193
FXUS63 KGRR 161740
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
140 PM EDT Wed Apr 16 2025

.KEY MESSAGES...

- Dry Wednesday/Thursday

- Potential for Severe Thunderstorms Friday

- Weekend weather unclear; could feature more rain

&&

.DISCUSSION...
Issued at 345 AM EDT Wed Apr 16 2025

- Dry Wednesday/Thursday

Early morning clouds are expected to scatter and evaporate as large
scale ridging and subsiding air takes hold. The top of the diurnal
mixed boundary layer will be tapping into very dry air so afternoon
relative humidity away from Lake Michigan should drop toward or
below 30 percent, particularly in mid/central Michigan. Fire
concerns are elevated today and Thursday given paltry rain amounts
in some areas on Tuesday and overall fairly dry weather the past two
weeks, with a gentle northwest breeze today and moderate south-
southeast breeze tomorrow. The wind direction tomorrow will keep
many areas close to Lake Michigan warmer and drier than usual.

- Potential for Severe Thunderstorms Friday

The threat for storms appears focused in two parts on Friday: early
morning and late afternoon/evening.

The arrival of warmer air and better moisture around 850-700 mb
Thursday night ascending an elevated warm frontal surface may
produce showers during Thu evening or early in the night, then
steepening 700-500 mb lapse rates arriving very late Thursday night
into early morning Friday may yield up to 1000 J/kg MUCAPE to
maintain any convection that may be moving our way from Wisconsin.
Forecast soundings around 5 AM Fri suggest moderate CAPE/buoyancy
through the -10 to -30 C layer and about 30-40 knots of effective
shear between updraft base and top. This does appear to be an
isolated hail threat, possibly severe depending on what
organizational mode the convection takes on. If storms do occur
early Fri morning, they will based on unstable air aloft (elevated)
and therefore a cold Lake Michigan is not a factor.

While still outside the window of most convection-allowing short-
term models, many runs of the global models and ensembles have been
limiting convection during much of the day Fri under capping from a
warm elevated mixed layer. Southwesterly winds should help advance a
a surface warm front into Lower Michigan, with temperatures climbing
into the 70s favored in the southern half of Lwr MI. Along the slow-
moving surface cold frontal system draped from western Ontario to
the central Plains, a surface low-pressure wave may move from
eastern Iowa toward Wisconsin or northern Illinois into Michigan
late in the day, providing some impetus for convection under a
strengthening 500 mb jet. Scattered elevated storms could develop in
northern Michigan, while surface-based storms are more favored in
southern parts of the state with around 1500 J/kg of CAPE
anticipated. Plenty of deep-layer shear will still be present,
supporting the risk of hail and damaging winds.

- Weekend weather unclear; could feature more rain

Previous discussion... Weather for the weekend is low confidence at
this time due to uncertainty in the sfc front position. It looks
like it may remain close enough to MI to keep clouds and a shower
risk going into Saturday, especially in srn sections. Some
indication the front may even come back north in the second half of
the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 140 PM EDT Wed Apr 16 2025

VFR weather prevailing through 18Z Thursday. Northwest winds 10 to
20 kts becoming light and variable tonight then increasing out of
the southeast to 10 to 20 kts by Noon Thursday.

&&

.MARINE...
Issued at 345 AM EDT Wed Apr 16 2025

Surface high pressure ridge will move over Lake Michigan by mid day,
so the hazardous winds and waves for small craft are expected to
subside by late morning. The high pressure and strong heating of the
land will sustain a northwesterly 10-15 knot lake breeze for much of
the day.

Winds on Thursday will be mainly offshore/southeasterly, and gusts
off the land may peak around 25 knots. Hazardous waves for small
craft will start to build late Thursday and persist into Friday.

A thunderstorm could cross Lake Michigan early Friday morning, and
there is a chance of it dropping hail. Additional storms are likely
to develop in the region later in the day Friday, though it is
questionable to what extent the nearshore waters will be affected.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Meade/CAS
AVIATION...Meade
MARINE...CAS