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000
FXUS63 KGRR 181940
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
340 PM EDT Thu Apr 18 2024

.KEY MESSAGES...

- Rain tonight, and slight chance lightning in southern Mich.

- Cooler Friday night, near freezing

- Multiple chances of frost/freeze next week

- Only chance of rain Monday night-Tuesday

&&

.DISCUSSION...
Issued at 340 PM EDT Thu Apr 18 2024

- Rain tonight, and slight chance lightning in southern Mich.

Virga and areas of light rain this afternoon will give way to a
period of rain overnight as a wave of low pressure along a cold
front tracks into the southern Great Lakes region. HREF and NBM QPF
indicate a good potential for at least 0.25 inches of rainfall, with
a narrower stripe of 0.50 to 0.75 inches possible from southwest
Michigan to the Saginaw valley. The HRRR and NAM-Nest soundings
around I-94 this evening have MUCAPE greater than 100 J/kg
originating from moist parcels at 800-750 mb underneath fairly steep
lapse rates that extend up to 500 mb (-20 C). So this elevated
convection in southern Michigan could produce a few lightning
flashes and rumbles of thunder, with the threat ramping up after
sunset.

- Cooler Friday night, near freezing

With increasing vulnerability to temperatures below 32 degrees among
blossoming vegetation, Friday night will kick off about a weeklong
pattern with threats of frost/freeze conditions (not every night of
the next seven, but several of them). Temperatures are expected to
drop into the mid 30s Friday night as progressively colder air is
transported into the area among westerly winds. However, a pressure
gradient will still exist given the center of high pressure being
back in the northern Plains, so this may keep the surface winds up
enough to prevent a sharper temperature inversion from becoming
established near ground level.

- Multiple chances of frost/freeze next week

One concern that is growing for the colder spell this weekend, and
cooler temperatures behind the Tuesday system are some frost/freeze
possibilities. Recent coordination with agricultural interests
including Michigan State extension, reveals that fruit trees are
entering a vulnerable period in their growth for most of the area,
except along the U.S.-10 corridor. The beginning of the growing
season has been coordinated with surrounding offices.

We are looking at multiple nights where temperatures will drop to
and a little below freezing. The nights with these concerns include
Saturday and Sunday nights, and Tuesday and Wednesday nights. We are
way too far out in time right now to fine tune details. This will be
something we will continue to monitor and issue headlines for if
needed as we come up to those conditions.

- Only chance of rain Monday night-Tuesday

The weather is looking very quiet for the area from this weekend
through next Thursday. The only caveat to this will be a system that
is forecast to move through the area from late Monday into Tuesday.

Behind the system that moves through on Friday, the area will be
under the influence of NW flow aloft downstream of the upper ridge
expected to be parked for a few days over the Plains. We will be
under increasing subsidence, with a cool flow from the North and
Northwest into early next week. This period should be rather dry
also, with a good deal of sunshine during the days. This may start
to create some Fire Weather concerns by later in the weekend after a
couple of dry days.

The one system that looks to affect the area after Friday`s system
will be a short wave that moves through late Monday through Tuesday.
This energy will have its origins from the upper low off of the
Western U.S. coast. It will tamp down the weak ridge over the
Plains, and move through the area. Right now, it does look like it
will have a sufficient low level feed of moisture from the Gulf to
provide good rain chances for the area at that time. I would expect
the rain amounts to end up a little less than advertised at the
moment with a lot of dry air that is expected to have to be
overcome.

Once the system on Tuesday moves out, we will see the upper ridge
over the Plains become re-established as a large upper low complex
moves in over the western portion of the CONUS. This will set us
back up with drier conditions as heights build for the rest of the
week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 149 PM EDT Thu Apr 18 2024

This forecast period can be summarized by being bookended by VFR
at the beginning and ending for the period, with IFR in the middle
with rain.

We are mainly seeing mid clouds expand and thicken up over the
area this afternoon. Some sprinkles have been observed at KMKG
coming out of that mid deck. This scenario looks to continue
through the rest of the afternoon and into the early evening
hours.

We will then see rain become more widespread toward 00z this
evening at KMKG, and then spread to the east. There will be a
couple of hours of transition as VFR conditions go to IFR with low
clouds moving in and reductions in visibilities. This looks to be
a generally 6-8 hour period, before rain tapers off.

Some low clouds will linger after the rain ends, and then
clearing should take place toward or just after daybreak Friday.
Winds will become a bit gusty from the NW Friday morning, with
gusts up to 30 knots expected.

&&

.MARINE...
Issued at 340 PM EDT Thu Apr 18 2024

Hazardous waves for small craft will likely build up between South
Haven and Whitehall early Friday morning and expand farther north
and south for at least a portion of the day. Will extend the small
craft advisory northward to Manistee and take it into Friday evening
based on latest model guidance. There may be a slight lull in the
conditions Friday night, mainly near South Haven, but the small
craft advisory may also need to be extended through the day
Saturday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 6 AM to 8 PM EDT Friday for LMZ844>849.

&&

$$

DISCUSSION...CAS/NJJ
AVIATION...NJJ
MARINE...CAS