Portage, Michigan 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

548
FXUS63 KGRR 160459
AFDGRR

Area Forecast Discussion National Weather Service Grand Rapids MI
Issued by National Weather Service Detroit/Pontiac MI Issued at 1258
AM EDT Fri May 16 2025

.KEY MESSAGES...

- Severe Thunderstorms this Evening

- Another Round of Strong to Severe Storms Friday Evening

- Windy Saturday, Cooler into Next Week

&&

.DISCUSSION...
Issued at 331 PM EDT Thu May 15 2025

- Severe Thunderstorms this Evening

Tonight between 8pm and 2am a round of thunderstorms is expected to
move through the area bringing with it the potential for damaging
winds, large hail, and tornadoes.

The main source of lift for these storms will be with the occluded
front with upper level divergence aloft along with some convergence
in the wind fields at 850-700mb. Instability has been gradually
building through the day with Mixed Layer CAPE values currently in
the 1500 to 2000 J/kg range across much of the area.
Thunderstorms are beginning to develop along the front in Minnesota
and are expected to expand south along the front as it moves east
this afternoon and into this evening. 0-1km storm relative helicity
values peak ahead of the front in the 100 to 150 m2/s2 range and
around 200 m2/s2 from 0-3km. Paired with 30 to 40 knots of effective
shear and Most Unstable CAPE values around 2000 J/kg this will be a
prime environment for rotating updrafts, hence the increased hail
and tornado risk.

There is some variability in the storm mode as it heads into western
Michigan. Individual storms are expected to develop in Wisconsin and
may become clustered or form a line as they move across the lake
into western Michigan. If storms take a more linear formation, then
winds and quick spin up tornadoes would become the main concern. Any
isolated or clustered supercells would be more likely to produce all
hazards, especially hail.

With the risk for severe storms late this evening, make sure you
have a way to receive warnings and a plan in place to take shelter
when warnings are issued.

- Another Round of Strong Friday Evening

Most of Friday will be dry with instability building through the
day. Steep lapse rates and ample dry air causing an inverted V in
soundings will set the stage for an environment favorable for
downbursts. Lift provided by the positive vorticity advection
swinging around the 500mb low will move in around 8pm Friday causing
showers and thunderstorms to develop. Similar to Thursday evening
storms taper off shortly after midnight.

- Windy Saturday, Cooler into Next Week

The same low mentioned in the previous section will gradually track
across the northern Great Lakes Saturday. Lingering showers
associated with the low should clear early Saturday afternoon, but
as the rain and lower clouds clear, gusty westerly winds will
develop. Some gusts to 40 mph are possible during the afternoon.
Calmer conditions move in for the remainder of the weekend with
highs in the 60s and lows in the 40s and 50s.

&&

.AVIATION /06Z TAFS SATURDAY THROUGH 06Z SUNDAY/...
Issued at 1258 AM EDT Fri May 16 2025

Strong large scale subsidence in the wake of the thunderstorm
complex will support mainly clear skies through the duration of the
night and Friday morning. Daytime heating will boost mixing depths
resulting in south winds gusting over 20 knots at times by Friday
afternoon. The passage of an upper level short wave and late day
instability will support a good chance for thunderstors Friday
eveing, primaily in the 01Z to 06Z time frame.

&&

.MARINE...
Issued at 331 PM EDT Thu May 15 2025

A round of thunderstorms is expected between 8 and 10pm with the
potential for large hail, damaging winds, and waterspouts. Rough
water is expected with this round of storms with southwest winds
moving in behind the line. A second round of storms moves in Friday
night with the potential for storms to be strong with gusty winds
being the primary concern. The low pressure system responsible for
these rounds of storms will move through the northern Great Lakes
Saturday bringing with it a tight pressure gradient and gusty
Westerly winds. A Small Craft Advisory is in effect for Friday
through Saturday evening due to winds and high waves.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 11 AM Friday to 2 AM EDT Sunday
for      LMZ844>849.

&&

$$

DISCUSSION...RAH
AVIATION...SC
MARINE...RAH