Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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208 FXUS63 KGRR 200224 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 1024 PM EDT Wed Jun 19 2024 .KEY MESSAGES... - Temps not as Hot Thursday north - Risk for Strong/Svr Storms this evening - Very warm and windy Saturday - Thunderstorms Friday through Sunday, especially Saturday night - Dry and somewhat more comfortable Sunday through Tuesday && .UPDATE... Issued at 1013 PM EDT Wed Jun 19 2024 Scattered convection continues overnight. Isolated storms have moved northward towards the I 94 corridor. While the hot and humid air mass over SW Michigan provided enough instability for storms earlier this evening, they were stifled due to the divergent pattern and weak shear. There is a boundary that extends from Big Rapids to the southwest that has a persistent line of showers. There were some storms along this boundary over mid Lake Michigan. However, not expecting for these showers to intensify, especially since there is weak shear, it is nighttime and the -10C is at 20K feet. && .DISCUSSION... Issued at 303 PM EDT Wed Jun 19 2024 - Temps not as Hot starting Thursday north Heat index values are well into the 90s at this time so we made no changes to the headline through the remainder of today. However, looking at Thursday, there has been a trend for a southward shift of the axis of hottest air then. This supports dropping the Heat Advisory for Thursday for the region north of a line from Ottawa County to Ingham County. We already issued the NPW reflecting this change. If this southward trend continues, further changes may be needed the headline area. - Risk for Strong/Svr Storms this evening DCAPE around the area is favorable for gusty winds with any stronger storms. ML CAPE values are over 1000 J/kg so decent instability is in place. Effective bulk shear values have dropped off and are under 25 knots. So far the low level convergence has been very weak, but there are some indications that it may increase somewhat near and southeast of a Kalamazoo to Lansing line. If convection can develop in that region, we will be monitoring it for possible strong wind gusts and locally heavy rain. It`s worth noting that the main PWAT axis is shifting east of the CWA so that dry air advection may limit the overall convective coverage. The latest HRRR only has a few isolated cells over the next few hours, but it does suggest some outflows from any cells that do develop. - Very warm and windy Saturday Highs Saturday could vary substantially from northwest to southeast, ranging from around 80 near Ludington to mid/upper 90s near Jackson. Winds from the southwest Saturday afternoon will be quite strong; although the official forecasts indicate gusts in the 20-30 MPH range, expect model trends to bump this closer to the 30-40 MPH range based on some of the more recent guidance. Mixing should be substantial. A broadening spread of dewpoint forecasts among the ensemble members hints at the mixing process and also suggests uncertainty about how much drying will occur. The effects of heat additionally may be curtailed by the development of thunderstorms in the afternoon/event (see following section). - Thunderstorms Friday through Sunday, especially Saturday night As noted in the previous discussion, we will have a surface boundary providing a source of weak low level convergence. Very flat ridging aloft will offer episodic opportunities for minor PV maxima to zip across the region and allow storms to fire and then cluster into MCSs. Timing details are uncertain but activity will generally be guided by the diurnal convective instability cycle. Timing for the main round of thunderstorms looks increasingly locked into Saturday night. A deeply mixed boundary layer ahead of this activity will elevate the chances for damaging winds as this activity moves through. Additionally, the upper shortwave trough looks sharper and farther south than before. Hence, QC forcing could be more substantial than previously thought. These ingredients point to growing severe potential for this timeframe. - Dry and somewhat more comfortable Sunday through Tuesday As noted before, confidence is good for a dry Monday given sharp shortwave ridging aloft behind the aforementioned trough. PoPs return Tuesday night into Wednesday along with unseasonably warm temperatures. Predictability for temperatures should be fairly good, but advertised PoPs probably can be better attributed to climatology rather than forecast certainty. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 717 PM EDT Wed Jun 19 2024 Scattered/isolated thunderstorms were over parts of Southwest Lower MI to start the period. They will approach several TAF sites early this evening with KJXN and KBTL appearing to have the highest probability to seeing a storm. It will remain unstable through the evening so close monitoring will be needed given the random development. A cold front will be dropping down from the north bringing a wind shift. Moisture will be abundant along this frontal zone. This front will dissipate over the area tomorrow and could serve as a source for new storm development, especially after 18z. Low clouds and some fog may impact the TAF sites later tonight into Thursday morning but I was not confident enough to go widespread IFR as some models suggest may happen. For now most sites will feature a period of MVFR conditions. && .MARINE... Issued at 303 PM EDT Wed Jun 19 2024 Based on the lowering winds and waves, we cancelled the beach hazards and small craft advisory a little early. Saturday it looking like the next big wind/wave event. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Air Quality Alert until midnight EDT tonight for MIZ050-056-057- 064-071. Heat Advisory until 8 PM EDT Friday for MIZ056-057-064>067- 071>074. MARINE...None. && $$ UPDATE...Ceru DISCUSSION...MJS/TJT AVIATION...MJS MARINE...MJS