Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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172 FXUS63 KGRB 211120 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 620 AM CDT Fri Jun 21 2024 Updated aviation portion for 12Z TAF issuance .KEY MESSAGES... - Thunderstorms with locally heavy rain are likely this afternoon and tonight. The rain could produce small stream and urban flooding, especially north of highway 8. - Strong to severe thunderstorms are possible Saturday with damaging winds the main threat. Heavy rains may cause some rivers to rise to near flood stage. - More showers and thunderstorms are expected Monday night into Tuesday, mainly for northern and northeastern WI. && .DISCUSSION... Issued at 425 AM CDT Fri Jun 21 2024 Short Term...Today...Tonight...and Saturday Jet energy moving through the southwest upper flow will induce rising motion over a warm front that was over northern Illinois early this morning. There are two or three discernible jets that will move across the region this afternoon and tonight. The first one will produce overrunning showers and thunderstorms that will arrive in central and northcentral Wisconsin by afternoon and in the east by evening. the activity will continue tonight. An inch or two of rain is possible, especially north of highway 8. Some Urban and small stream flooding is possible the severe threat looks to be rather low, as the storms are not surface based and the air aloft is warm. Cool outflow from the storms this afternoon and tonight will likely keep the warm front south of the forecast area through much of Saturday, though the ECMWF and Canadian bring it into the Fox Valley during the afternoon. Another jet streak will move across the area Saturday, and produce additional showers and storms. In addition to additional heavy rain, there could be some severe storms along and just north of the warm front. Damaging winds look to be the main threat, though a tornado cannot be ruled out. Long Term...Saturday Night Through Thursday Persistent high pressure over the southern US keeps an active sub- tropical jet running across the northern CONUS. There will be several chances for rain and thunderstorms through the extended period as multiple short-waves cross the region. At this point the flooding concern through the long term is rather broad brush, but will become more focused to areas that see the greatest rainfall amounts Friday and Saturday. Saturday night through Sunday...Areas of heavy rain and scattered strong to possibly severe thunderstorms are likely to be ongoing into Saturday night as a cold front sweeps across the region and a low pressure system starts to depart to the east. Strong wind gusts are expected to be the main hazard with any thunderstorms by this time period. The axis of heaviest rain by this time will likely have shifted to east-central WI. By the time the rain is all said and done much of the region will likely be looking at 1.5-2.5 inches of rain, with locally higher amounts where the strongest convection ends up. A short-wave following behind the main departing system is progged to dig south over northern WI Sunday returning a chance for showers and a few thunderstorms. With colder air moving in aloft low-level lapse rates are forecast to steepen Sunday which along with decent elevated instability and strong effect shear could sustain a few stronger thunderstorms, mainly for northeast WI where ascent is expected to be maximized. Monday through Tuesday...Ensemble guidances is in decent agreement with a brief mover over the area Monday keeping much of the day dry. Temperatures and humidity will be on the rise Monday as return flow develops during the afternoon. The next period worth watching for possibly strong thunderstorm is Monday night into Tuesday morning across north and northeast WI as a low-level jet develops ahead of an approaching short-wave. Forecast sounds do show strong elevated instability and deep-layer during this period, however, a fairly strong cap is expected to develop during the overnight hour limiting the window for any convection. If any storms do develop strong winds, hail and heavy rain would be the main hazards. Rest of the Extended...With a ridge building over the northern Rockies during the middle part of next week the region will reside in northwest flow regime bring cooler temperatures back to the region. The northwest flow may also bring a drier air mass to the region which would limit the precipitation chances middle to end of next week. && .AVIATION...for 12Z TAF Issuance Issued at 620 AM CDT Fri Jun 21 2024 Flying conditions will deteriorate today as showers and thunderstorms move in from the west. The activity is north of a warm front that will be moving slowly north from northern Illinois. Periods of showers and thunderstorms will continue tonight and Saturday, with significant rain possible. Ceilings will fall to IFR from west to east during the late afternoon and early evening hours today, with IFR conditions in most areas tonight and Saturday. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....RDM/GK AVIATION.......RDM