000
FXUS63 KGRR 201804
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
204 PM EDT Sat Apr 20 2024

.KEY MESSAGES...

- Cold Tonight and Breezy Sunday

- Showers/Isolated Storms Tuesday

- Frost/Freeze Potential Wednesday night

&&

.UPDATE...
Issued at 900 AM EDT Sat Apr 20 2024

Shortwave currently passing through has touched off some light
snow showers or flurries north of I-96, so have updated the
forecast to reflect that. The shortwave will move off by early
afternoon and will end the flurry threat by 18Z however
considerable clouds may linger, especially east of Highway 131,
due to H8 cold pool/thermal trough of -8C overhead.

&&

.DISCUSSION...
Issued at 307 AM EDT Sat Apr 20 2024

- Cold Tonight and Breezy Sunday

The weekend will be dominated by cool weather as cyclonic flow
within upper-level troughing dominates the forecast. The current
pocket of mid-level cloud associated with a 500mb shortwave will
exit this morning. However, skies will be partly sunny today given
an expected BKN diurnal cumulus field this afternoon.

Attention then turns to tonight where a risk of a frost/freeze
exists as temperatures fall into the low to mid 30s and winds calm
to near or below 5 mph. In coordination with DTX and IWX, have
elected to defer possible frost/freeze headline decisions to the day
shift.Reason being (like the HRRR/RAP) which seem to be initializing
well on cloud cover tonight suggest we may not clear out which would
be needed for temps to truly fall. Given this uncertainty in cloud
cover headlines will not be issued with this forecast package.

Sunday will then be breezy with wind gusts around 25 mph. Given
several days of a dry airmass and RH values forecast to fall into
the 30s, this leads to fire weather concerns given the available
fuels. Conditions Sunday may approach red flag warning criteria as a
result. Otherwise look for plenty of sunshine with highs rebounding
into the 50s as the upper-level cold pool begins to shift east.

- Showers/Isolated Storms late Mon Night into Tuesday

Models continue to advertise a strong mid level shortwave digging
into the Great Lakes region during this time.  PWAT values climb up
close to an inch as this wave moves in. Models do show two periods
of deeper moisture. The first is later Monday night into Tuesday
morning. That is when the nose of a 50 knot low level jet arrives.
This feature, combined with falling mid level heights could lead to
some shower activity.  The next round of precipitation occurs in the
late afternoon into early evening hours.  This is when the mid level
trough axis arrives.  Models are generating generally weak
instability as this feature moves in.  The GFS shows the most
instability with values over 500 J/kg.  Deep layer shear will be
diminishing as this wave moves in but mid level lapse rates will
be steepening up.  Thus if we do see some convection, small hail may
be a concern. We will maintain the shower/isolated storm risk for
this period.

- Frost/Freeze Potential Wednesday night

On the backside of this departing wave, another surge of cool air
drops down from Canada.  This time, the winds will be from the
north northeast going into Wednesday night.  We could end up with
clear/clearing skies and little protection from the lakes warmth
given this flow.  It appears this will be a cold night for much of
the region with a high potential for sub freezing temperatures, even
for the lakeshore region. Ensemble temperature trends are a little
lower with this model run as well. We will maintain a forecast that
features much of the area dropping into the upper 20s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 203 PM EDT Sat Apr 20 2024

Widespread diurnal stratocumulus has developed today with cold
temperatures and troughing aloft. Bases in general are VFR, but
there is some MVFR ceilings near Lake Michigan at times early this
afternoon. We feel VFR will be the dominant category the next 24
hours. Clouds will be fairly widespread the rest of this afternoon
and most of tonight. Clouds will initially be stratocumulus but
will be aided and backfilled by clouds moving in from Wisconsin.
All of these clouds will have bases in the 3500 feet to 5000 feet.
We expect at least partial if not complete clearing late tonight
after 10Z to 12Z.

Winds will be gusty through sunset from the west at 10 to 20
knots. Speeds will become light around 5 knots overnight before
increasing again on Sunday after 14Z.





&&

.MARINE...
Issued at 307 AM EDT Sat Apr 20 2024

The current small craft advisory is verifying well with numerous
shoreline stations and buoys reporting small craft advisory
conditions. Conditions hazardous to small craft are expected through
late afternoon so will leave the current advisory as is. Latest
guidance trends towards conditions on Sunday remaining below small
craft advisory levels. However, increasing winds and waves Monday
into Wednesday may create conditions that are hazardous for small
craft.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for LMZ844>849.

&&

$$

UPDATE...Meade
DISCUSSION...MJS/Thomas
AVIATION...Duke
MARINE...Thomas