Marine Weather and Tide Forecast for Chambersburg, PA

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

2) Shower and thunderstorm chances continue through the end of the work week.

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.

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.
Thurs s/b 4-5F hotter than Wed, 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. The dewpoint forecast is likely not to change with the availability of sunlight, and could actually be bumped up a tiny bit with the occasional precip making the ground and boundary layer more moist. So, the Apparent Ts/Heat Indices for Thurs and Fro still poke above 100 for a few towns around Harrisburg/Lancaster/York on Thu. Fri should be 1-2F higher, and likely to necessitate a Heat Advisory issuance for a few counties. Worries for Extreme Heat Warning are low, with hardly a pixel or two of 105+ showing up in the AppT grids that day.
With some uncertainty (50%) as to just not large in magnitude,how many 100s show up on Thurs aftn, we plan to hold off on any heat products for this package. However, Fri looks like a good bet (70%) for a few counties, if not all the Lower Susq and nearby counties, to reach the 100 heat index mark. But, neither day instills a 50% chc confidence of 105+, and therefore no Extreme Heat Watch at this point.

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KEY MESSAGE 2: Shower and thunderstorm chances continue through the end of the work week.

Welcome rain for the most part this week, beginning very soon, and lasting until a cold frontal passage (CFROPA) later Fri and Fri night. 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. For the time being, SPC has all of the CWA in the 15% (eventual slight) risk category. The repeated shots of sct-nrms SHRA/TSRA over the next 3-4 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.

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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. 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 /00Z WEDNESDAY THROUGH SUNDAY/
The southerly flow regime continues this TAF period with moisture continuing to flow into Central PA. PWATs remain in excess of 1.5" across Central PA. This will allow multiple chances of rain regionwide tonight with some instability in the SW possibly allowing for -TSRA to develop in the 07Z-10Z timeframe at KJST and KBFD. As instability builds tomorrow, scattered thunderstorms are likely to move through the region in addition to showery activity. Have reflected forecast guidance's latest thinking on timing with PROB30s for -TSRA tomorrow afternoon in latest TAF package.

Ceilings will lower west to east through tonight as the corridor of moisture moves overhead. A period of IFR is likely (70-80% chance) across the higher terrain (KBFD/KJST)
overnight and into the morning (primarily 07Z-17Z), with MVFR favored elsewhere. For Wednesday afternoon, HRRR members range from widespread VFR outside of the Laurels (HRRR) to MVFR- IFR holding regionwide (3km NAM). The current TAF package leans toward gradual improvement to MVFR across KBFD/KJST by 17Z Wednesday, with MVFR continuing into the early afternoon elsewhere given the moisture axis being overhead at that time.
In the wake of convection during the afternoon, a return to VFR is possible.

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.