textproduct: State College

WHAT HAS CHANGED

* Focusing on 2 areas of enhanced instability for convection over the next several hours - across the Laurel Highlands and along/east of a line from KHZl to KMUI. * Revised dewpoints downward and temps upward a bit over the S for this aftn and evening. * Freezing/sub-freezing temps look even more likely Monday and Tuesday mornings.

KEY MESSAGES

1) A few areas of showers and possible highly localized/beneficial moderate rainfall amounts of 0.25 to 0.50 inch across the Laurel Highlands and just east of the I-81 corridor east of KMDT.

2) Heat and humidity decrease Friday and Saturday, but remain above normal due to the arrival of some cooler air and NW winds.

3) Cold front brings precipitation and a break from summer heat late this weekend. Freezing temps likely or even certain for Monday night.

DISCUSSION

KEY MESSAGE 1: A few areas of showers and possible highly localized/beneficial moderate rainfall amounts of 0.25 to 0.50 inch across the Laurel Highlands and just east of the I-81 corridor east of KMDT.

10 PM Update...the greatest SFC-925MB CAPE has become focused INVOF KJST and also just to the east of the I-81 corridor east to the KMDT area.

A slight ramp up in llvl-based CAPE has occurred across the Central/Southern Mtns of PA after a total void in instability late this afternoon/evening within a region of broad llvl divergence/lower SFC TDs in the 40s.

The convergence of old outflow boundaries from late afternoon convection and a nose of higher PWAT air advecting NE up the Ohio River Valley ahead of a compact mid/upr level shortwave will support another 2-3 hours of moderate to briefly heavy showers from SW PA to near KIDI and KJST.

The second area of convection, supported by some nwd advection of higher low level Theta-E air off the Chesapeake Bay and upslope flow into the Western Poconos will also support about another 3 hours of SHRA and perhaps a few low-topped TSRA.

To the north of I-80 the air is rain-cooler and more stable with any sfc based CAPE being less than 300 j/kg with broad llvl anticyclonic flow.

Another unseasonably mild night under plenty of clouds with lows ranging from around 50F across the NW Mtns to the low 60s in the Southern Valleys.

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KEY MESSAGE 2: Dry, windy conditions through early this evening make a risk of rapid wildfire spread. Risk decreases Friday.

Dewpoints have dropped slightly lower than expected - thus far - and may drop more, esp over the S. Temps also running a little higher than forecast (1-2F). The result is that much of the srn tier is AOB 30pct RH and some locations may eventually dip into the teens. The wind is on track. The fine fuels and wind speeds are not critical enough to warrant a fire weather watch/red flag warning. Conditions improve tomorrow (Fri) as wind decreases and temps cool and dewpoints lower a little. Thus, the RH should be higher on the whole for the srn tier.

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KEY MESSAGE 3: Heat and humidity decrease but remain above normals due to the arrival of some cooler air and NW winds.

The trough coming through tonight and a weak cold front on Friday will knock temps down 5-10 degrees between today and tomorrow, but they will still be well above normal. We go up a few degrees on Saturday as southerly flow increases ahead of an approaching cold front.

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KEY MESSAGE 4: Cold front brings precipitation and a break from summer heat late this weekend. Freezing temps likely or even certain for Monday night.

The next round of showers and storms will come along and ahead of a cold front Sat - Sat night. SPC has introduced a SLGT severe weather outlook for eastern OH and western PA Sat, with the MRGL risk area clipping our western zones. The showers will probably linger in the Se half of the area Sunday morning despite the front clearing our zones by 12Z. The significant cold front ushers in much, much colder air beginning Sat night. In fact, there could even be some snow showers on Sunday. While it will be getting near freezing in the NW half of the CWA Sunday night/Mon AM. Sunday night may be too windy for frost to form, but some places will get down to freezing anyway, which could necessitate a freeze warning being issued at some point. Most certain time frame for sub-freezing temps is Monday night/Tues AM. Latest forecast (as of Thurs aftn) drops temps just a deg more than last cycle for everyone = down below 32F, and as cold as 20 in the northern tier.

Temps rebound Tue afternoon and remain mild into the middle of next week. But, colder air could sneak back in from the N/NE in the second half of the week.

AVIATION /02Z FRIDAY THROUGH TUESDAY/

A line of scattered showers and thunderstorms will track eastward across central PA through 03Z Friday as a shortwave trough moves over the region. Instability/moisture diminishes with eastward extent, where clearing skies and dry antecedent conditions have allowed for low-level moisture to mix out more efficiently, leading to a transition to -TSRA at KBFD to primarily -SHRA across all other terminals. TEMPO groups for KIPT/KMDT/KLNS account for brief WNW wind gusts to around 25 kts associated with this band of weakening showers/storms, with an inverted-V profile helping to perpetuate these downdraft wind gusts even as the band weakens. Scattered showers are moderately likely (40-50% chance) to linger across KBFD through 07Z Friday due to lake-enhanced moisture, and across KJST through 11Z due to upslope.

Northwesterly offshore flow in the wake of this evening's showers/storms will transport low-level moisture from Lake Erie to the Northern Tier (KBFD) and Laurel Highlands (KJST) later tonight. At KBFD, the LAMP suggests a moderate to high (60-80%) chance of MVFR ceilings through 18Z, with IFR being moderately likely (30-50% chance) between 07Z-15Z. Moreover, there is a low chance (10-20%) of LIFR per the LAMP, though upstream observations will need to be monitored to increase confidence. Further south at KJST, MVFR ceilings are moderately likely (40-60% chance) primarily between 11Z-17Z as winds shift northwesterly. A SCT025 layer was added between 02Z-11Z is to hint at a low chance (10-20%) of MVFR ceilings due to upslope showers, though certainty is too low to prevail MVFR.

In the morning and early afternoon on Friday, a 850 mb temperature minimum associated with the trough will be passing overhead, which when combined with daytime heating and sufficient moisture with dew points in the 50s, spotty showers are possible. This possibility was reflected by PROB30s at KAOO/KUNV/KMDT/KLNS where the HREF had 30% PoPs between 14Z-15Z and 18Z. After 18Z, VFR is highly likely (80-90% chance) to prevail due to subsidence on the backside of this shortwave, with drier air filtering in from the north.

Outlook...

Sat-Sun...Restrictions in SHRA possible, transition to SHSN across higher elevations overnight Sunday night into Monday morning.

Mon...Lingering SHSN possible early across N PA with restrictions possible; otherwise, dry conditions with VFR prevailing.

Tue...Dry with VFR conditions prevailing early, rain showers move south-to-north across the area during the afternoon and evening hours.

CLIMATE

MDT and IPT both had high temps of 87F on Wed Apr 15, which ties the record at MDT (set in 1941) and breaks the record of 86 at IPT (set in 1994). IPT also broke the Apr 15 record for warmest daily low temp, where 58F broke the record of 57F set in 2023.

Record high daily mins are possible again today and tomorrow. Here's a look at the records as they stand right now, with a * indicating that the forecast is equal to or greater than the record.

Record High Daily Max / Daily Min...

For Harrisburg: April 16: 89 (2002) / 63* (1941) April 17: 91 (2002) / 60* (2002)

For Williamsport: April 16: 90 (2002) / 55* (2023) April 17: 91 (2002) / 58* (1896)

CTP WATCHES/WARNINGS/ADVISORIES

None.