textproduct: State College

WHAT HAS CHANGED

* Minor modifications to today's rainfall chances as a decaying line of showers moves into an increasingly dry airmass. * Nudged high temperatures down Sunday-Tuesday to account for known biases in the NBM forecast in patterns like this.

KEY MESSAGES

1) Decaying line of showers to bring light rain across western PA this afternoon, a sprinkle or two possible farther east.

2) Hottest temperatures of the year so far forecast early next week with highs pushing into the 80s and 90s.

3) Strong cold front to bring showers and storms with potential for severe weather on Wednesday followed by cooler temperatures.

DISCUSSION

KEY MESSAGE 1: Decaying line of showers to bring light rain across western PA this afternoon, a sprinkle or two possible farther east.

A broad area of low pressure drifting across the Southern Great Lakes is bringing showers and storms to the Ohio Valley early this afternoon. As that complex of showers moves east, it will encounter an increasingly dry airmass with surface dewpoints in the 40s and sufficiently dry air aloft. Hi-res guidance agrees that the extent and intensity of ongoing showers will wane with eastward extent, though the longevity of showers remains in question. At the very least, isolated showers/sprinkles are possible mainly across the western half of PA and rainfall amounts will remain well below 0.10". Farther east, virga will be the most likely occurrence, but a few sprinkles cannot be ruled out. Cloud cover and showers in western PA should also help temper afternoon highs, so the latest forecast package includes temperatures a couple degrees lower than the previous forecast across western PA.

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KEY MESSAGE 2: Hottest temperatures of the year so far forecast early next week with highs pushing into the 80s and 90s.

Daytime high temperatures will be steadily rising today through early in the upcoming week. An anomalous upper ridge building over the east-central U.S. will deliver summerlike conditions this weekend through the first part of the new week. High temperatures will peak in the mid 80s to lower 90s Monday and Tuesday and could challenge daily records in some locations.

Manual adjustments were made to lower NBM maxT by a few degrees due to a known systematic bias correction issue identified during the shoulder seasons. This should also keep max heat indices under 100F for the most part. However, heat risk impacts will continue to be monitored given the quick ramp- up in heat and limited acclimation time. Dewpoints over 60s for many will also feel quite humid relative the recent cool stretch we've had.

With a boundary just to our north and sufficient instability in the warm sector over PA, there will be a chance for some stray afternoon and early evening showers or thunderstorms Monday and Tuesday. Most folks should stay dry, but thunderstorms could drop a quick 0.10 to 0.25" of rain in any one location.

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KEY MESSAGE 3: Strong cold front to bring showers and storms with potential for severe weather on Wednesday followed by cooler temperatures.

A cold front (accompanied by more widespread and frequent showers/t-storms) brings relief next Wednesday into Thursday. Some of the AI guidance paints an elevated risk of severe weather across the Lower Susquehanna Valley on Wednesday afternoon in the presence of peak heating and sufficient shear/CAPE profiles. The magnitude & location of the severe threat will be highly dependent on cold front passage timing. The eventual risk area will highlight locations along and downstream of the cold front between 12PM and 8PM on Wednesday.

In the wake of the cold front, high pressure will build in for a short time. Temperatures are forecast to cool down back to historical/climo averages for late May for the end of next week. Early indications show the aforementioned cold front stalling out south of the Mason-Dixon line late in the week and lifting back north across PA as a warm front into Memorial Day weekend. The current outlook favors a cooler/wetter stretch through the holiday weekend thanks to that warm front, but there is still a lot of time for the forecast to trend in a more favorable/optimistic direction for the holiday weekend that marks the unofficial start of summer.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Mainly high scattered clouds moving over central PA this afternoon. VFR clouds will increase and thicken west to east as a shortwave tracks across the northern periphery of high pressure off the east coast & its concurrent ridge Saturday afternoon/evening, bringing scattered showers with any flight restrictions looking to be brief and unlikely outside of any locally heavier showers. A few thunderstorms may develop across the Allegheny Plateau (KBFD/KJST) where the HREF mean MUCAPE values are around 100 J/kg. As precipitation progresses eastward, height rises and increasingly stable conditions will result in a downtrend in activity. Confidence is still too low to mention lightning in the TAFs for this afternoon given a downtrend of lightening activity as these showers approach from eastern Ohio.

As the shortwave exits our area during the evening, ridging will continue to build across the region, resulting in residual low-level moisture becoming trapped beneath a subsidence inversion. As a result, MVFR/IFR restrictions are possible to develop after 22Z-23Z Saturday across the Allegheny Plateau (KBFD/KJST), with lesser confidence in evening/overnight flight restrictions east of the Alleghenies due to downsloping flow and less antecedent moisture. It will be a tricky fog forecast given the dry air in place currently, but if a good rain shower comes over a terminal that will increase the chance of fog development for Sunday morning.

Outlook...

Sun-Tue...Mainly VFR. Patchy A.M. fog. Isolated P.M. shower or t-storm possible.

Wed...Restrictions probable through Wednesday night with showers/storms along CFROPA.

CLIMATE

Daily record max temperature for May 18-19:

5/18 5/19 Harrisburg 94 in 1962 95 in 1962 Williamsport 95 in 1962 96 in 1996 Altoona 91 in 1962/1996/2017 92 in 1996 Bradford 85 in 1962 85 in 1962 State College 92 in 1962 92 in 1934/1996

Please note: State College Co-operative observations are a 24-hour summary taken once per day around 7 AM (7AM-7AM). Therefore, a max temp occurring in the daylight or late in the day is usually reported in the _next_ day's observation. Also, the same min temp may be reported on two consecutive days if the min occurs at observation time.

CTP WATCHES/WARNINGS/ADVISORIES

None.