textproduct: State College

WHAT HAS CHANGED

* Flood Watch expanded to include portions of south-central Pennsylvania until 1 AM tonight. * Severe Thunderstorm Watch in effect for south-central and southeastern Pennsylvania until 8 PM this evening. * Increasing chance for some tornadoes across southeastern Pennsylvania this evening.

KEY MESSAGES

1) Severe thunderstorms capable of damaging winds and heavy rainfall remain possible today into this evening. An isolated tornado will also be possible.

DISCUSSION

KEY MESSAGE 1: Severe thunderstorms capable of damaging winds and heavy rainfall remain possible today into this evening. An isolated tornado will also be possible.

Current radar outlines a cluster of showers and thunderstorms generally south of a FIG-UNV-AVP line, generally moving east- southeastward as we move into the mid-afternoon hours. Some breaks in the clouds during the late morning and early afternoon hours has allowed for destabilization across this region, which will allow for some severe potential throughout the rest of the evening hours. Recent SPC Mesoanalysis maximized SBCAPE in the 1500-2000 J/kg range across southeastern Pennsylvania, which is more than sufficient for some strong-to-severe thunderstorms in the near-term. Shear remains the main question mark with regards to a more organized severe threat with recent bulk shear values topping out closer to 20 knots across south-central PA. Despite this lack of shear, damaging winds will remain possible over the next couple of hours and into the evening as more shear is present closer to southeastern Pennsylvania. Of note, 0-1 km shear on most recent SPC Mesoanalysis appears to be sufficient for some tornadic potential as storms move further southeast into slightly more favorable shear. In fact, the best storm (as of 1PM EDT) across Huntingdon County did show some history of rotation, and will be moving into a slightly more favorable environment this afternoon. Given all these factors, Severe Thunderstorm Watch will be in effect for portions of southern and eastern Pennsylvania until 8PM this evening.

Heavy rainfall signal remains concerning for much of central Pennsylvania with some gauge reports in Clearfield County and Cambria County reporting fairly widespread 3-5" and have prompted two Flash Flood Warnings this afternoon. HREF PWATs near the 90th percentile will warrant some watching of areas that receive either multiple rounds of thunderstorms and/or slow-moving thunderstorms this evening. Overall, cells appear that they will be moving, thus locations would ultimately need to receive multiple rounds of this heavy rainfall in order for problems to arise. That being said, instantaneous rainfall rates between 2-4"/hr could cause some issues in the more urbanized areas of southeastern Pennsylvania and the rolling hills of south-central Pennsylvania where water will drain towards the lowest locations. Thus, have expanded the Flood Watch a tier of counties northward and eastward to grab the south-central mountains to cover this threat.

Given the dual hydrology and severe threat this evening, have multiple ways to receive warnings! If outdoors this afternoon and evening, continue to monitor your local forecasts and have plans in case a severe storms and/or flooding occurs. Some storms will produce damaging winds and/or potentially a tornado this afternoon/evening, while some will not. Flooding will also be possible this evening given the ample rainfall rates, so if you are located near smaller streams and/or urbanized areas, be sure to monitor nearby surroundings if a thunderstorm moves over your area.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Current forecast package will deal primarily with -SHRA and TSRA moving across the forecast area through late evening. Very moist atmosphere is currently producing MVFR to IFR conditions in heavy rain and embedded TSRA. There is plenty of instabiity but cloud cover from earlier has kept any of the storms from becoming severe so far. This instability should be realized into stronger storms over southern PA and would affect KMDT and KLNS. Skies have cleared in this area and sun will help kick start convection which will likely be severe by late afternoon.

Convection will wane later this evening as a cold front pushes through the region. A result of the heavy rain will be a lowering of conditions to MVFR and possible IFR in BR or FG. Confidence is low to medium (20-40%) of any one location dropping to LIFR at this time, so made a mention of a drop to MVFR with VCFG wording overnight.

Behind the front on Wednesday, a BKN stratus layer will slowly dissipate through the day on a breezy to gusty northwest wind. Gusts will approach 20-25KTS during the afternoon.

Outlook...

Thu...Improving conditions with lingering -SHRA. Patchy AM fog.

Fri...Mainly VFR.

Sat and Sun...Patchy AM fog. Restrictions possible in isolated showers/storms mainly in the south.

CTP WATCHES/WARNINGS/ADVISORIES

Flood Watch through late tonight for PAZ026>028-035-036- 056>059-063>066.