Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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461
FXUS63 KGRR 291506
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1106 AM EDT FRI JUL 29 2016

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 328 AM EDT Fri Jul 29 2016

A developing surface low over central Indiana today will will
remain nearly stationary most of the day today before finally
starting to move slowly east toward western Lake Erie by Sunday
morning. This will bring showers and isolated thunderstorms today
into Saturday mostly near and south of I-96. Easterly winds will
bring in cooler temperatures so highs will only get to around 80
today and Saturday. The system moves far enough to our east to
allow clearing on Sunday. A warming trend sets in for Monday
through Thursday. Thursday will be the next chance for showers and
thunderstorms.

&&

.UPDATE...
Issued at 1105 AM EDT Fri Jul 29 2016

Our main focus this morning in on the heavy rain potential for
later this afternoon and this evening across the far srn portion
of the CWFA. Showers and a few storms that have been affecting the
area this morning have been trending down over the past couple of
hours. This is likely due to some short wave ridging building in
for a short time.

We are expecting an uptick in showers and storms later this
afternoon and this evening around the Chicago area and likely
east along I-80. A short wave trough near the Quad Cities will be
slowly rolling east. A bit better instability down south will help
these to develop. The uncertainty lies in how far north would the
line get as the short wave approaches. The potential looks to be
that it would get to the I-94 row of counties.

We are not really expecting any severe weather with these storms,
just heavy rain potential. SB CAPE`s are expected to only
increase to around 1000 j/kgs with less ML CAPE`s. Effective shear
will be rather low with 20 knots or less expected. The lack of
wind aloft in the cloud layer will keep storms moving slow, and
heavy rainfall to be likely even with PWATs not outrageous around
1.5 inches. This has been the case already yesterday and this
morning around the region.

&&

.SHORT TERM...(Today through Sunday)
Issued at 328 AM EDT Fri Jul 29 2016

The short story is showers and isolated thunderstorms are expected
into Saturday night. Once the band of rain near Route 10 is done
(by 8 am or so) most of the rain will be near and south of I-96
this afternoon into Saturday night. It will be cooler thanks to
Canadian high pressure trying to build south from central Canada
into this area. I am thinking by Sunday this system should be far
enough to the east to allow for some clearing to take place.

We have a complex weather pattern for the next few days which
means my confidence in where and when it will rain is not all that
high. There is currently a shortwave on the southern branch of the
polar jet over Nebraska and Kansas that is heading eastward. While
that is happening there is also a northern branch of the polar jet
shortwave currently over Alberta that is heading southeast. That
system is being forced southeast by a much stronger system
currently in the Gulf of Alaska. The Gulf of Alaska system closes
off into an upper level low by Saturday night while the southern
branch upper wave tries to close off over Lower Michigan in about
the same time frame. A the surface we have a weak surface low in
Indiana that is actually waiting for the system from Nebraska to
catch up with it before it moves eastward.

This would mean Southwest Michigan is in the relativity cooler
air north of the primary polar front which is now somewhere near
I-80 or just south of that. This also means processioned east
winds below 800 mb today and through 600 mb Saturday. Thus the
area of showers over central Lower Michigan early this morning
should reform between I-80 and I-94 by this afternoon. That area
of showers may drift north as far as I-96 Saturday. Some locally
heavy rain is possible as precipitable water values are still high
(near 1.5 inches). That means any shower will be slow moving into
Saturday night.

Finally the entire system will be booted eastward as the northern
branch upper low deepens and digs. That Will build an upstream
ridge over the central CONUS forcing the system near us to move
east. This should mean clearing by Sunday afternoon will
temperatures near normal.

.LONG TERM...(Sunday night through Thursday)
Issued at 328 AM EDT Fri Jul 29 2016

The sfc high over the Great Lakes will move east by early next week
and a warm front will approach Lower Michigan Tuesday. Chances for
showers/storms will increase but still remain fairly low...20-30
pct. A building upper ridge and a warm front north of the cwa will
result in temperatures making a run at 90 again mid to late week. It
will also tend to produce a cap that will limit storm development
too. From Sunday night through Thursday, much of the time will be
dry, but instability from the hot humid airmass will necessitate low
chc pops.

The best chance at showers/storms will come Thursday night when a
cold front approaches. This front will also arrive with some upper
support too, so we`ll keep an eye out for the possibility of strong
storms.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 743 AM EDT Fri Jul 29 2016

Overall expect VFR conditions to hold through Sunday. There will
be some slow moving showers and thunderstorms around into Saturday
morning but timing when the visibility falls below 3 miles this
far out in time is not easy so I put VCTS and called it good.


&&

.MARINE...
Issued at 328 AM EDT Fri Jul 29 2016

Persistent northeast winds of 5 to 15 knots will push waves off
shore into Saturday night. I see no real threat for a small craft
advisory from this system as a result of that.


&&

.HYDROLOGY...
Issued at 200 PM EDT Thu Jul 28 2016

Wednesday`s thunderstorms in Central Michigan produced isolated
totals over 2 inches according to radar estimates, which fell in SW
Mecosta County and west-central Clare County. While much of the
water was likely absorbed, some has run off into the rivers of the
Muskegon Basin. Slight rises are noted at Evart and Oak Grove, and
that is all that is anticipated.

Showers and storms this afternoon and evening could produce locally
heavy rainfall totals as well. This time the greatest risk is
farther west and south... focused closer to Lake Michigan and in
areas south of I-96. Weak vertical shear will produce slow-moving
pulse-type storms which will propagate off their own outflow, as is
currently occurring in N Illinois and S Wisconsin. Surface dew
points are still in the mid 60s, and there is greater moisture in
the upper levels versus yesterday. Precipitable water values are
around or over 1.5 inches which is about the 90th percentile for
late July. Brief heavy downpours are expected with today`s storms
but localized areas may see a more persistent duration of rainfall
where updrafts persist. Therefore localized heavy rainfall amounts
will have to be monitored, which may lead to temporary urban and
poor drainage flooding.

&&

.GRR Watches/Warnings/Advisories...
MI...None.
LM...None.
&&

$$

UPDATE...NJJ
SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...99
AVIATION...WDM
HYDROLOGY...CAS
MARINE...WDM



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