Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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610 FXUS63 KGRR 201748 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 148 PM EDT Fri Sep 20 2024 .KEY MESSAGES... - Showers/Storms Today, Sunny Saturday - Likely showers and possible thunderstorms Sunday and Sun Night - Unsettled next week, likely not as warm, some rain chances && .UPDATE... Issued at 1155 AM EDT Fri Sep 20 2024 A cold front is situated north/south over Eastern Wisconsin extending southward into Northern Illinois. The front is advancing our direction and will move out over Lake Michigan this afternoon. Ahead of the front surface dew points have surged into the middle 60s to around 70. A narrow ribbon of 850mb dew points extends northward up Lake Michigan with values around +14C. The ribbon of low level moisture (surface to 850mb) is narrow and will advance into Western Lower Michigan this afternoon. High resolution model data indicate that MUCAPE values will increase into the 1000-1500 j/kg range in spots. DCAPE values are already around 1000 j/kg which is fairly sizable for this time of year. DCAPE to this level makes damaging winds the primary threat today. There is enough low level curvature in model hodographs that there will be some storm organization (0-1km SRH of 75 m2/s2). Deep layer shear (0-6km) is on the order of 30-40 knots today. So, at max heating this afternoon, hail is also a threat today, but a notch below the wind threat. We remain in a SPC marginal and that seems like the correct category at this point. HREF and machine learning model data suggest our western CWA, near and west of highway 131 will be the main area in play today. This is due to the fact that the moisture does not surge east until it is well after sunset. Again, threat area is in the west and the main time frame for strong to severe storms being from noon to 800pm. Within that time frame 300pm to 600pm will likely contain our strongest storms. We are not expecting widespread severe weather but some isolated to scattered wind/hail reports will certainly be possible today. && .DISCUSSION... Issued at 340 AM EDT Fri Sep 20 2024 - Showers/Storms Today, Sunny Saturday We finally have some rain in the near term forecast. Latest sfc analysis shows a weak cold front over western WI slowly moving east. Radar is showing a slow moving band of showers beginning to move over the western part of Lake MI, then extending southwest into northwest IL. Most of the lightning associated with this was in far southeast WI and northern IL. As the front moves east today into Lower MI, we`ll see rain chances increase quite a bit. Moisture is progd to increase substantially just ahead of the front this afternoon. Precipitable water values jump up to nearly 2 inches and surface dewpoints climb into the upper 60s as a diurnally strong LLJ noses into the southwest cwa. Concurrently, the left exit region of the upper jet will nose into the southwest cwa this afternoon which will provide decent lift within the column. More lift will be created by the short wave moving across the lake this afternoon. Sfc-3km lapse rates are impressive too...8c/km. SBCAPE values in the 1200-1700 j/kg range coupled with bulk shear values 35-40 kts suggest a potential severe threat as cells organize. Forecast soundings show a very impressive inverted V below 750 mb. Corresponding DCAPE values aoa 1200 point toward a wind threat. SPC has expanded the marginal risk to include the western three rows of counties along/south of I-96 and includes Grand Rapids, Kalamazoo and Battle Creek. The main threat window is from roughly 18-00z. Storms will diminish as the front moves through. Partly cloudy skies are expected tonight followed by sunny weather Saturday. Little, if any, cooler air is expected behind this front. Dewpoints in the mid 60s will result in humid conditions Saturday. - Likely showers and possible thunderstorms Sunday and Sun Night Ensembles are consolidating around likely showers with some thunderstorms centered between the latter part of Sunday through Sunday night. An upper-level low will migrate from the Desert Southwest to the Central Plains on Sunday. Ahead of it will be a surface frontal zone from the Central Plains to the Western Great Lakes, connecting to a northern-stream low over Northwest Ontario. A weak surface triple-point low over Michigan on Sunday with perturbations in the upper-level flow ahead of the Central Plains low should serve as the focus for precipitation here as warm and moist air in the low-mid levels is advected toward Michigan on the back side of a southeast US high. There is a large spread in the high temperature solutions for Sunday depending on when the showers start, with a range between mid 70s and mid 80s being plausible around Grand Rapids, more likely on the warmer side farther southeast toward Jackson. Deep-layer moisture will be present late Sunday and PWAT climbs to 1.5 to 2.0 inches, making locally heavier rain amounts over 1 inch possible especially if any instability can be realized. Area-averaged rain amounts look to be around 0.5 inches. - Unsettled next week, likely not as warm, some rain chances Additional chances for rain may linger Monday night into Tuesday as the main upper-level disturbance over the Central Plains on Sunday moves toward the Great Lakes, though shearing out and weakening as it gets incorporated into the northern stream. The ECMWF ensemble offers more rainy solutions than the ensembles of the GFS and CMC. From there, predictability in the day-to-day weather mid-late week is rather low as a chaotic interaction between northern and southern stream wave patterns unfolds over North America. The median temperature solutions among the ensembles offers highs generally in the lower 70s and lows in the 50s, which is closer to normal for the time of year, perhaps slightly warmer than normal. The ECMWF ensembles are tending cooler than the GFS and CMC ensembles, many of them creating another blocking pattern with a large upper-level low sitting over the Great Lakes. Some solutions among the GFS and CMC ensembles offer more ridging and a warming trend in temperatures later in the week. No matter what happens, there is quite a variety of QPF solutions out there, anything from staying completely dry, to having a day of heavier rain, to having multiple days of lighter showers. So, unsettled is the word, but at least it`s more like autumn. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 148 PM EDT Fri Sep 20 2024 Main changes this update were to tidy up timing of thunderstorms based on radar presentation of storms marching across southwest Lower MI at this time. As noted before, VFR should prevail this afternoon with possible brief and localized incursions into borderline IFR visibility restrictions under the stronger storms. Wind gusts have not been overly strong yet, but the potential still exists. LIFR ceilings/vis still expected later tonight due to either fog or clouds a few hundred feet above ground. && .MARINE... Issued at 340 AM EDT Fri Sep 20 2024 South winds will increase ahead of the cold front today. Waves will respond by increasing to around 2 feet. It`s possible that we could see waves up to 3 feet, but likely only briefly. The front isn`t strong and winds on the back side of the front will be fairly light. Thunderstorms are expected along the front today, which will pose a threat to mariners and beach goers. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ UPDATE...Duke DISCUSSION...04/CAS AVIATION...TJT MARINE...04