Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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636 FXUS63 KGRB 130339 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1039 PM CDT Fri Jul 12 2024 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - Isolated showers/storms will be possible through this evening. While severe weather is not expected, any storms could produce locally heavy downpours. - A few rounds of strong to severe storms are possible from Saturday through Monday. The mostly likely times are Saturday night, and again Monday afternoon/evening. However, there is still high uncertainty. - Warmer and more humid conditions arrive Saturday through Monday. Heat index values in the 90s are expected over central, east central and parts of far northeast WI Sunday into Monday. && .DISCUSSION... Issued at 401 PM CDT Fri Jul 12 2024 Short Term...Tonight and Saturday Main forecast concerns for the short-term will revolve around severe weather potential and building heat and humidity. Best timing for anything severe looks to be late Saturday evening and into the overnight, although CAMs are currently showing a variety of different scenarios that could play out. Showers/storm potential this afternoon/evening... Mid-level capping and dry air intrusion from high pressure situated over Michigan`s lower peninsula have kept shower activity at bay through the early afternoon. However, decent moisture and instability (~2,000 J/kg CAPE) have sustained a mid-level cu field across the entirety of the CWA. Some pulse showers/storms may be possible into this evening, although dynamics don`t look too impressive given the absence of any surface features or triggers to focus convection. This being said, anything that does go up may have the potential to produce brief heavy downpours given PWATs of 1.25" and weak storm motion. Fog/smoke... Some patchy ground fog will be possible tomorrow morning, especially in areas if/where any rain falls today. Some elevated Canadian wildfire smoke was also observed across the upper Midwest, enough to create hazy conditions. While areas of thicker smoke will remain off to our northeast, even a thin layer of smoke may be enough of a hindrance to prevent us from reaching convective temperatures later this afternoon and evening. Saturday severe weather potential... Most likely solution currently shows a complex of storms coming down from the northwest late Saturday evening into early Sunday morning, mainly before 12Z. Main source of discrepancy between CAM guidance centers around shower/storm activity earlier in the day as weak shortwave energy treks across the Great Lakes. The HRRR solution favors convection developing off to our west along the axis of greatest instability Saturday afternoon before dissipating as it encounters drier air over central Wisconsin. Other short-range models show more isolated/pop-up activity during peak heating. There are some questions as to whether or not this will hinder the east/southeast progression of the MCS later in the evening, with the most recent suite of CAM guidance favoring a slightly later arrival than was shown this morning. Current thinking is that any organized storms would be hard to come by earlier in the day due to the influence of dry air from departing high pressure and low- level capping. Inverted-V soundings indicate locally damaging winds as the primary severe threat for Saturday evening, while mid-level lapse rates and wet bulb zero heights favor hail as a secondary threat. Otherwise, persistent southerly flow looks to make this weekend rather unpleasant as temperatures climb into the low 90s and a surge of low-level moisture brings dewpoints up into the low 70s. The combination of heat and humidity will result in heat indices potentially reaching the mid 90s tomorrow, and flirting with heat advisory criteria on Sunday. A persistent southeast lake breeze will keep temperatures along the lakeshore slightly cooler. Long Term...Saturday Night Through Friday The focus is on the first 48 hours or so of the long-term forecast with the potential for multiple rounds of convection. Heat and humidity will also be a concern during this time. Saturday night...Already starting out with high uncertainty at 00Z/Sun. Some indication thunderstorms or a convective system could be moving across the area at this time, or it could have dissipated already (as discussed above). Regardless, attention will quickly turn to a compact shortwave coming through Saturday night. This is more likely to generate a thunderstorm complex across northern Minnesota Saturday evening, which tracks into Wisconsin overnight (timing window anywhere from 9PM to 6PM across the forecast area). Placement is uncertain, but generally favors a southeast movement across most of the forecast area. Damaging winds would be the main threat, with smaller potential for large hail and localized heavy rain/flooding. Sunday...A morning break from convection, however the heat and humidity build during this time. Dew points rise into the lower 70s for most of central and east-central WI (in the mid-60s across north- central WI). High temps forecast in the mid 80s to low 90s will yield heat index values as high as 95 to 100 across central and east-central WI. However, temps could be impacted slightly by any lingering clouds or smoke aloft, along with any isolated thunderstorms. While most models favor a cap holding off any thunderstorms, and there is limited upper level dynamics, will want to watch for any meso scale interactions which could lead to some isolated thunderstorm potential. A lot of CAPE will be in place, so anything which does develop could become severe. Monday...Once again looking at significant moisture with dew points in the low 70s, and afternoon highs in the low 80s, and increasing instability. There are more dynamics at play, leading to somewhat higher confidence for thunderstorms. A surface cold front, along with mid-level shortwave, and RRQ of upper jet will all serve as larger scale lifting mechanisms. Timing and details are less certain given the prior activity. Tuesday through Friday...Quieter and more seasonable temperatures settle into the region. && .AVIATION...for 06Z TAF Issuance Issued at 1039 PM CDT Fri Jul 12 2024 VFR conditions are expected tonight and Saturday. Satellite imagery indicates some Canadian wildfire smoke is aloft, which may cause some hazy conditions at times. Surface winds are expected to be light through the TAF period. Thunderstorms are possible Saturday afternoon and evening as a thunderstorm complex develops across Minnesota and tracks east through the region. CAMs models are in poor agreement regarding exact timing of this complex; however, there appears to be enough of a chance (30 percent) across central and north-central Wisconsin to include a PROB30 group in this set of TAFs for AUW/CWA/RHI. Further east confidence is too low to include at this time. Any thunderstorms would likely reduce conditions to MVFR or IFR at times. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Goodin/KLJ AVIATION.......Kurimski