textproduct: State College

WHAT HAS CHANGED

* Increasing odds for a noticeably cooler and rather wet start to Memorial Day weekend * SPC expanded the marginal risk for severe thunderstorms this afternoon into the Lower Susquehanna Valley.

KEY MESSAGES

1) Summerlike heat wave peaks today with record challenging temperatures in the mid 80s to mid 90s

2) Scattered strong to severe thunderstorms are possible across central PA later today/tonight and south central PA Wednesday afternoon

3) Major cooldown arrives Thursday and lasts into Memorial Day weekend with widespread soaking rainfall likely Friday and Saturday

DISCUSSION

KEY MESSAGE 1: Summerlike heat wave peaks today with record challenging temperatures in the mid 80s to mid 90s

Another scorcher today with max temps 85-95F approaching daily records (see climate section). Max heat indices are fcst to reach the upper 90s, just shy of heat advisory criteria across the LSV. Record high minimum temps are also on the table for tonight. The one area of uncertainty regarding this temperature forecast is the showers and thunderstorms that have developed over the Laurel Highlands early this afternoon. These may help to keep high temperatures in the low to mid 80s for the areas that see these showers and storms.

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KEY MESSAGE 2: Scattered strong to severe thunderstorms are possible across central PA later today/tonight and south central PA Wednesday afternoon

Widely scattered showers and thunderstorms will continue to pop up through the rest of the afternoon in a moderately unstable airmass. Wind shear is very weak (0-6 km shear is generally less than 20 knots), so any cells that develop may struggle to maintain themselves for long. That being said, steep low-level lapse rates and dry mid-levels may support a few strong to marginally severe downbursts in any longer-lived storms. The best thermodynamic environment for downbursts is over the Lower Susquehanna Valley, with DCAPE values exceeding 1000 J/kg this afternoon, but dry air may limit the coverage of storms in that part of the forecast area. SPC has expanded the marginal risk of severe storms to highlight this potential.

A weakening line of showers and thunderstorms is expected to move into northwest PA during the late evening ahead of a cold front. SPC has maintained a slight risk of severe weather across northwest PA as this line may produce strong to severe wind gusts as it moves through. The line will quickly encounter a more stable environment as it progresses southeastward, so the threat for severe wind gusts should quickly decrease as it moves farther into the forecast area.

Convective focus shifts to south central PA on Wednesday with narrow plume of weak to moderate CAPE fcst ahead of the aforementioned cold front. Steep low level lapse rates may support a few instances of strong to severe winds. The magnitude & location of the severe threat will be highly dependent on CFROPA timing and amount of cloud cover. SPC MRGL risk (level 1/5) covers the southeast half of the CWA where max temps are fcst btwn 80-90F.

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KEY MESSAGE 3: Major cooldown arrives Thursday and lasts into Memorial Day weekend with widespread soaking rainfall likely Friday and Saturday

Relief from the early season heat wave arrives in earnest on Thursday with max temps 20-30 degrees lower vs. Wednesday across the southeast half of the forecast area. Strong high pressure slides from the Lower Great Lakes to off the New England coast to close out the week. Model data shows the high channeling southward down the east-side of the Appalachians, setting up a cold air damming type event heading into Memorial Day weekend. We may not be bearish enough on daytime temperatures. It also appears probable that periods of soaking rain will dampen at least the start of the holiday weekend. WPC 48hr QPF amounts are quite wet with a widespread 1-2.5 inches fcst from 12Z Friday to 12Z Sunday.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

VFR conditions are expected to prevail for this hot afternoon. Winds have become gusty with daytime heating and ridging building from our south, gusts could reach up to 25kts. Convection is already being observed over the southwestern airfields, but forcing is weak making coverage and strength of these storms uncertain for the remainder of today. Instability is high enough that any storm that does develop will likely contain lightning until after sundown. The most likely airfields to be impact by these storms through 00Z will be JST and AOO.

Tonight, the low pressure system over the Great Lakes will approach from the northwest. This will cause ceilings to lower and bring additional rain chances from northwest to southeast for the day on Wednesday. MVFR ceilings are likely ahead of the cold front at BFD, JST, AOO, UNV, and IPT. The only site likely to experience IFR ceilings will be BFD early tomorrow morning, elsewhere should remain MVFR or better.

Lastly, LLWS mentions were included at our northern terminals (KBFD/KIPT) through the evening/overnight hours as 900 mb winds increase to around 40 kts due to pressure gradient between low pressure over Southern Canada & Bermuda High over the Western Atlantic.

Outlook...

Wed...Restrictions probable through Wednesday night with widespread showers/storms along CFROPA.

Thu...Lingering showers across S PA, restrictions possible.

Fri...Restrictions likely with rain moving in from south to north, lasting into Fri night.

Sat-Sun...Restrictions likely with rain.

CLIMATE

The following climate sites across central Pennsylvania set new daily high temperature records for May 18th:

* A record high temperature of 92 degrees was set in Altoona, breaking the previous record of 91 degrees initially set back in 1962. This record was tied two times previously, back in 1996 and 2017. * A record high temperature of 86 degrees was set in Bradford, breaking the previous record of 85 degrees set back in 1962.

** Note: The high temperature for State College will be reported around 7AM on 5/19. This is due to the State College CO-OP observations running 7AM-7AM. Thus, today's (5/18) high temperature will be reported in tomorrow's (5/19) observation. This record, for reference, is 92 degrees set in 1962.

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Abnormally warm temperatures will continue on May 19th, thus multiple sites could challenge daily record high temperatures.

Daily Maximum High Temperatures for May 19th: * Harrisburg: 95F set in 1962 * Williamsport: 96F in in 1996 * Altoona: 92F set in 1996 * Bradford: 85F set in 1962 * State College: 92F set in 1934, tied in 1996.

CTP WATCHES/WARNINGS/ADVISORIES

None.