textproduct: State College

WHAT HAS CHANGED

* Lowered today's temps a few degrees in the east and lowered Thursday's dewpoints slightly from NBM based on pattern and collaboration. * Upped PoPs for Sat (vs previous fcst) due to increased confidence.

KEY MESSAGES

1) Much warmer today and even warmer Thursday and Friday. Numerous light showers this morning, many more on Friday.

2) Wet start to the weekend.

DISCUSSION

KEY MESSAGE 1: Much warmer today and even warmer Thursday and Friday. Numerous light showers this morning, many more on Friday.

A warm front over the state this morning will be followed by the parent low pressure area as it drops southward. The low won't drop all the way through Central PA until late in the day. The numerous, but light and disorganized showers will continue into mid-morning as they move southward. The precip is progged to dry up and may not amount to more than a sprinkle for folks S of US Route 22 - other than the Laurel Highlands where it should rain up to a quarter of an inch. Some clearing will try to happen late morning/afternoon as the thickest clouds slide to the south. Clouds could hang tough in the east, and have nudged temps down a few degs in the Poconos and E of Lebanon and Lancaster. Just enough forcing will remain and afternoon instability and a weak enhancement to orographic forcing could touch off isolated SHRA near and just N of State College and Williamsport, which would drift SE if they do form. The wind becomes northerly this evening after the low and it's attendant cold front (more like a dry line) pass. This should dry it out again.

Thursday looks dry and much warmer, jumping about 10F over today. Everyone but the far NErn mtns will be in the 70s. An 80F is not out of the question close to the MD border. This warmth and associated deep mixing should help dewpoints/RHs get a little lower than NBM guidance. Min RH should be close to 30 pct if not lower Thurs. NAM is an outlier with much higher dewpoints and cloud cover and slightly cooler temps in the west. Have discounted for now. The front that passes through late today/early tonight will return northward over western PA on Friday.

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KEY MESSAGE 2: Wet start to the weekend.

The lift over the front and daytime instability will generate sct SHRA/TSRA in the western half of the CWA on Friday. The cold air tries to hang on over the east and may even back in a bit as the upper low over New England presses southward along the coast. This will keep the warm front in place or nudge it to the west as a backdoor cold front. The energy diving down from the N/NW and perhaps an enhanced LLJet will bring in more moisture, and generate more SHRA/TSRA Fri night. Sat looks pretty wet for everyone with numerous SHRA/TSRA. QPF in the 0.5-1.00" range is a solid forecast for many Fri night and Sat. Some model divergence is seen for Sunday, though. Confidence is low in how fast the precip moves through. So, Sunday may hold sct SHRA, and PoPs in the 20-35% range are being kept for the time being.

Next storm system approaches for Tues, resulting in a medium confidence and medium to high PoPs. Will leave those high PoPs in for now, but model/EPS difference exists on evolution of the storm/mass fields.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

Timing of flight restrictions has continued to be delayed due to dry antecedent air mass (T/Td spreads around 10 degrees C) ahead of showers associated with low pressure moving across the Great Lakes. The HRRR/RAP models were favored for timing of boundary layer saturation over other guidance (e.g. NAM/NAM3km) as its recent runs have been more representative with timing of lowering ceilings at KBFD. A period of IFR at KBFD remains possible (40-50% chance) between 13Z-16Z, though the likelihood & timing of IFR will be better resolved as the low clouds on model guidance materialize over the next 6 hours. On the other hand, while the HRRR was used for timing of initial MVFR, I favored a HRRR/NAM blend at KJST where westerly upslope flow may result in MVFR stratocumulus between 13Z-18Z Wednesday. Likelihood of MVFR ceilings decreases with south & east extent, remaining borderline (20% chance) for our central terminals (KUNV/KIPT) with the HRRR trending drier, then unlikely (<20% chance) over our southeastern terminals (KMDT/KLNS).

Prevailing VFR is likely (80-90% chance) after 18Z as W/NW winds aloft advect drier air over the region. With that said, a few showers may develop where the low pressure center passes overhead in the afternoon, with a PROB30 for P6SM -SHRA included at KIPT between 19Z-23Z.

Lastly, LLWS mentions were included at KJST/KAOO through 11Z-13Z with 850 mb winds between 40-50 kts. These concerns will diminish after 13Z as daytime heating mixes out the boundary layer and weakens the nocturnal LLJ.

Outlook...

Thu...VFR. Partly cloudy and rainfree.

Fri...Light rain possible. VFR to MVFR.

Sat-Sun..Periods of rain with MVFR to IFR conditions.

CLIMATE

A handful of record lows were set/tied on Tues morning (April 21st):

Harrisburg's 29 broke the previous record of 30 degrees set back in 1956.

Altoona's 24 broke the previous record of 26 degrees also set back in 1956.

Williamsport tied it's record of 25 degrees set back in 1925.

CTP WATCHES/WARNINGS/ADVISORIES

None.