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LMZ521 Green Bay South Of Line From Cedar River To Rock Island Passage And North Of A Line From Oconto Wi To Little Sturgeon Bay Wi- Green Bay South Of Line From Oconto Wi To Little Sturgeon Bay Wi- 632 Am Cdt Mon May 20 2024
Today - SE wind 5 to 10 kts. A chance of light showers and Thunderstorms. Waves 2 ft or less. Tonight - NE wind around 5 kts backing nw after midnight. A chance of light showers and Thunderstorms in the evening. Patchy fog. Waves 2 ft or less. Tuesday - E wind 10 to 15 kts. Patchy fog in the morning. A chance of rain showers. Waves 2 ft or less. Tuesday night - SE wind 10 to 20 kts veering S 15 to 25 kts after midnight. Gusts to around 30 kts. Rain showers and a chance of Thunderstorms. Waves 1 to 3 ft. A small craft advisory may be needed.

LMZ500
No data

FXUS63 KGRB 201145 AFDGRB

Area Forecast Discussion National Weather Service Green Bay WI 645 AM CDT Mon May 20 2024

Updated aviation portion for 12Z TAF issuance

KEY MESSAGES

- Showers and thunderstorms are likely today. Isolated strong or severe storms with gusty winds and hail are possible south of Highway 10 this afternoon.

- Confidence is increasing in potential for impactful severe weather and localized flooding across much of the forecast area Tuesday afternoon and evening. Damaging winds, large hail, and a few tornadoes will be possible with stronger storms.

DISCUSSION
Issued at 414 AM CDT Mon May 20 2024

Short Term
Today
Tonight...and Tuesday

An upper trough moving from Iowa to northern Lake Michigan will produce showers and elevated thunderstorms today. The associated surface low should track south of the forecast area, and winds will have a component from over the cool waters of Lake Michigan.
This should keep surface based convection mainly to our south. A few models have the warm front getting into the far southern part of the forecast area, and if so there could be some strong or marginally severe storms with hail and strong winds in the afternoon. Highs today will be close to normal.

The showers should exit the lakeshore area in the early evening, with dry weather overnight. Wet ground and dewpoints in the low 50s may make for ground fog after midnight.

A strong, negatively tilted 500mb trough will approach the area Tuesday. At the surface, an intensifying surface low will move from northern Kansas to northwest Wisconsin. The surface warm front is expected to stay to our south until early evening. This will likely prevent surface based convection from initiating, but there will be showers and elevated convection north of the front.
Precipitable water values are at all time records for this time of the year, so very heavy rain and local flooding is possible, especially in the afternoon and evening. There is also some potential for large hail with the stronger storms in the afternoon. The greatest threat of severe storms looks to be Tuesday evening as the warm front moves north into our area.

Long Term...Tuesday Night Through Sunday

Focus for the long-term remains on severe weather and heavy rainfall concerns Tuesday evening with the passage of a robust system. Confidence is increasing in potential for impactful severe weather and localized flooding across much of the forecast area as more models come into agreement about the anomalous nature of the driving surface low. The SPC currently highlights central Wisconsin in a Day 2 enhanced risk for severe weather.

Tuesday evening storm chances... Dynamics with this system continue to look favorable for a severe weather setup Tuesday afternoon and evening as a negatively tilted shortwave swings to our northwest. During this time, the attendant surface low is expected to deepen to ~983 to 990 mb (once every ten year or more climatological return interval) as it ejects up from the central Plains. Timing of this system paired with the track of the warm front will be critical in optimizing instability. An earlier arrival during peak heating Tuesday afternoon may be enough to maximize CAPE values along/north of the warm sector, allowing for severe storm development as far north as the Highway 29 corridor.
This solution would potentially spell out a broken cap scenario across portions of east-central Wisconsin Tuesday night where a mean southeast wind at the surface advects a stable lake boundary onshore. Otherwise, the assembly of other convective ingredients may be enough to overcome limited surface-based instability. An open Gulf will enable the transport of surface dewpoints in the mid to upper 60s into central and east-central Wisconsin, resulting in a broad swath of 1.5 to 1.8" PWATs. Additionally, deep layer shear profiles (0 to 6 km shear of 50+ knots) and low- level helicity (200 to 400 0-1 km SRH, 400 to 500 0-3 km SRH) may support some discrete, rotating cells Tuesday night. Strong rotating storms may be capable of producing tornadoes, which bears emphasis. Sufficient lapse rates (~8 deg C/km) and a 50 to 60 knot low-level jet would also indicate large hail and damaging winds as potential hazards. Suspect that the damaging wind threat will decrease to the east given a lake-induced inversion and, hence, lower likelihood of stronger winds aloft mixing down to the surface. Heavy rainfall and localized flooding will continue to be a concern, although highest QPF will likely remain off to our west where the deformation zone is. We remain under a marginal to slight risk for receiving excessive rainfall.

Rest of the extended... The remainder of the extended sees the current pattern quieting down slightly as mid-level flow begins to flatten out toward the end of the week. Next chances for precip look to arrive Friday into Saturday, although models are struggling with timing and amounts this far out.

AVIATION
for 12Z TAF Issuance Issued at 645 AM CDT Mon May 20 2024

An upper level disturbance will bring showers and MVFR conditions in most areas today. Thunderstorms are possible, mainly south of a AUW to GRB line. The activity will end from west to east during the late afternoon and early evening hours.

IFR conditions are expected in most area overnight, due to wet ground and moist low level air. Surface winds will be light.

A strong storm system moving from Iowa to northwest Wisconsin will bring showers and thunderstorms to the area Tuesday and Tuesday night. The activity will arrive from southwest to northeast. IFR conditions are expected once the precipitation arrives. There is a possibility of severe storms during the evening hours south of a AUW to GRB line.

GRB WATCHES/WARNINGS/ADVISORIES
None.

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