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000
FXUS63 KGRR 171150
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
750 AM EDT Wed Apr 17 2024

.KEY MESSAGES...

- Scattered potential strong storms today

- Period of Rain Thursday Night

- Cooling off for the Weekend

&&

.DISCUSSION...
Issued at 342 AM EDT Wed Apr 17 2024

- Scattered potentially strong storms today

Latest Nam3km seems to have a pretty good handle on current radar
activity, which would suggest two periods of storms today. The
first begins now with the storms over the lake and then another
round by 18z.

Radar shows scattered convection developing from near LDM
southwest across the lake near the warm front. Quite a bit of
lightning occurring over the lake, but storms have generally be
below severe levels to the west. The storms were moving northeast,
which should continue given boundary layer flow. An area of
steeper mid level lapse rates near 7c/km was noted over east
central WI and that area is progd to move northeast toward LDM
withing the next few hours.

HRRR soundings show a pronounced inversion near 900 mb with
limited instability north of the warm front. Thus, we don`t
anticipate the convection to bring down the stronger winds aloft.

This area of convection will move end by 12z leaving several hours
of dry weather, especially across the southern cwa; regional
radar shows a dearth of echos over northern IL/northwest IN at
this time.

Instability will ramp up later this morning, especially over the
southern cwa behind the warm front. HRRR/Nam3km/HREF show another
round of convection developing around noon over the lake. Forecast
soundings shows SBCAPE in the 1500-2000 j/kg range and the
aforementioned models point toward a broken line of storms
developing along the lake shore around 16z and then moving east,
exiting the cwa by late afternoon. Given the strong shear coupled
with the frontal lift and ambient instability, a few of the
storms could be strong to severe with strong winds the main
threat.

Tonight will be mostly dry as the upper trough moves east, but the
next system developing in the mid Missouri Valley will already be
taking aim on the Great Lakes, so cloudy skies are expected
Thursday and an afternoon shower isn`t out of the question as an
upper low noses into the region.

- Period of Rain Thursday Night

As a frontal zone tracks in from the west later Thursday and into
Thursday night a wave develops along it, just south of the CWA. A
southerly low level jet over the OH Valley will advect moisture into
the frontal zone. PWAT`s climb up to around an inch as the front and
associated wave move through. There is even some weak elevated
instability. Ensemble QPF values have been trending up. As a result,
we will increase the POPs, higher QPF and feature a low risk for a
few storms.

- Cooling off for the Weekend

Colder air advects in from the northwest behind the departing front
Friday into Saturday.  Temps at 850 mb lower down to -7 to -9 deg C
on Saturday.  That will likely keep the daytime highs in the 40`s.
Then with a potential for some clearing Saturday night and starting
off the evening only in the 40`s, we will likely see below freezing
temperatures for Sunday morning. The low level thermal trough will
be east of the CWA Sunday so it won`t be quite as cool as Saturday
but still well below normal.  However with a surface high over the
area Sunday night, good radiational cooling is expected.  Thus a
risk for overnight lows to drop down to near freezing  will exist,
especially for inland sites.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 741 AM EDT Wed Apr 17 2024

A cold front will be pushing eastward through the TAF sites later
this morning and through the early afternoon hours. There is
higher than normal confidence that we will be seeing at least
scattered thunderstorms develop along the front as it tracks
through. These were included in the 12z issuance. By 20z the
majority of the activity should be east of KLAN and KJXN.

There is a southerly low level jet overhead to start the period.
This feature may lead to low level wind shear until the surface
winds swing around to the southwest and increase. Occasional gusts
over 25 knots are looking likely by late morning and into the
afternoon. Any stronger storm could yield even higher winds.

Ahead of the front there was an area of MVFR clouds. Those lower
clouds are expected to move into the TAF sites over the next
couple of hours.

&&

.MARINE...
Issued at 342 AM EDT Wed Apr 17 2024

The Gale Warning will end at 6 AM and be replaced by a Small Craft
Advisory that will run through tonight. The Small Craft Advisory
south of Grand Haven will be extended through tonight as well as
winds behind the cold front will create hazardous conditions for
small craft.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for LMZ844>849.

&&

$$

DISCUSSION...MJS/04
AVIATION...MJS
MARINE...04