textproduct: State College

WHAT HAS CHANGED

* Severe Thunderstorm Watch #492 in effect until 01Z Sunday

* Expanded the Flash Flood Watch (until Midnight EDT tonight) north to cover all of the CWA with the exception of the Northern Tier Counties - based on this mornings MCS and additional several rounds of storms with very heavy rainfall rates this afternoon and evening as a warm front over the Alleghenies moves slowly across Central PA

* Tornado risk is still 5% chc of a tornado w/in 25 miles of a point), but the conditional intensity group 1 has been introduced S & E of Harrisburg

* Damaging wind gust threat is high (45% chc of wind damage w/in 25 miles of any point) over almost all of the CWA

KEY MESSAGES

1) Severe thunderstorms with damaging winds and locally very heavy rainfall are possible from mid-day into late this evening. Tornadoes also possible.

2) Sunday & Monday quiet but Tues looks active again with severe and flood risks returning.

DISCUSSION

KEY MESSAGE 1: Severe thunderstorms with damaging winds and heavy rainfall are possible through early this evening. A few tornadoes area also possible with the strongest storms.

Little change from the previous disc regarding the severe TSRA potential this afternoon/evening as the one-two punch of a warm front/cold front helps to focus several rounds of convection.

Based on the number of afternoon TSRA that impacted ELK county and adjacent parts of the northern Mtns (along with the rather extensive coverage of ground- priming TSRA that blossomed into a MCS this morning), we thought it would be prudent to expand the Flood Watch (through 04Z Sunday) a layer of counties further north.

Previous...

SPC has kept the Enhanced risk of severe weather expectation for all of Central PA today. They have also added a special (conditional intensity group or CIG) hatching to SErn PA (including areas S & E of Harrisburg. The CIG designation means that tornadoes that do develop in that area could be stronger than just EF0/1. A reasonable maximum intensity of EF2-strength tornadoes is possible today - again, mainly in SE PA.

Early showers/thunderstorms two will occur near/along a pair of warm fronts moving north and east through the area this morning and early afternoon.

The vort max supporting the initial rounds of mid to late morning convection will be moving east of the Susq Valley early this afternoon, followed by some cloud thinning and recovery with pockets of MDT-STG Sfc-Based instability developing for the afternoon and evening hours. This will help to ramp up the threat for SVR TSRA and increase rainfall rates within the very juicy airmass.

The timing of that vort max and the SHRA/TSRA it helps generate look like they will be early enough in the day that ample heating should then occur as the main warm front pushes into the srn counties and across the CWA late this morning and early this afternoon. As the CAPEs climb, and the wind profiles/hodographs become rounded, the risk for deep convection that would also take advantage of the high helicity to produce supercells increases. This convection could start a couple hours earlier than climatologically-favored timing...more like a noon kickoff than early- to mid-aftn. The risk for discrete, spinning cells is highest over the Lower Susq, but is possible anywhere across the CWA with the warm front(s) and varied boundaries from earlier rain and differential heating playing roles to make the near storm llvl/blyr environment favorable. 70F+ dewpoints will arrive in the S with the first warm front, and the LCLs should be low and favorable for tornado development. The Sig Tor Parameter (STP) is highest over the SE, esp in Lancaster Co. Lower (l-m60s) dewpoints hang on across the N for much of the day.

The NW will have the next-best chance for severe storms to develop. However, the increasing moisture and moderate-high shear and moderate-high CAPEs will mean we'll have to watch all of the CWA for storm initiation during the early and mid afternoon hours.

Some of the storms could also be supercells away from the SE - at least early in the event. The wind profile becomes more- aligned/straight hodographs that would be more favorable for storm clusters or a broken line, esp if a pre-frontal trough materializes.

The cold front moving down from the NW should arrive late in the day and could continue to provide lift for storms in the NW half of the area into the evening. But, it may arrive in most places too late (after stabilization from other/earlier precip) to make a wide threat for severe wx to occur along it as it passes. Expect precip along the cold front to weaken and fall apart the deeper it gets into the CWA (into the central mtns).

If there is a chance for a bust or weakness in the severe forecast it is over the central mtns/ridge-and-valley region. Potent storms are likely/certain in the SE and the NW, but are less certain in the Laurels and ridge and valley counties.

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KEY MESSAGE 2: Sunday & Monday quiet but Tues looks active again with severe and flood risks returning. A drying trend is expected Sunday with noticeably lower humidity allowing for a pleasant day and relatively cool night. However, latest RRFS shows some near sfc smoke returning to CPA from the Great Lakes. This could result in reduced air quality and hazy sky conditions.

Additional bouts of rain/t-storms are possible into the workweek. The first of which will be around Tuesday (Day4) as a well-developed synoptic storm moves through the Great Lakes and NE CONUS. SPC has been highlighting a 15% prob (equivalent to SLGT risk, level 2/5) for Central PA. Forcing/support for this medium range severe outlook appears fairly robust and would take stock in this risk manifesting at shorter ranges. WPC is also highlighting Tuesday for heavy rainfall and possible flooding with the synoptic system generating multiple chances for rain with high atmos moisture in place.

Temperatures will trend near to slightly below historical averages through late week into month-end with some noticeably cooler nights possible.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

Aviation concerns continue to revolve around convection across northwest and south central PA through the afternoon hours. To a lesser extent, FU continues across the region, dropping visibilities to MVFR and IFR.

The first area of convection will move out of the southern part of the state by late afternoon and focus will rthen shift back to northern PA as a cold frontal boundary sags through the state.

Confidence is about 40% that convection and associated -SHRA will combine with low level smoke to bring down visibilities to LIFR for a time this evening and overnight.

Expect conditions to improve Sunday morning as the cold front pushes out low level smoke, and convection moves south of the area. Conditions will improve from IFR to VFR by mid morning with the last hold outs being any locations that had dense fog overnight.

Outlook...

Mon...Mainly VFR.

Tue-Wed...Restrictions possible in showers/t-storms with cold FROPA expected between Tuesday/Wednesday.

CTP WATCHES/WARNINGS/ADVISORIES

Flood Watch until midnight EDT tonight for PAZ010>012-017>019- 024>028-033>036-041-042-045-046-049>053-056>059-063>066.