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Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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729 FXUS63 KGRB 140859 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 359 AM CDT Wed Aug 14 2024 Forecast discussion for routine morning forecast issuance .KEY MESSAGES... - Patchy fog, locally dense, may result in hazardous travel conditions for this morning`s commute. - Showers and thunderstorms will impact the area from late tonight through Friday night. Periods of moderate to heavy rainfall are expected, which could lead to localized flooding in urban and low-lying areas. - Conditions hazardous to small craft will be possible on the Bay and Lake on Thursday and Thursday night. Dangerous swimming conditions are also expected on Lake Michigan beaches. && .DISCUSSION... Issued at 359 AM CDT Wed Aug 14 2024 Short Term...Today...Tonight...and Thursday Today...light winds and clear skies will allow for patchy fog to expand before sunrise, with locally dense fog in some river valleys and other low-lying areas. This may cause locally hazardous travel concerns for the early morning commute. The low clouds and fog will burn off/mix out shortly after sunrise. Otherwise, a quiet mid-August day is on tap for the region as weak high pressure slides across the Great Lakes. Some high clouds will drift in from the west through the day, along with daytime cumulus later this morning and afternoon, mainly away from the lake. Expect dry conditions for most/all of the area, with the one exception being near the MI border, or possibly near the Bay, as a spotty shower will be possible this afternoon along the lake/bay breezes. Less instability than yesterday, generally under ~500 J/kg, plus a weak cap just above 10,000 feet, mixing into dry air, little to no upper support and weak shear, will make it difficult for any shower/storm to form. If a shower were able to form, it would be pretty short-lived and very isolated with a brief heavy downpour and wind gust (inverted-V soundings) being the only minor concerns. Thinking the better convergence will be in the U.P., and with the chance in any one spot under 10%, will stick with a dry forecast. It will be a warm day with highs climbing into the low to mid 80s for most of the area, with slightly cooler readings near the shores of Lake Michigan. Humidity levels will be pretty comfortable with dewpoints mainly in the 50s to around 60. Tonight/Thursday...dry conditions are expected through around midnight, then look for rain to begin to spread into parts of central and north-central WI overnight as the initial push of WAA/ isentropic lift and shortwave energy arrives. A secondary shortwave and slug of deeper moisture (PWATs of 1.7-2.0") arrives Thursday morning, helping spread this area of rain across the entire area. Most of the rain looks to be light to moderate, but embedded areas of heavy rain are expected at times as soundings showing very deep moisture, up to ~40,000 feet and warm cloud depths, making for efficient rain production. Chances for thunder will be low with the initial WAA precip (but some rumbles are certainly possible), as only some weak elevated instability will be present. Better chances for thunder will be later in the day as more unstable air advects in from the southwest, but the activity looks to be more isolated/scattered in nature. Severe weather threat will be low, but as CAPE builds to ~1000 J/kg, some strong storms are possible with gusty winds and hail. Latest rainfall amounts have come down, with most spots staying under an inch during the day on Thursday. Some locally higher totals are possible associated with any storms in the afternoon, but the storms are expected to have enough movement to limit the heavy rain threat in any one spot. So while some locally heavy rain will be possible, the lower totals, plus several days of mainly dry conditions, should keep any flooding threat low through sunset. Southeast winds will become a little gusty, especially across eastern WI with gusts to around 30 mph expected. Lows will drop into the 50s over northern WI and remain in the 60s further south. Highs will be cooler than the past few days due to the clouds/rain, with most spots holding in the upper 60s to mid 70s. Long Term...Thursday Night Through Tuesday This portion of the forecast will start out wet as low pressure crosses the region from Thursday night through Friday night. Then after the low moves out, relatively seasonable late summer weather returns through the middle of next week. Rainfall trends: Low pressure will slowly move across northern Wisconsin on Thursday night and Friday before exiting over northern Lake Michigan on Friday night. Ahead of the low on Thursday night, an axis of instability (perhaps up to 1000 j/kg of elevated cape) will lift northeast across central to northeast WI. Precipitable water values will remain high at 1.8-1.9 inches during the evening, so think locally heavy rainfall will remain possible in this area. Forecast soundings indicated cloud layer winds of 20-30 kts, which is significantly higher than models were showing yesterday. As a result, think the forward storm motion will keep the flash flooding threat rather low. A few strong storms cannot be ruled out though, which could produce small hail. The chance of showers and storms will continue at times on Friday as as the low slowly meanders east across the state. Will likely see an increase of coverage of the precip as instability builds with the heat of the day. Deep layer shear of 20-25 kts looks rather marginal for severe weather, but a moist profile and pwats around 1.5 inches will aid in rainfall efficiency of the showers. Light showers will likely stick around on Friday night into Saturday morning before ending. Rest of the extended... Northwest flow takes over after the system departs later on Saturday. No significant systems are forecast to impact the region from Sunday through next Wednesday. Temperatures should remain near normal. && .AVIATION...for 06Z TAF Issuance Issued at 934 PM CDT Tue Aug 13 2024 Skies will be mostly clear tonight and Wednesday. Areas of ground fog are possible late tonight, especially in places that received rain showers the last few days. Any fog that materializes should dissipate by 13z Wednesday. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Bersch/MPC AVIATION.......RDM