textproduct: State College

WHAT HAS CHANGED

* Severe weather risk level upgraded to Slight (2 out of 5) Thursday & Friday * Held off on Heat Advisory issuance owing to lingering uncertainty with PM storms Thursday & Friday

KEY MESSAGES

1) Increasing heat and humidity will bring increased risk for heat-related illnesses Thursday, and especially on Friday.

2) Daily shower and thunderstorm chances continue through the end of the work week with damaging winds possible.

3) A refreshing decrease in humidity is a good bet this weekend with high pressure moving in after a cold front.

DISCUSSION

KEY MESSAGE 1: Increasing heat and humidity will bring increased risk for heat-related illnesses Thursday, and especially on Friday.

Heat and humidity still on tap for the second half of the week. Thursday should be 4-5F hotter than Wednesday, and Friday will add another 1-2F. We're still keeping daytime temps a shade cooler than NBM mean guidance, leaning heavily on the reliable ol' MOS guidance and multi-office ensemble of forecasters. The main driver of that change is the expected highly-variable cloud cover and occasional shots of precip. Warm front does look like it will nudge precip northeastward on Thurs, and we could have enough sunshine to knock out the forecaster bias. So, the changes are minimal in magnitude, especially when considering the dewpoints, as well. Dewpoints are progged int the low 70s across much of the area, which will feel quite uncomfortable. The forecast Apparent Ts/Heat Indices for Thursday and Friday still poke above 100 for a few towns around Harrisburg/Lancaster/York on Thursday. Fri should be 1-2F higher, and may necessitate a Heat Advisory issuance for a few counties. The main forecast uncertainty is whether or not we'll see well-timed storms on Thursday or Friday afternoon, which would keep temperature below criteria. Because our forecast just barely touches 100F both days and there remains uncertainty about storm coverage/timing, we will continue to hold off on any heat products for this package. LREF calibrated apparent T guidance keeps the probability of 100+ F heat index values capped around 20% both Thursday and Friday afternoon in the far southern tier (lower northwest), so we are comfortable to continue to message hot/humid weather but not yet sound the Advisory alarm.

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KEY MESSAGE 2: Daily shower and thunderstorm chances continue through the end of the work week with damaging winds possible.

Underneath a broad ridge of high pressure, a couple shortwaves will drift across Pennsylvania over the next few days. Sufficient instability in the presence of warm & moist air along with variable amounts of shear will lead to multiple rounds of showers and thunderstorms. This will be a typical summer-like pattern with many storms on radar and lingering questions about how strong they'll get. If multicell clusters can develop large enough cold pools, an elevated threat of damaging wind gusts may materialize in the form of an MCS. Such a scenario cloud produce a swath of more significant wind damage (most likely on Thursday).

A cold frontal passage (CFROPA) will occur on Friday. Dynamics in the form of a digging upper trough, high available moisture suggest severe thunderstorms are possible. But, the small ridge that pumps up overhead late Thu could be a setback for how widespread the threat will be. SPC has maintained the Slight Risk for Day 3 across all of the CWA. Biggest question is timing/speed of the cold front. A faster progression could mean the cold front clears our area before peak heating (lower threat of damaging winds), while a later arrival could spell more concern for severe weather. The best mid- and upper-level winds will be well to the north of PA precluding any higher severe risks.

The repeated shots of scattered to numerous SHRA/TSRA over the next 3 days is likely to yield lower FFG thresholds and some flash flooding would become possible, too. But, it has been very dry recently, and everything is growing and thirsty, ready to soak up as much as it can. As always, too much of a good thing can be bad. We'll keep an eye on the soil saturation and other factors that could lead to flooding. Back-to-back-to-back Marginal risks of excessive rainfall are in place through Friday.

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KEY MESSAGE 3: Temperature relief not large in magnitude, but a decrease in humidity is a good bet with high pressure after the cold front Sat/Sun.

The cold front that passes through Fri/Fri night will sweep out the humid air, but really for only one or two days. RHs dip into the 30s each afternoon in the mountains, and lower 40s elsewhere over the weekend. However, the max temps don't get below normal over the weekend. Regardless, more sunshine and less humidity will feel a bit refreshing. Perhaps on days 7 & 8 we'll be a little cooler, back to normal maxes (75-83F) thanks to a zonal flow aloft and clouds/precip arriving/passing through. CFROPA also an increasing possibility sometime Mon or Tues of next week, and could bring another bout of severe weather and/or heavy rain.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Scattered showers across much of central Pennsylvania with a more organized cluster closer to MDT/LNS expected to continue moving eastward across the area this morning. Recent observation within the precipitation shield about to impact MDT/LNS suggest not as low ceilings compared to when these showers tracked over JST/AOO with slightly drier low-level air in place across southeastern Pennsylvania. Recent GLAMP/NBM model guidance trended towards low-end VFR and this seems like the leading solution (~60-70% confidence); however, cannot completely rule out some brief MVFR restrictions at MDT/LNS through ~14-15Z with IFR unlikely at this time.

Recent HREF probabilities for precipitation markedly increase across W PA closer to 15Z, with recent guidance also outlining a fair amount of instability. This trend outlines a pretty good shot of some thunderstorms, especially in the 16Z-22Z timeframe moving west-to-east across the forecast area. Greatest concern with respect to TSRA will be coverage which continues to warrant PROB30s at this time due to just the low confidence in these storms impacting the airfields directly. In the FWIW category, it does appear at this time that highest potential is generally UNV/IPT/MDT/LNS but strongest returns could stray with not the best run-to-run consistency in model solutions. As precipitation tapers off, will need to watch for patches of clearing for some IFR/LIFR potential overnight, as winds do trend downward; however, too low of confidence to tank airfields more than MVFR at this juncture. Recent NBM/HREF guidance paints the western terminals (BFD/JST) as the highest probability for IFR/LIFR restrictions, which coupled with RAP model soundings warrants some lowering of ceilings after 06Z Wednesday due to ample low- level moisture in place.

Outlook...

Thu...Restrictions possible in scattered -SHRA/-TSRA.

Fri...Restrictions expected as FROPA brings line of -TSRA.

Sat...Mainly VFR.

Sun...VFR favored early, with restrictions possible as FROPA brings chance of -SHRA/-TSRA later in the day.

CTP WATCHES/WARNINGS/ADVISORIES

None.