Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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517 FXUS63 KGRR 170006 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 806 PM EDT Sun Jun 16 2024 .KEY MESSAGES... - Thunderstorms Tonight - Hot, humid with occasional thunder this week && .UPDATE... Issued at 802 PM EDT Sun Jun 16 2024 Radar at 00Z has scattered convection moving across Lake Michigan. These storms are forming on, and being strengthened by, the outflow from a mesoscale convective vortex (MCV) moving to the northeast from Illinois. The big question remains how intact will the storms be as they move from the lake into and across southern Michigan. Latest CAMS continues to show scattered thunderstorms moving through Lower Michigan through the first half of the overnight period from this outflow. Another round of showers and storms will form overnight towards morning coinciding with MCV`s trek to the east. The latest CAMS have consistently had the potential for heavy rain across the I 96 corridor, especially through Kent and Ionia counties. PWATS continue to be in the 1 to 2 inch rain and the model soundings show an almost completely saturated atmosphere early Monday morning. There is a concern that mid level winds will be weaker overnight and the MCV will slow down allowing for a longer duration of showers. There is still some questions as to where the heaviest rain will fall but given the trends there is the potential for 1 to 2 inches along and north of the I 94 corridor and along and south of the I 96 corridor with locally heavier amounts in and around thunderstorms. && .DISCUSSION... Issued at 305 PM EDT Sun Jun 16 2024 - Thunderstorms Tonight The showers and storms across the central and northern zones will move east this afternoon and should be moving east and out of the forecast area this evening. Then we watch an MCV moving through northern Illinois and southern Lake Michigan into our southwest zones with the potential to bring locally heavy rain, hail and high winds overnight across the southern half of the forecast area. SPC marginal risk covers this threat area. - Hot, humid with occasional thunder this week The MCV propagates east and out of the forecast area by Monday morning but is expected to leave behind an outflow boundary across the southern forecast area that could focus some convection Monday afternoon. The HREF has apparently picked up on this idea and now has 70 to 80 percent probabilities of 0.1 QPF in this area on Monday afternoon. We painted an area of higher POPs there but kept them in the 25 to 35 percent range given lack of confidence in the specifics. The Heat Watch was not changed and it still appears that this could be a long duration event where the cumulative effect of high daytime and night time temperatures could necessitate a warning being issued even without warning criteria being met on any one day. There remains uncertainty on extent of convection on Tuesday afternoon, but the the biggest uncertainty remains on Thursday and Friday as a cold front or convective outflow could modify temperatures across the northern half of the forecast area. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 725 PM EDT Sun Jun 16 2024 The first item to watch will be a gust front moving off Lake Michigan and toward the terminals in the 01z-03z time frame. This gust front is an outflow boundary from earlier storms across Northern Illinois. It will cause a wind shift from southerly to westerly or WSW winds for a short time, potentially gusting to around 35 kts especially MKG/GRR/AZO. A shower is possible along this boundary, and perhaps a brief thunderstorm. After this boundary moves through, some additional showers and thunderstorms could develop especially closer to 06z. These could impact just about any of the terminals but those with the higher likelihood are GRR and LAN. Several runs of high resolution models have indicated some localized heavy rain rates, thus some reduced visibilities to IFR may occur in the 06z-12z window, but low confidence exists at any given terminal. && .MARINE... Issued at 305 PM EDT Sun Jun 16 2024 No changes to the SCA or CFW. Current forecast on track with southerly winds driving some higher waves from Grand Haven north with structural and longwave currents making swimming dangerous. Winds and waves could be locally higher in thunderstorms this evening south of Muskegon. Winds and waves will decrease slightly on Monday as the sfc pressure gradient slackens a little, but the night shift can evaluate this and determine if small craft and swim conditions will improve enough for the SCA and CFW to expire at 2 am or have to be extended. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement until 2 AM EDT Monday for MIZ037-043-050- 056. Air Quality Alert from midnight tonight to midnight EDT Monday night for MIZ037-043-050-056-064-071. Excessive Heat Watch from Monday afternoon through Friday evening for MIZ043>046-050>052-056>059-064>067-071>074. MARINE...Small Craft Advisory until 2 AM EDT Monday for LMZ847>849. && $$ UPDATE...Ceru DISCUSSION...Ostuno AVIATION...Hoving MARINE...Ostuno