textproduct: State College

WHAT HAS CHANGED

Augmented wording for Sct TSRA with gusty winds across the Lower and Middle Susq Valley this afternoon and early this evening.

KEY MESSAGES

1) A few gusty thunderstorms possible across portions of the Lower and Middle Susquehanna Valley this afternoon and evening.

2) Turning more humid and unsettled early in the upcoming week with the highest probability for showers and thunderstorms Monday afternoon and evening and again on Tuesday and Tuesday night.

DISCUSSION

KEY MESSAGE 1: A few gusty thunderstorms possible across portions of the Lower and Middle Susquehanna Valley this afternoon and evening.

A weak upper level disturbance embedded in the NW flow aloft will race SE and over a favorable zone of convergence along a lee trough that will be situated across the Middle and Lower Susq Valley this afternoon/early evening.

Surface dewpoints in the low to mid 60s convergence and the passage of the left exit region of a 90 kt jet max over Ohio and SW PA late this afternoon will provide the fuel and favorable larger scale UVVEL for some pulse and small clusters of strong to potentially svr multicell TSRA across the I-81 and RT 11/15 corridors in southern PA within a fairly narrow corridor of sfc-based CAPE between 1000-1500 J/KG between 18Z-00Z today/Monday. SPC has this relatively small portion of our CWA in a MRGL Risk for SVR TSRA today.

KEY MESSAGE 2: Turning more humid and unsettled early in the upcoming week with the highest probability for showers and thunderstorms Monday afternoon and evening and again on Tuesday and Tuesday night.

A warm front will be moving across western PA Monday afternoon/evening, followed by a rather potent/digging mid/upper level shortwave and surface low. This shortwave will help to amplify a slow-moving upper trough over the Mid Atlantic Region as we head into the midweek period.

Fcst dewpoints are not all that high for late July, and there has been a slight downward trend with fcst dewpoints over the last few days.

Main things supporting showers and storms is a deepening low, and a large scale pattern that supports significant directional and speed shear in advance of a quite anomalous 995-1000 mb sfc low moving over or just north of PA on Tuesday. Hodographs Tuesday have a broad loop, but winds in the lowest KM are fairly light, so we're leaning to the storm type being pulse, but with 0-2KM helicity values peaking around 150 M2/S2 Tuesday, we could see some transient supercells over Central and Ern PA during the afternoon.

Previous disc...

The timing of the favorable lift associated with the thermally indirect, left front quad of the 80+ KT jet max moving across the Ohio Valley, WVA and southern PA Tuesday afternoon and evening will help to enhance a low level lee trough and convection along it. Scattered strong to severe TSRA are likely Tue afternoon and evening over at least the eastern half of the CWA with a second line of TSRA likely along the cold front across NW PA Tuesday afternoon.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Current LIFR conditions at BFD are progged to continue through 13-15Z Sunday before prevailing VFR with widespread VFR conditions likely (> 80%) to continue elsewhere through 00Z Monday. The only real potential for sub-VFR conditions in this timeframe would after 21Z with some potential for isolated moderate SHRA/TSRA in the vicinity of IPT (40-60% probabilities) and MDT (20-40% probabilities) through 03Z Monday. Have trimmed back mentions at LNS compared to earlier packages due to some hi-res solutions trending back on precipitation mentions at this airfield, but there remains a non-zero chance in SHRA/TSRA making a push towards this airfield, generally between 01-03Z Monday if this is to occur. Main restrictions would be limited to MVFR at all of these airfields, with IFR looking unlikely within SHRA/TSRA for the majority.

After 06Z Monday, quite the range in guidance thus slightly lower confidence in the TAF package. Recent GLAMP/NBM model guidance stand out as fairly pessimistic guidance with respect to low ceilings and visibilities. Looking at 10th percentile members of the HREF guidance; however, outlines these restrictions being limited to BFD/IPT and *potentially* MDT which tracks with RAP model soundings that outline the most plentiful low-level moisture. Given this, have went with a forecast that trends closer to the RAP/HREF solutions at this time. There does remain a fair amount of uncertainty this far out; however, and more widespread rainfall would lean towards the GLAMP/NBM solutions and will need to be monitored more closely this afternoon/evening.

Outlook...

Mon...Restrictions likely in afternoon -SHRA/-TSRA. Patchy overnight fog possible.

Tue...Restrictions likely with scattered -SHRA/-TSRA.

Wed-Thu...Restrictions possible as -SHRA/-TSRA becomes less numerous. Patchy overnight fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.