Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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228 FXUS63 KGRR 041753 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 153 PM EDT Thu Jul 4 2024 .KEY MESSAGES... - Warm Independence Day With Slight Chance of a Shower - Scattered Thunderstorms Friday, Low Confidence of Severe Potential - Showers Possible Saturday, Unsettled Pattern Continues Next Week && .UPDATE... Issued at 1154 AM EDT Thu Jul 4 2024 Surface high pressure is located across the Great Lakes region as we approach midday. Aloft, a mid level shortwave is moving toward our area from Illinois. Rain showers precede this shortwave over Illinois and Indiana. The current thought is the going forecast has things covered well as only very small chances for spotty showers exist. The 12Z HREF continues to show some isolated rain shower activity this afternoon and early evening, but there is not a consensus within the members of the HREF on precipitation occurring. HRRR model soundings this afternoon show some mid level moisture with very little to almost zero CAPE. Below the mid level moisture, rather dry air exists down to the surface. The dry air will work on any precipitation moving our direction and will likely keep us precipitation free. So, we expect partly sunny to mostly cloudy skies with some virga or perhaps an isolated light shower or a few sprinkles. We think most areas will remain dry. High temperatures will be in the 80s with light winds. All said, a forecast for the fourth that is pretty typical for Southwest Lower Michigan. && .DISCUSSION... Issued at 350 AM EDT Thu Jul 4 2024 - Warm Independence Day With Slight Chance of a Shower Temperatures are expected warm up into the mid 80s today with light winds. The cloud cover forecast for late morning through afternoon has become a little more pessimistic, as there may be several hours when it is mostly cloudy. Also feel compelled to broad brush a slight chance of showers everywhere south of Whitehall to Mount Pleasant for the afternoon and evening. A midlevel shortwave trough, augmented by overnight storms in Kansas, is likely to spread cirrus and altostratus over Lower Michigan during the late morning and afternoon. This cloud deck will likely contain ice and may precipitate as virga or sprinkles, but would also serve as ice nuclei feeders to the diurnal cumulus that might develop underneath. Whether the models are overdeveloping cumulus underneath the midlevel clouds remains to be seen, but zones of surface convergence may be a focus for some development, and also reasonably steep lapse rates suggested in the models above the LCL up to the altostratus cloud deck would allow cumulus to attain decent depth. The number of convection-allowing models among the 00Z suite producing scattered light rain showers between afternoon and dusk, perhaps even an isolated downpour, seemed too great to ignore. On the plus side, the chance of lightning does appear to be rather small given the CAPE and better updraft strength would be in the lower/warmer part of the cloud. You shouldn`t be rained out of your holiday outdoor activities unless you are really unlucky. - Scattered Thunderstorms Friday, Low Confidence of Severe Potential A round or two of scattered showers/storms is likely between Friday morning and afternoon, but uncertainty in the evolution of the responsible low-pressure system makes it difficult to ascertain if there is much threat of severe weather. A broad synoptic-scale trough over the northern Plains today should deepen a little as it progresses east, closing into a 500 mb low over Minnesota tonight. By 8 AM Friday, a surface to 850 mb low should be located somewhere over Wisconsin with a quasi-stationary or warm front extending east across Lower Michigan. Models differ greatly in the position and strength of the surface low Friday morning, and the strength of the pressure field near the frontal triple point over Lower Michigan by mid day. The surface to 850 mb wind fields around this low also vary greatly among each model, resulting in substantial differences in vertical shear profiles/hodographs, some of which would support severe weather, including a tornado, and some of which would not. This will have to be monitored closely, though, given the reasonable magnitude of 0-6 km bulk shear, 0-3 km CAPE, and total MLCAPE advertised by most models. - Showers Possible Saturday, Unsettled Pattern Continues Next Week As we enter this holiday weekend there remains quite a bit of uncertainty headed in the mid to long range pattern. The upper level jet stream should remain over the region Saturday through Sunday. This should provide enough dynamics that, coupled with mid to low level moisture and with the instability brought by a positively tilted trough, should allow for a chance of showers Saturday. Any showers should be scattered and light QPF associated given lack of deep moisture. The operational models and ensembles diverge on a signal into early next week, however there remains chances for showers and storms especially Monday into Tuesday. There remains a low confidence in timing. Max Temperatures look to remain below normal through the upcoming week. This should make the week quite comfortable, depending on humidity this week. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 150 PM EDT Thu Jul 4 2024 VFR conditions will prevail at all TAF sites through the valid period. Mid and high level cloudiness are expected to prevail today and tonight, and a few light showers will be possible at AZO/BTL/JXN through 00Z. Winds will remain light and variable through tonight. Attention will then turn to a risk for showers and thunderstorms on Friday, thanks to daytime heating and a surface trough approaching the region from the west. A few of these storms may produce hail and gusty winds, particularly west of the US-131 corridor during the early afternoon. && .MARINE... Issued at 350 AM EDT Thu Jul 4 2024 Light winds and waves 1 foot or less for Independence Day. On Friday, scattered thunderstorms could cross Lake Michigan between morning and afternoon. Somewhat stronger winds could build up waves and perhaps make the swim risk moderate, though there is more variability than typical among the models regarding the exact wind speeds and direction through the day. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ UPDATE...Duke DISCUSSION...CAS/Ceru AVIATION...Smith MARINE...CAS