Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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876 FXUS63 KGRR 051602 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 1202 PM EDT Fri Jul 5 2024 .KEY MESSAGES... - Strong Storms Possible This Afternoon - Unsettled Weather At Times This Weekend - Showers and thunderstorms most probable Monday Night/Tuesday - Generally dry with seasonal temperatures starting Wednesday && .UPDATE... Issued at 1200 PM EDT Fri Jul 5 2024 A compact yet potent shortwave can be clearly seen in both model data and water vapor imagery over Wisconsin in the mid levels. This shortwave is pushing to the east and will drive into the area this evening. Down at the surface weak low pressure is centered out over Central Lake Michigan west of Big and Little Sable Points. A warm front stretches to the east from near Fremont to Mount Pleasant to Saginaw. A weak cold front stretches south down the lake from the low and can be seen in visible satellite imagery as a fine line in the what is likely a line of cumulus. The front is approaching the Lake Michigan shoreline now in Western Lower Michigan. Dew points in the warm sector of the system (which is the bulk of our forecast area) are in the middle 60s. Surface based CAPE values have reached 1,000 j/kg. Combine the low level moisture with what should be cooling mid levels associated with the shortwave and we are looking at instability values increasing with SBCAPE nudging further into the 1,000 to 2,000 j/kg range. We expect a diurnal increase in both shower and thunderstorm activity this afternoon as the upstream shortwave provides synoptic scale lift. The convection will likely be favored near both fronts (warm front lifting further into Central Lower and the cold front sweeping east over much of the area). That said while looking at the last few runs of the HREF, storms will be more likely over Central Lower Michigan near the warm front. There are chances for isolated severe thunderstorms this afternoon and evening due to a combination of increasing instability and deep layer shear values near and even above 40 knots due to strong mid and upper level jets in the area. Bottom line we are not expecting widespread organized severe weather, but isolated severe storms are certainly possible between 100pm and 800pm. Hail to around 1 inch in diameter and wind gusts to around 60 mph will be the main threats. && .DISCUSSION... Issued at 250 AM EDT Fri Jul 5 2024 - Strong Storms Possible This Afternoon and Evening Ample moisture across the area will lead to patchy fog this morning, dissipating shortly after daybreak. At the same time, a dying cluster of showers and storms arrives mainly impacting our northwest CWA just after daybreak. The focus then turns to this afternoon as a robust 500mb trough arrives from the west, driving an occluding surface low across the CWA. Guidance is consistent in redeveloping one or more clusters of convection along a cool front after about noon. At this point, significant occlusion looks to hold off until this evening allowing MLCAPE values to climb north of 500 J/kg and MUCAPE north of 1000 J/kg across the area, however uncertainty in the extent of destabilization does exist across guidance. One thing to monitor going into the daytime hours is the potential for locally better instability if sunshine can break through, as with dewpoints in the 70s it wont take much for instability to overachieve. With 35+ knots of deep layer shear, storm organization is likely. Greatest coverage of storms is expected north of M46 where an upper-level vorticity maxima and enhanced forcing from the triple point provide better lift, though storms are possible CWA wide. With no significant low-level jet and by extension weak low-level shear, along with the moderate nature of instability, severe weather will not be widespread. However, DCAPE values north of 800 J/kg and low-level lapse rates exceeding 7C/km will support a damaging wind threat and cold mid-level temperatures will also add a localized hail threat in any storms that develop this afternoon. This is covered by the marginal risk across the CWA. Coverage and intensity of storms will diminish quickly after sunset as diurnal heating is lost. - Unsettled Weather At Times This Weekend Going into Saturday, most of the area will be in weak cyclonic flow as upper-level troughing lingers overhead to start the day. This brings the chance of isolated diurnally driven showers near and east of US127, especially north of I96. However, much of the area is expected to remain dry. The weak cyclonic flow keeps temperatures seasonally cool Saturday with highs in the low to mid 70s. There is some uncertainty in the Sunday night and Monday timeframe regarding precipitation chances, as there is a signal for a minor leading PV max ahead of a meridionally phased and positively tilted upper shortwave trough located farther upstream. At this time an upward transition from 20 percent to 30-40 percent PoPs during this time seems warranted. - Showers and thunderstorms most probable Monday Night/Tuesday There is reasonably good certainty with this part of the forecast given the previously mentioned upper shortwave trough, whose characteristics tends to be associated with good predictability. Several consecutive runs of the ECE have shown a large majority of members indicating precipitation during this time, which supports the idea of good predictability. Given that this will be a seasonably deep upper trough, we will need to prepared for the possibility of severe weather and even localized flooding. - Generally dry with seasonal temperatures starting Wednesday Low precipitation chances (below 20%) seem justified at least starting out given we will have confluent northwest flow aloft which will help orient a surface high over the western Great Lakes. Guidance PoPs remain low all the way into Friday. This latter part might be a bit optimistic given that, as noted previously, we will be beneath upper troughing and the surface high will have moved east, yielding southeasterly return flow and a concomitant increase in low level moisture. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 754 AM EDT Fri Jul 5 2024 We are starting to climb out of IFR/MVFR visibility restrictions on observations and guidance and this will continue to be the case with the possible exception of MKG which will still be under the influence of a marine airmass and nearby precipitation. Expectations for showers and thunderstorms have not changed much from the previous update. Storm motion would be to the northeast at 30 to 40 knots with gusts above 40 knots possible. Window for VCTS should be on the order of an hour or less and we hope to refine forecast timing accordingly in subsequent updates. && .MARINE... Issued at 250 AM EDT Fri Jul 5 2024 Light winds and a moist airmass will bring patchy fog over the waters through mid-morning. Otherwise, with no notable wind in the next few days, Small Craft Advisories and Beach Hazards Statements are not expected. Winds under 10 knots this morning become southwesterly at 10-15 knots later today before becoming northwesterly late Friday as a cold front moves through. Scattered showers and isolated storms will affect the nearshore waters north of Grand Haven this morning. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ UPDATE...Duke DISCUSSION...TJT/Thomas AVIATION...TJT MARINE...Thomas