textproduct: State College

WHAT HAS CHANGED

* Higher confidence in timing of most thunderstorms (2 PM through 11 PM)

KEY MESSAGES

1) Strong to severe storms expected for Central PA this afternoon and evening.

2) Rare SPC Day 5 outlook for SVR TSRA potential across much of PA on Thu 6/18

DISCUSSION

KEY MESSAGE 1: Strong to severe storms expected for Central PA this afternoon and evening.

High clouds will cover most of PA this morning. The filtered sunlight may take an hour or two longer to heat things up today. The temperatures should still get near convective temp in the early afternoon in the west. Storm motion will bring these into Central PA while additional but isolated storms pop up over the CWA. A cold front will be sweeping across the CWA later today and early tonight. That front will be the main focus for thunderstorms late this aftn and this evening. A pre-frontal trough may develop a few storms in the more-humid region of the Lower Susq in the afternoon, but models have backed off somewhat on this possibility with the 00Z runs. Time-lagged ensembles still hold onto some hope for this, but the trend is drier and for fewer storms in the air over the central mountains and Susq Valley. A cap aloft will probably stifle deep convection in those areas before 5 PM EDT until heights start to fall and lapse rates steepen.

The dewpoints in the SE are already a little higher than the rest of the CWA, and will rise even more through the day, but high moisture (Td in the m-u 60s) will also pool ahead of the front. Deep-layer (0-6km) shear ramps up to nearly 45KT and CAPE could touch 2000J/kg in the S and more than 100 in the N. Hodographs get slightly curved before the front, and the NAM generates helicity over 300 in the late aftn and early evening - just ahead of the front - in many places. So, supercells are possible for a brief window. Mostly, the storms will be multi- cell clusters. Wind damage will be the most probably threat. Hail less so, and tornadoes even lower. However, the LCLs will be low which could make it easier for tornado formation should discrete cells develop. The front picks up speed in the evening, and should push all the storms past BFD close to 8 PM, and through LNS around 11 PM.

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KEY MESSAGE 2: Rare SPC Day 5 outlook for SVR TSRA potential across much of PA on Thu 6/18

The nose of a very strong, 40+ KT swrly 850 mb jet 50+ KT at 700 mb (both being 3+ sigma on the standardized anomaly scale), the the right entrance region of a 100 KT 250 mb jet and PWATS around 1.5 inches (with the potential of a EML surging in from the west ahead of a strong sfc cold front) will bring the ideal synoptic setup for a severe weather outbreak here in the CWA. As always, even with the moisture, instability and shear present, such strong forcing could produces plenty of clouds and early day convection to stabilize the atmos and decrease the potential for significant afternoon severe weather. We'll be monitoring this potential closely as we head into the new week.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Very high (> 80%) confidence in VFR conditions continuing through 18Z Sunday with current observations outlining clear skies across the entire Commonwealth as of 10Z with mid-to-high level clouds encroaching on the area and some radar returns west of the region. Dry air will initially make it to where very little of this falls to the ground; however increasing moisture will potentially bring some shower/storm activity between 17Z-21Z across SE PA (MDT/LNS). Slightly higher confidence on this occurring; however, the bulk of it based on hi-res model guidance strays outside of the airfield but still close enough to warrant mentions of VCSH given closer proximity of the showers. Current window for VCSH may be a tad too wide at MDT/LNS; however, future TAF cycles can fine-tune that two-hour period based on near-term trends closer to 18Z.

Model guidance continues to agree on more widespread shower/storm coverage along a cold frontal passage, thus have amped up mentions in the 12Z TAF cycle. Timing on the 06Z TAF package looked decent with hi-res model guidance continuing to hone in on the two-hour windows in the previous cycle, thus have increased confidence at all airfields enough to include TEMPOs at all airfields outside of MDT/LNS. If anything, hi-res model guidance might be tracking a tad on the slower end; however, would like to see more consistency before throwing down one-hour windows on TSRA mentions at this time horizon. TSRA will likely bring a drop in visibility, but not expecting widespread lower ceilings that warrant longer-term mentions in the TAF. After the cold frontal passage, guidance continues to give some mixed signals, likely due to some potential for clouds scattering out with dry air behind the cold front. At this time, have went towards more optimistic guidance that keeps some cloud coverage and/or a light breeze which will promote VFR conditions; however, cannot rule out IFR/LIFR ceilings if low-level dry air and lighter winds prevail. Recent NBM probabilistic guidance indicates a 50:50 split in these conditions, so either solution is viable and will need to be further assessed in the 18Z/00Z TAF packages.

Outlook...

Mon...Mostly VFR with isolated -SHRA possible.

Tue...Mostly VFR with isolated -SHRA/-TSRA possible.

Wed...Higher chances for -SHRA/-TSRA, restrictions more likely.

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

CTP WATCHES/WARNINGS/ADVISORIES

None.