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Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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183 FXUS63 KGRB 151753 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1253 PM CDT Mon Jul 15 2024 Updated aviation portion for 18Z TAF issuance .KEY MESSAGES... - Isolated to scattered storms expected this afternoon and evening. Some storms could be severe. Primary hazards are damaging winds and hail. - Heat index values in the upper 80s to middle 90s across central and east central WI. - Mainly dry weather expected mid-late week into next weekend. Temps drop below normal Wednesday, but will be around normal rest of the time. && .DISCUSSION... Issued at 421 AM CDT Mon Jul 15 2024 Short Term...Today...Tonight...and Tuesday Main forecast concerns will be determining shower/storm potential today and if the severe weather risk. Last day of the heat and humidity with max heat indices in the mid 80s to mid 90s. Some possible dense fog and smoke impacts as well. Convective Trends & Severe Wx/Flooding Chances: A dry start to the day is on tap for the area as one large MCS tracks into MI and the OH Valley and another one pushes across MN. Quite a bit of debris clouds from the MCS to our west is expected, which will likely limit surface heating for a time this morning (and if the HRRR is right could bring a few sprinkles this far east), with additional clouds developing later in the day possibly further holding down instability. This, along with the exact placement of each shortwave, will lead to some uncertainty on how this afternoon will play out. The more organized severe weather event (an eventual MCS) is forecast to be to our south with the southern shortwave/outflow boundary, but severe storms will still be possible in our area after 18z ahead of the cold front and lake breeze dropping south, as bulk/effective shear increases to 30-50 kts and lapse rates increase, supporting the wind/hail threats. There is still some uncertainty how widespread the storms will be, as just how unstable we will get and if some dry air and capping will inhibit storm development. CAMs in decent agreement (especially compared to previous days/events) showing isolated to scattered activity in the afternoon and early evening, with some bowing segments as the event unfolds. So expecting at least isolated coverage. Moisture will be a concern over the far north as dewpoints may not get into the mid 60s. Inverted-V soundings will support winds being transported to the surface. The tornado threat is very low, with LCL heights over 1000 ft and questions regarding instability and low level moisture. But the tornado chance peaks in central WI in the mid to late afternoon when LCL heights slightly lower and SRH maxes out, and if any discrete cells can form and become well organized. SPC has kept most of the area in a marginal risk for severe weather, with the slight clipping the southern counties. With the isolated/scattered nature of the storm activity, the overall flood threat will be low/reduced. Plus, most of the activity looks to have decent forward speed. However, favorable heavy rain environment with PWATs around 1.5" would support heavy rain producers. If any storms that can align and move over the same location, it could bring a flooding threat, especially in areas that saw recent heavy rain (specifically parts of the Fox Valley where 1-hour flash flood guidance remains around one inch). While most of Tuesday is looking dry, models hinting at some isolated shower activity in the mid morning through mid afternoon as a weak front crosses the area and an upper trough drops in the Great Lakes. GFS is advertising stronger cold front pushing, with more widespread shower activity. Suspect we might need to increase PoPs during the day. Heat & Humidity: One more day of warm and humid conditions today. Will stick close to the best performing guidance from yesterday (BCONSALL) which has highs in the 80s for most of the area. Could get a 90 if we get a little more sun than expected in/just west of the Fox Valley as 925mb temps support low 90s. A little relief is expected near the shores of Lake Michigan as onshore winds are expected this afternoon. Dewpoints off to a higher start compared to the previous mornings with low to mid 70s across much of central and east central WI. These look to remain in place for most of the day, producing heat index values in the low to mid 90s. Drier air will arrive in tonight. with CAA will ushering in cooler air for Tuesday with highs in the 70s across north-central WI and in the low 80s over the south. Fog / Smoke Trends: With clear skies, light winds, a steep low-level inversion, dewpoints in the 60s to mid 70s and recent rains, fog developed across much of the area overnight, mainly across central and northern WI. The patchy fog, locally dense, should continue to expand through around sunrise then will lift/burn off. Can`t rule out a little patchy fog late tonight into Tuesday morning, but stronger winds just off the surface and slowly lowering dewpoints should limit fog formation. Still no fog on Lake Michigan, but will continue to monitor as winds turn south this morning and the highest dewpoints arrive with water temps still in the 60s. Northern buoy is reported 68/68 so would imagine we are not far away from some fog forming...just need a little more moisture. In between the areas of convection across northern and southern WI yesterday, GOES GeoColor showed areas of wildfire smoke across Lake Superior, Upper MI and WI. HRRR/RAP showing the smoke, some near or at the surface, continuing across Lake Superior, Upper MI and northern/eastern WI. Have added some haze to the sky grids today to account for this. A separate area of mainly elevated smoke will approach from the west through the day and spread across the region into Tuesday. Could have some air quality issues at times, along with possibly some visibility reductions if the thicker/lower smoke makes it here. Long Term...Tuesday Night Through Sunday The upper pattern will shift in the extended, bringing a broad upper ridge just west of Wisconsin, extending over most of western and central CONUS. This will set up a period of strong northwesterly flow for our region, bringing in drier and cooler conditions for the middle of the week through early next weekend. This is not to say we will be completely dry, as a few shortwaves may yet bring some isolated to scattered convection, but no widespread rainfall or severe weather is anticipated. High temperatures during this time will drop into the 70s for Wednesday before gradually moderating back towards the upper 70s to middle 80s by next weekend. && .AVIATION...for 18Z TAF Issuance Issued at 1252 PM CDT Mon Jul 15 2024 Isolated to scattered showers and storms are possible early this afternoon through this evening as a cold front sweeps across the forecast area. Stronger storms could produce gusty winds, hail, and heavy rain. Flying conditions may briefly lower to high MVFR with any storms, especially across the western TAF sites. Forecast confidence remains low on exact timing and location of showers/storms, although did opt to add in TEMPO groups when thunder appears most likely. Expect southwest winds to ramp up ahead of the cold front this afternoon, gusting between 15 and 20 knots, before veering to northwesterly behind the front late this evening. Models continue to show a plume of near surface smoke skimming northern Wisconsin as a thin layer of elevated smoke builds in from the west. Will continue to hold off on adding in lower vsbys until we start to see impacts. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch/Uhlmann AVIATION.......Goodin