textproduct: Cleveland

WHAT HAS CHANGED

The threat of severe storms has trended up for Northwest Pennsylvania with an Enhanced Risk (level 3 of 5) for Erie County PA. This includes a threat of damaging winds and potential for a tornado.

KEY MESSAGES

1) Several rounds of thunderstorms are possible through midday on Friday. Thunderstorms may be strong to severe at times, including a threat of heavy rain and flash flooding.

2) Hot and humid conditions are forecast to continue on Thursday. Heat index values are forecast to be in the upper 90s for another day in Northwest Ohio with upper 80s to lower 90s forecast in Northeast Ohio.

3) Cooler and drier weather is expected for the holiday weekend.

DISCUSSION

KEY MESSAGE 1... A broad ridge remains in place across the Mid- Mississippi and Ohio Valley Regions. Hot and unstable conditions remain in place with dewpoints in the low 70s and ML CAPE values of 2500-3000 J/kg. Temperatures of 10C at 700mb can be a good indicator of a capped environment and this threshold extends to just north of Mansfield in the west but south of Youngstown in the east. So far a lack of forcing has been a limiting factor for thunderstorms this afternoon but that could change this evening.

The active storm track extends around the periphery of the ridge from Iowa across Central Lower Michigan to New York. We are seeing some cu trying to build across the snowbelt region with isolated to scattered storms possible in this area between 4-8 PM. The first couple showers that have developed have struggled to grow vertically. We will have to see if this initial activity has strong enough updrafts to be severe. Later this evening, the shortwave crossing lower Michigan will move towards New York and expect the potential for scattered strong to severe storms to move south into Northwest Pennsylvania, likely extending westward into Ohio. Effective shear values will increase to 40+ knots this evening coupled with backed flow in the low levels downwind of Lake Erie. The SPC has raised the severe weather outlook across New York and extending into Erie County PA to an Enhanced Risk (level 3 of 5) while a Slight Risk (level 2 of 5) remains from nearly Toledo to Youngstown. Main time frame for organized severe storms looks to be in the 7 PM - 11 PM window, then settling south across Northeast Ohio overnight. A few of the CAMs show another push of stronger storms late tonight moving southeast along the boundary but confidence is low in that occurrence.

The overnight activity will set the stage for Thursday as the ridge axis shifts slightly westward with northwest flow aloft. Instability will be very high again and expect to see scattered showers and thunderstorms develop with daytime heating during the afternoon. This could initially be focused from northwest to southeast across the eastern counties of Ohio but think we will see an expansion of storms across much of northern Ohio during the evening. Still many options for how this will evolve but the potential for severe thunderstorms and heavy rain will continue. PW values of around 2 inches will support heavy rainfall. Good coverage of thunderstorms are likely to persist Thursday night as a stronger shortwave trough finally crosses northern Ohio. Showers with scattered thunderstorms may linger into Friday but will become focused across Northwest Ohio by Saturday as the overall longwave pattern transitions to northwest flow with a trough over New England.

KEY MESSAGE 2...

Temperatures will trend slightly cooler on Thursday as the boundary settles south into the area. The hottest area will be along the I-75 corridor where highs will still be near 90 and heat index values will reach the upper 90s. Given the potential for scattered showers and thunderstorms along with partly cloudy skies, a Heat Advisory is not planned for Thursday. This could change overnight if chances of precipitation trend lower in the west or if more sun is expected. Will see how the convection plays out first before deciding if we need to an advisory for a few more counties.

KEY MESSAGE 3... High pressure will build south out of Canada on Saturday and bring cooler and drier conditions through the Holiday weekend. High temperatures will return to near normal values in the upper 70s to near 80. Dewpoints in the upper 50s to low 60s will bring lower humidity. A lower chance of precipitation will continue along the I-75 corridor on Saturday with dry conditions in the forecast Sunday through Tuesday.

AVIATION /06Z Thursday THROUGH Monday/

Ongoing convection moving in off Lake Erie continues for CLE/ERI, and could move into the CAK/YNG area. Terminals further west likely not to have to deal with this activity. Storms have histories of 25-40kt wind gusts and expect this to persist for the next couple of hours in NE OH/NW PA. After convective activity subsides after 08Z, mid level cloud deck remains through the night CLE-MF and terminals east. Cumulus field develops again Thursday with TSRA potential after 16Z, mainly for TOL/CLE/ERI/CAK/YNG. Could see another round after that towards the end of the 06Z Friday time frame, and may see this populated in the 12Z set of TAFs later this morning for later Thursday night.

Outlook... Non-VFR possible in rounds of showers/storms through early Friday.

MARINE

The main marine concern over the next 36 hours will be several rounds of showers and thunderstorms across the lake with the potential for strong, damaging wind gusts and large hail. For broader conditions outside of the storms, the lake is ahead of a cold front this afternoon and will have southerly flow until the cold front passes on Thursday evening. Some ridging will build from the north for Friday and into the weekend and allow for some elevated north to northeast flow. This flow could allow for waves in the 2 to 3 ft range over the weekend, which would be a problem for some moderate swimming risk for the western and central basin beaches.

CLE WATCHES/WARNINGS/ADVISORIES

OH...Heat Advisory from 2 PM this afternoon to 8 PM EDT this evening for OHZ003-006>008-017>019-027>031-036>038-047. PA...None. MARINE...None.