nrn tier of counties to allow it to be mostly snow anytime by the middle of the day. Some accums of <=1" should occur on the hills of the north, but that will be along a 48-hr stretch. Some snow may mix into the mostly-rain showers in the central mtns and Laurels Mon night. Dry and cold Tues with low RHs (m20s-30s), and a little warm up on Wed will result in even lower RHs (20s).

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Lingering IFR to LIFR conditions were noted (as of 16Z) near and to the north of KIPT, just to the north (on the cool-side) of a slowly northward moving warm front.

Elsewhere within the warm sector, low clouds mixed-out and cloud bases have greatly lifted to 6 KFT AGL or higher and will remain that way for the bulk of the time through 22-23Z. Afterward, scattered SHRA/TSRA will be developing over the Scent Mtns and moving/expanding NE across the Middle Susq Valley.

PROB30s for -TSRA are maintained for these sites, with HREF mean SBCAPE values around 1000 J/kg suggesting sufficient instability for thunderstorm development given the moisture/lift in place.

Flight conditions will likely deteriorate again after 00Z Sunday as low-level moisture transport increases ahead of an approaching cold front. Widespread MVFR ceilings are probable (60-70%), with IFR ceilings being possible as well (30% chance). Moreover, a backdoor cold front is expected to nose into the Lower Susquehanna Valley (KMDT/KLNS), and potentially reach as far west as KUNV by around 03Z, which could reinforce flight restrictions. While instability will be waning overnight, HREF mean MUCAPE values of around 100 J/kg suggests isolated thunder may be possible with the frontal passage. A PROB30 for -TSRA was included at KBFD between 05-08Z, given that MUCAPE values further diminish thereafter. LLWS is also a concern overnight as 850 mb winds increase to 40-50 kts over a decoupled boundary layer, with LLWS mentions included at KBFD/KMDT/KLNS at this time.

Outlook...

Sun...Restrictions possible with showers and thunderstorms ahead of and along a strong cold front.

Mon...Breezy and cooler with MVFR N/W and VFR elsewhere, with scattered rain/snow showers possible.

Tue...Mainly VFR with a few rain/show showers.

Wed...VFR.

CLIMATE

Here are the warmest high and low temperatures recorded for April 4th at area climate sites:

SITE LOW HIGH Harrisburg 60/1974 | 82/1999 Williamsport 62/1981 | 84/1921 Altoona 61/1974 | 80/1950 Bradford 50/1981 | 72/1981 State College 57/1999 | 81/1910

CTP WATCHES/WARNINGS/ADVISORIES

None.