Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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292 FXUS63 KGRR 210446 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 1246 AM EDT Sat Sep 21 2024 .KEY MESSAGES... - Chance of showers and storms this evening - Another chance for showers and storms Saturday night - Likely showers and possible thunderstorms Sunday and Sun Night - Unsettled next week, more seasonal temperatures, some rain possible && .UPDATE... Issued at 1244 AM EDT Sat Sep 21 2024 We issued a Dense Fog Advisory for the entire cwa until 10am. Widespread dense fog was developing and should persist throughout the night as higher dewpoints coupled with additional low level moisture from rainfall Friday produce ideal conditions. && .DISCUSSION... Issued at 337 PM EDT Fri Sep 20 2024 - Chance of showers and storms this evening Certainly not widespread rain this afternoon, but a few areas picked up some measurable amounts over Southern and Western Lower Michigan. Additional rainfall is expected this evening as a cold front moves in off of Lake Michigan. Dew points have come up into the middle to upper 60s across Western Lower Michigan with MUCAPE values now pushing towards 2000 j/kg. Around 1100 j/kg of DCAPE is in place currently, so previous thinking that the possibility of strong to locally severe storms exists remains valid. Deep layer shear per the SPC meso page shows 40 knots nosing into areas near and southwest of Grand Rapids. We continue to think that showers and storms will blossom potentially near MKG/BIV and GRR and then move to the south and east as we head through the evening. Wind remains the biggest threat with hail being a notch below that threat. The most widespread precipitation is expected this evening, becoming scattered overnight before moving east by 09Z or so. - Another chance for showers and storms Saturday night An upper shortwave trough moving from the Northern Plains into Canada on Saturday will push a cold front towards Lower Michigan from the west Saturday night. The front will be moving well away from its source of synoptic scale lift associated with the wave aloft. The fact that the front will be coming through in the overnight period, that its running away from the upper wave and that it will be moving into dry air does not bode well for widespread precipitation. We may see some scattered showers late moving towards the Western Lower Michigan shoreline, but not expecting much more than that. - Likely showers and possible thunderstorms Sunday and Sun Night As noted before, ensembles have been very honed in on a solution of shower and thunderstorm chances increasing during the day Sunday and remaining high through Sunday night. A sharp upper trough will break cyclonically over the extreme northwestern Great Lakes region. So, although the strongest upper forcing will be displaced significantly to our north, there will be an expansive surface front extending from the Western Great Lakes into the Central Plains (as noted previously). Per latest SPC guidance, severe weather is not anticipated given the unfavorable placement of upper forcing, but anomalously high PWATs certainly support localized heavier rainfall. As also noted previously, there is relative uncertainty in maximum temperatures Sunday due to questions about thunderstorm evolution and associated diabatic cooling effects. To illustrate, the ECE ensemble features an interquartile (25-75 percentile) range of temperatures between 75 and 84 degrees F, with highs in the lower 70s also quite plausible. - Unsettled next week, more seasonal temperatures, some rain possible No changes to previous thinking. Rain chances are pretty well assured Monday into early Tuesday. After this, we are looking at increasingly unphased flow aloft. By mid-week there is evidence of a southern stream cutoff low originating in the southwestern CONUS region amidst very light flow and whose evolution is highly uncertain. Predictability with this type of setup is extremely low in general and the offered forecast is a bit of a hedge with low precipitation chances each day. What is more certain is a reduction in temperatures closer to seasonal normals during the course of the week. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 608 PM EDT Fri Sep 20 2024 Primarily VFR conditions this evening will gradually deteriorate to IFR/LIFR overnight due to development of fog and low clouds. These conditions will linger through mid morning Saturday before gradually improving to VFR. Winds overnight and Saturday will be rather light aob 10 kts. && .MARINE... Issued at 337 PM EDT Fri Sep 20 2024 Waves are near 2 feet at most of the nearshore buoys associated with some stronger winds last night. Also, winds this morning with a line of showers and storms helped build a wave field. The winds are expected to be fairly light, 15 knots or less, tonight into Saturday, so waves should settle below 2 feet. Saturday night the southerly winds increase a bit ahead of our next front, with conditions that could approach Small Craft Advisory criteria. Sunday is our next good chance for winds and waves as north flow takes hold behind the cold front. 2-4 footers look plausible on Sunday with the chance for high than that. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Dense Fog Advisory until 10 AM EDT this morning for MIZ037>040- 043>046-050>052-056>059-064>067-071>074. MARINE...None. && $$ UPDATE...04 DISCUSSION...Duke/TJT AVIATION...Laurens MARINE...Duke