270 FXUS63 KGRR 191049 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 649 AM EDT Sat Sep 19 2026 .KEY MESSAGES... - Periods of rain through tonight, locally heavy at times - Cooler period through the week with small chances of rain && .DISCUSSION... Issued at 326 AM EDT Sat Sep 19 2026 - Periods of rain through tonight, locally heavy at times The rain event we have been focusing on for the last few days is now underway. A long line of showers here, extends all of the way back into Northern Iowa where storms are present. This line is the edge of the elevated warm front where the low level jet is surging deep moisture over it. Precipitable waters pushing 2 inches is at the core of this moist air mass advecting in. The rain here so far has been on lighter side with limited low level moisture, and with the best forcing focused west of the area so far. This rain is at the leading edge of the low level jet, and we will see the more concentrated rain push in between now and sunrise. There is plenty of thunder to our west, but we are not expecting that to be as widespread as it is currently. Instability progs like MU CAPE show the elevated instability gradient staying mostly SW of the forecast area, except the far SW corner of the area. Severe weather potential is quite low, however this area is most likely to see the heavier convective bursts under the stronger updrafts and highest pwats. We could end up needing some Flood Advisories with the heavier bursts. This first batch should be pushing out of the area by noon. We will see the intensity of the rain diminish, but we do not expect a complete break in the rain for the afternoon. The models are indicating that some semblance of a low level jet will remain, just not as strong of a core as this morning. This overrunning will continue, and keep rain possible, even if it is just drizzle with lower moisture becoming plentiful. We expect the rain to increase in coverage and intensity once again toward and after sunset as another low level jet core increases ahead of the main sfc low. This lasts into the early overnight hours before the elevated front/baroclinic zone, and low level jet move south of the area. Again, not expecting much thunder with elevated instability fairly weak. Heavy rain is once again the main threat, especially after the rain from earlier priming the ground and rivers/streams. The highest potential for this is where the Flood Watch is across the SW corner of the forecast area. River forecast points will also see good increases. There has been some uncertainty whether Sunday sees residual rain or not. It looks like the best chance of any rain beyond sunrise on Sunday would be early, and would move out quickly as the front sinks south of the area. Clouds are likely to hold in most of the day. - Cooler period through the week with small chances of rain This wet system for this weekend will move out, but we are not looking at any extended dry period behind it. We will see a weak short wave ridge pass overhead on Monday, which may keep all of the area dry. We quickly see an upper low complex over the Upper Midwest rotate over the area, and merge with additional upper level energy to become a bigger complex. This complex then looks to sit over the area from Tuesday through the end of the week. Timing and exact track of any weak short waves rotating around it will be problematic as is the case always with upper lows. What we do know is that is will keep a cooler air mass over the area, likely keeping temperatures in the 60s. Timing of showers will be a little more favorable in the afternoon hours when some diurnal instability can help with the development. This looks to hold pretty much through Friday, before the larger upper ridge can push this system east of the area. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 649 AM EDT Sat Sep 19 2026 Band of rain is affecting most of the forecast area and the terminals as of 1045z. Storms are trying to make it over the area also, having occurred near KBIV and then diminishing as they continue east. MVFR is becoming more prevalent as low level moisture is increasing significantly. The current rain and embedded storms are forecast to move out by about 16-17z. Not sure if the rain will end, or if we see drizzle continue with a great deal of low level moisture staying in place. This scenario is likely to occur from 16-17z until 22-23z when the next likely batch of showers and embedded storms move in. The storms will be most prevalent south once again, and will last longer than the northern terminals. Low clouds and fog will hold in through the end of this forecast package, with LIFR likely. && .MARINE... Issued at 326 AM EDT Sat Sep 19 2026 We continue to watch the winds and waves today through this evening, but will continue with no headlines at this time for today. Offshore flow will dominate with winds staying just below criteria for today. Limited mechanical mixing today due to the clouds and rain will keep winds in check on this side of the lake. Sunday looks like a better shot of needed a Small Craft Advisory, mainly due to the winds. The flow will remain offshore, but will increase enough to likely need a Small Craft Advisory with winds up around 30 knots. Waves will remain 1 to 4 feet with the limited fetch. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Flood Watch through Sunday morning for MIZ064-065-071>073. MARINE...None. && $$ DISCUSSION...NJJ AVIATION...NJJ MARINE...NJJ

NWS GRR Office Area Forecast Discussion