Severe Thunderstorm Warning 74, NWS Grand Rapids, MI

Area Forecast Discussion
National Weather Service Grand Rapids MI
641 AM EDT Tue Aug 6 2024

.KEY MESSAGES...

- Showers and Storms Likely Today

- Dry Weather Favored Wednesday through Friday

- Scattered Showers Possible Over the Weekend

&&

.DISCUSSION...
Issued at 219 AM EDT Tue Aug 6 2024

- Showers and Storms Likely Today

Scattered showers and storms are ongoing this morning with more
widespread activity arriving in the next few hours as a decaying
squall line exits Wisconsin fed by the low level jet. The primary
concern is locally heavy rainfall. PWAT values of 1.5-2 inches
this morning and enhanced forcing from a mid-level FGEN axis are
the meteorological drivers. Currently the heaviest rainfall is
forecast south of M20. Widespread rainfall of 1-2 inches is
expected with locally higher amounts possible in training storms
or areas of better rainfall efficiency. The HREF LPMM supports
this with isolated pockets of 3+ inches across the CWA. In terms
of flooding, rivers should remain below flood stage though a few
more responsive streams may make a run at action stage. Overall,
lower soil moisture content suggests areal flooding will not be
widespread. However localized flooding is possible and flood
advisories may be needed for areas with the heaviest rainfall,
especially in our urban areas.

A low threat of gusty winds exists given high PWATs and deep layer
shear of 30+ knots. Main concern is near to shortly after sunrise as
the low-level inversion around 2kft weakens, mainly near I94 where
around 1000 joules of CAPE builds in. Overall the threat is low
given the elevated nature of any storms, but not zero so we'll be
watching these storms as they move through.

Rain chances slowly diminish after 18z Tuesday and dry conditions
are expected for Wednesday as high pressure moves into the area.

- Dry Weather Favored Wednesday through Friday

Still favoring a dry forecast for the mid to late week period. Dry
northeastly low level flow will be in place Wednesday into Thursday.
Highs in the upper 70s are expected along with low humidity. Medium
range guidance is pushing a cold front through the region Thursday
night into Friday, though temperatures won't immediately feel much
cooler yet. Currently NBM POPs are only about 10% with this front,
so will keep mention of rain out of the forecast for now.

- Scattered Showers Possible Over the Weekend

Will introduce some scattered showers into the forecast for the
weekend, mainly from I-96 to the north. ECE/GEFS ensemble mean 500
mb heights over the weekend show deep upper troughing along with
cooler than average 850 mb temperatures as low as 8C. ECE
probability for 0.01" or more, a good proxy for measurable rain, is
higher than one would expect at 60-90% for areas along and west of
US 131 (and north of I-96) Saturday into Sunday. The 24hr QPF
chiclet chart for LDM shows the vast majority of members producing
light showers. While not a washout, the likelihood for some
scattered rain showers in a cold air advection regime seems to be
going up a bit for the weekend period. NBM POPs are 20-30% at this
time, which seems reasonable, though higher POPs may eventually be
introduced as medium range guidance clarifies the depth of upper
troughing. Highs will likely max out in the 70-75 range, possibly
upper 60s along the lakeshore.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 635 AM EDT Tue Aug 6 2024

Overall forecast remains the same, with mainly minor refinement of
timing based on radar trends. Given the robust cell over southern
Lake Michigan have added TSRA at BTL and AZO. Thunderstorms will
impact all terminals through 15-16z. Gusts of 20-30 knots are
possible in heavier storms, and gusts above 40 knots cannot be
ruled out if the storms over the lake hold together. Cigs will
fall to IFR with IFR and lower visbys as these thunderstorms roll
through. Cigs will be slow to improve, not climbing until late
afternoon into the evening.

&&

.MARINE...
Issued at 219 AM EDT Tue Aug 6 2024

A weakening squall line will move across the waters over the next
few hours. Cloud to water lightning is likely and locally higher
winds and waves are possible. Increased easterly winds are likely
behind this line with gusts to around 25 knots. Will be issuing a
Small Craft Advisory for all nearshore zones from 09z today to 06z
Wednesday for the gusts. Easterly winds keep flow offshore so waves
are not expected to be hazardous to small craft/no Beach Hazards
Statement is needed. After this, hazards not expected for the rest
of the work week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 2 AM EDT Wednesday for LMZ844>849.

&&

DISCUSSION...Hoving/Thomas
AVIATION...Thomas
MARINE...Thomas

Hazardous Weather Outlook...CORRECTED
National Weather Service Grand Rapids MI
235 AM EDT Tue Aug 6 2024

MIZ037>040-043>046-050>052-056>059-064>067-071>074-070645-
Mason-Lake-Osceola-Clare-Oceana-Newaygo-Mecosta-Isabella-Muskegon-
Montcalm-Gratiot-Ottawa-Kent-Ionia-Clinton-Allegan-Barry-Eaton-
Ingham-Van Buren-Kalamazoo-Calhoun-Jackson-
235 AM EDT Tue Aug 6 2024

This hazardous weather outlook is for portions of southwest Lower
Michigan.

.DAY ONE...Today and Tonight

Thunderstorms are expected through this afternoon. Widespread
rainfall of 1-2 inches is likely with locally higher totals
possible. Nuisance flooding is possible in spots. A low chance of
damaging wind gusts exists near and south of I94 near sunrise
through early afternoon.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Hazardous weather is not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed through this afternoon.

LMZ844>849-070645-
St Joseph to South Haven MI-South Haven to Holland MI-
Holland to Grand Haven MI-Grand Haven to Whitehall MI-
Whitehall to Pentwater MI-Pentwater to Manistee MI-
235 AM EDT Tue Aug 6 2024

This hazardous weather outlook is for portions of southwest Lower
Michigan.

.DAY ONE...Today and Tonight

Thunderstorms with heavy downpours and frequent cloud to water
lightning are likely through this afternoon. Locally higher winds
and waves are possible with any storms.

Winds will be hazardous to small craft from early
Tuesday morning to early Wednesday morning.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Hazardous weather is not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed through this afternoon.