Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211909
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
305 PM EDT Mon Aug 21 2017

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine

.SYNOPSIS...
Issued at 305 PM EDT Mon Aug 21 2017

A low pressure system and cold front will bring rain and
thunderstorms late this evening and overnight. Showers will linger
into Tuesday morning until frontal passage. It will turn much
cooler and less humid behind this system for mid to late week.

&&

.SHORT TERM...(This evening through Wednesday night)
Issued at 305 PM EDT Mon Aug 21 2017

Primary short term fcst concerns involve determining convective
potential through tonight into early Tuesday.

The approaching cold front will cause development of rain and
thunderstorms late this evening and overnight. The most likely
time frame for convection will be from around 05Z Tue - 12Z Tue.
Severe wx is unlikely but a few strong to marginally severe storms
are possible given sufficient deep layer shear on the order of
30-40 kts in conjunction with moderate instability. Ample forcing
will come from the approaching front and llj and there is plenty
of low level moisture in place.

Several factors going against severe wx potential include
unfavorable cold frontal timing and lack of stronger instability.
Locally heavy rainfall is also possible overnight due to strong
1000-850 mb moisture transport and precipitable water values
approaching two inches. However only isolated urban/small stream
type flooding issues are anticipated given the progressive nature
of the convection overnight.

Showers and isolated non severe storms will linger into mid to
late Tuesday morning prior to fropa with decreasing cloud cover
and breezy conditions Tuesday afternoon after fropa. Skies will
gradually clear Tuesday night as a much drier airmass gradually
advects in. Rather extensive diurnally driven cu will develop
during the day Wednesday before skies clear again Wednesday night.

Regarding late this afternoon and early evening a few showers and
thunderstorms will continue to develop mainly east of US-131 where
moderate instability (ml cape values around 1500 j/kg) has developed
along with some sfc convergence with the lake breeze. None of this
activity will be severe but brief heavy rainfall is possible.

.LONG TERM...(Thursday through Monday)
Issued at 305 PM EDT Mon Aug 21 2017

As we have been writing about for nearly a week, and the models have
persistently forecast, a significant and prolonged cooler then
normal temperature pattern is on the way. It could well be that most
locations in SW MI will not see highs at or above 80s till at the
middle of next week if then. Actually the GEFS ensembles keep highs
below 80 through Sept 5th.  The 12z ECMWF has highs around 80 on the
2nd of September. Clearly we are in for a prolonged period of colder
than normal temperatures.

The overall upper air pattern favors a longwave trough through most
of eastern Canada and the northeastern CONUS through the middle of
next weekend. We do have a strong system in the north Pacific that
comes from northern Russia... which crosses the dateline later
Monday into early Tuesday. That should kick some upper wave energy
out of the system currently in the Gulf of Alaska. At some point
early in the following week this will give should give us our next
significant precipitation event but the models are all over the
place as to when and how this happens. One thing for sure, not big
warm ups any time soon.

I added showers to our NE CWA on Thursday afternoon and early
evening as a northern stream shortwave, assoicated with the coldest
pool, of air moves into Michigan. There is enough instability and
just enough low to mid level moisture that I do think we will see
scattered afternoon showers north and east of Grand Rapids. Other
than that it may remain dry in this area until beyond Monday the
28th.

This second cold push may just be cold enough that areas north and
east of Grand Rapids could have lows in the mid to upper 30s Friday
morning and maybe even Saturday morning. In the coldest locations
like Leota, I would not be surprised if there is a touch of frost
Friday or Saturday morning.

My bottom line to this is cooler than normal temperatures and little
in the way of meaningful precipitation from Thu through Mon (and
beyond).

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 135 PM EDT Mon Aug 21 2017

For the most part, expect VFR conditions until the pre-frontal
convection moves through the area in the 06z to 09z times frame.
I then expect a band of heavy rain to sweep across the TAF stasis
the storms just ahead of the front move through the area. Some
sites may have a period of brief IFR Behind the line of storms
there will likely be a period of rain showers (no thunder) then
finally the front will come through, bring the winds to the west
or Northwest and there will be low MVFR cigs until around early
afternoon as the drier air moves in.

&&

.MARINE...
Issued at 305 PM EDT Mon Aug 21 2017

Will issue a small craft advisory from early Tuesday morning
through Wednesday afternoon. Winds will veer to the west to
northwest and increase Tuesday which in conjunction with cool air
advection will result in a significant increase in wave heights
from early Tuesday through the day. Wave heights will remain
hazardous to small craft through Wednesday afternoon. Thx for
coord on small craft headline IWX/APX.

&&

.HYDROLOGY...
Issued at 305 PM EDT Mon Aug 21 2017

Area rivers are around normal levels for the time of the year and
well below bankfull. Though we are going to get localized heavy rain
of up to an inch tonight into Tuesday morning, most areas will not
see even minor river flooding. Some water covered roads are
possible. No significant precipitation is expected Wednesday through
Sunday, so any rises on rivers will have time to recover.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Tuesday morning through Tuesday
     evening for MIZ037-043-050-056-064-071.

LM...Small Craft Advisory from 6 AM Tuesday to 6 PM EDT Wednesday for
     LMZ844>849.

&&

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...WDM
AVIATION...WDM
HYDROLOGY...63
MARINE...Laurens



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