


Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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448 FXUS63 KGRR 180752 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 351 AM EDT Sat Oct 18 2025 .KEY MESSAGES... - Periods of Showers/Storms Through Sunday with Locally Heavy Rain, Windy Sunday - Another Period of Rain and Wind Monday Night through Tuesday - Unsettled Weather Wednesday through Friday && .DISCUSSION... Issued at 351 AM EDT Sat Oct 18 2025 - Periods of Showers/Storms Through Sunday with Locally Heavy Rain, Windy Sunday No noteworthy changes to the forecast are needed compared to yesterday`s day shift forecast. A few light showers have made it to the NW corner of our forecast area this morning. Much more widespread showers and scattered thunderstorms are currently over the SE half of Wisconsin as of 3 am EDT this morning. This area of rain seems to be directly correlated to the 40+ knot low level jet centered over that area. These showers and storms are expected to move over our area toward and just after daybreak as the low level jet axis shifts eastward over the area. The showers and storms over IA are likely the area that will move overhead. The storms with this should not initially be capable of much beyond rain and lightning as these storms are elevated in nature. They could strengthen a bit before moving out as they may be able to tap some additional instability that may build with some diurnal heating. We expect additional showers and storms to develop this afternoon with MU CAPEs building to 500+ J/kg. This will likely be across the SE corner of the forecast area as some of those areas may just miss out on the morning convection, and some better instability will build. In addition to better instability, it will become more sfc based, and it will have good deep layer effective shear values of 40- 50 knots, and some decent low level shear values with 30 knots of the low level jet remaining over the SE corner of the area. Can not rule out some localized wind with DCAPEs 700-800 J/kg, or some small, ll hail with mid level lapse rates around 6.5-7 C/km. The stronger deep layer shear could help some of the storms rotate aloft if enough instability can sync up with the stronger shear. We will then watch the heaviest of the rain move in by late evening, and continue through a good portion of the night. The sfc low will be driven by the long wave trough coming in. At the same time, we will see a short wave ride up along the front, and induce a nice band of FGEN just NW of the sfc low due to the interaction. This plethora of forcing will be acting on an atmosphere that will have 200-250% of Precipitable water values normal for this time of the year. This looks to squeeze out a good 1-3 inches, with the heaviest axis between GR and Lansing. Please see the Hydrology discussion below for more details. The heavy rain will move out by Sunday morning, but we are looking at another full day of steady rainfall to occur over the area. This will be the case with the long wave trough coming in and interacting with the residual moisture.. - Another Period of Rain and Wind Monday Night through Tuesday After a break Sunday night and much of Monday, we are looking at another period of showers developing Monday night. This system will be coming in from the NW, vs. the system that will be coming in from the SW tonight. Initially moisture will be limited coming in from the NW. A long trajectory of low level flow will begin tapping Gulf moisture once again, and will be absorbed by low. The winds with this system will be somewhat similar in magnitude with a strong pressure gradient developing on the stronger sfc low. The difference with this system is it will drag a bit more of colder air in over the relatively warmer waters of Lake Michigan. 850 mb temperatures will be dropping to as low at -2C, yielding delta t`s in the mid to upper teens. This scenario will likely kick up some lake effect rain showers for the lakeshore counties with a flow from the NNW. - Unsettled Weather Wednesday through Friday Wednesday will see some residual lake effect rain showers earlier, before they should diminish through the day. Some weak ridging will build in for Thursday. The long wave flow will feature more troughing over the region lingering from the systems passing through. We could see another short wave dive in for Friday, bringing another threat of rain showers. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 142 AM EDT Sat Oct 18 2025 This forecast period is starting out with mainly high clouds in place at all of the terminals. Showers are starting to form over Lake Michigan and are ongoing over Wisconsin. This axis of shower activity with some isolated storms will arrive at KMKG by 11z or so, and will continue to move across most of the rest of the forecast terminals, except maybe staying north of KJXN. MVFR conditions are likely, and there may be some periods of IFR with the heavier showers/storms. This first batch will move out by about 18z. A few showers may linger, but there will be a relative lull in the more widespread shower activity. Later in the afternoon and into the evening, the I-94 corridor is likely to see additional scattered showers and isolated storms develop. Then beginning around 03z, widespread rain will move over the I-94 corridor and spread north through 05-06z. IFR conditions are forecast to develop in this widespread rain. Some isolated storms will remain possible. && .MARINE... Issued at 351 AM EDT Sat Oct 18 2025 We will be upgrading the Gale Watch to a Gale Warning for Sunday and Sunday evening. Gales (34+ knots) look to be a certainty on the backside of the low as it departs Sunday morning. The highest winds will likely be Sunday morning as the colder air on the backside of the system rushes in, and coinciding with the pressure fall/rise couplet. Winds and waves will subside a bit for Monday, before coming up significantly on Tuesday with the next system. This system looks to bring Gales to the nearshore waters also. Obviously a bit of time to monitor that. && .HYDROLOGY... Issued at 318 PM EDT Fri Oct 17 2025 Significant rainfall is expected over the next several days across much of Lower Michigan. The total amount of rain we are expecting continues to slowly nudge upwards as forecast details come into focus. The rain will be spread out over several rounds, primarily spanning Saturday and Sunday, with the heaviest rain currently expected before/during sunrise on Sunday. Overall averages across much of the area will probably end up in the 1 to 2 inch range, with some multi-county streaks of 2-3"+. The good news is that because we`re in a drought, the rivers have plenty of room to put this water. Some of the midsize rivers in the area like Sycamore Creek (Holt), Buck Creek (Grandville), and the Looking Glass River (Eagle) are the most likely to see rises close to bankfull, but no river flooding is expected unless this storm seriously overperforms our expectations. It would probably take 4"+ over a fairly large area to begin flooding the midsize tributaries mentioned above. Nevertheless, with water levels currently running lower than normal due to the drought, notable within-bank rises are expected across most of the area. Anyone with personal property near the waters edge leftover from the summer should prepare for rising water. While river flooding isn`t a big concern, if some slow-moving heavy rain bands park over some of our urban areas we could see some localized minor flooding develop in the classic low spots and other poor drainage areas. It`s hard to pin down which urban areas would be most at risk of this happening, but recent model trends suggest the eastern edges of our area are most at-risk, which would include Lansing and Jackson areas. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for LMZ844>849. Gale Warning from Sunday morning through Sunday evening for LMZ844>849. && $$ DISCUSSION...NJJ AVIATION...NJJ MARINE...NJJ HYDROLOGY...AMD