


Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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601 FXUS63 KGRR 111750 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 150 PM EDT Fri Jul 11 2025 .KEY MESSAGES... - Periods of showers/storms into Saturday evening - Dry and Warm later Sunday through Tuesday - Active period of convection possible Wednesday and Thursday && .DISCUSSION... Issued at 345 AM EDT Fri Jul 11 2025 - Periods of showers/storms into Saturday evening Lots of complicated activity expected in the first 36-48 hours of the forecast for the area, with multiple features interacting over the region. The first feature is a MCV/upper wave that is approaching the area from the WSW this morning. This feature was responsible for convection earlier toward the Quad Cities and east to near Chicago, before diminishing after daytime heating was lost. The MCV is still going strong, is is now starting to ramp up shower and storm activity with it as the low level jet is ramping up ahead of it and starting to nose into the area. We expect a fairly solid area of showers and an embedded line of storms to take shape, and move across at least the southern two thirds of the forecast area through early this afternoon. The strength of this convection this morning will be limited by a lack of good instability ahead of it, as the wave is on instability gradient. In addition, this feature coming through in the morning will not allow much diurnal instability to develop as it moves through. Would not be surprised for it to strengthen a bit as it exits the area early this afternoon. Initially the area will have been worked over, and limited heating for the first few hours of the day will suppress temperatures a bit. However, we will see the instability start to spread up over the area with limited heating this afternoon, and for the warmer and more humid air to make a run this way. This will be the result of the system coming in Saturday starting to increase the upper flow from the SW. Sitting on the instability gradient and it making a move north will allow for a few showers and storms to pop this afternoon. Any storm that forms has the potential to become strong with CAPEs up over 2000 J/kg and deep layer shear values as high as around 30 knots. Microbursts will be possible, and some hail with mid level lapse rates a little high in the mid 6C/km range. We will be waiting for more organized convection, likely later this evening and overnight. The short wave/MCV out in Iowa this morning will be steered a bit to the NE with the more organized synoptic short wave coming in from the Upper Midwest. The low level jet ahead of it is expected to fuel an additional wave of convection. There seems to be some general agreement with the bulk of the convection expected to affect the NW section of the forecast area. The uncertainty with this is how the line may propagate ahead of it. Some of it could run along the instability gradient over the forecast area. The uncertainty tonight then continues into Saturday, and will affect how additional convection fires. The early morning convection is likely to work the atmosphere over, and set up a boundary over/near the area. This will be the focus of development Saturday afternoon as synoptic scale forcing arrives ahead of the upper wave. Where the atmosphere can remain unaffected my convection/outflow boundaries, instability could build to over 3000 J/kg Saturday afternoon. The further east, the better the instability with temps approaching 90 and dew points in the 70s. The mid level winds increasing with the wave will also support better shear of around 25 to 30 knots, and an increased severe potential. The SPC Day 2 outlook has this captured well. The other threat with storms through the period will be the potential of locally heavy rainfall. The atmosphere will have PWATS up around 2 inches. Any training of cells, or prolonged storms over any area could cause some local flooding. - Dry and Warm later Sunday through Tuesday The sfc front should be through the forecast area by 12z Sunday morning, ending all of the storm threat and widespread rainfall. The upper trough will be still poised to move through, so clouds and a small chance of a residual shower may linger. We should see conditions improve through the day on Sunday with the passage of the trough. We then see upper ridging build over the area late Sunday and hold through Tuesday. This will bring the area dry conditions. Temperatures will initially drop just a little bit initially, but it will only be a few degrees. The humid air mass on Saturday will be replaced by dew points about 10 degrees lower to make it less humid by Monday. The heat will build though with the ridge in place and then warmer 850 mb temps moving in. - Active period of convection possible Wednesday and Thursday The upper ridge will then build east by Wednesday, allowing lower heights to settle in with an upper trough that will approach the area. This trough will be rather broad in nature, and a front will end up dropping over the area and stalling out as it becomes parallel to the upper flow. Short waves are forecast to ride along this front over the area, and could bring multiple rounds of showers/storms to the area by later Wednesday and continue through Thursday. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 150 PM EDT Fri Jul 11 2025 Going more pessimistic on ceilings this afternoon as there is a lot of moisture in the lower atmosphere and many areas of scattered to broken MVFR cloud bases that may not fully mix away. Scattered shower development also can`t be ruled out through this evening. The chance of an isolated thunderstorm prior to 02 Z is below 30 percent but not zero. Thunderstorms crossing Lake Michigan may head toward MKG and possibly GRR between 03 and 07 Z per the 12z HREF. && .MARINE... Issued at 345 AM EDT Fri Jul 11 2025 We are cautiously optimistic in saying that we may not need any marine headlines for a few days. We do see some increased wind aloft move in starting this morning, and lingering through Saturday. However with the temperature above the sfc of the lake being quite a bit warmer, the stable conditions over the lake should prevent this from coming down (outside of storms). There is not really much colder air behind the system on Sunday to kick up winds/waves either. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...NJJ AVIATION...CAS MARINE...NJJ