textproduct: State College

WHAT HAS CHANGED

*No significant changes at this time.

KEY MESSAGES

1) Trending more humid with heavy downpours and localized flash flooding possible late this week into the weekend

2) Severe thunderstorms are possible Thursday and Friday afternoons/evenings

3) Heat index could approach 100F Thursday and Friday SE of Harrisburg

DISCUSSION

KEY MESSAGE 1: Trending more humid with heavy downpours and localized flash flooding possible late this week into the weekend

Weak sfc low (1018 MB) near KDCA, with an ill-defined warm front extending NE along the I-95 corridor and an inverted trough into the Lower and Middle Susq Valleys will be the focus for several waves of heavy showers and a few TSRA through early tonight, while another sfc low(s) (1017 MB) over Central and Southern Ohio will bring similarly slow-moving and training showers/and a few TSRA, mainly west of the RT 219 corridor.

12 hour POPS across our Laurel Highland zones will be 60-70% and in 70-80+% range across the Lower Susq Valley.

Most of the CWA is in WPC's MRGL risk of Excessive Rainfall (Flash Flooding) for the next 5 days (with the entire state in a MRGL risk for Fri and Sat).

We expect PWATS to climb to +1-2 Sigma with corresponding sfc dewpoints rising into the 70s over the S and U60s in the N and stay there right through at least Tuesday.

PWATs seem to peak this afternoon around 2.25" in the SE. Then they don't get that high again (daily peaks of 2" in the SE, but 1.7" in the NW). A pattern shift to a more- zonal flow aloft should occur over the weekend, but a cold front and/or dry air is not in store for us.

The confidence in flooding is low due to confidence in placement of the heaviest rain being low. Certainly, the wettest places from over the weekend will sit higher on the list, and there are some hints that the SErn third or so of the area will be most at- risk for repeated shots of heavy rain. But, no flood watch are expected for today, and we'll only get a better feel for each new day as we see where the previous day's precip has been heaviest.

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KEY MESSAGE 2: Severe thunderstorms are possible Thursday and Friday afternoons/evenings

MRGL risk for severe wind gusts exist on Thurs (and likely Friday, too) over the SE half of the area per the latest SPC outlooks (new DY3 not out at press time). The shear is less than modest, but CAPE does climb into the 1000-2000 J/kg each of those aftns. However, the CAPE is long/skinny, which is more indicative of heavy rain threat. If we can get some dry air in the mid levels either day, and the small height falls on Friday can help jack up the instability and DCAPEs, any of our storms could make a strong to severe gust. ------------------------------------------------------

KEY MESSAGE 3: Heat index could approach 100F Thursday and Friday SE of Harrisburg

The heat index does get very close to and may exceed 100 degrees across a small portion of our SE zones using the current NBM guidance temps/dews. However, clouds and the anticipated showers/storms will keep the HI lower - even if just a degree or two which would make the difference in advy/no. The temps won't be all that abnormal, it will be the dewpoints which would take us over. At this point, we'll hold off on heat advisories.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Surge of low level moisture from south to north will likely bring mainly MVFR ceilings across the Lower Susq Valley and points east for the rest of the daylight hours, where low level convergence zones associated with weak troughs (and a warm front near the I-95 corridor will combine with the highest Precipitable Water values to focus lower convective cloud bases.

Cigs most likely trend back to higher end MVFR to low-end VFR for the late afternoon across the Laurel Highlands. Best odds for a brief heavy downpour and localized MVFR/IFR visibility appears to be in the KMDT/KLNS vicinity.

Confidence is generally low regarding precip trends, but locations from KAOO to KUNV and KIPT should stay predominantly dry through much of the late afternoon/evening hours. Humid and stagnant airmass tonight suggests lowering flight cats toward MVFR with areas of IFR into early Thursday morning.

Outlook...

Thu-Sun...Pockets of fog in the morning. Scattered showers/isolated t-storms with heavy downpours and localized restrictions.

CTP WATCHES/WARNINGS/ADVISORIES

None.