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FXUS63 KGRB 211958 AFDGRB

Area Forecast Discussion National Weather Service Green Bay WI 258 PM CDT Tue May 21 2024

Forecast discussion for routine afternoon forecast issuance

KEY MESSAGES

- Impactful strong to severe storms are anticipated across portions of the area this evening. The areas most likely to see severe weather will be central to east-central Wisconsin.
Damaging winds, large hail, and a few tornadoes will be possible with the stronger storms.

- Heavy rainfall through this evening may also pose a flooding concern, especially as areas of central WI have already received an inch of rainfall.

- Showers and thunderstorms possible at times Wednesday and Thursday, mainly across northern Wisconsin.

- Better chance for widespread showers and thunderstorms Friday into Friday night.

DISCUSSION
Issued at 258 PM CDT Tue May 21 2024

Short Term...Tonight and Wednesday

Initial convection ahead of the warm front

The first round of convection is on its way this afternoon, with showers and a few thunderstorms developing north and ahead of the warm front. These initial storms remain capable of producing moderate to heavy rainfall, some small hail, and the occasional wind gust up to 30 to 40 mph.

The bigger concern will be a second round this evening, as a second round of strong storms is brought in along a surface cold front. This line is already in development on radar in western Iowa this afternoon, and is expected to steadily cross towards and into Wisconsin by the early evening. Although instability remains on the more modest side in our area, with MUCAPE around 1000 J/kg, the remaining dynamics remain impressive. Bulk shear and helicity values remain incredibly high, and wind fields support these storms to be quite fast moving once they arrive in the evening. Therefore, one of the primary concerns will be strong wind gusts reaching the surface. Additionally, low LCL heights and high helicity values associated with these storms will support tornado-genesis, especially in portions of western to central Wisconsin. North and east of these areas the tornado threat quickly drops off, but again wind is expected to be the predominant concern, with gusts up to 60 to 70 mph possible at times.

Finally, localized flooding can't be discounted with these storms either, as some portions of central WI have already reported near an inch of rainfall from this morning and although the next round of storms will be fast moving, they will also bring additional heavy rainfall.

The main uncertainties between this afternoon and the evening remain the norther extent of the warm front and whether any additional development precedes the arrival of the second line.
Clearing on satellite evident in southern Minnesota and Wisconsin do support this front to begin lifting towards central Wisconsin by this evening, which would also support stronger storm development. If any storms do develop ahead of the main line as well, they too may ingest enough ambient shear and helicity to be a severe concern, but these will be more localized than the longer, stronger line.

Finally, as the system departs, dry conditions are expected to return to the region fairly quickly overnight. Winds however will remain gusty, with some gusts up to 25 to 35 mph still possible through the overnight into Wednesday. Cooler conditions follow for tomorrow.

Long Term...Wednesday Night Through Tuesday

Zonal flow is expected to persist over the region through this forecast period with a few embedded shortwave/troughs bringing chances for showers and storms at times. Timing out these shortwave/troughs will be the main forecast challenge.

The greatest confidence for showers and storms appears to be Friday into Friday evening from a cold frontal passage. Models suggest marginally severe parameters will be present as the cold front moves through, but it is too early to determine the full severe potential at this time.

High pressure will follow this cold front for Saturday into Saturday night, leaving dry conditions in the forecast. Models are not in agreement with the next chance for showers and storms due to disagreement with the evolution of two mid-level shortwaves riding along the zonal flow late this weekend. This translates to chances for showers and storms from Sunday through Tuesday, but anticipate the timing to become more confined over the coming days.

AVIATION
for 18Z TAF Issuance Issued at 108 PM CDT Tue May 21 2024

VFR to MVFR conditions will be in place for the afternoon and evening, but IFR/LIFR conditions are possible at times as showers and thunderstorms cross the area. Initial convection early this afternoon is not likely to be severe, but will have thunder and the potential for small hail or a brief wind gust.

As we get into the late afternoon and evening hours, storm strength and potential increases, with a strong round of storms expected to cross Wisconsin in the 23Z-05Z timeframe. Timed this thunderstorm round with a set of TEMPO groups for the area TAF sites for now. Damaging winds, large hail, and a few tornadoes are possible with these storms. After the storms pass, active weather should return for the late overnight but winds will likely remain gusty. The passing system will also bring a period of LLWS that will likely last through the overnight hours.

GRB WATCHES/WARNINGS/ADVISORIES
None.

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