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FXUS63 KGRB 012332 AFDGRB

Area Forecast Discussion National Weather Service Green Bay WI 632 PM CDT Wed May 1 2024

Updated aviation portion for 00Z TAF issuance

KEY MESSAGES

- Another round of showers and storms is expected Thursday afternoon through Friday morning. Best chance for thunderstorms will be Thursday afternoon and evening with small hail and gusty winds the main impacts with any storms that develop.

- Due to recent and upcoming rainfall, rivers and streams will be on the rise late this week. Some rivers are forecast to reach bankfull stage by Friday or Saturday.

- Additional showers are possible Saturday into Saturday night with more showers and thunderstorms Monday night through Wednesday. This will keep river levels elevated well into next week with some rivers remaining at or above bankfull.

DISCUSSION
Issued at 228 PM CDT Wed May 1 2024

Short Term...Tonight and Thursday

As decaying low pressure continued to exit to the north this afternoon, weak high pressure building in its wake began to erode a lingering mid-level cloud deck across central and east-central Wisconsin. West winds continued to ramp up behind the cold front this afternoon with some assistance from daytime mixing and 850 mb cold air advection, with widespread gusts between 30 and 35 mph observed across the forecast area.

Main concerns for the short-term will be pinpointing rain/storm chances for tomorrow as well as potential for patchy fog across the north tomorrow morning. A surface low currently over New Mexico is progged to make its way into the upper Mississippi Valley by Thursday evening, bringing rain chances to the forecast area beginning Thursday morning. Warm air advection precip is expected to arrive from the southwest out ahead of a warm front Thursday morning before lifting northeast across the forecast area and becoming more widespread Thursday afternoon. Most short-range models are showing a strong signal (~60 to 70% probability) for at least half an inch of storm total QPF across our western counties, decreasing further east. Locally higher amounts (between 0.75 and 1") may be possible in areas that receive heavier rainfall, especially given strong poleward moisture transport and an open Gulf. For this reason, can expect river/stream levels to be on the rise through the end of the week with a marginal/slight risk of excessive rainfall through Friday morning. Localized flooding may also be possible in low-lying or urban areas.

Thunder chances... Thunder potential for tomorrow doesn't look too impressive, with most convective elements looking to remain to our south. However, an elevated thunderstorm with some gusty winds cannot be ruled out Thursday evening with MUCAPE values in the 300 to 400 J/kg range across east-central Wisconsin.
Additionally, any storms that do get going would have to contend with a robust surface inversion due to easterly flow advecting a stable lake airmass onshore.

Fog potential... Some radiation fog is possible tomorrow morning across the far north, although confidence is currently low. Light and variable winds overnight combined with brief windows for partial clearing tomorrow morning may be favorable for development of patchy ground fog, although its southern extent is still uncertain. Moreover, low temperatures tonight in north-central Wisconsin are expected to dip down between a favorable four to five degrees below this afternoon's lowest dewpoint temperature.

Long Term...Thursday Night Through Wednesday

An active weather pattern to continue as a series of systems moves across the CONUS into next week. The first system to exit northeast WI Friday morning. A weaker system may bring showers on Saturday, but models disagree on the extent of precipitation. A stronger system is forecast to arrive Monday night and may linger over the area through Wednesday. Depending on rain amounts, hydro concerns will remain an issue as a number of rivers in northeast WI are already near bankfull. Temperatures will be at or above normal through the period.

Thursday night and Friday...
Showers/isolated thunderstorms to already be underway across northeast WI at the beginning of Thursday night as a cold front pushes across the area. Instability appears meager at best, but 0-6km shear is 50 kts or greater, thus cannot rule out some thunderstorms to persist through much of the night. Severe potential does remain minimal due to the lack of instability.
Additional rainfall amounts of one-quarter to one-half inch can be expected which would keep river levels elevated or even past bankfull. Min temperatures to be in the lower to middle 40s north, upper 40s to lower 50s south. Other than a few light showers holding on to far eastern WI early in the morning, the rest of Friday will be dry as weak high pressure moves into western sections of the Great Lakes. Clouds will be on the decrease with mostly sunny skies by the afternoon. The return of sun is forecast to bump max temperatures up into the middle 60s to lower 70s.

Friday night and Saturday...
Quiet conditions to prevail Friday night as the high pressure moves across the Great Lakes. Mostly clear skies through the evening will gradually give way to increasing clouds later at night as a modest mid-level shortwave trough pushes across the northern Plains. The initial lack of clouds in conjunction with relatively light winds, will send min temperatures down into the upper 30s to lower 40s north, middle to upper 40s south. This shortwave trough shifts into the Upper MS Valley on Saturday with the main embedded shortwave moving into northern MN. Stronger forcing to be focused over northern sections of the Great Lakes, but instability is essentially nil. There is also less moisture available compared to Thursday night, thus will have chance pops in the forecast for Saturday with no thunder and less QPF amounts.
Max temperatures Saturday to range from the lower 60s north- central WI/near Lake MI, middle 60s to around 70 degrees elsewhere.

Saturday night and Sunday...
Any light shower activity would end Saturday evening over eastern WI with the next high pressure building southeast across the Upper Midwest. Look for clouds to gradually decrease from west to east later Saturday night with mostly clear skies everywhere by daybreak. This high pressure moves into the Great Lakes region on Sunday providing for mostly sunny skies and warm temperatures.
Readings to range from the lower 60s near Lake MI, middle 60s to around 70 degrees inland.

Sunday night and Monday...
Quiet conditions to persist through Monday as the high pressure shifts into the eastern Great Lakes. It appears enough dry air to remain in place to keep any precipitation from the next system to our west and south. There will be an increase in clouds on Monday, but enough filtered sun to keep max temperatures in the lower 60s lakeside, upper 60s to lower 70s inland.

Monday night through Wednesday...
Models continue to struggle with the eventual movement of a nearly-vertically system over the northern Plains and various frontal boundaries toward the middle of next week. Trying to time precipitation chances this far out in time is futile due to vast model solutions. Therefore, have followed the blended solution which keeps chance pops in the forecast all the way through Wednesday. Max temperatures for both Tuesday and Wednesday would be tricky depending on precipitation trends. For now, anticipate readings at or above normal.

AVIATION
for 00Z TAF Issuance Issued at 631 PM CDT Wed May 1 2024

VFR conditions are expected overnight, but showers and thunderstorms will produce IFR/MVFR conditions as they arrive during the late morning and early afternoon hours Thursday. Poor flying conditions are likely Thursday night as showers and scattered thunderstorms continue.

GRB WATCHES/WARNINGS/ADVISORIES
None.

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