229
FXUS63 KGRR 151551
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1151 AM EDT Mon Jul 15 2024

.KEY MESSAGES...

- Showers and storms move off to the east this morning

- Another round of strong/severe storms this evening/tonight

- Mainly dry after Tuesday

&&

.UPDATE...
Issued at 1113 AM EDT Mon Jul 15 2024

Much of the Grand River basin saw 1 to 2 inches of rain overnight
and many of the smaller streams are elevated. Another round of
storms with heavy rain are anticipated tonight. Runoff will be
efficient. As a result, we issued a flood watch for the region
near and south of I-96, from Muskegon to Lansing. Also, river
levels are elevated, especially around Jackson. The added rainfall
tonight could push them up to near flood stage.

As for the severe weather tonight, there has been a slight
northward shift of the axis of strongest convection in the morning
model runs. Thus the southern half of the CWA looks to be at risk
for a fast moving line of storms with damaging winds. There is now
good consistency amongst the models on this scenario and the 12z
HREF supports this as well. The 12z ABR sounding showed a deep
EML with 45 to 55 knot westerly flow 700 mb through 400 mb. The
latest upstream VWP`s 500 mb pressure level plots are showing the
50 knot winds around 500 mb persisting at KFSD and KMPX. From
SPC`s meso page, the current and forecast Effective Bulk Shear
values are shown to be strong as mid to upper level wave seen on
WV imagery over the upper Plains moves into the Great Lakes Region
tonight. MU CAPE values were already up to 3000 J/kg in southern
WI and over 4000 J/kg in IA. Visible imagery shows strong heating
will continue through the afternoon. The southwesterly low level
jet is shown to strengthen ahead of the convection through the
evening. Based on all this, confidence for a line of storms with
damaging winds has increased for southern parts of Lower MI
tonight.



&&

.DISCUSSION...
Issued at 408 AM EDT Mon Jul 15 2024

- Showers and storms move off to the east this morning

A very long lived mesoscale convective vort is moving through the
forecast area this morning. This vort was formed last night in the
northern plains and it reinvigorated convection yesterday evening
on the Mississippi near the IA/WI line. It has not produced severe
weather in our area, but it has produced flooding and a lot of
lightning. We remain concerned about flooding in the Lansing metro
area early this morning as the ground there has been saturated
from recent rains and this activity has been producing 1-2 inch
per hour rainfall rates. We do have areal Flood Advisories and
Warnings that carry into the 800am to 1000am time frame. Flooding
should recede fairly quickly this morning as it is more urban and
small stream related. The showers and storms will likely clear to
the east out of the GRR forecast towards 12z.

In the wake of this activity we expect some clouds to linger much
of the morning over at least half the forecast area from near and
just north of I-96 southward. This will put a lid on how much heat
we see today as the clouds do not clear out until towards midday.

- Another round of strong/severe storms this evening/tonight

The next focus is on our next round of thunderstorms which could
be strong to severe. The HREF 4 hour max reflectivity which does
well most of the time shows a surge of convection moving in after
800pm. It sweeps through the forecast area between 800pm and
600am. This round looks to hit many of the same areas as the
current round (near and south of I-96). There is a signal for some
severe weather in the same area given quite a few updraft helicity
swaths moving through the area. MUCAPE values hit 2000-3000 j/kg
along with 30-40 knots of deep layer shear so we will be
monitoring once again for severe weather. We are in a slight risk
from the SPC for the southwest half of the area (MKG to GRR to
LAN) and points to the south. Can envision the slight line being
bumped a bit north today, possibly up towards Alma to include
points south down U.S. 127.

- Mainly dry after Tuesday

The front will progress south on Tuesday as an upper trough
settles into the area. We have chance pops for showers in the
forecast across the south on Tuesday. We may see some scattered
showers on Wednesday with the upper trough, otherwise much of the
rest of the week looks dry with high pressure settling in.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 750 AM EDT Mon Jul 15 2024

Lingering showers and areas of IFR moving out by 15Z or so,
leaving dry/VFR conditions for the afternoon. Another round of
thunderstorms arriving after 00Z this evening although confidence
is low regarding the exact timing, coverage, and severity of the
storms. It is possible we will see a bow echo with wind gusts at
or above 50 knots at some point between 00Z and 06Z tonight. Also
heavy rainfall associated with the storms could bring some IFR
cigs/vsbys. This potential will be fine-tuned in later TAF
updates.

&&

.MARINE...
Issued at 408 AM EDT Mon Jul 15 2024

An overall fairly slack gradient is in place through Tuesday
night, so wave height overall will be on the low side. The
exception is the potential for a short duration burst of wind
tonight associated with thunderstorms expected to sweep across
Lake Michigan. Thunderstorms look likely this evening over the
lake, which will likely kick up a wave field. This is one of our
top problematic scenarios as the wind is short lived, but can be
strong with overnight thunderstorm complexes and low level jets.
We could be looking at waves that over perform into Tuesday
morning. As of right now we have 2-4 footers in the forecast and
we will have to see tonight what actually sweeps across the lake.

Our next time frame of concern will be on Wednesday in a north
flow behind a frontal passage. NAM winds look to be in the 10-20
knot range which should keep us just below advisory. The GFS
however is showing more 15-25 which would push us into a Small
Craft Advisory and Beach Hazards Statement. We will be watching
for more consensus in speeds as this event closes in.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 9 PM EDT this evening through late tonight for
     MIZ050-056>059-064>067-071>074.
MARINE...None.

&&

$$

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