Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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678 FXUS63 KGRR 200553 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 153 AM EDT Thu Jun 20 2024 .KEY MESSAGES... - Temps not as Hot Thursday north - 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. - 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 /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 153 AM EDT Thu Jun 20 2024 VFR prevailing much of the time over the next 24 hours although brief periods of MVFR/IFR are possible if and when any of the widely scattered showers and storms impact a terminal. The most likely timeframe for thunderstorms is later today between roughly 20Z and 02Z, and mainly near and south of a GRR to LAN line. MKG should be excluded from the thunder threat later today due to a stable onshore/lake breeze. Scattered showers and storms weakening and ending after sunset then the main concern will become areas of fog and stratus developing overnight, especially where rain occurred. && .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...Heat Advisory until 8 PM EDT Friday for MIZ056-057-064>067- 071>074. MARINE...None. && $$ UPDATE...Ceru DISCUSSION...MJS/TJT AVIATION...Meade MARINE...MJS