textproduct: State College

WHAT HAS CHANGED

* Higher confidence in timing of most thunderstorms (2 PM through 11 PM). Increased POPS for SHRA/TSRA layer this morning and this afternoon across the Wcent and Northern Mtns.

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. SPC's late morning upgrade added a higher wind threat across NW PA (similar to the elevated, 30% wind threat near and to the SE of the I-81 corridor in PA.

Plenty of multi-layered WAA clouds will cover the NW half of PA late this morning and early this afternoon with a high probability for a 30-90 minute period of showers associated with an area of showers (and possibly a rumble of thunder) mainly to the north of a line from KJST to KUNV and KIPT.

This initial area of convection is linked to a relatively weak mid/upper level vort max lifting NE across the region ahead of the main show (i.e. an anticipated line of strong to severe TRSA that will sweep SE across the CWA this afternoon and evening).

This lead area of convection shouldn't intensify much, if any, as its moving over an area with plenty of cloud cover and SBCIN of 200-500 j/kg.

In contrast, partly sunny skies should last into the mid afternoon hours near and to the east of I-81. SFC Based CAPE is with no CIN is already up into the 1000-1500 J/KG range across the Lower Susq - south of KMDT and KLNS thanks to the better sfc heating and injection of higher llvl moisture (Sfc Tds in the mid 60s) north off the Chesapeake Bay. 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 /18Z SUNDAY THROUGH FRIDAY/

Upper level shortwave energy is slipping through western PA early this afternoon ahead of potent cold frontal boundary. Expect just a glancing blow from this feature with only VCSH generally across northern PA through the next few hours.

VFR to MVFR conditions will remain until the onset of showers and convection moves steadily across the state through the late afternoon and evening hours. Some timing diffrences in the short term high resolutions models, but the trend is for convection to enter northwest PA in the late afternoon and exit southeast PA during the late evening. As with any convection scenario, expect wind shear, heavy rainfall/wet runways and lightning as storms cross any airfield.

Once the storms exit and the frontal boundary passes after midnight, conditions will improve to MVFR then VFR overnight with winds shifting to the northwest near 10KTS.

With cold air aloft on Monday, a BKN cloud layer will form but it will remain VFR. An isolated -SHRA is possible after this forecast period - especially over northern PA.

Outlook...

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.