Area Forecast Discussion
Issued by NWS Green Bay, WI
Issued by NWS Green Bay, WI
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541 FXUS63 KGRB 060855 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 355 AM CDT Sat Jul 6 2024 Forecast discussion for routine morning forecast issuance .KEY MESSAGES... - Flooding will continue along the Little Wolf River downstream of the Manawa Dam. Do not drive across flooded roadways if traveling through this area. Other rivers and streams will continue to run above bankfull due to recent rainfall. - Isolated showers and thunderstorms are possible this afternoon. Any storms will be slow moving and could produce locally heavy rainfall. Most areas will remain dry though. - A few thunderstorms on Sunday afternoon could be strong with gusty winds, small hail and heavy rain. At this time severe weather is not expected. On and off chances for showers and thunderstorms will continue Monday through Wednesday. Although the severe weather threat remains low, locally heavy rainfall will be possible. - A warmer/drier summer pattern may be in store toward the end of the week as upper-level flow flattens out. Multiple consecutive days of dry weather may occur. && .DISCUSSION... Issued at 355 AM CDT Sat Jul 6 2024 Short Term...Today...Tonight...and Sunday Today and tonight...Low that caused the heavy rain and flooding on Friday is heading across the lower Great Lakes early this morning. Ridge of high pressure is drying conditions out. With the dry air and clear skies, temps have fallen as low as the upper 40s north- central. Patchy ground fog where temps are the coolest and where rainfall yesterday has kept ground saturated. Other than some scattered-broken stratocu early on, skies will be mostly sunny this morning. Daytime heating will then result in building cumulus by midday as convective temps remain quite low. With highs in the upper 70s to around 80 degrees inland, upwards of 1000 J/kg MLCAPE will build by early afternoon. Though forcing is weak there is a hint a weak mid-level wave will move in during peak heating to trigger scattered showers and storms. Most of that activity would be from central to north-central WI, but there are still hints that lake breezes could also generate isolated storms. Overall, there will be plenty of dry time today, but cannot rule out a pop-up shower or storm from these mechanisms. No severe weather with very weak shear, but that also means storm motions will be slow again as MBE vectors are hardly 5 kts. Unlike yesterday though PWAT is quite low, barely over 1 inch. Locally heavy downpours will be possible given the slow moving nature of the storms. Any showers fade quickly this evening with loss of daytime heating. Rest of tonight will be dry. Risk of ground fog looks lower tonight as min temps will mainly stay above the crossover temp. MOS guidance not picking up on any fog either, even as typical favored spots such as RHI. Sunday...Shortwave trough over the plains shifts to northern MN while sfc low follows similar track. Weak front with the system will still be lagging behind, entering MN mid to late afternoon. A pre-frontal surface trough tied to weaker lead shortwave will bring some showers with minimal instability for thunder through the morning. Greatest chances for showers through noon will be over central WI. This activity makes it to northeast and east- central WI early afternoon. One idea is for the convection to strengthen as daytime heating (highs mid 70s west to around 80 east) boosts MLCAPES to at least 1000J/kg. Another possibility is these initial showers weaken while moving through, then with heating that occurs later there could be resurgence of more sfc based showers and thunderstorms mid to late afternoon. Given MLCAPEs 1000-1500J/kg and our area being within modest mid-level jet (enhancing effective shear to at least 30 kts) there is a risk for at least isolated stronger storm could occur with gusty winds and hail both possible, along with heavy rain. Long Term...Sunday Night Through Friday Focus for the first part of the extended remains on periods of heavy rain/storms Sunday through Tuesday. Best potential looks to be Sunday afternoon through Monday as a surface low tracks across the upper Mississippi Valley and through the Great Lakes. An end to the current wet pattern may then be in sight toward the end of the week as upper-level flow flattens out and high pressure makes its way across the US/Canada border. Sunday evening through Tuesday... A persistent troughing regime will bring several chances for rain/storms to the forecast area as weak impulses circulate through the broader synoptic pattern. The first of these impulses is progged to arrive Sunday afternoon, which may be enough to generate some storms with help from low- level moisture transport and accompanying instability (nearing 2,000 J/kg). Additional support may be provided by placement under the left exit quad of an upper-level jet. Some strong to marginally severe storms may develop Sunday afternoon and evening given decent deep-layer shear exceeding 30 knots. Otherwise, more diurnally-driven showers/storms will be possible Monday through Wednesday. Storm coverage will likely be more widespread on Monday due to low convective temperatures (mid to upper 60s). Rest of the extended... A warmer/drier summer pattern may be in store toward the end of the week as ridging over the west coast shifts east and flattens the upper-level flow. Expect temperatures during this time to be stagnant in the upper 70s to low 80s, running slightly above average for this time of year. && .AVIATION...for 06Z TAF Issuance Issued at 1037 PM CDT Fri Jul 5 2024 Clearing skies and wet ground could result in areas of dense ground fog or low clouds late tonight. The areas most likely to have dense fog or low clouds late tonight should be north of a Merrill to Iron Mountain line, and possibly the area west of a Green Bay to Oshkosh line and east of a Clintonville to Waupaca line. If low clouds or fog materialize, it should dissipate by 13z. A few hours of MVFR ceilings are possible between 13z and 16z Saturday, otherwise VFR conditions and light surface winds are expected. There could be a stray shower or thunderstorm in the afternoon or early evening west of a Iron Mountain to Oshkosh line. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....JLA/Goodin AVIATION.......RDM