textproduct: State College

WHAT HAS CHANGED

SPC expanded the MRGL RIsk for SVR TSRA areas today (Western half of the CWA) and for tomorrow (approx 90 percent of the CWA, except for the Southern Tier of the state).

KEY MESSAGES

1) Upper level low pressure swinging SE from the upper Glakes to southern Ontario will lead to high temps trending cooler by a few to several degrees each day through Monday along with periods of showers and isolated to scattered gusty thunderstorms.

DISCUSSION

KEY MESSAGE 1: Upper level low pressure swinging SE from the upper Glakes to southern Ontario will lead to high temps trending cooler by a few to several degrees each day through Monday along with periods of showers and isolated to scattered gusty thunderstorms.

Significant amplification of a longwave trough will occur over the next several days across the Glakes, Mid Atlantic region and Central Appalachian with 500 mb height falls of 60-100 meters and mean 850-700 mb layer temps of around 6 deg C from this evening to Monday morning.

Abundant sunshine expected across the CWA North and West of the Lower Susq Valley with max temps this afternoon generally between 75-80F.

The initial lead shortwave and right entrance region of a 65 kt upper level jet trailing a zone of very sharp upper diffluence over the western 1/3 of PA late today/early tonight will support development of scattered strong to severe TSRA across the western 1/3 to 1/2 of our CWA with the likely evolution/upscale growth of multicell clusters into a bkn line of TSRA with its axis near the RT 219 corridor between 00Z and 03Z Sunday where 0-1KM EHI will be in the 1-1.5 M2/S2 range and 0-3 KM Bulk shear will be average 20-25 KTS with some pockets just above 30 kts across the NW Mtns near KBFD at 03Z Sunday.

HREF MU CAPE Values do drop off fairly quickly from their late afternoon/early evening peak of 1200-1500 J/KG across western PA, and the severe threat will be diminished by 02z across the Mtns of Western PA. The eastern half of PA will get the gradually weakening convection later this evening/overnight.

For Sunday, the best larger scale/deep layer cooling aloft will occur across the eastern half of the state and that will be the focus for the SVR threat. MU CAPE will be lower by around 500 J/KG but deep layer swrly shear will be 35-40 kts across our eastern CWA. Therefore, SPC's MRGL risk covers the better threat for gusty winds with the storms across the eastern half of PA on Sunday with some isolated occurrences of pea to marble size hail in the taller storms Sunday.

The cyclonic flow may keep showers downwind of Lake Erie into Monday. Daytime highs will be near to slightly below the historical average for late August, making it feel more like mid September especially Sunday into Monday. SPC trimmed the MRGL back a bit to the west on Saturday, which should feature early day clouds and a few showers spreading from SE to NW during the day. Drier/less humid conditions are likely to prevail during the first half of next week.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Highly variable cigs and vsbys for the mid to late morning in radiational/valley fog regime with vsbys ranging from 1/4SM to 2SM+ in just a few miles. In any event, all TAF sites in Central and Western PA will be MVFR or better by 14Z today, while KLNS may stay MVFR w/respect to CIGS for the entire daylight hours today.

While VFR in a SCT/BKN stratocu cloud deck will be the prevailing conditions today, some isolated showers are possible in the afternoon. Chances for this are highest at MDT and LNS with spotty morning activity possible as well. This activity may lead to bounces between VFR and MVFR throughout the day tomorrow in the southeast, with greater restrictions more likely at LNS.

This showery activity will be ahead of a cluster of storms moving through in the evening/early night along and ahead of a cold front. Have updated PROB30s for -TSRA to reflect most confident timing at the current moment. HREF ensemble and NBM trending later in storm timing in their most recent runs. Should a strong core cross paths with an airfield, brief IFR restrictions are possible.

Outlook...

Sun...Restrictions possible, mainly NW in SHRA. Improvement elsewhere.

Mon-Wed...Patchy morning fog, otherwise mainly VFR conditions return early next week.

CTP WATCHES/WARNINGS/ADVISORIES

None.