textproduct: Cleveland

WHAT HAS CHANGED

A Day 1 SPC Marginal Risk (1 of 5) for severe weather has been expanded through the entire area tonight, and a Slight Risk (2 of 5) has been added to NW Ohio. A Day 1 WPC Slight Risk (1 of 4) for Excessive Rainfall has been slightly expanded into more of NW Ohio tonight, and a Day 3 Slight Risk for Excessive Rainfall has been expanded to most of the area Tuesday.

KEY MESSAGES

1) Briefly quiet and less humid today followed by multiple rounds of showers and thunderstorms tonight through Wednesday. Confidence is low on the timing, location, and intensity of each round of storms, but rainfall totals could add up in some areas, and some of the storms could produce severe weather.

2) Turning gradually drier, cooler, and less humid Wednesday night through Saturday.

DISCUSSION

KEY MESSAGE 1... A weak cold front will continue to settle south toward the Ohio River this morning as weak surface high pressure builds across the southern Great Lakes. This will allow dew points to run about 10 degrees cooler today making for noticeably less humid conditions, but temperatures will remain very warm with highs in the mid/upper 80s again.

The well advertised very active pattern will begin tonight as the surface high drifts east and allows the front to lift back north as a warm front in response to a mid/upper shortwave dropping into the Upper Midwest. This shortwave will be one of many that will progress through a WNW mid/upper flow pattern between a broad ridge covering the southern Plains and Mid Mississippi Valley and broad trough over southern Canada and the Great Lakes. Each shortwave will interact with the frontal boundary as it becomes quasi-stationary across the southern Great Lakes and Ohio Valley regions, bringing multiple rounds of convection tonight through Wednesday. Unfortunately, the forecast has become even more uncertain in terms of the timing, location, and intensity of each round of convection since each thunderstorm complex (MCS) will move the location of the front and also lay down outflow boundaries which will affect the evolution and track of future complexes. Despite the high uncertainty and resulting low confidence forecast, attempted to describe some individual periods below.

Tonight and Monday morning: This period is trending somewhat more impactful, and the Day 1 SPC Marginal Risk (1 of 5) for severe weather has been expanded through the entire area tonight, and a Slight Risk (2 of 5) has been added to NW Ohio, along with a small expansion of the WPC Day 1 Slight Risk for excessive rainfall into more of NW Ohio. The initial shortwave moving into the Upper Midwest tonight will ignite an MCS over the western Great Lakes this evening, where a 30-40 knot low- level jet will focus strong warm/moist/theta e advection into the frontal boundary. The past few runs of the HREF members and RRFS have struggled with the evolution and track of this MCS (and will continue to do so in this entire pattern), but consensus is becoming more aligned with the idea that it will move at least into NW Ohio late tonight and possibly roll through the lakeshore counties in the pre-dawn hours Monday morning along the retreating frontal boundary. Instability will be weaker tonight and early Monday morning compared to the source region for this complex to the west, so the most intense portion of the system may dive south through Illinois and Indiana, but slowly increasing low-level moisture and instability through the night coupled with increasing mid-level flow ahead of the approaching shortwave should be enough to support some damaging wind risk, and PWATs rising back to 1.5 to 2 inches definitely support torrential rainfall with any thunderstorms. So, potential is growing for thunderstorms late tonight and early Monday morning to have decent organization and produce some scattered wind damage and heavy rainfall.

Monday afternoon and Monday night: This period is trending drier since the greater coverage and organization of the convection early Monday morning should push the boundary back southwest and also lay down outflow boundaries. This will likely cause new convection to initiate farther southwest Monday evening and Monday night. The shortwave will continue to progress through the central Great Lakes Monday afternoon and evening, with a 30-40 knot low-level jet response focusing warm/moist/theta e advection into the front and outflow boundaries. This will ignite another MCS Monday evening or early Monday night, but consensus is growing that the bulk of the activity will be confined to areas along and south of a Findlay to Mount Vernon line, and some HREF members even have it entirely southwest of our CWA. While the threat of organized convection is decreasing for most of our area Monday evening through Monday night, noteworthy deep layer effective shear of 35-40 knots along with the low-level jet aligned increasingly parallel to the main boundary supports both severe weather (mainly damaging winds but a couple of tornadoes) and torrential rainfall/flash flooding from backbuilding convection wherever it develops and tracks. It will be important to continue to check the latest forecast for Monday evening and Monday night since the convection will be significant wherever it ends up being.

Tuesday and Tuesday night: Uncertainty becomes really high by the time we get to this period of the forecast, but the synoptic frontal boundary remaining quasi-stationary and wavering around the southern Great Lakes and Ohio Valley combined with what appears to be one or two additional shortwaves will ignite additional scattered convection during peak heating and MCS clusters at night. Whether the convection ends up south of our CWA (possible scenario due to all of the outflow from previous convection) or into parts of our area, excessive rain totals and flash flooding will be a growing concern because this will be the third day of potential heavy rain. The expansion of the WPC Day 3 Slight Risk for excessive rainfall into most of the area Tuesday and Tuesday night is reasonable based on the uncertainty in location but potential for rainfall accumulations to be high by the time this period arrives depending on where all of the individual rounds end up.

Wednesday and Wednesday night: The shortwave train should finally flatten the southern CONUS ridge enough by this period that the trough across the Great Lakes and Northeast CONUS will finally be able to deepen enough to begin to push the active boundary south of the area. One more day of convection and heavy rainfall is possible over or just south of the CWA Wednesday before the drier air begins to work in Wednesday night.

KEY MESSAGE 2... The deepening mid/upper trough across the Great Lakes and Northeast CONUS Wednesday night through the start of the weekend combined with Canadian high pressure at the surface gradually sliding down across the Great Lakes and pushing the frontal boundary into the Ohio River Valley will yield drier, cooler, and less humid conditions. Highs will be in the mid to upper 70s by Friday and Saturday with lows in the 50s away from the lake.

AVIATION /18Z Sunday THROUGH Friday/

The easy part of the forecast is that away from thunderstorms, VFR conditions are expected with south-southwest winds generally 10kt or less...outside of a northerly lake breeze at ERI this afternoon. The more challenging portion of this forecast is the evolution of one or two clusters of convection that are expected to approach and/or move through parts of the airspace this evening through Monday morning.

As of 18z, one cluster of thunderstorms extends from central Lake Michigan to just northwest of Chicago and is moving east- southeast at 25-30kt. This cluster of storms is not modeled particularly well. The 15z RRFS offered the best simulation of its current structure and was relied upon to aid in timing across parts of northern Ohio between 0-5Z. Confidence is not extremely high in this first cluster bringing TS to any particular terminals yet so opted for prevailing VCSH with a PROB30 for TS impacts (which includes gustier winds at TOL, FDY and MFD), though confidence is increasing by the hour and will AMD as able to reflect increasing confidence over the coming hours. Confidence is lower at impacts to ERI and YNG, precluding a PROB30 group there. Guidance suggests another cluster of storms will quickly develop to our west-northwest tonight and track east-southeast towards the vicinity of our terminals early Monday morning. Confidence in this round occurring is decently high so have prevailing -SHRA at all sites with VCTS at some, but confidence in the exact timing and location is low. A combination of the 12z NAM Nest, 12z HRRR, end of the 16z HRRR, and end of the 15z RRFS was used to approximate timing of the early Monday round. However, confidence in when and where any TS impacts occur is lower, so utilized a PROB30 group for TS impacts with lower vsby early Monday morning across all terminals. A period of more tranquil conditions should follow for at least several hours on Monday once early activity exits.

Outlook...Periods of non-VFR conditions are expected through Thursday as several rounds of scattered showers and thunderstorms move across the airspace. Some storms could be strong to severe during this timeframe.

MARINE

Northwest flow is present over the lake this morning with a cold front exiting the region. High pressure will briefly build over the lake today and flow will shift to the southwest by this afternoon. For tonight through Wednesday, the weather pattern appears fairly unsettled with several weak low pressure systems moving over or just south of the area. These systems will allow for southwest to west flow to persist through the middle of the week and will bring periodic shower and storm chances. There may be some marine headlines needed with the storm chances over the lake. A cold front will cross the area by Thursday and high pressure will build into the region toward the end of the week, allowing for northwest flow to take over. There could be some 2 to 3 ft waves with the cold frontal passage, which may lead to a brief moderate swimming risk.

CLE WATCHES/WARNINGS/ADVISORIES

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