textproduct: Grand Rapids

KEY MESSAGES

- Rain event Saturday with placement still somewhat uncertain

- Diminishing rain chances Saturday night and Sunday

- Cooler pattern much of next week with chances of rain showers

DISCUSSION

Issued at 336 AM EDT Fri Sep 18 3366

- Rain event Saturday with placement still somewhat uncertain

After a gloomy and drizzly day for most of the area on Thursday, today is shaping up to be a better day before rain chances increase once again tonight.

The front that has been just south of the forecast area over the last couple of days has moved further south, taking its moisture with it. This is the result of a stronger push of drier air coming in from the NE with strengthening high pressure over Canada. This will be short lived however.

A long wave upper low dives over the Western U.S. by Saturday, and helps to establish an upper flow from the WSW. We will see upper short waves eject, and help to lift the warm front back northward toward the area. The main feature is a 35 knot low level jet ahead of the wave and intersecting the front to bring an area of fairly widespread rain.

Even though we are approaching this event, the details are far from certain yet. This is because we really have multiple forcing mechanisms trying to affect the area, and interacting with each other. The low level jet will be transporting precipitable water values of 2+ inches over the area through Saturday morning. Just as the low level jet bis relaxing and starting to move out, we see some upper level jet dynamics in the form of RRQ dynamics lead to some additional mid level fgen working on the moist air mass. There is also a trough coming through at 700 mb leading to more upward forcing.

Some models are placing the heavy rain over the I-96 corridor, while others are placing it over the far SW corner of the forecast area. It is tough to pick one or the other, but the larger scale synoptic models are likely to handle this better than the mesoscale models given the multiple forcing features.

The good thing is that the impacts will be limited from this event. Thunder will be possible, but will be elevated, so severe weather is not likely. Rain could be locally heavy, but it not expected to be too heavy over a widespread area, so no headlines are needed at this time.

- Diminishing rain chances Saturday night and Sunday

The multiple forcing features, and how they interact with each other create the main uncertainty with the forecast. This includes some of the model solutions/ensemble members holding some rain over Southern Lower into Sunday. The departing low level jet and upper trough pushing through should tend to push the moisture and rainfall out a bit more given the scenario.

- Cooler pattern much of next week with chances of rain showers

The upper trough comes in over the area as the main long wave trough over the west is pushed toward the area. This upper trough encounters additional energy and has been shown for multiple runs to close off an upper low over the region. If this indeed happens, we will be looking at a cool, and unsettled weather pattern much of next week. Unfortunately, the development and then subsequent movement of the complex is one of the most uncertain things to try and forecast. The good thing is it can be summarized well with cooler temps and a chance of rain each day, but it will not rain all of the time.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 639 AM EDT Fri Sep 18 2026

We are looking at VFR conditions dominating this forecast period, even with rain coming in late in the period. All of the clouds that were over the terminals just before 06z have cleared out of the area to the south. Some light patchy ground fog has tried to develop. We have a tempo group in at KJXN through 13z.

Clouds will gradually thicken up and lower for the second half of this forecast period. Light rain will become possible after 00z Saturday. The rain looks light enough with limited moisture below 4k ft that visibilities should mostly stay VFR through 12z. Conditions will deteriorate after this forecast period.

MARINE

Issued at 336 AM EDT Fri Sep 18 3366

We are looking at a tame period regarding winds and waves for most of today, before they ramp up a bit ahead of the next system. Fortunately, the winds will be mostly offshore and warm air advection which will limit what is realized at the sfc. Right now, winds will be marginal at best for a Small Craft Advisory. Being offshore in nature, will hold off on any headlines for now.

We could see some better chances for headlines late Saturday behind the front, but it is again too marginal to jump on any headlines yet.Sunday and Monday do look like some windy days with quite a bit of wave action. Small Craft Advisories are likely, with some Gales possible.

GRR WATCHES/WARNINGS/ADVISORIES

MI...None. MARINE...None.