000
FXUS63 KGRR 170541
AFDGRR

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

.KEY MESSAGES...

- Chance for Severe Weather Wednesday

- Light Rain Possible Thursday Night

- Cooling Trend With Dry Weather into the Weekend

&&

.UPDATE...
Issued at 1003 PM EDT Tue Apr 16 2024

Wind gusts so far have not exceeded 40 mph (ASOS/AWOS) in the
Wind Advisory area with main limiting factor being low level
inversion/lack of deeper mixing. However the strongest winds are
still progged to occur 03Z-06Z and just had a local 47 mph gust
report at East Grand Rapids, so will keep the advisory going for
now. Showers have arrived but only trace amounts so far due to low
level dry air. The more widespread/measurable rainfall and sct`d
thunder arrives overnight once the current convection over WI/IL
gets here (albeit in a weakening mode).

&&

.DISCUSSION...
Issued at 346 PM EDT Tue Apr 16 2024

- Chance for Severe Weather Wednesday

The main focus of the forecast will be on the threat for severe
weather tomorrow.

Clouds will be on the increase this evening with showers breaking
out most likely after 800pm. Showers and a few embedded
thunderstorms are expected tonight. Regarding the thunderstorm
chances tonight, the HREF is not overly bullish on 50dbz echoes
which normally does quite well as a proxy for thunderstorm chances
and coverage. Most unstable CAPE values are a bit higher though
in today`s model runs, now in the 500-1000 j/kg range. So, some
embedded storms tonight, but nothing severe is expected.

The main focus is on thunderstorm and severe chances for
Wednesday. It still appears the southern half or so of the CWA
breaks into the warm sector of the system on Wednesday. Most
unstable CAPE values are still forecast to reach the 1,000-2,000
j/kg range with deep layer shear on the order of 30-40 knots.
850mb LLJ wind speeds are still 30-40 knots with a mid level jet
near 50 knots. Shear and instability are both adequate for severe
weather. All hazards are essentially in play, but it is not a slam
dunk event given the likelihood for morning clouds and
precipitation. Still, with a warm sector in the area and both a
warm front and cold front moving through the CWA we will need to
be prepared for the potential of all hazards. Strong winds to
50-60 mph would be the most likely threat with hail being further
down the threat list followed by an isolated tornado.

In terms of precipitation and hydro concerns we are looking at
0.75 to 1.25 inches across the forecast area. These values should
not produce concerns for us as all hydro sites in the area are
within bankfull at the start.

The low will move off to the east Wednesday night and showers and
storms will come to an end.

- Light Rain Possible Thursday Night

Conditions turn dry Thursday after the Wednesday surface low travels
northeast. Rain chances will return Thursday afternoon as a large
mid-level low moves through southern Canada and a frontal zone
tracks through lower Michigan. Best chances for rain will be across
southern Michigan and will clear east by daybreak Friday.

- Cooling Trend With Dry Weather into the Weekend

Temperatures will be on a downward trend into the weekend as the mid-
level low moves into Southern Hudson Bay and an 850mb thermal trough
settles in over the region. Coldest 850mb temps down to -5 to
-8 arrive Saturday resulting in peak temperatures in the 40s to 50s.
Overnight lows in the 30s are expected Friday night through Sunday
night with Saturday night the coldest with lows at or just below
freezing. Temperatures will begin to trend upward through the start
of next week as the thermal trough pivots east. Conditions will be
dry under surface high pressure with good amounts of sunshine Friday
through Monday. Rain chances increase Monday night into Tuesday as a
clipper moves through the state.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 130 AM EDT Wed Apr 17 2024

An area of showers and isolated thunderstorms will be lifting
northeastward through the TAF sites through 10z. The instability
is forecast to weaken, thus we did not feature thunderstorms in
the TAF sites through the remainder of the night. A cold front
pulls in from the west late Wednesday morning into the mid
afternoon hours. There will be sufficient instability around for
scattered thunderstorms to form. For now we have a narrow window
in the TAF sites when we are forecasting the highest potential for
these storms to happen.

With a southerly low level jet around through the remainder of the
night and east to southeast surface winds, low level wind shear
may exist. We therefore continued to feature this in the TAF
forecasts. Occasional surface wind gusts will still top 25 knots
at times through 00z Thu.

The dry air in the low levels has limited any sub VFR impacts,
however that will change as moisture continues to stream ahead of
the front. Once the front clears the TAF sites, conditions should
climb back to VFR during the afternoon.



&&

.MARINE...
Issued at 346 PM EDT Tue Apr 16 2024

Small Craft Advisories are currently in effect along our Lake
Michigan shoreline as wind has increased this afternoon. We made a
small modification in that the northern zones, north of Grand
Haven, to move into a Gale Warning a bit earlier now starting at
800pm this evening. A strong core of wind will be in place this
evening with gales looking likely out of the east. The strongest
winds will likely occur between 800pm and midnight.

Winds will very to the southeast tonight becoming south on
Wednesday. Conditions will remain hazardous to smaller craft on
Wednesday and Wednesday night before winds and waves subside.
Throughout the duration of this event waves will never really get
above 4-5 feet in the nearshore waters as the strongest winds
(this evening) will be offshore. By the time the winds come around
to onshore Wednesday night the winds will be subsiding.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 2 AM EDT early this morning for MIZ037>039-
     043>045-050-051-056-057.
MARINE...Small Craft Advisory until 6 AM EDT early this morning for
     LMZ844>846.
     Gale Warning until 6 AM EDT early this morning for LMZ847>849.

&&

$$

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