textproduct: Cleveland

WHAT HAS CHANGED

No significant changes with this update. Rainfall amounts late Saturday into Sunday remain similar to the previous forecast.

KEY MESSAGES

1) Rain showers will slowly increase Saturday, with periods of heavy rain possible Saturday night into Sunday morning. The heaviest rain is expected over the lakeshore counties and also in NW Ohio where localized flooding is possible.

2) Cool and unsettled weather is expected next week.

DISCUSSION

KEY MESSAGE 1... Canadian high pressure over the northern and central Great Lakes this afternoon through early tonight will briefly keep the active frontal boundary to our south near the Ohio River. However, the current showers and thunderstorms over Illinois and Wisconsin are a sign of what's to come for the weekend. This activity is tied to the first in a series of mid/upper shortwave troughs that will progress across the southern Great Lakes tonight through Sunday. Increasing isentropic ascent and right entrance forcing beneath a subtle upper jet streak will generate showers near Lake Erie and NW Ohio by Saturday morning, but abundant low-level dry air will likely cause much of this initial activity to fall apart. Kept chance to slight chance POPS in the morning and afternoon for low impact, scattered showers.

Better potential for more of a widespread, heavier rain will start to arrive Saturday evening and especially Saturday night through Sunday morning as the next and much stronger mid/upper shortwave crosses the southern Great Lakes region. An 80+ knot upper jet streak rounding the base of the shortwave trough and upper diffluence will aid in a slightly deepening surface low tracking from NE Iowa Saturday afternoon through southern Ontario Saturday night/early Sunday. The frontal boundary will lift back north into the region ahead of this low Saturday afternoon and Saturday night, likely becoming quasi-stationary near the south shore of Lake Erie. This will bring the rich low-level moisture characterized by 70+ F dew points back into the area. The combination of strong warm/moist advection and isentropic ascent north of the lifting frontal boundary, upper diffluence and right entrance forcing south of the aforementioned jet streak tracking over Lower Michigan, and the moisture rich airmass will lead to a band of steady rain. There will be a "sweet spot" where the strongest jet dynamics align to tighten mid-level frontogenesis, and that is where an embedded narrower strip of heavier convection is expected to line up. Most of the region will see 0.50 to 1.00 inch of rain late Saturday through midday Sunday, but where the embedded band of heavier convection sets up could see 1-2 inches (locally higher). PWATs of 1.75 to 2 inches will be in the 90th to 99.5th percentile of daily climatology, and this combined with deep warm cloud layers supports efficient rainfall rates. The frontal boundary will be aligned fairly parallel to the mid/upper flow, so will need to watch for training convection where this narrow band sets up. This could lead to rises on area rivers, creeks, and streams as well as flooding of typical flood prone area. The WPC Slight Risk for Excessive Rainfall still looks good for both Saturday and Sunday (Level 2 of 4) in NW Ohio since consensus among RRFS and HREF CAMS are showing greatest consistency over that area, but this heavy rain swath could also extend across the lakeshore counties of NE Ohio and Erie County, PA. Held off on a Flood Watch because there is still plenty of time to see future CAMS pinpoint the exact location of the convective band, but a Watch could be considered in the next couple of forecast cycles.

The area will dry out from north to south Sunday afternoon and Sunday night as a strong cold front pushes south of the region behind the exiting surface low and Canadian high pressure tries to build in from the northern Great Lakes.

KEY MESSAGE 2... The first true stretch of Fall weather will arrive Monday and persist through next week as a broad mid/upper trough across the Great Lakes becomes blocked to the south of an anomalous mid/upper ridge building over most of Canada. This will result in a closed (cut off) low eventually developing and pinwheeling between the Ohio Valley and Mid Atlantic regions mid to late week to the south of a strong Canadian high over eastern Canada. This pattern will result in cool NE low-level flow and breezy conditions, along with periods of rain showers. Details for each day are very uncertain at this time, but the main message is cool with periods of showers all week. Highs will generally be in the low/upper 60s with lows in the 50s.

AVIATION /18Z Friday THROUGH Tuesday/

VFR conditions expected this afternoon and evening with mostly sunny skies due to high pressure over the Great Lakes region. Winds will be out of the north around 8-10 knots this afternoon.

Low pressure approaches from the southwest, with a series of mid- level impulses impacting the region starting Saturday. Showers and thunderstorms are most likely to impact Northwest Ohio Saturday morning, with showers and storms potentially getting into north- central and Northeast Ohio later Saturday afternoon/evening. Non-VFR visibilities are likely within heavier showers/thunderstorms.

Outlook...Non-VFR possible with showers and occasional thunderstorms through Tuesday.

MARINE

As high pressure builds south across Lake Erie today, a brief period of elevated northeasterly winds of 10-15 knots will impact portions of the western and central basins. Given the onshore flow, wave of 1- 3 feet along the lakeshore is expected with up to 4 feet possible at times. As a result, there is a moderate risk for rip currents through late morning. As the high pressure becomes dominant this afternoon, northeast flow will weaken to 5-10 knots and allow for waves to diminish to 1-2 feet. These conditions will persist into Saturday morning before winds shift and become easterly at 10-15 knots ahead of an approaching low pressure system. As this system approaches the southern lakeshore Sunday morning, increased winds from the northeast to 15-20 knots will gradually build waves to 3-5 feet. Given a deepening gradient, these winds are expected to increase into Monday, possibly approaching Gales by Monday afternoon. These strong winds will be highly dependent on the exact placement of the low pressure system, but either way at minimum a Small Craft Advisory and Beach Hazards Statement will be needed for much of Lake Erie from Sunday through at least Tuesday. By midweek, model guidance begins to disagree quite a bit resulting in less confidence.

CLE WATCHES/WARNINGS/ADVISORIES

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