Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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271 FXUS63 KGRR 191700 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 100 PM EDT Thu Sep 19 2024 .KEY MESSAGES... - Mostly sunny and hot today, chance of storms Friday - Cooler next week with chances for showers or storms, most likely Sunday night into Monday && .DISCUSSION... Issued at 332 AM EDT Thu Sep 19 2024 - Mostly sunny and hot today, chance of storms Friday One more day of abundant sunshine and hot temperatures as the blocking pattern begins to break down. Mixing will bring down drier air aloft again today which will keep the humidity in check. Highs in the mid 80s are expected again. Increasing clouds tonight will limit how much temperatures fall. I`d expect lowest temperatures...upper 50s...to occur over the eastern cwa. A short wave over North Dakota will lend some support for the surface low over the upper MS Valley today as it tries to push a weak cold front eastward. Given the downward diurnal instability trend late tonight, anything that moves across the lake will likely be showers around sunrise. Instability will rise during the day Friday and SBCAPE values increase to around 800-1200 j/kg...highest over the southern cwa. As the frontal boundary moves eastward, it will encounter the higher instability and a broken line of showers/storms may develop. Bulk shear values rise to 35-40 kts across the far southern cwa after 18z giving rise to the possibility of some organization and potentially a few strong/severe storms. CAMs forecast soundings Friday afternoon show the classic inverted V signature suggesting that downdraft winds will be the greatest threat. - Cooler next week with chances for showers or storms, most likely Sunday night into Monday The spread among the ensembles remains rather large with varied plausible outcomes in the day-to-day weather next week, but overall it is a pattern that supports daytime highs more often in the 70s and occasional showers or thunderstorms. The complex interaction between a northern jet stream wave pattern which is fast-flowing, amplified, and progressive against a southern stream which is showing a tendency for blocking may be a culprit behind the lesser predictability toward the middle of next week. There is still medium confidence for rain, potentially over 1 inch in spots and the best chance of rain in quite a while, late Sunday into Monday. This is as an upper-level low over California and the Desert Southwest moves toward the Midwest while becoming more incorporated into the northern stream. Northward transport of moisture in the lower atmosphere ahead of this low and a frontal zone over the region would support showers, and an assortment of ensemble members have enough instability to support a chance of thunderstorms. The 00Z run of the ECMWF ensemble provides a 40-60 percent chance of 0.50+ inch rain totals in 24 hours between Sunday evening and Monday evening. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 100 PM EDT Thu Sep 19 2024 Much of this forecast period will see a continuation of the conditions over the past few days with mostly sunny/mostly clear skies and lighter winds. We will see some rain chances later in the period, with some potential MVFR conditions with the rain chances. Once again, mainly high clouds in place at the beginning of the forecast period. Cumulus clouds are starting to form, but will remain VFR. The cumulus clouds will be more concentrated for the eastern terminals. Those cumulus clouds will once again dissipate after sunset. Fog is not likely tonight with a light wind around 4-5 kts remaining at the sfc, and better winds 1-2k ft agl. We will see some mid level clouds start to move in just before sunrise Friday at KMKG and spreading east. Can not rule out a sprinkle with this, but the very dry lower levels should keep the sfc dry. The better chance of showers and isolated thunder will arrive at the western sites after 16z or so. The eastern sites will not see this potential until after the end of this period. Some MVFR will be likely with the showers and thunder, with some isolated IFR possible also. Will stick with the MVFR wording for now, and fine tune the forecast in subsequent issuances. && .MARINE... Issued at 332 AM EDT Thu Sep 19 2024 A weak pressure gradient will result in onshore winds during the afternoon for the next few days. A weakening cold front may push a broken line of showers and/or storms across the lake late tonight and Friday morning. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...04/CAS AVIATION...NJJ MARINE...04