textproduct: Cleveland

WHAT HAS CHANGED

Confidence has increased that a cluster of strong to severe storms with gusty winds, some hail, and heavy rain will move west to east across the area late this afternoon and evening. Another round of storms with some severe weather potential and a better potential for heavy rain and localized flooding is still expected tonight. The rest of the forecast and messaging remains the same through the weekend, as additional rounds of showers and storms are expected.

KEY MESSAGES

1) A cluster of strong to severe thunderstorms will sweep across the region late this afternoon and evening. The main impact will be gusty winds and heavy rainfall, but hail and a couple of brief tornadoes are possible too.

2) A second round of strong to severe thunderstorms and heavy rainfall is expected tonight. Heavy rainfall is the bigger concern with this overnight round, with widespread amounts of 1 to 1.25 inches possible through Wednesday, and locally higher where training occurs. This could cause flooding of urban, low- lying, and other flood prone areas.

3) Lower impact system Thursday and Friday, with occasional showers and storms and a return of very warm temperatures.

4) A stronger system brings a greater potential for severe storms and heavy rainfall late Saturday into Sunday.

DISCUSSION

KEY MESSAGE #1: Quasi-zonal flow continues across the southern Great Lakes region this afternoon overtop a flat mid/upper ridge over the SE CONUS. An active stationary front remains draped across Lower Michigan and southern Ontario along the northern edge of this ridge, and this boundary continues to serve as the focus for a series of shortwaves and associated weak surface lows.

One shortwave and associated mid-level speed max, characterized by 700 mb flow over 50 knots, is currently crossing northern Indiana and SW Lower Michigan. This shortwave and its associated 40+ knot low-level jet is driving a Mesoscale Convective System (MCS) that will move into NW Ohio by 19Z. The downstream environment across northern Ohio and western PA has sufficiently destabilized this afternoon to support severe weather due to a rapid uptick in shear expected as the wave approaches. Current mesoanalysis shows MLCAPE of 500-1000 J/kg, with around 1500 J/Kg of SBCAPE, beneath a stout EML plume (7 to 7.5 C/Km mid- level lapse rates). Westerly Deep layer shear will increase to 40-50 knots ahead of the wave, with vectors oriented perpendicular to the north-south oriented line of convection on the front edge of the MCS, and this combined with the moderate instability will support forward propagation of the MCS late this afternoon and evening and associated wind risk. PWATs of 1 to 1.2 inches (90th percentile of climatology) will also lead to brief heavy rain as the complex progresses across the region. While wind and heavy rain are the biggest hazards, low-level (0-1 Km) shear increasing to 20-30 knots and effective SRH increasing to 250-300 m2/s2 could support a couple of brief tornadoes embedded within the main line or with any semi discrete cells that develop ahead of it. The aforementioned steep mid-level lapse rates also support occasional large hail, especially with any semi discrete cells. The overall highest severe weather risk appears to be east of I-71 this evening, where higher instability has built up the past few hours. This is especially the case for damaging winds given higher low-level lapse rates of 7 to 7.5 C/Km leading to higher DCAPE in NE Ohio and NW PA.

In terms of timing, the convection should reach the I-75 corridor by 20Z, the I-71 and I-77 corridors by 22Z, and should be over NW PA by 23 to 00Z.

KEY MESSAGE #2: After a short break during the mid evening hours, the shortwave and associated surface low exiting east across the eastern Great Lakes tonight will start to push the stationary front SE across the region as a cold front. Another line of convection is expected to develop along and ahead of the cold front as large scale forcing for ascent increases. Shear will remain moderate to high overnight with the presence of an enhanced low-level jet, so a few severe storms are possible (mainly wind and a brief tornado or two) where surges and bows occur in the QLCS. However, the atmosphere will be pretty worked over from the earlier evening convection, and this combined with the overnight timing should limit the severe weather threat.

The bigger concern overnight is heavy rainfall and flash flooding. The front will slow in forward progress as the night progresses and will become aligned parallel to the W to WSW low and mid-level flow. This combined with the 35-40 knot low- level jet will lead to training convection within the QLCS/backbuilding. On top of that, PWATs will increase to over 1.25 inches (near the daily max of sounding climatology) with Integrated Vapor Transport (IVT) in the 90th to 97th percentile, and this all combined with a skinny CAPE profile all points to swaths of very heavy rainfall and efficient rainfall rates. A widespread 1 to 1.25 inches of rain is likely through Wednesday morning, but locally higher totals of 2 to 3 inches+ are possible where training occurs. This could lead to flash flooding of low-lying, urban, and other prone areas, as well as potential river flooding if enough falls in a basin.

Much of the area will try to dry out Wednesday as the front temporarily sags into southern or central Ohio and Canadian high pressure over the northern Great Lakes tries to influence at least the north half of our region. However, a swath of showers will continue north of the frontal boundary where isentropic ascent will be persistent, so the southern and central counties (south of I-80) may hold onto off and on light rain much of the day. If the front is slightly farther north or south, than the location of the swath of showers will change. The overall impact and message Wednesday is that it will be a cold and damp day, with highs held into the mid 40s to low 50s. Showers will definitely increase Wednesday evening and Wednesday night as the front slowly lifts back north. KEY MESSAGE #3: The next mid/upper shortwave ejecting across the Plains Thursday will spawn another surface low that will lift into the northern Great Lakes by Friday morning. The front should continue lifting north Thursday morning ahead of this developing low, so most of the leftover showers should be north of the region by midday. This will set up a mainly dry and very warm afternoon in the warm sector. As the low tracks from the northern Great Lakes through northern Ontario Friday, the front will sag back SE as a cold front. However, it is looking increasingly likely that the boundary will stay NW of the region as it stalls in response to yet another developing low in the Plains. This points to slightly higher coverage of showers and a few thunderstorms Friday compared to Thursday, but any heavy rain or severe threat should be isolated, so lower impact than the mid week convection. Temperatures will soar into the upper 70s to near 80 Thursday and Friday. KEY MESSAGE #4: The next surface low mentioned above developing in the Plains Friday will lift across the Great Lakes Saturday and early Sunday. This low will be stronger and will be the final wave to kick this old frontal boundary out of the region as it's associated with a strong mid/upper trough digging into the Great Lakes. Given the better dynamics and strength of this system, severe thunderstorms and heavy rainfall are possible late Saturday and Saturday night, depending on the timing. This will be monitored. Much colder air will settle into the region behind this system for early next week.

AVIATION /00Z Wednesday THROUGH Sunday/

Highly convective environment this evening into tonight with rounds of TSRA expected for a few more hours before a cold front finally sinks southward across northern Ohio and northwestern Pennsylvania. Expect observations to fluctuate in and out of showers and storms. As far as convective winds go, FDY has the best chances for strong wind gusts to 40kts or higher as that area has not seen much in the way of activity. Further north and east, expecting gusts to be limited to the 30-35kt range in storms as the surface air is a bit more stable now after the first complex earlier this evening. After the cold front comes through, ceilings to drop to IFR and remain there pretty much through the duration of the TAF. Rain will be off and on Wednesday in these low ceilings, so restrictions through much of the forecast should ultimately be expected one way or another.

Outlook...Non-VFR likely in showers and low ceilings Wednesday afternoon. Additional periods of non-VFR conditions possible in showers and thunderstorms Thursday through Saturday, though appears most likely on Saturday.

MARINE

Elevated southwest winds of 15 to 20 knots will persist across Lake Erie into tonight. Though winds may occasionally gust up to 25 knots, believe most of the higher winds should be confined along the nearshore given the cold lake surface temperatures. Thus, have opted against a Small Craft Advisory at this time. Winds will briefly shift towards the north behind a cold front on Wednesday, 10 to 15 knots, increasing to 15 to 20 knots and gradually becoming east to northeast Wednesday night into Thursday. May need a brief Small Craft Advisory Wednesday night into Thursday morning with the onshore flow near 20 knots. Winds will return to the southwest as the cold front lifts back north as a warm front late Thursday into Friday, 15 to 20 knots.

Another item of concern this afternoon and evening will be the threat of strong winds greater than 40 knots associated with a line of thunderstorms moving west to east across Lake Erie.

CLE WATCHES/WARNINGS/ADVISORIES

OH...None. PA...None. MARINE...None.