Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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000
FXUS63 KGRR 270730
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 AM EDT Tue Jul 27 2021

LATEST UPDATE...
Synopsis/Discussion/Marine

.SYNOPSIS...
Issued at 329 AM EDT Tue Jul 27 2021

- Some thunderstorms into mid morning today

- Excessive rainfall and strong storms Wednesday night

- Unsusual chilly early next week proceeded by storms Saturday



&&

.DISCUSSION...(Today through next Monday)
Issued at 329 AM EDT Tue Jul 27 2021

- Some thunderstorms into mid morning today

The short story is I expect the thunderstorms crossing Lake
Michigan now to dissipate to rain showers before sunrise. The
trailing area of storms will also try to make a run at the I-96
area during the mid to late morning but it too will weaken to rain
showers and dissipate.

As for the details... There area two area of thunderstorms
heading this way early this morning. The lead area of convection
is currently crossing Lake Michigan. There seems to be an MCV over
NE WI with this area of storms. The second area that I believe
will be more significant to us is the convection that was near
Duluth and is moving south south east currently. That is where the
cloud tops are cooling the most and were the RAP model shows the
stronger upper wave. Looking at the surface, the area of storms,
now south of Duluth is in an area of good surface convergence. The
RAP model 1000/850 moisture transport shows that second area has
stronger support longer. So I believe that area will become the
area that will try to make it into the I-96 area by mid morning.
The area now moving over the lake should dissipate as it runs int
more stable air and less support. Even the area that trails it
will run out of support by mid morning since the upper wave will
be moving east of our area by then.

So, the result of all this is that the shortwave causing all this
convection will drive a cold front south today into tonight. Due
to surface heating a low may develop (shown by HRRR and RAP ) west
of Saginaw by mid afternoon. That will focus any new convection
over the thumb area. Subsidence behind the upper wave and southwest
winds off Lake Michigan all work to keep most of our CWA dry once
the remnant showers dissipate by midday.

With the front being pushed south of here tonight we will be in an
area of surface divergence and upper convergence. That will remain
so through most of the day Wednesday. So it should largely be dry
over our CWA tonight and through the daylight hours of Wednesday.

- Excessive rainfall and strong storms Wednesday night

We have collectively been writing about the system Wednesday night
into Thursday morning for more than a week now. The timing has
sped up some since we started writing about this system but it
remains a potent system for this area, even through it is coming
through the area during the middle of the night.

WPC has put this area in an slight risk for excessive rainfall.
SPC has us in a slight risk for severe storms. We have a frontal
wave that tracks southeast across southwest Michigan during the
early morning hours of Thursday. There is decent dynamics for this
wave as we have a 95 knot 300 mb jet driving this event. The NAM
has a 65 knot low level jet feeding the frontal wave. The SPC SREF
has 2000 to 2500 j/kg of MU cape into Southwest Michigan even at 2
am in the morning Thursday. The equilibrium level at 2 am over
GRR is over 45,000 ft (or over 200 mb). The precipitable water
values on the NAM rise above 2.5". The SPC SREF mean is more like
1.7 to 2.0". Model sounding show deep saturation as this wave
moves through. All of this points to heavy rainfall, localized
flooding, and at least isolated severe storms (wind damage). The
timing of this would be late evening into the early morning hours
of Thursday.

- Unsusual chilly early next week proceeded by storms Saturday

Once the frontal wave is through the area we get a large surface
high to move into the area. We will be on the anticyclonic side of
the polar jet Friday. So we should be cooler but with no
precipitation.

By Saturday a 120kt jet streak from north central Canada dives
southward to help split the polar vortex into two centers. One
over southern Hudson Bay and the other over eastern Russia. The
shortwave driving this will bring showers and thunderstorms to
the area Saturday. Then it will be unusably chilly into at least
Tuesday due to the polar jet actually being south of this area


&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 147 AM EDT Tue Jul 27 2021

We continue to watch a line of strong thunderstorms, which at
0530z extends from the central Upper Peninsula of Michigan into
north central Wisconsin. Our in office timing tool shows the
storms reaching extreme northwest Mason County by 0930z. The
question then becomes just how far south of that do they get? Our
in office thinking is the low level jet axis, which is forecast to
remain north of Route 20, should keep the storm mostly north of
that overnight. It does not now appear that we will develop nearly
as strong of a outflow from the storms either. So I did not shift
the winds at MKG in the 06z TAF like we have been showing for the
past several TAF forecasts for MKG. I do have VCSH for the I-96
TAF sites in the 11z to 15z time frame as some left over showers
could make it that far south.

So, other than that left over showers reaching to the I-96 TAF
sites in the 11z to 15z time frame, I am not expecting
thunderstorms to impact our TAF sites during the daylight hours of
today. That is due to the upper shortwave, sparking the
convection should be moving away from the area by midday today.
The next upstream shortwave to cause more convection is currently
to far away to impact this area before Wednesday night. There is
enough dry air below 700 mb today that is should inhibit storm
new storm development. I will admit given there will be a frontal
boundary just north of this area today, it would not be out the
question we could see a few thunderstorm develop north and east of
Lansing in the afternoon but if they do they should dissipate
shortly after sunset.

Tonight (Tuesday night into Wednesday morning) should see mostly
clear skies and light winds.


&&

.MARINE...
Issued at 329 AM EDT Tue Jul 27 2021

The cold air coming in will likely result in small craft
condition for the near short by Thursday afternoon.



&&

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

$$

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


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