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LMZ543 Expires:202404270100;;426805 Fzus53 Kgrb 262131 Nshgrb
nearshore marine forecast national weather service green bay wi 430 pm cdt Fri apr 26 2024
for waters within five nautical miles of the shore on lake michigan
lmz542-543-270100- sturgeon bay to two rivers wi-two rivers to sheboygan wi- 430 pm cdt Fri apr 26 2024
.small craft advisory in effect through late Saturday night - .
Rest of this afternoon - SE wind 10 to 20 kts. A chance of rain showers. Waves 2 to 4 ft. Tonight - SE wind 15 to 25 kts. Light rain showers in the evening. A chance of Thunderstorms. A chance of rain showers after midnight. Waves 3 to 5 ft building to 4 to 7 ft early in the morning. Saturday - S wind 15 to 25 kts. A chance of rain showers. Waves 4 to 7 ft. Saturday night - SW wind 10 to 15 kts veering N after midnight. Gusts to around 25 kts. A chance of light showers and Thunderstorms. Waves 4 to 7 ft subsiding to 1 to 3 ft. Sunday - NE wind 10 to 20 kts. Rain showers. Waves 1 to 3 ft building to 2 to 4 ft in the late morning and afternoon.

LMZ500
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FXUS63 KGRB 261917 AFDGRB

Area Forecast Discussion National Weather Service Green Bay WI 217 PM CDT Fri Apr 26 2024

Forecast discussion for routine afternoon forecast issuance

KEY MESSAGES

- Several systems will bring periods of showers and thunderstorms tonight through mid-week. Storms Sunday into Sunday night could produce periods of moderate to heavy rain, resulting in chances for isolated flash flooding in urban and low lying areas.

- It remains possible that conditions will support a few strong to severe storms from mid-Saturday afternoon through early Saturday evening over northeast Wisconsin. Large hail and damaging winds will be the primary threats.

- Conditions hazardous to small craft will be possible at times tonight through the weekend on the Bay and Lake.

DISCUSSION
Issued at 216 PM CDT Fri Apr 26 2024

Short Term...Tonight and Saturday

The latest RAP analysis and satellite imagery show a Rocky Mountain cyclone developing over Nebraska with a warm front extending east across Missouri and far southern Illinois. Strong isentropic ascent via 40-50kt low level jet are pushing showers into far southwest Wisconsin early this afternoon. However, these showers are running into a very dry airmass associated with a receding Hudson Bay high where PWATs remain 25% of normal over far northeast parts of Wisconsin. The strong southerly flow remains expected to overwhelm the dry air, and push some these showers into central and east-central WI by mid to late afternoon. As the low tracks northeast and across the region over the next 24 hours, precipitation trends and severe weather potential remain the primary forecast concerns.

Precipitation trends tonight into Saturday morning...Showers will continue to surge northeast across the region this evening.
Elevated instability will eventually move into the region, but not until late evening into the overnight. Some models depict elevated instability reaching up into the 500-700 j/kg range, but with effective shear values under 20 kts, most likely looking at sub- severe storms that could produce small hail. As the low level jet lifts into Upper Michigan, precip will trail off late overnight into Saturday morning across the region. Most of the region will remain shrouded in clouds until midday.

Severe potential on Saturday afternoon...As the low pressure system moves into the Lake Superior region on Saturday afternoon, it will leave behind a trailing cold front across northeast Wisconsin. This cold front could become the focus for strong or severe storms starting around mid to late afternoon.

A conditional severe weather event remains in the cards.
Uncertainties revolve around how destabilization and mid-level capping evolves in the afternoon. Low levels are forecast to remain quite moisture laden, but models insist partial clearing taking place ahead of the front by early afternoon. If clearing occurs, models are likely under doing surface temps and mixing which could lead to temps reaching into the middle 70s and dewpoints in the low 60s (instead of mid 60s like some models depict). If this were to occur, mixed layer instability of 1500 j/kg is possible, which modified soundings indicate would be sufficient to scrape by a mid-level cap for convective initiation by 21z.

The strength of the cap and mid-level dry air are also problematic as mid-level subsidence will be occurring behind a departing shortwave. Models frequently underdo the strength of the cap.
However, convective allowing models (CAMS) appear to be indicating that the cap and mid-level RH of 40-50% will be sufficient to prevent widespread deep/severe convection from developing. Most CAMS show little in the way of thunderstorm redevelopment on Saturday afternoon and precip amounts have trended down over the past 24 hours.

That said, it's hard to ignore the prospects of destabilization combined with effective shear values upwards of 40 kts due to strong winds through a deep layer. If storms develop, brief cyclonic curvature to the hodographs suggest discrete storms and all hazards would be possible including damaging winds, large hail, and possibly a tornado or two in the late afternoon hours before wind fields become more unidirectional. But dry air entrainment and mid-level capping will likely make the threat of severe storms isolated in coverage, if at all. It appears the greatest risk will occur over and just west of the Fox Valley where the most solar insolation is likely to occur and further removed from a more stable marine layer near Lake Michigan.

Long Term...Saturday Night Through Friday

An active pattern containing a train of shortwaves and several accompanying rain/storm chances will persist through the duration of the extended. Main focus will be on excessive rainfall risk with a Sunday/Monday system trailing behind Saturday's cold front.
Periods of moderate to heavy rainfall associated with this system may result in localized flash flooding. Attention will then turn to a mid-week system that clips the upper Midwest, bringing additional storm chances to the forecast area.

Saturday night through Monday... The severe weather threat should begin to wind down Saturday evening as the cold front drops to our south and encounters stable air near the lake. A brief lull in precip will then set in Saturday night before next chances for precip arrive Sunday morning. A southern stream trough will spin up a surface low over the Texas and Oklahoma panhandles before trekking into the Upper Mississippi Valley by Monday morning. Rain is expected to arrive to the forecast area Sunday morning out ahead of the warm front as it lifts north before becoming more widespread through the afternoon and evening. Potential for severe weather doesn't look too impressive given a robust capping inversion and insufficient airmass recovery time after Saturday's system. Any storms that do pop up would likely be elevated in nature, with gusty winds and small hail the biggest threats.
Current ensemble guidance currently seems to be targeting portions of central and north-central Wisconsin to receive the bulk of the rain (~0.6 to 1" 24-hour QPF). Given an open Gulf and 1 to 1.5" PWATs being ingested by the surface low, suspect that rainfall amounts may trend on the higher end of what is being shown.

Tuesday through Wednesday... Better chances for thunderstorms look to arrive with a Tuesday/Wednesday system. Another piece of closed upper-level energy will ride the US/Canada border through the beginning of the week, placing a surface low over Alberta that will skim the upper Midwest by Wednesday morning. Models are still struggling to resolve precip timing, but a broad overview seems to suggest that some convective elements (MUCAPE, low-level lapse rates, surface forcing due to cold FROPA) may come together enough to warrant thunderstorm potential. However, it is still too far out in the forecast period to ascertain storm severity.

AVIATION
for 18Z TAF Issuance Issued at 1236 PM CDT Fri Apr 26 2024

A very dry airmass will prevent precipitation from arriving until mid afternoon over central Wisconsin to early evening over far northeast Wisconsin. Periods of showers remain expected to surge northeast across the region for most of the night. Some thunderstorms are possible during the late evening into the overnight hours. Highest probabilities for storms is over central Wisconsin. Breaks in the precipitation are then expected on Saturday morning.

Ceilings are expected to deteriorate as a warm front lifts across the region tonight into Saturday morning. Widespread IFR and some LIFR ceilings are forecast late this evening into Saturday mid- morning. Vsbys may also fall to MVFR in the heavier showers or storms.

LLWS is expected region-wide tonight and Saturday morning as robust low- level winds off the surface arrives atop of strong low-level inversion.

GRB WATCHES/WARNINGS/ADVISORIES
None.

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