Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 281801

Area Forecast Discussion
National Weather Service Grand Rapids MI
201 PM EDT THU JUL 28 2016


Issued at 324 AM EDT Thu Jul 28 2016

A large Canadian high pressure system, currently centered near
Lake Winnipeg, will slowly move south and east into Sunday.
Normally this would result in mostly clear skies and cooler
temperates for Southwest Michigan. However in this case the
cool air is to shallow and so we will see periods of showers and
thunderstorms into Saturday night. By Saturday most of the rain
will be near and south of I-96. Highs will remain in the 80s and
lows mostly in the 60s.


Issued at 1133 AM EDT Thu Jul 28 2016

No real changes needed to the fcst this morning. A band of mainly
light rain showers was going north of the CWFA, while an isolated
shower was between Big Rapids and Mt. Pleasant earlier. These
areas of showers have been diminishing.

We are expecting a few showers and storms to develop this
afternoon, mainly along and south of I-96 where better instability
will be present. Models are indicating that we could see CAPEs of
around 2000 j/kg along and south of I-96. Shear values are
expected once again to be on the low side, generally below 20
knots so severe weather potential is on the low side. Main threat
would be some locally heavy rainfall with a slow movement expected
with the storms.


.SHORT TERM...(Today through Saturday)
Issued at 324 AM EDT Thu Jul 28 2016

The main issue with this forecast is figuring out when, where and
if we will see showers and thunderstorms. There is a large
Canadian high pressure system, which at 3 am was centered near
Lake Winnipeg. The polar jet`s southern branch dives south from
Idaho into Nebraska, then head east toward Indiana before it turns
back to toward Lake Erie this morning. Over time Canadian high
sinks south and east and is over Michigan by Saturday morning.
This would normally clear our skies but in this case we have a
series of shortwaves on the southern branch of the jet that will
result in a trough over the western Great Lakes. That will allow
some over-running of the shallow cold air of the Canadian high.
The result of all this is the band of showers/thunderstorms will
slowly sink south over the next 2 days. This means that Saturday
may not be as dry as we had through it would be.

Another aspect to this is with northeast winds as the high builds
in today, we could end up with lake breeze convergence along the
coast and that would mean evening thunderstorms (not severe).

High and low temperatures should be a touch cooler Friday and
Saturday due to more clouds and cooler air overhead. I would
think we should still be able to get into the lower 80s each day
even so.

My bottom line to all this is variable cloudiness with scattered
showers and thunderstorms into Saturday.

.LONG TERM...(Saturday night through Wednesday)
Issued at 324 AM EDT Thu Jul 28 2016

The models have been showing the late week evolution of the upper
pattern from a zonal flow to a broad trough across the Great Lakes
for a few days. They have also been showing a dominant sfc high over
Quebec ridging westward across Michigan as well. It doesn`t look
like much pcpn will be generated under this setup, in part because
most of the weak short waves are progd to have already moved east of
the cwa by Saturday night. Instead, we`re more likely to see
afternoon cu and dry wx from Sunday through Tuesday. It`s at that
point that the Quebec high moves east and a warm front moves
northeast toward Lower Michigan. We should see an increase in
coverage of pcpn Tuesday into Wednesday as the warm front moves
through and south flow draws higher moisture northward and
precipitable water content increases to around 2 inches.

Highs will remain above normal...mainly in the mid to upper 80s
through the period.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 200 PM EDT Thu Jul 28 2016

Cumulus/convection has been slow to develop today but all
indications are that at least widely scattered convection can be
expected between roughly 20z and 03z. The best prospects for
tstms/CBs due to sfc convergence looks to be along I-94 including
AZO/BTL/JXN, and also along the lakeshore including MKG.

Scattered tstms will weaken/diminish after sunset leaving
primarily scattered rain showers out of a mid level deck of clouds
overnight into Friday morning. Some tstms could redevelop on
Friday south of I-96 but that would probably be after 18z.

The radiation fog potential looks low tonight due to the
expectation of considerable clouds. However if skies clear
overnight where pockets of heavier rain occur later today then we
could see some local IFR fog developing. This probability is too
low however to mention in any of the TAFs at this time. We will
have to wait and see if any heavy showers/tstms directly impact
any of the terminals later today.


Issued at 324 AM EDT Thu Jul 28 2016

Little in the way of marine issues seen at this time. The large
Canadian high pressure system will cause mostly northeast winds
into Saturday. The wind and waves will remain below small craft
criteria through the weekend.


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...


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