284 FXUS63 KGRR 211811 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 211 PM EDT Sat Aug 21 2021 LATEST UPDATE... Aviation .SYNOPSIS... Issued at 339 AM EDT Sat Aug 21 2021 - Convection possible today into tonight - Warm with periodic convection into next weekend && .UPDATE... Issued at 923 AM EDT Sat Aug 21 2021 Have issued a Beach Hazards Statement and Small Craft Advisory for the area north of Whitehall starting late tonight and continuing all day Sunday. Winds will shift northerly and increase to 15-25 kts overnight, building waves to 3 to 6 feet. These marine headlines may need to be expanded southward, but will review the 12Z guidance before making that call. It is more of a slam dunk north of Whitehall than to the south, so figured we'd get the headlines there out now and expand later today if needed. Either way, there will still be at least a moderate swim risk south of Whitehall on Sunday with north winds of 10-20 kts and waves of 2-4 ft. && .DISCUSSION...(Today through next Friday) Issued at 339 AM EDT Sat Aug 21 2021 - Convection possible today into tonight The basic question today is if and when there will be convection. First through, a short answer, convection is possible (not likely) this afternoon near and east of US-131 associated with warm advection and some lake breeze convergence. Then we have the frontal convection that would come through tonight. It is looking more and more like that will not get much past US-131 if it gets that far. So, I am forecasting between 8 am Saturday and 8 am Sunday most of the CWA will not see measurable rain. Normally this would be easy, we have a cold front heading into very warm and humid air. The frontal timing through is that it comes through our forecast area tonight. That is not the issue through that will keep it from raining. There are many issues but the primary issue is the shortwave energy heads well north of our CWA tonight. That happens because we have an upper low that was over us but is now east of us and there is a series of strong Pacific shortwaves crossing the North Pacific that will try to make a western trough (but that trough will be shallow). Even so that process is causing rising heights over most of the central CONUS, starting now and continuing into Tuesday. We have a very sharp upper ridge over us tonight (not good for storms to reach here). The temperature rises around 2c between 700 mb and 300 mb between 8 am this morning and 8 pm this evening. That much warming at mid and high levels is also bad for convection. So I am thinking, like what the HREF from the 8 pm run shows, that most of the measurable precipitation will have a hard time making it east of US-131 early tonight. Most of the area east of US-131 will very likely see NO PRECIPITATION tonight. All of our hi-res models (RAP, NAM, NAMNEST,HRRR,HRW NSSL,HRW ARW,HRW FV3, and HREF) all support this idea. Oddly through the HREF, FV3, ARW, NSSL and NAMNEST show the warm advection will help set off a few afternoon thunderstorms near US-131 early this afternoon that will head toward US-127 by mid afternoon. These will be widely scattered so most will stay dry here too. One other odd aspect to all of this, the frontal trough stalls over eastern Lower Michigan (that closed upper low stalled to our east with HENRI in it by then) Sunday. That set up decent surface convergence over eastern Lower Michigan in the afternoon. There is surely more than enough instability around too (1000 to 1500 j/kg of MU cape) too. So we will see some thunderstorms in the I-69 area Sunday afternoon. - Warm with periodic convection into next weekend Overall we are in an weather pattern that will change little over the next week or so. As I wrote above, a series of North Pacific shortwaves dry to develop an upper trough over the Pacific Northwest into west central Canada through this coming week. That builds heights over the central CONUS or at least holds them were they are (much higher than normal/ 1.5 to 2.5 standard deviations from the mean at 700 mb and 500 mb) through the week into the following weekend. So, that being so, one would not expect much cooling. From time to time a shortwave tracking along the polar jet will set off convection and bring us the risk of thunderstorms. Timing of this with this sort of patten is questionable at best. The ensemble means on both the GFS and ECMWF show some preference for convection in the Wednesday time frame and also for next Sunday but for each day next week, at least few members of those ensemble show some rainfall. The forecast for the next week is warm and humid with periodic thunderstorms but most of the time it will be dry. && .AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 158 PM EDT Sat Aug 21 2021 Clouds are beginning to form, however they will remain VFR through most of the afternoon and into early evening. A line of convection will continue to move eastward this afternoon and evening. The best chance for thunder will be north of Muskegon. There is still a chance for scattered showers as the system moves through after 23Z. MVFR cigs and vsbys are possible in light rain showers with patchy fog possible into Sunday. && .MARINE... Issued at 339 AM EDT Sat Aug 21 2021 There will be a push of cold air behind the front tonight. The GFS is every aggressive with this and suggest 20 to 30 knot winds near Big and Little Sable points around midnight. All of the other models show some variation of this but not as strong of winds. This wind core heads to the western side of Lake Michigan by 5 am. So if this really happens we may need a Small Craft Advisory for our northern 2 marine zones early Sunday morning. We will let the day shift look at this before we issue any such headlines. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement from Sunday morning through Sunday evening for MIZ050-056-064-071. Beach Hazards Statement from late tonight through Sunday evening for MIZ037-043. LM...Small Craft Advisory from 2 AM to 8 PM EDT Sunday for LMZ848-849. Small Craft Advisory from 11 AM to 8 PM EDT Sunday for LMZ844>847. && $$ UPDATE...Meade SYNOPSIS...WDM DISCUSSION...WDM AVIATION...Ceru MARINE...WDM