Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 280734
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
334 AM EDT Wed Jul 28 2021

LATEST UPDATE...
Synopsis/Discussion/Marine

.SYNOPSIS...
Issued at 334 AM EDT Wed Jul 28 2021

- Strong to Severe storms Possible Early Thursday

- Stronger cold front brings showers Saturday

- Prolonged period of cooler weather



&&

.DISCUSSION...(Today through next Tuesday)
Issued at 334 AM EDT Wed Jul 28 2021

- Strong to Severe storms Possible Early Thursday

It continues to look like Southwest Michigan will have a
significant threat of a cluster of thunderstorms, possibly even a
derecho moving through this area during the early morning hours of
Thursday. The setup for this is better than we typically see for
Southwest Michigan. We have tandem jet cores, one on the northern
stream and the other on the primary polar jet that come in phase
and will significantly deepen the eastern trough. As the shortwave
tops the western upper ridge and coming down the into the eastern
trough it will strengthen. That of course cause a mid and low
level response. Which comes together over northern MN and WI by
late this afternoon. The mix layer and most unstable cape in that
area will be over 3000 j/kg. The low level jet becomes unusually
strong for this time of year and will increase from around 30
knots when the storms develop over WI/MN but will increase to
around 45 knots, aimed at central Lake Michigan by around 2 am
Thursday. There is strong deep layer shear due the jet cores
coming close to this area at that time. Also the cape is deep,
model sounding show the EQ near 45,000 ft over our area during the
early morning hours of Thursday. The storms will ride the cape
gradient southeast into Southwest Lower Michigan by midnight.

The storms may be individual cells when they first develop in
MN/WI but by the time the get to Michigan they should have merged
in some sort of linear feature, meaning gusty winds will be the
most likely mode of hazard risk in our area. There is also the
threat of an area with 2 to 4 inches of rain along the track of
the MCS, which means localized flooding is also possible. These
storms will be moving quickly but with the precipitable water near
2 inches, we could see rainfall rates more than high enough to
result in areas with more then 2 inches of rain. An isolated
tornado would not be out of the question, but that depends on
where the surface wave actually tracks. Seems the risk for that
would be south and west of Grand Rapids.

So the real question is not, will there be these strong to severe
storms early Thursday morning, but where will they actually be?
As typical of our numerous models that forecast these things the
track is anywhere from NE- WI toward N IL to TVC to DTW. It seems
to me the real question is where does the surface low track? Just
about all the models have the strong storm just east of the track
of the surface low. The ECMWF is way north compared to the other
models with this feature. Most of the other models have the
surface low tracking from southern WI to near Kalamazoo. If that
is the track of the surface low, we would have the risk of
isolated tornadoes. There would be just enough low level shear
and with a low LCL and decent low level cape, that is a real
possibly.

At this point my best guess is the greatest risk of strong to
severe storms will be south and west of Grand Rapids. I do thing
locally heavy rain is likely west of US-131 (more or less).

Once the storms roll through I would expect some clearing by
midday Friday. While the cold front comes through Thursday morning
the actual cold air does not get in here until Friday morning.

- Stronger cold front brings showers Saturday

As I have been writing about all this week, we are seeing a major
change to our overall weather pattern take place. This is actually
are response of the northern hemisphere wave pattern to MJO
tropical convection, which is now in phase 6. That supports cooler
than normal temperatures over nearly all of the eastern United
States. This will take several shortwaves to get done. The first
one will cause the thunderstorms early Thursday. The next one
comes through our area in Saturday. Likely we will see more
showers and maybe a thunderstorm. The GFS brings the upper level
low over Michigan Sunday (fastest run). That would mean enough
instability in the afternoon for inland showers and thunderstorms
on Sunday into Monday.

- Prolonged period of cooler weather

We have more shortwaves diving into this eastern trough, so the
cool weather and periodic showers will continue most of next
week. This is supported by all of our medium range models and
there ensembles a well as the MJO teleconnections.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 146 AM EDT Wed Jul 28 2021

I expect solid VFR conditions with light and variable winds
through most of the day today. This is thanks to the surface high
moving through the area today.

The specter of thunderstorms at our TAF sites rises toward
midnight (04z) at MKG and (05z) GRR. The other TAF sites will
likely not see any storms until after 06z. These storms may well
have gusty winds (>35 KT gusts) and at brief heavy rain (IFR vis)
as they move through. The the lowest condition would likely not
last more than 5 to 10 minutes at any one station. Conditions will
quickly recover to VFR after the storm line moves through.


&&

.MARINE...
Issued at 334 AM EDT Wed Jul 28 2021

I was thinking we would need a small craft advisory and beach
hazard statement on Thursday as the cold air came in behind the
first cold front. I am beginning to question that now as it
actually seems the cold air does not come in until Friday. We
will have to look closer at this in later forecasts. At this point
it is to uncertain to issue any sort of Marine headline for that
yet.



&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

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


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