117 FXUS63 KGRR 162325 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 725 PM EDT Wed Apr 16 2025 .KEY MESSAGES... - Dry During The Day Thursday - Severe Storms Possible Friday - Lower Confidence Weekend Forecast, Showers/Storms Possible && .DISCUSSION... Issued at 305 PM EDT Wed Apr 16 2025 - Dry During The Day Thursday Conditions will stay dry through the daylight hours, with clouds increasing through the day. Southerly winds will keep things quite dry with RH values falling into the upper 20s to 30s in the afternoon. Combined with south winds gusting to around 20 mph in the afternoon, fire danger will be elevated Thursday. - Severe Storms Possible Friday Friday will feature the potential for two rounds of severe storms. One in the early morning, and one in the evening. Thursday Night into Friday morning will feature showers and storms associated with a warm frontal boundary lifting north across the area. A strengthening low-level jet will advect warm air and steepen mid-level lapse rates and produce around 1000 J/kg MUCAPE. Forecast soundings from the RAP taken from MKG and BIV near 5am Friday show decent HGZ CAPE as well as 30-45 knots of effective shear. This would be enough to bring the potential for an isolated hail threat if any storms occuring Thursday in Iowa survive long enough to reach West Michigan. Soundings show storms Friday Morning would be elevated (rooted near 850-800mb) meaning cold lake Michigan water will not be a relevant factor. Much of the day Friday looks to be dry as a stout elevated mixed layer will be in place near 850 mb. This will pair with mid-level height rises on the order of 2-4 dam much of the day to suppress convection. During this time southerly winds will lead to a warm front moving north with mid 50s to low 60s dewpoints south of it. Highs south of the front look to be well into the 70s. This will lead to a conditional threat of severe weather Friday evening into the early overnight hours, coinciding with the arrival of a slow-moving cold front from the west. Forcing from the cold front in combination with mid-level height tendencies going neutral brings the potential for convection to develop in the latter half of the evening into the early overnight hours. Conditional on convection developing, the environment will be favorable for storms to quickly become severe. Mid-level lapse rates on the order of 8 to 9 C/km will translate to 1000-1500 J/kg of CAPE. Deep layer shear will be present in abundance with deep layer shear values in in excess of 45 knots. Storms across Southern Michigan will be surface based while storms across northern- lower will be elevated. At this range, the primary hazards in any storms that develop will be large hail and damaging winds. As previously noted, severe weather will be conditional on a favorable overlap of frontal timing/cap erosion and instability so expect fine tuning of the forecast over the next 48 hours as we move into CAM range. - Lower Confidence Weekend Forecast, Showers/Storms Possible The synoptic pattern becomes less clear into the weekend, as questions arise about the possible development of a cutoff low across the western CONUS. Cluster analysis shows that the uncertainty lies in how a northern stream mid-level wave interacts with the western CONUS wave affecting the extent of mid-level forcing. Periods of showers are possible through the weekend, particularly across southern Michigan closer to a front that stalls over the Ohio Valley. Several different guidance suites then bring a low pressure system and associated rain chances to West Michigan in the Sunday Night into Monday timeframe, though the exact track and strength of this feature remains uncertain. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 720 PM EDT Wed Apr 16 2025 VFR conditions expected through the period due to high pressure overhead. Wind will become gusty from the southwest Thursday afternoon ahead of the next system that will arrive Friday. && .MARINE... Issued at 305 PM EDT Wed Apr 16 2025 Winds increase into Thursday, peaking around 25 knots near Big and Little Sable Points. Southeasterly/off shore flow will keep the greatest waves offshore initially. Late Thursday into Friday winds veer to southerly causing waves hazardous to small craft to build closer to the western shoreline. Waves likely diminish below Small Craft Advisory levels in the Friday Night/Early Saturday timeframe. The other marine concern is for two main rounds of thunderstorms. The first arrives early Friday morning, where a low chance of hail and/or brief gusty winds exists. A second round of storms is possible Friday evening, though it is questionable whether Lake Michigan will be affected or not. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...Thomas AVIATION...04 MARINE...Thomas