230 FXUS63 KGRB 191143 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 643 AM CDT Sat Jun 19 2021 Updated aviation portion for 12Z TAF issuance .SHORT TERM...Today...Tonight...and Sunday Issued at 301 AM CDT Sat Jun 19 2021 The main concern in the short-term portion of the forecast will be increasing chances for showers and thunderstorms for Sunday. Today and tonight: Any lingering showers/isolated thunderstorm and cloud cover associated with the passing shortwave, will quickly exit off to the east of the area this morning. High pressure is expected to quickly build in from the northwest and become centered overhead this afternoon into tonight. This will allow for calm winds and mostly clear skies across northeast Wisconsin. Most locations will see high temperatures in the mid to upper 70s to around 80. Overnight lows are forecast to range from the mid and upper 40s north to the mid and upper 50s south. Sunday: The aforementioned high pressure center is expected to shift east of the area late Sunday morning into the afternoon. As this happens, southerly winds are expected to increase as a low pressure system approaches from the northern Plains. The low is expected to steadily deepen as an extensive upper-level trough axis digs into the Upper Midwest. There are some differences in the model solutions as to how strong the surface low is and exactly where the low will trek. The NAM has the low a bit stronger and takes the trek across the southern WI/northern IL border. The GFS/Canadian/EC have the surface low a bit weaker and less defined directly across northern Wisconsin. Either way, deep layer moisture is expected to increase across the entire state as warm air advection and moisture advection increases. Isentropic upglide on the WAA will help to increase precipitation coverage by late Sunday morning and especially into Sunday afternoon from the southwest. The overall evolution is a little murky as the meso models have two rounds of convection. The first round is progged to develop over IA in the morning before lifting northeast through souther WI. That area of convection could impact east-central WI by early afternoon. Then it looks like there would be a break ahead of the approaching cold front later in the afternoon into the evening. Shear values are expected to increase to around 30-40 kts, but instability looks to be fairly meager with mid-level lapse rates only on the order of 5.5 to 6 C/km through Sunday afternoon. It only looks like around 400 J/kg of instability is being painted out across the area, and that is close to the highest values from guidance. This may be a result of limited recovery time between the first round of convection and the second. Unless lapse rates steepen and instability values increase due to a larger break in rounds of precipitation, the risk of severe weather looks to be fairly low. PWAT values are expected to increase to around 1 to 2 inches, so any thunderstorms that do occur, would have the potential to produce heavy rainfall. Most locations across northeast WI can expect between 0.5 and 1.0 inches of much need rainfall from this system through late Sunday night. High temperatures are forecast to be in the 70s for much of the area. .LONG TERM...Sunday Night Through Friday Issued at 301 AM CDT Sat Jun 19 2021 The main story for the first half of next week to be a deep upper trough that will dig across the Great Lakes region and usher in much cooler air with temperatures bottoming out on Monday with readings 10 to 20 degrees below normal. This trough departs at mid-week as a weakening upper ridge moves across the central CONUS. This will allow for temperatures to rebound back above normal by next Thursday. There appears to be two good chances for precipitation with the first chance coming Sunday/Sunday night with the upper trough and the second chance coming late Thursday into Friday morning with a cold front/shortwave trough. Sunday night and Monday... Showers and thunderstorms to be ongoing Sunday evening as a surface low/cold front begin to pull east, while the trailing upper trough arrives later Sunday night. As the better lift and moisture gets shunted east during the night, expect precipitation to become more scattered in nature. Anticipate a healthy rainfall overall with totals from Sunday through Sunday night generally in the three-quarters of an inch to a little over one inch. Min temperatures to range from the middle to upper 40s far north- central WI, to the upper 50s east-central WI. Models show individual mid-level shortwaves moving through the upper trough on Monday and the question is whether any precipitation will get generated or simply bring a healthy dose of clouds. It appears that any shower activity would be sporadic and may be nothing more than sprinkles due to the dry air in place. The bottom line is that the vast majority of northeast WI will be dry on Monday with max temperatures in the upper 50s north, middle 60s to around 70 degrees east-central WI. Monday night and Tuesday... The exit of the shortwave and loss of daytime heating should allow for quiet conditions Monday night. High pressure located to our south over the mid-MS Valley will help bring mostly clear skies and light winds to northeast WI. These conditions, when coupled with a rather cool air mass overhead (8H temperatures of +2 to +5C) will bring a cool night to the area. Min temperatures to range from the middle to upper 30s north (patchy frost?), to the upper 40s along Lake MI. Models hint at another shortwave to drop southeast into the region on Tuesday and bring an increase in clouds along with a slight chance of showers. Once again, most locations will remain dry through the day. Max temperatures Tuesday to reach the lower to middle 60s north, middle 60s to around 70 degrees south. Tuesday night and Wednesday... Some of the showers could linger into Tuesday night, then the process repeats itself on Wednesday as the last of the shortwaves moves through the upper trough which will begin to shift east. A more south-southwest flow at low-levels of the atmosphere will begin to pull warmer air into WI on Wednesday which will bring max temperatures up into the lower to middle 70s north-central WI, mainly upper 70s over central/east-central WI. Wednesday night and Thursday... As the upper trough departs Wednesday night, a weakening upper ridge will move across the central CONUS allowing for a modest zonal flow to develop over the western Great Lakes. Models are still getting a handle on the movement of the next cold front/ shortwave trough on Thursday. The ECMWF has sped up this system and is now closer to the GFS solution. Therefore, have increased the chance of precipitation a bit from the previous forecast. Max temperatures by Thursday will jump above normal with readings in the upper 70s to around 80 degrees north/near Lake MI, lower to middle 80s elsewhere. Thursday night and Friday... Precipitation chances would then increase for Thursday night into early Friday as the cold front/shortwave trough sweep east into the Great Lakes region. Much too early for specifics yet on severe potential, but any rain would be beneficial, especially over east-central WI where drought conditions still exist. Max temperatures on Friday to range from the upper 70s to around 80 degrees north/lakeside, lower to middle 80s central/east-central WI. && .AVIATION...for 12Z TAF Issuance Issued at 643 AM CDT Sat Jun 19 2021 As a potent shortwave exits the area this morning, expected the showers and thunderstorms to end from west to east. Some of the remaining thunderstorms may produce small hail and heavy rain at the ATW/GRB/MTW TAF sites. This may result in MVFR conditions at times early in the TAF period. Otherwise, high pressure will build across northeast Wisconsin through the day into tonight, bringing VFR conditions with light winds. && .GRB WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM.....Cooley LONG TERM......Kallas AVIATION.......Cooley