Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 270506
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
106 AM EDT Thu Apr 27 2017

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 325 PM EDT Wed Apr 26 2017

A few showers and thunderstorms can be expected through Thursday
afternoon as a cold front moves through. Conditions will dry out
for a short period late Thursday through early Friday. Low
pressure will approach on Friday and bring a chance of rain. A
much better shot at widespread rain will come late Saturday
through Sunday as another low pressure area tracks across Lower
Michigan.

&&

.UPDATE...
Issued at 105 AM EDT Thu Apr 27 2017

Since we have an area of showers and thunderstorms (I just head
thunder here) moving north northeast in a band, I updated the pop
and weather girds to show 90 % pops for this area moving
northward. The HRRR pop looked usable with a little modification.
I do no expect much behind this but we will have brief heavy
rain. There is not enough instablility for more than generic
thunderstorms.



UPDATE Issued at 1028 PM EDT Wed Apr 26 2017

Lowered POPs and QPF overnight and into Thursday morning. Models
have been trending drier and last few runs of the HRRR and other
high res models show bulk of precip missing Lower Michigan to the
west tonight and tomorrow. Severe threat also appears to be rather
low overnight despite greater than 50 knots of 0 to 6 km shear, as
instability is lacking.

Instability appears to be limited on Thursday as well but we Will
have to watch for narrow line of convection late Thursday morning
or afternoon that could have strong gusty winds as indicated by
the hi res NMM/ARW.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 325 PM EDT Wed Apr 26 2017

Our focus with this forecast package is on the waves of
showers/storms through Thu, and the chance of rain coming back in
for Fri/Fri night.

There is a dying line of showers over Central Lower that is at the
tail end of an upper vort moving NNE away from the area. This seems
to be just a bit too far north to tap 1000 j/kg of low level CAPE
over the srn portion of the area. This line seems to be some what of
a weak cool front that extends down S of the area. We can not rule
out another shower/storm popping up, but we do not expect widespread
showers/storms until after 00z tonight.

We are expecting a wave of showers/storms to arrive after 00z from
the SSW that are currently in Srn Il. These are associated with
another upper/sfc wave that will move NE through the evening. The
associated low level jet will nose up into the area and strengthen.
The sfc based instability that we see now will diminish, and this
wave will feature mainly elevated instability with it. This will
limit the severe potential with it. There is some decent shear with
healthy wind profiles up the column. The decent wind profiles will
keep an isolated wind threat in, if it could punch through the
stable near sfc layer. Heavy downpours will be possible too with
PWATS up around 1.5 inches.

We will likely see a lull in the rain for late tonight and early Thu
as the nose of the low level jet moves out, and the sfc low remains
to our west. Heavy widespread rainfall looks to stay over WI. We
will see the main cold front push through late Thu morning and early
Thu afternoon. This will likely develop a bkn line of
showers/embedded storms ahead of it with some diurnal heating taking
place before it pushes through. Forecast soundings do not not
support much of a severe weather threat with thin CAPE profiles. The
wind aloft however keeps a threat in before the front moves through.
The front looks like it should clear the CWFA by 19z Thu. A few
showers will remain in the afternoon to the NW as the upper wave
moves through.

We should end up dry Thu night with rain chances then increasing on
Fri again. The current upper system affecting us moves out, and the
leading short wave with the next upper system approaches. Low level
moisture is not exciting, but there may be just enough for a few
light rain showers to make it to the ground into Fri night. Thunder
chcs look too low with this to include in the forecast for now.

.LONG TERM...(Saturday through Wednesday)
Issued at 325 PM EDT Wed Apr 26 2017

Only small changes were made to the going grid forecast, as the
models are in good agreement and fairly steady. Active weather is
expected through much of the long range forecast.

Showers will be ongoing across Southern Lower Michigan Saturday,
with thunderstorms becoming possible Saturday night through Sunday
night. Widespread rain, could be heavy at times through the weekend.
Showers will continue into Tuesday.

Rainfall totals of one to two inches are likely Saturday night
through Sunday night with locally higher amounts possible.
Temperatures should be fairly close to normal through the long range
forecast period.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1159 PM EDT Wed Apr 26 2017

A mix of vfr/mvfr conditions should gradually deteriorate to
mainly mvfr overnight due to low clouds and scattered rain
showers. Isolated thunderstorms may also affect mainly the KAZO
and KGRR terminals during the next couple of hours.  Conditions
will then remain mainly mvfr Thursday morning and early afternoon
due to low clouds and scattered showers.

A line of showers and perhaps isolated thunderstorms will move
through early Thursday afternoon right along the cold front. Conditions
will then improve to vfr mid to late Thursday afternoon after the
frontal passage.

South to southwest winds will increase to around 15 to 20 kts by
late Thursday morning prior to frontal passage and will veer to
the west and increase to around 15 to 20 kts with gusts to near 25
kts after fropa. Conditions will then remain vfr through Thursday
evening.


&&

.MARINE...
Issued at 325 PM EDT Wed Apr 26 2017

We are extending the Small Craft Advisory out through 06z Fri
morning with this forecast package. The initial advisory was mainly
for the winds out ahead of the system. We are now to the point that
the winds behind the system on Thursday need to be dealt with. These
will be the strongest Thu afternoon, and the waves will come down
enough finally Thu night.


&&

.HYDROLOGY...
Issued at 325 PM EDT Wed Apr 26 2017

Confidence continues to grow that we will have an increasing flood
risk as we head into the weekend and next week. The GFS, GEM, and
ECMWF all have been indicating a pattern that will produce
significant rainfall for our region, likely on the order of 2" to 3"
or more. When that much rain falls across the entire reaches of the
major basins - Grand, Muskegon, and Kalamazoo - there will be some
significant rises. Trends will continue to be monitored.

Smaller rivers and streams are going to be highly responsive to this
type of rain, with abrupt rises possible. Residents along smaller
rivers and streams should be aware of the risk for these types of
changes late into the weekend and early next week.

At this point it is too early to tell exactly how high the rivers
will get, but flood stage may be exceeded at many locations
potentially.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 2 AM EDT Friday for LMZ844>849.

&&

$$

UPDATE...WDM
SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...63
AVIATION...Laurens
HYDROLOGY...Hoving
MARINE...NJJ



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