Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 252340
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
740 PM EDT Tue Sep 25 2018

LATEST UPDATE...
Mesoscale Discussion

.MESOSCALE DISCUSSION...
Issued at 731 PM EDT Tue Sep 25 2018

At 2330Z KGRR radar trends show a classic bow echo about 20 miles
west of South Haven moving ene with significant evacuation of low
level reflectivity on the back side of the bow as a result of the
rear inflow jet.

23Z mesoscale analysis shows a favorable environment for severe
wx this evening near to mainly south/southeast of the I-96
corridor where the Craven/Brooks sig svr parameter is up to around
40000 with ml cape values of around 1000-1500 j/kg and around a
1000 j/kg of dcape. Severe thunderstorm wind gusts seem possible
if not probable with this area of convection from around South
Haven ene to Lansing given the history of the convection... radar
trends and 23Z mesoanalysis.

&&

.SYNOPSIS...
Issued at 1259 PM EDT Tue Sep 25 2018

A strong cold front will pass through this area around midnight.
This will result a showers and a few strong thunderstorms this
evening ahead of the cold front. Behind the front after midnight
will become breezy and colder. Temperatures will fall to around 50
by morning but struggle to rise to near 60 Wednesday afternoon.
Some light lake effect rain showers are possible Wednesday during
the day. A second cold front will come through the area Thursday
night with a few more light rain showers possible into Friday
morning. Lake effect rain showers will likely continue into
Saturday morning behind that cold front. Then a wave on the front
will bring rain back to the area Sunday and that will continue
into Monday. Temperatures will likely be cooler than normal
through this period.

&&

.DISCUSSION...(This evening through next Tuesday)
Issued at 327 PM EDT Tue Sep 25 2018

The short term issue is the threat of strong to severe storms this
evening into early tonight. Wednesday the issue is lake enhanced
light rain showers. Then we have another northern stream cold
front early Friday and finally stalled front south of us late in
the weekend that may bring us periods of showers and thunderstorms
that could result in areas of heavy rainfall.

The severe storm threat remains for this evening as a strong cold
front pushes into a subtropical moist airmass in front of it.
There is a double jet core heading our way. There is a northern
stream jet entrance region over northern lower while a southern
stream jet core travels over southern lower mid evening. The
result of this is significant deep shear and strong dynamics but
narrow but deep cape. We are thinking isolated severe storms are
likely this evening. The hi-resolution models seem to be backing
off on showing strong cells this evening in our CWA but given the
dynamics of this set up and the deep moisture, it seems to me we
still have a real threat. So I have likely pops with thunderstorms
for this early tonight.

Behind the cold front the wind flow becomes anticyclonic by mid
morning but there is still enough moisture in the 925 to 850 layer
and given the 850 temps are nearly 20c colder than that lake
surface, it would seem light rain showers from lake effect are
likely. I am not sure we will get much measurable rainfall but
there will be showers around. I added pops for that tomorrow.

There is a chance of showers later Thursday night into Friday
morning with the next cold front but most of that should be north
of Muskegon.

Finally we still get that front to stall just south of Michigan
over the weekend and it looks likely we will see overrunning rain
Sunday into Monday. There is enough instability for thunderstorms
with this event.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 713 PM EDT Tue Sep 25 2018

One round of showers and storms were pushing east of KLAN and
KJXN. Meanwhile a larger area of showers and storms were crossing
Lake MI. These storms will likely cross the TAF sites from west to
east this evening. Gusty winds may accompany any stronger storms.

There is an area of low clouds behind the front that is triggering
the storms over Lake MI. These low clouds will likely spill in
behind the front impacting the TAF sites later tonight and through
much of Wed morning. As drier air filters in Wed afternoon...for
now it looks like VFR conditions will return.


&&

.MARINE...
Issued at 1259 PM EDT Tue Sep 25 2018

I issued a beach hazard statement for overnight into Wednesday due
to the strong winds and high waves expected. With the 1000 to 850
mb layer cooling over 15c in 12 hours there is little question in
my mind there will be enough wind to cause those sorts of waves.
For the same reason I will continue the Small Craft Advisory as
is.


&&

.HYDROLOGY...
Issued at 1259 PM EDT Tue Sep 25 2018

Steady or slowly falling stages were observed across most West
Michigan rivers over the past 24 hours.  Rainfall was generally
less than one tenth of an inch except in eastern Isabella and Clare
counties where around one quarter inch was observed. Another area of
heavier rain...from one quarter to one half inch... fell in southern
Calhoun and Jackson counties south of Interstate 94.

Another round of showers and thunderstorms is expected across Lower
Michigan this afternoon and tonight as a cold front moves through
the region.  Ensemble model output suggests a good chance of one
inch of rain in the I-69 corridor...with lower chances to the north
and west.  The Kalamazoo and upper Grand Rivers would be the most
likely areas to experience strong within bankfull rises.  It does
not appear likely that any rivers in West Michigan would rise above
flood stage as a result of this precipitation event.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from midnight EDT tonight through
     Wednesday evening for MIZ037-043-050-056-064-071.

LM...Small Craft Advisory from midnight tonight to 8 PM EDT Wednesday
     for LMZ844>849.

&&

$$

MESOSCALE...Laurens
SYNOPSIS...WDM
DISCUSSION...WDM
AVIATION...MJS
HYDROLOGY...MWS
MARINE...WDM


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