-- Discussion containing changed information from previous version are highlighted. --
937
FXUS63 KGRR 041143
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
743 AM EDT Tue Jun 4 2024

.KEY MESSAGES...

- Scattered thunderstorm chances today through Wednesday

- Cooling down for the end of the week and possibly through the
  weekend

&&

.UPDATE...
Issued at 723 AM EDT Tue Jun 4 2024

Showers and even a few thunderstorms have been tracking through
the region north of Muskegon early this morning. Based on radar
trends, we did raise the POPs and QPF for that region and updated
the zones and nearshore forecast.

&&

.DISCUSSION...
Issued at 336 AM EDT Tue Jun 4 2024

- Scattered thunderstorm chances today through Wednesday

Three main atmospheric features will provide chances for isolated
storms today and tonight and a likelihood of at least scattered
thunderstorms early morning to mid afternoon Wednesday.

First, affecting west-central Lower Michigan today, is ongoing
elevated convection in WI associated with narrow 850 mb moisture
transport across IL/WI and a midlevel effective front extending
west of a remnant mesolow near Muskegon county. Convection has not
been making it into Michigan yet as 850 mb moisture transport is
much weaker and instability is low. CAMs suggest the focus of
isolated shower/thunderstorm development during the morning will
shift northeast into west-central/northwest Lower Michigan, as a
transition to diurnal surface-based mixing with southerly winds
occurs, and broad upper-level ridging amplifies over the lower
Great Lakes which will lead to warming midlevels and increasing
convective inhibition with southeastward extent. Any storms that
develop will exist in a weakly sheared environment. Today should
be a very warm and somewhat humid day for many, easily the warmest
day until at least mid June.

Next, affecting west/southwest Lower Michigan late this evening
through tonight, a compact 500 mb low over Missouri will move
toward southern Lake Michigan tonight while weakening as it
becomes incorporated into the flow ahead of the main feature, an
advancing Upper Midwest trough. Southerly flow at 850-700 mb will
increase during the night with some moisture transport, though our
area will be in the divergent part of the developing speed max.
HREF MUCAPE mean of 250-500 J/kg may support some convection,
though not all member models develop storms before the early
morning hours of Wednesday.

The highest chance of scattered thunderstorms for the whole area
will be Wednesday morning through mid afternoon, as a sharp upper-
level trough over the Upper Midwest approaches and a surface cold
front passes through. The timing of this, relatively early in the
day, does mellow the threat, but growing surface- based CAPE to
500-1000 J/kg and 25 knots of 0-3 km shear by mid day could
support a marginal severe wind threat for mid-southern to
southeastern Michigan (including Lansing-Marshall areas and east).
Briefly heavy rainfall rates may occur as deeper environmental
moisture will be present through the potential convective layer.

- Cooling down for the end of the week and possibly through the
weekend

Persistent cold air advection is advertised by the models through
Friday as low pressure stalls out over northern Lake
Huron/southeast Ontario. Ensemble temperature trends are lower in
most models for Thursday and Friday and the mean max temp values
support several degrees below normal for these two days. With
several mid level vort maxes arriving Wednesday night and into
Thursday, we could see at least scattered showers.

As we go through the weekend, the models are showing some spread
with the duration and strength of the stalled out low over the
Great Lakes Region. The ECMWF has mid level heights rising as we
go through the weekend, but the GFS actually deepens the low and
sinks it southward into Lower MI. The Canadian is sort of in
between the two. We will continue to feature below normal
temperatures as that is supported by the latest ensemble
forecasts.

With an unseasonably deep low over the northern Great Lakes
Region and a tight pressure gradient on the south side of this
feature, gusty winds can be expected at times into the weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
-- Changed Discussion --
Issued at 735 AM EDT Tue Jun 4 2024

Radar trends show the convection just north of KMKG still
persisting. Overall the track has been east northeast with those
cells so we are still expecting the thunder risk to remain north
of KMKG over the next hour or two. After that, much of the area is
expected to be dry through the day with only an isolated
shower/storm during peak heating times this afternoon. Tonight
though, the moisture increases and that will lead to showers
moving in. It`s now looking like the ceilings will lower down into
the MVFR category later tonight and it may get close to IFR by
this time Wednesday. There will be some instability around later
tonight so an isolated storm could develop but we were not
confident enough on the coverage warranting adding thunderstorms
to the forecast at this time.
-- End Changed Discussion --

&&

.MARINE...
Issued at 336 AM EDT Tue Jun 4 2024

A south breeze today will build 1 to 3 ft waves, highest near the
Sable Points. Prevailing winds and waves will provide a low swim
risk today, then low to moderate Wed. Swim risk will be moderate
or approaching high Thu-Fri 0amid west winds with cooler air. Today
through Wed morning, other hazards to boaters and beachgoers
would be from isolated to scattered thunderstorms.

&&

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

&&

$$

UPDATE...MJS
DISCUSSION...MJS/CAS
AVIATION...MJS
MARINE...CAS