Severe Thunderstorm Warning 54, NWS Grand Rapids, MI

Area Forecast Discussion
National Weather Service Grand Rapids MI
743 AM EDT Sun Jul 19 2020

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 327 AM EDT Sun Jul 19 2020

-Thunderstorms likely today

-Fair Weather Monday

-Stormy midweek

&&

.DISCUSSION...(Today through next Saturday)
Issued at 327 AM EDT Sun Jul 19 2020

--Thunderstorms likely today--

The timing of the convection keeps getting delayed and it now
looks like the storms will not be moving into this area until
around sunrise. There is still a considerable amount of
instability to work with but the bulk shear is for the most part
less than 20 knots. Based on our timing tool the line of storms
would now get into our northwest coast around 6 am. The latest
runs of the HRRR and ESRL HRRR show the line falling apart as it
moves onshore actually. Then more convection develops midday from
around Alma to near Holland and that convection moves east across
the rest of the area during the afternoon. It would seem if that
is how it works out what severe threat that we have would be in
the I-69 area during the afternoon.

So my thinking is the severe threat for our western and northern
counties this morning is rather low due to the lack of bulk shear.
Also having storm come through around sunrise is not a good time
for severe storms unless you have strong dynamic forcing with it
does not seem at this point we do. Even so it is not out of the
question through given the high values of instability (like mix
cape near 2000 j/kg). The more likely threat would be as I wrote
in the above paragraph, is in the I-69 area early to mid afternoon
assuming the second line of storms actually forms.

Once the cold front comes through skies will clear from west to
east, that more than likely would be in afternoon west to early
evening east. It will cool off tonight and dew points will fall
back from the mid 70s this morning to mid 50s north to lower 60s
south by tomorrow morning.

--Fair Weather Monday--

Shortwave ridging and a surface high moving overhead by afternoon
should result in a rather pleasant summer day in Southwest Lower
Michigan featuring lower dew points, cooler afternoon
temperatures (lower 80s) and winds around 10 mph.

--Stormy midweek--

The next upstream Pacific system tracks into our area in the
Tuesday into Wednesday time frame. Since the polar jet is
actually farther south with this system Wednesday (over I-96
compared to Upper Michigan today) this would suggest we do not get
into the hot air this time. It would seem to increase the risk of
more widespread convection. So I still believe most of the area
has a better chance of getting meaningful rain from the Tue/Wed
system than the one today.

Beyond that we get another shortwave ridge and cooler air but then
the upper ridge seems to come back by the next weekend, that would
bring hot air back into the region but that's not a sure thing
yet.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 743 AM EDT Sun Jul 19 2020

I am expecting the two lines of thunderstorms to merge between
I-94 and I-96 by 13z and that area of convection will likely
impact all of our TAF sites between 13z and 16z. After the storms
clear the TAF sites I would expect VFR conditions to return.
Meanwhile, as the storms move through expect locally IFR in the
heavier rain. Winds will gust to around 25 knots, 35 knots is
possible as the storms get closer to JXN since by then day time
heating may help the storms grow stronger.

After 16z or so skies for the most part be VFR with cigs around
5000 ft. Skies should clear by 21z or so and winds will diminish
after sunset.

&&

.MARINE...
Issued at 327 AM EDT Sun Jul 19 2020

We are not planning to change our headlines at this point. Our
ongoing forecast looks good in that we will have west winds behind
the front so we should see significant waves and winds in the 15
to 25 mph range behind the cold front this afternoon. Winds and
waves will let up after midnight so tomorrow should be a nice
beach day.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this evening for MIZ037-043-050-
     056-064.

LM...Small Craft Advisory until 8 PM EDT this evening for LMZ844>849.

&&

SYNOPSIS...WDM
DISCUSSION...WDM
AVIATION...WDM
MARINE...WDM

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