textproduct: State College

WHAT HAS CHANGED

* Increased rain chances across the northern tier this afternoon. * Level 2/Slight severe thunderstorm risk introduced for Tuesday (9/1)

KEY MESSAGES

1) Light rain north, scattered thunderstorms southeast today 2) Late summer heat+humidity to close August and open September 3) Bouts of heavy rain & severe storms possible Tuesday- Thursday

DISCUSSION

KEY MESSAGE 1: Light rain north, scattered thunderstorms southeast today

An ill-defined front will continue to drift into Central PA this afternoon, bringing some warm advection showers across the northern tier. Although showers are expected to weaken with eastward extent as they encounter some drier air, the radar presentation is showing no signs of weakening and light rain is becoming increasingly like along and north of I-80 through the afternoon.

Meanwhile, we are starting to see some convection initiation in the Lower Susquehanna Valley. 500 to 750 J/kg of MLCAPE will provide sufficient fuel for a few taller showers to develop and perhaps some lightning. As we head into the overnight hours, a wave of energy drifting across the Great Lakes will bring more widespread rain chances into the overnight hours. All in all, rainfall totals north of I-80 will be between a quarter and half inch, with some locally higher amounts possible. Farther south, up to a quarter inch is possible but less than a tenth of an inch is most likely.

KEY MESSAGE 2: Late summer heat+humidity to close August and open September

Strong upper ridge expanding east-northeastward from the south central Plains through the TN Valley into the southern Mid Atlantic and Southeast states will deliver a stretch of above normal late summer warmth and very humid conditions to close out August and kickoff September. Forecast max/min temperatures are projected to rise several degrees above the historical average Sunday-Thursday with the largest departure from normal (+10-20F) found in midweek overnight lows. The late summer heat is forecast to peak Tuesday and Wednesday with max heat indices approaching advisory levels in the mid to upper 90s across parts of south central PA. The well above normal temperatures combined with high humidity and very warm overnight lows will result in level 2 (moderate) to level 3 (major) HeatRisk values Tuesday and Wednesday. Increasing troughing across the Northeast U.S. should drop temperatures back toward seasonal climo by Friday/Saturday.

KEY MESSAGE 3: Bouts of heavy rain and severe storms possible Tuesday-Thursday

A series of mid level impulses tracking eastward along the northern periphery of the elongated 500mb ridge will interact with anomalous moisture/instability and strong flow aloft to produce a few episodes of heavy rainfall and possible severe weather through late week. There are typical timing and amplitude differences of the smaller scale disturbances that will need to be resolved, but the overall convective signal from operational/ensemble/ML/AI guidance is supportively bullish. There is a growing signal for severe storms and heavy rain on Tuesday through Thursday.

SPC introduced a level 2 severe t-storm risk for D3/Tuesday and D5/Thursday and maintained the 15% or level 2 equivalent risk for D4/Wednesday. WPC continues to cover parts of CPA with level 1 excessive rainfall outlooks which may be refined/upgraded as the convective details become more clear and confidence in timing and location increases. The environment will be characterized by impressive CAPE (on the order of 2-3k J/kg) and sufficient shear. A corridor of strong storms is likely somewhere across upstate NY into PA each day through the week, with the initiating boundary each day being highly dependent on the extent/intensity of storms the day before. The battleground between a hot/humid/dry day and perhaps multiple rounds of strong thunderstorms will likely be draped somewhere across Central PA this week, so those with outdoor plans should continue to monitor the forecast closely.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Ceilings will gradually lower across northern Pennsylvania through the night as showers increase in coverage in association with a wave moving across the Great Lakes. Some guidance, including the GLAMP, show MVFR or IFR ceilings developing at BFD overnight, but confidence is low on this occurring. The HREF shows a less than 20 percent chance of ceilings dropping to IFR and soundings from the HRRR suggest that VFR ceilings will persist through the night. Visibility restrictions appear more likely at BFD, however, as rain develops. MVFR or IFR visibility is expected at times into the morning. Outside of BFD, confidence is low on how far south and east showers make it tonight. IPT has the next highest chance of seeing rain, but it is certainly possible that all TAF sites see at least a brief shower.

VFR conditions are favored for much of the day on Monday, though additional showers are expected to develop during the late morning and early afternoon. Confidence on timing and placement is low, but IPT would be most likely to see impacts from rain. Winds will generally be less than 5 knots.

Outlook...

Tue-Fri...Scattered SHRA/TSRA with brief restrictions possible; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.