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Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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287 FXUS63 KGRR 160053 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 853 PM EDT Mon Jul 15 2024 .KEY MESSAGES... - Severe weather risk tonight - High pressure builds in for Thursday and Friday && .UPDATE... Issued at 839 PM EDT Mon Jul 15 2024 Bottom Line up Front: Tornado watch issued for Allegan, Van Buren, Barry and Kalamazoo counties due to the strong line of convection moving through Northern Illinois and southern Wisconsin. Supercells capable of producing tornadoes are associated with this mesoscale convective system with a strong LLJ, strong updraft helicities and copious mixed layer CAPE. Unlike last evening, this system has strong dynamical forcing and a strong shear environment. Couple this with lower LCL`s and an environment capable for producing tornadoes exists. Outside of that threat, damaging straight line winds are also possible. While this system is moving quickly, flooding is possible especially due to antecendent conditions in southern Michigan and due to high PWATs and 1 to 2 inch per hour rainfall rates. && .DISCUSSION... Issued at 308 PM EDT Mon Jul 15 2024 - Severe weather risk tonight The atmosphere continues destabilize ahead of the approaching mid to upper level wave in the upper MS Valley. Surface based CAPE values ranged 2500 J/kg here in MI to near 7000 J/kg in portions of IA and IL. Effective bulk shear values were maximized upstream in WI, closer to the wave and just north of the instability corridor. VWP`s from upstream radars still show 50 to 55 500 mb winds, supporting the stronger deep layer shear. The 12z KABR sounding showed a deep EML, which are commonly seen with higher end severe weather events. Convection should develop in IA in the next few hours, which is supported by the latest high res models. The initial convection should evolve into bowing linear complex as it tracks east southeast. Damaging winds are the main risk with the complex here in MI. As the low level jet ahead of it strengthens this evening, some of the deeper instability should advect up into Lower MI supporting the complex to impact southern parts of Lower MI. How far north the strongest wind reach into the state is somewhat uncertain but near and south of I-96 is still looking like the region of greatest risk for high winds. The period of 02z to 07z seems to be the timeframe of greatest risk. - High pressure builds in for Thursday and Friday Much more stable conditions arrive Thursday and into Friday as a large Canadian high pressure system builds in. The steady cool air advection ahead of it will lead to lower than normal temperatures. Subsidence associated with this system will act to dry out that atmosphere and lead to limited cloud cover. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 734 PM EDT Mon Jul 15 2024 Only significant changes with this update are to adjust timing of thunderstorms based on trends in guidance later by about 12 hour. Thunderstorms arrive after near 03z along the lakeshore and move east rapidly with IFR or lower visibilities and winds of 40-50+ knots likely in the strongest portion of the storm. Confidence in the greatest impacts is highest a AZO and BTL. Some uncertainty into whether the strongest winds and lowest visibilities extend north into the GRR/MKG and east into the LAN/BTL terminals so amendments are likely as radar trends become apparent, however high confidence in thunder at GRR/LAN/MKG. MVFR cigs follow the storms before lifting to VFR Tuesday afternoon. && .MARINE... Issued at 308 PM EDT Mon Jul 15 2024 A line of storm is likely to track through the marine zones tonight. Commonly these systems stir up the winds and waves. The impacts may linger into Tuesday. We increased the waves to show generally 2 to 4 footers, and it would not take too much increase in the winds to push those wave values to 4 feet or higher. No headlines at this time but close monitoring will be needed as BHS and SCA`s may be need for later tonight into Tuesday. We will have to monitor the advancing high pressure system for Wednesday into Thursday which can sometimes lead to hazardous beach and boating conditions. && .HYDROLOGY... Issued at 308 PM EDT Mon Jul 15 2024 Not too dissimilar from what happened last night, models are consistently suggesting a swath or two of 1-to-2.5 inch rain amounts falling very quickly with storms tonight. The heavy rain from the remnants of tropical cyclone Beryl last week, plus areas of heavy rain last night, make it increasingly likely that any given location`s soil in southern Michigan will not be as capable of absorbing the rain it normally could. Any locations which receive 2 inches of rain tonight can flood roads in low-lying spots, even up to car roofs in some cases (such as 28th & Division in Grand Rapids). Small creeks and streams would be the first to rise. Buck Creek in Grandville, Sycamore Creek in Holt, and the Grand River at Jackson are some of our forecast points that particularly vulnerable, but really any river`s headwaters could flood quickly. The main-stem rivers including at Lansing, Ionia, Marshall, and Comstock/Kalamazoo, could see a new push toward higher crests in the coming days, but only if a large portion of their upriver drainage basins receive substantial rain. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch until 4 AM EDT Tuesday for MIZ050-056>059-064>067- 071>074. Tornado Watch until 2 AM EDT Tuesday for MIC005-015-077-159 MARINE...Tornado Watch until 2 AM EDT Tuesday for LMZ844-845-876-878 && $$ UPDATE...Ceru DISCUSSION...MJS AVIATION...Thomas MARINE...MJS HYDROLOGY...CAS