textproduct: Cleveland

WHAT HAS CHANGED

Showers and thunderstorms have trended a little more widespread early this morning. A Slight Risk for Excessive Rainfall (level 2 of 4) has been added for Monday and Monday night.

KEY MESSAGES

1) Widespread showers and thunderstorms with heavy downpours early this morning, then scattered showers and thunderstorms ahead of a cold front late this afternoon and evening.

2) Strong to severe thunderstorms and heavy rainfall are possible Monday and Monday night, with periods of heavy rain possibly lingering Tuesday.

3) Turning gradually drier, cooler, and less humid Wednesday into next weekend.

DISCUSSION

KEY MESSAGE 1... The familiar pattern remains this morning characterized by a broad mid/upper trough over south central Canada and the northern Plains into the Great Lakes and a broad mid/upper ridge over the Desert Southwest and southern Plains. This is keeping active WNW mid/upper flow in place across the southern Great Lakes and Ohio Valley overtop a quasi-stationary east-west front.

The next in the train of reinforcing shortwaves is moving into western Ohio early this morning. RAP analysis hints at a weak surface wave tied to this mid/upper shortwave that is traversing the frontal boundary, and the boundary is retreating northward this morning in response. Weak warm air/theta e advection and 500-1000 joules of MLCAPE in the presence of a very moist environment is triggering a large area of convection along the slowly retreating front this morning. This is being aided by upper diffluence and mid- level PVA, so expect showers and thunderstorms to be widespread again this morning as they slowly lift east and northeast. Given PWATs near 2 inches, skinny CAPE, and high freezing levels with deep warm cloud layers, torrential rainfall will accompany the convection, so will need to monitor for localized flooding this morning, particularly if heavy rain occurs in spots that saw it yesterday. Much of the lakeshore from Erie and Lorain Counties through central and southern Cuyahoga County measured 1 to 3 inches of rain since Thursday night, so these areas will be vulnerable if the convection can build farther north this morning.

This morning activity will exit NW PA by about 14Z, and expect a lull with quite a bit of sunshine into this afternoon allowing temperatures to warm into the mid/upper 80s ahead of the trailing cold front. This front is trending a few hours slower since it parallels the mid/upper flow but should sink across the region this evening. Pooling low-level moisture along the boundary (dew points 70-75 F) beneath a plume of steeper mid-level lapse rates around 7 C/Km will combine with the surface heating to yield strong instability this afternoon and evening. RAP and NAM forecast soundings suggest near 3000 joules of MLCAPE will be possible by mid to late afternoon, and with just enough deep layer effective shear for loose convective organization (0-6 Km shear around 25 knots), could see a few strong to severe storms with damaging winds late this afternoon and evening. The main question is convective coverage. Forecast soundings show quite a bit of dry air, and while this could aid downburst potential (DCAPE near 1000 joules), it may be too dry to get a lot of storm coverage. The thinking is that there will be a broken line of convection that will move ESE across the area in the 21-03Z timeframe, with a few widely scattered cells possible ahead of the line.

KEY MESSAGE 2... Weak surface high pressure will briefly build in tonight and Sunday, pushing the front to near the Ohio River and allowing somewhat less humid air to filter in (dew points in the low/mid 60s), however, the front will retreat back north as a warm front Sunday night in response to a much more potent mid/upper shortwave dropping into the Upper Midwest. This will bring the rich low-level moisture back into the region for Sunday night and Monday and possibly fuel an organized severe weather and heavy rainfall event.

Confidence continues to be low in terms of how convection will play out Sunday night through Monday night, so this set up has a high ceiling but also high bust potential. As the potent shortwave drops into the Upper Midwest Sunday evening, a 30-40 knot low-level jet and northeast periphery of a stout EML should trigger a severe MCS that will propagate ESE into northern Illinois, northern Indiana, and Lower Michigan along the strong instability gradient. This MCS will be outrunning the best instability and upper support by the time it reaches northern Ohio overnight Sunday night, but instability will be slowly increasing as the frontal boundary retreats north, so expect it to affect northern Ohio in some fashion Sunday night, with potential for gusty winds and heavy downpours. Best chance for this will be in NW Ohio. The bigger impact from this MCS will be how it affects convective redevelopment Monday. The shortwave is expected to continue east across the central and southern Great Lakes Monday with a resultant increase in mid- level flow yielding 40-50 knots of deep layer effective shear. This mid-level jet will encourage a low-level jet response by Monday evening to possibly ignite a new MCS over southern Lower Michigan, Lake Erie, and northern Ohio ahead of a cold front, but the location and strength of this development depends on how much cloud debris and showers from Sunday night linger into Monday. If clouds and leftover showers are widespread into Monday afternoon, instability will be reduced, and the new convection may be less organized or develop outside of our area. However, if the Sunday night activity clears out quickly Monday morning allowing for strong destabilization, then a potent severe weather and heavy rainfall set up could materialize Monday evening into Monday night. The strong shear and potentially strong instability could support widespread and significant wind damage, as well as embedded tornadoes if the higher ceiling can be reached, and a persistent westerly nocturnal low-level jet oriented increasingly parallel to the sagging cold front by Monday night also supports backbuilding and training convection with an associated flash flood risk. A WPC slight risk for excessive rainfall has been introduced for most of the area Monday and Monday night, and the area also remains in an SPC severe weather outlook.

The front looks to be slow to sink south Tuesday as it remains aligned parallel to the WNW mid/upper flow, and additional shortwave energy and remnant outflow boundaries interacting with it in the continued moist environment could support additional thunderstorms and heavy rainfall, so maintained high POPS through Tuesday.

KEY MESSAGE 3... The broad mid/upper trough will continue to slowly deepen across the Great Lakes and Northeast CONUS Wednesday into next weekend as a series of shortwaves gradually force the southern CONUS ridge into the 4 corners region and help the longwave trough to carve out as a closed low becomes established near Hudson Bay. This will continue to push the active frontal boundary slowly southward, so have gradually decreasing chances for showers and thunderstorms mid to late week. This will also allow progressively cooler and less humid conditions to filter in, with highs in the 70s by Thursday and Friday and lows dipping into the 50s in some areas.

AVIATION /18Z Saturday THROUGH Thursday/

MVFR ceilings have been loathe to scatter out this afternoon, especially locations east and south, following this morning's shower and thunderstorm activity. A cold front will approach from the northwest later today, ushering in some fairly transient showers and thunderstorms. All terminals can expect to see a period of two to three hours with thunderstorms on station, after which light winds and VFR conditions will persist into the nighttime. Toward daybreak, brief radiational fog (i.e., Sunrise Surprise) is expected to develop at nearly all locations. Dry, VFR conditions are expected through much of Sunday.

Outlook...Periods of non-VFR conditions are are expected Sunday night through Thursday, along with scattered showers and thunderstorms, with the passage of a couple frontal boundaries. Some storms could be strong to severe during this timeframe, especially on Monday.

MARINE

Southwest flow will maintain across the lake today ahead of a cold front. The cold front will cross the lake tonight bringing some scattered shower and storm activity to the region, followed by northwest flow on the lake tonight. A brief ridge will build into the region and return southwest flow to the lake on Sunday. A series of low pressure systems with shower and thunderstorm chances will move through the region on Sunday night through Tuesday. Winds on the lake will start southwest and become westerly on Tuesday. A weak ridge will reenter the region on Wednesday and bring light but variable flow over the lake. Overall, any marine headlines through the period will be tied to shower and thunderstorm activity and probably not winds/waves.

CLE WATCHES/WARNINGS/ADVISORIES

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