Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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431 FXUS63 KGRB 121153 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 653 AM CDT Wed Jun 12 2024 Updated aviation portion for 12Z TAF issuance .KEY MESSAGES... - There is a Marginal Risk of severe thunderstorms across north central and central Wisconsin late this afternoon and evening. Overall confidence in the severe weather threat is low, but a few of the strongest storms may produce gusty winds and hail. - Well above normal temperatures are forecast from Sunday into the middle of next week. Heat indices may surpass 90 degrees at times. - There are periodic chances for thunderstorms Saturday night through the middle of next week. It is uncertain if and when any of the storms would become severe. Locally heavy rainfall will be possible at times. && .DISCUSSION... Issued at 403 AM CDT Wed Jun 12 2024 Short Term...Today...Tonight...and Thursday Precipitation trends, severe weather potential this evening, warmer temperatures and gusty winds on Thursday will be the main forecast concerns. Surface analysis showed an ill-defined warm front situated from west central WI southeast-ward into far southern WI early this morning. WV imagery and upper air analysis indicated a seasonably strong jet streak extending from northern MT into northern MN. Subtle short-waves embedded in the fast zonal flow were moving through the Dakotas. Precipitation Trends and Severe Thunderstorm Potential: The surface warm front is expected to lift northeast across the forecast area this morning, with broad WAA developing across the region. A weak short-wave and RRQ of an 80-90 knot upper level jet streak will move through the region in the late morning and early afternoon, and should generate isolated to scattered showers and a few weak thunderstorms. Additional scattered thunderstorms are expected to develop as a 35-45 knot low-level jet increases ahead of an approaching cold front late this afternoon and evening. By this time, instability will be increasing across the western part of the forecast area, with MUCAPE reaching 1000 j/kg or a little higher. Deep layer shear will be 35 to 50 knots, which is supportive of storm organization. Negative factors for severe storms include possible earlier convection, a lack of significant upper level forcing, a later arrival time after peak heating, and marginal instability that wanes into the evening hours. Right now, think that SPC has the right idea with a Marginal Risk over the western half of the area, but overall confidence in how and where this will play out is low, as models show a multitude of possibilities. The overall threat of storms will continue to spread southeast across the forecast area this evening, then shift out of east central WI during the overnight period as the low- level jet veers west. The cold front will move through overnight into Thursday morning and instability will develop over our SE CWA in the late morning/midday. However, with west flow at 925/850 mb, low-level convergence will be shallow/weak along the boundary, leading to isolated storms at best. A short-wave trough and secondary cold front will impact northern WI in the afternoon, bringing scattered showers. Temperatures: Broad WAA will boost H8 temperatures to +17 to +18 C by the end of the day, which would typically support highs into the middle to upper 80s. However, increasing clouds and isolated to scattered showers in the late morning/early afternoon will hold readings back a bit. Went with highs in the upper 70s to middle 80s, but cooler lakeside. A mild night is in store as a weak cold front slowly moves through the region. Lows should be in the upper 50s and 60s. Despite CAA on Thursday, deep mixing should allow temperatures to reach into the middle 70s northwest to the lower to middle 80s southeast. Gusty Winds on Thursday: Deep mixing through 6-8k feet should tap into strong winds aloft, resulting in west winds gusting to around 30 mph in most areas. Gusts should not be as strong over Lake Michigan, due to more stable conditions over the relatively cool waters. Thus, expecting gusts to remain below the 25 knot threshold for a Small Craft Advisory. Long Term...Thursday Night Through Tuesday A 500mb trough will swing east of the area Thursday night which will result in a comfortable northwest flow pattern through Saturday morning. High temperatures on Friday should be close to normal with highs in the 70s. It will be cool again Friday night across the north with lows in the 40s in many spots. On Saturday, the 500mb flow flattens with the next system moving into portions of the area Saturday afternoon. The ECMWF was the slowest model with the rain not arriving until Saturday evening while the other models moved rain into portions of the area by late Saturday afternoon. The highest chances of rain will be across central Wisconsin. The prospects for thunder Saturday afternoon seem small as the best instability remains well to the south and west. On Saturday night, the models are indicating a 40 knot 850mb jet cranking up during the evening. Most unstable CAPE values were highest across southwest Wisconsin with values of at least 500 to 1,500 J/KG. Elevated thunderstorms are expected to be on the increase during the mid evening and overnight in response to the low level jet. Some stronger storms are possible across central Wisconsin as 0-6km shear values were from 35 to 40 knots. Probably more of a concern will the potential for locally heavy rainfall, especially if there is some training along the warm front. Sunday continues to be an interesting day as models continue to show lingering showers and storms through the morning, and possibly into the afternoon. With the ongoing convection and cloud cover, there is concern about the forecast high temperatures. We could end up being several degrees cooler if the clouds don`t break up in the afternoon. Also, the ECMWF has a more pronounced 500mb trough moving across the area that was not seen in the other models. This feature was probably a response to some convective feedback. For early next week, the ECMWF has the 500mb ridge building across the east coast with the area being in the ring of fire which means there will be periodic chances of thunderstorms through the middle of next week. The details and timing of the these storms this far out is murky due to the model differences. Any of these complexes of storms could bring the potential for stronger storms and locally heavy rainfall. Unseasonable warm to maybe hot conditions are likely early next week with the heat index values at times climbing into the 90s. && .AVIATION...for 12Z TAF Issuance Issued at 631 AM CDT Wed Jun 12 2024 Patchy fog was already mixing out at issuance time, so will not include any mention with the new TAFs. A couple areas of precipitation should impact the region today into tonight. An initial batch of showers and storms over MN will shift into the forecast area late this morning and impact the western TAF sites into mid-afternoon. Additional showers and storms are expected to develop ahead of a cold front later this afternoon into tonight. Have attempted to frame out the best 4-hour period for thunderstorms during the evening and early overnight hours at each TAF site. Once the storms have ended, there may be some potential for a brief period of MVFR ceilings over mainly NC WI late tonight. LLWS is expected to form later this afternoon into tonight, as a 35-50 knot low-level jet develops. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kieckbusch/Eckberg AVIATION.......Kieckbusch