textproduct: State College

WHAT HAS CHANGED

Increasing potential for severe weather on Monday.

KEY MESSAGES

1) Severe thunderstorms possible on Monday; heavy rainfall may lead to isolated instances of flash flooding.

2) Dry for the middle of the week, but rain chances return to the forecast by Thursday.

DISCUSSION

KEY MESSAGE 1: Severe thunderstorms possible on Monday; heavy rainfall may lead to isolated instances of flash flooding.

Steady rain will overspread the northern half of Pennsylvania tonight ahead of a surface low that will track to our northwest during the day on Monday. Locations along and north of I-80 may see up to an inch of rain into early Monday afternoon.

Farther to the south, a warm front associated with this low will lift northward into the afternoon, potentially making it as far north as I-80. Dew points will climb into the upper 60s to the south of the front and there should be enough breaks in the clouds to allow for at least modest destabilization. The 12Z HREF shows SBCAPE approaching 1000 J/kg over much of south- central PA and many of the CAMs show higher values than this. Thunderstorms developing in this environment during the afternoon will have 40 to 50 knots of shear to work with, which will support some severe weather potential. Damaging winds appear to be the main concern at this time, though fairly strong low-level shear (SRH in the 100-200 m2/s2 range) may allow for an isolated tornado to develop. SPC has expanded the slight risk northward into parts of southern PA to highlight the increasing damaging wind potential, and they have expanded the 2% tornado contour northward to encompass most of the area east of I-99 and south of I-80.

Heavy rainfall will also be a concern with any thunderstorms that develop Monday afternoon as PWATs are expected to be as high as 1.9 inches over the Lower Susquehanna Valley. This is around the 95th percentile for this time of year. A warm cloud depth in excess of 11000 feet and tall, skinny CAPE profiles will also support heavy rainfall. WPC has maintained a slight risk for excessive rainfall over roughly the southern third of the forecast area and isolated instances of flash flooding will be possible, particularly in urban areas or any locations that see repeated rounds of thunderstorms. The two main things going against a more widespread flash flood threat are the ongoing drought and the fast storm motions owing to the strong flow aloft.

KEY MESSAGE 2: Dry for the middle of the week, but rain chances return to the forecast by Thursday.

Much drier conditions are expected for Tuesday and Wednesday as high pressure builds into Pennsylvania. A slight warming trend is expected, with high temperatures expected to be in the 70s and low 80s through the end of the week. An upper level disturbance moving through the region on Thursday and Friday will bring the next chance for showers and thunderstorms to Central PA, though there are still some timing differences in the models as to when rain will be most likely. Temperatures continue to warm up into the weekend, with most of the area expected to see highs in the 80s by Sunday.

AVIATION /00Z MONDAY THROUGH FRIDAY/

A low pressure system approaching from the Ohio Valley will bring a gradual increase in moisture and cloud cover from west to east, with VFR expected to prevail at all terminals for tonight. A band of rain associated with warm air advection (WAA) ahead of the system will overspread north- central Pennsylvania (KBFD/KIPT/KUNV) between 06Z-15Z Monday, with convective -SHRA/-TSRA becoming more numerous through the morning/afternoon. Embedded thunderstorms are possible within the WAA rainband as overrunning warm/moist air introduces instability within the mixed-phase layer (~50 J/kg MUCAPE between -10C to -30C), enabling lightning production.

Ceilings will trend lower during the daytime on Monday as the low pressure center tracks over the Commonwealth & showers/storms grow more widespread, with lower confidence in ceiling restrictions across the Lower Susquehanna Valley (KMDT/KLNS) deeper into the warm sector. With that said, visibility restrictions will be possible in any heavier showers/storms given anomalously high precipitable water values (PWATs >1.5") supporting locally heavy rain. LLWS will also be a hazard due to a southerly/southwesterly LLJ ahead of the system, with higher confidence at KBFD/KJST at this time.

Lastly, Convection-Allowing Models (CAMs) suggest a line of thunderstorms may develop along a pre-frontal trough Monday afternoon, with potential impacts at KMDT/KLNS between roughly 18Z-21Z. Decent deep-layer wind shear (0-6 km shear around 40 kts) + adequate instability (MLCAPE between 750-1000 J/kg) will support organized convection with this trough, along with the possibility for gusty winds and perhaps some locally severe weather.

Outlook...

Tue...Lower chances of showers, mainly VFR.

Wed...Mainly VFR, watching for precipitation chances though in the evening/overnight.

Thu...Restrictions possible in another round of evening/overnight.

Fri...Restrictions possible in another round of daytime/evening showers.

CTP WATCHES/WARNINGS/ADVISORIES

None.