Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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000 FXUS63 KGRR 301109 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 709 AM EDT SAT JUL 30 2016 LATEST UPDATE... Update/Aviation .SYNOPSIS... Corrected at 346 AM EDT Sat Jul 30 2016 Periods of showers and isolated thunderstorms are expected today into Sunday. Some of the showers will be locally very heavy with the potential for localized flooding. It will be cooler too with highs only in the upper 70s to lower 80s both today and Sunday. The slow moving storm that is causing all of this rainfall will finally move out of the area by Sunday night. This will allow a large high pressure system to move over Michigan, clearing the skies and bringing warmer afternoon temperatures. A warm front will come through the area on Wednesday putting Southwest Michigan back into the hot and humid air. A cold front will move into the area late Thursday with the threat of thunderstorms, but not before highs reach around 90 degrees. && .UPDATE... Issued at 709 AM EDT Sat Jul 30 2016 I have updated the zones to have the more significant rainfall farther north through the day today. The deformation axis at mid levels stays from near Holland to Grand Rapids to Saginaw but the strength of the deformation increases significantly this afternoon as the upper wave from NE moves into the western Great Lakes. This also increased the upper divergence and given our precipitable water is forecast to be nearly 2 standard deviations from normal, I have to believe locally heavy rainfall is a real threat. So I have that in our forecast wording too. Once this NE shortwave reaches lower Michigan the lift should diminish so all of this should quiet down this evening. && .SHORT TERM...(Today through Monday) Corrected at 346 AM EDT Sat Jul 30 2016 The main issue today is the threat of localized flooding from slow moving heavy rain showers and thunderstorms. This threat really goes into Sunday as the system creating all of this will likely not move out of the area until Sunday night. We have precipitable water values near 1.75 inches this afternoon over a good part of our CWA. While that is happening, we have the deformation axis in the 500 to 700 mb layer from near MBS to GRR and not too surprisingly, we have decent upper divergence over this same area. All of this is caused by the next upstream shortwave (currently near Nebraska and Kansas) moving into the trough over the Great Lakes. That significantly increases the upper divergence and, in turn, lift. So I see bands of showers rotating from south to north through the CWA then stalling near the deformation axis into this evening. Afternoon heating will help increase the lift and strength of the showers. I do not at this time plan to issue a Flood Watch as it would seem any heavy rain will be rather localized. Still we will have to watch this. It will take until Sunday evening to finally start lifting this system out of the area. So that means I have extended the showers into Sunday. Sunday will feature decent instability, so thunderstorms are once again expected. Locally heavy rainfall is also possible. Ridging builds in Sunday night into Monday, so skies should clear and allow for some warming of the afternoon highs by Monday. .LONG TERM...(Monday night through Friday) Issued at 330 AM EDT Sat Jul 30 2016 The trend in the forecast for next week is warmer and drier with a fair amount of sunshine. In general, the upper level pattern will be defined by ridging through much of next week. 1000-500 mb thicknesses and 850mb temps will gradually climb Monday night through Thursday. Highs in the mid 80s should be common Tuesday and Wednesday, with upper 80s to around 90 degrees more likely for Thursday. There are a couple shots at precip next week. The first will be Tuesday into Tuesday night as a weak shortwave dives southeast embedded in broad northwest flow aloft. As the shortwave moves through Chicagoland into northern Indiana, a warm frontal boundary will be draped across Indiana and Ohio. Will feature highest PoPs (30 pct) across Southwest Lower. A better chance for precip will arrive Thursday night and Friday. A cold front is forecast to move through the state on Friday. This feature is associated with a closed upper low over Hudson Bay. A moisture-rich, humid airmass will be in place out ahead of the cold front as evidenced by a plume of higher theta e extending from the Gulf Coast states into Michigan. && .AVIATION...(For the 12Z TAFS through 12Z Sunday morning) Issued at 709 AM EDT Sat Jul 30 2016 MVFR cigs and vsbys are likely to continue in the I-94 TAFs and become IFR tonight as the showers and thunderstorms continue. The I-96 TAFs may see occasional MVFR / IRF in the heavier rain showers but the northeast wind bringing in drier air from the north should limit the low cigs during the day time hours. I would think all areas will have IFR/MVF cigs and vsbys overnight tonight as the showers should continue. && .MARINE... Corrected at 346 AM EDT Sat Jul 30 2016 Northeast winds of 5 to 15 knots will not cause much in the way of beach hazard conditions. However, the threat of locally heavy showers and thunderstorms will be an issue through Sunday. && .HYDROLOGY... Issued at 328 PM EDT Fri Jul 29 2016 Thunderstorms the past two days have primarily been heavy rain threats, and this will continue to be the case into Saturday. A slow moving trough with weak vertical shear will continue to promote isolated to scattered slow-moving storms. No substantial dry air can be found in the deep convective layer, and precipitable water values are still around 1.5 inches which is near the 90th percentile for late July climatology. Slow moving storms Thursday into Friday produced a number of isolated reports of rainfall greater than 2.50 inches, along with street and parking lot flooding several inches deep. We will continue to stay alert for the isolated flooding threat through Saturday. && .GRR Watches/Warnings/Advisories... MI...None. LM...None. && $$ UPDATE...WDM SYNOPSIS...WDM SHORT TERM...WDM LONG TERM...EBW AVIATION...WDM HYDROLOGY...CAS MARINE...WDM

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