Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 161808
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
208 PM EDT Fri Jun 16 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 320 AM EDT Fri Jun 16 2017

A few areas may get wet today and tonight with some storms
popping up across the area, but many areas will stay dry. The
better chance for thunderstorms comes later Saturday when some
severe weather is possible. Early next week looks cooler with
occasional chances for showers and perhaps some thunderstorms, but
with many dry periods as well.

&&

.UPDATE...
Issued at 1118 AM EDT Fri Jun 16 2017

Adjusted the timing of the scattered convection that is expected
today. Main time frame should be from now through about 20Z, with
the areas southeast of Grand Rapids seeing the highest coverage.
Will have 40 POPs there, meanwhile will have a dry forecast for
the NW CWA this afternoon.

The cloud cover will kept us from getting too unstable, so no
severe storms are expected. Drier air arrives later today, so will
feature a clearing trend from the NW after 21Z.

&&

.SHORT TERM...(Today through Sunday)
Issued at 320 AM EDT Fri Jun 16 2017

The main focus is on thunderstorm coverage today into tonight and
the threat for severe weather on Saturday into Saturday night.
Overall, the coverage of thunderstorms today will be low and the
threat for severe weather Saturday into Saturday night is
increasing but not certain as of yet.

There are some favorable factors both early this morning and into
the rest of the day for a few storms to pop up. A modest LLJ core
will develop this morning (25-30 kts) over the lake and spread east.
This occurs as some upper level divergence moves through. Deep layer
shear looks better today than yesterday, with 35-40 kts expected.
H1000-H700 wind convergence is noted on the GFS as we move
throughout the afternoon, possibly helping some convection grow. PW
values are respectable, in the 1.25"-1.50" range. However, high res
CAMs are yawning at the prospect for convection today, leading to
some speculation about coverage of storms. Kept POPs conservative
today, mainly focusing on both early morning and then late day
storm chances. As was the case the last several afternoons, many
locations won`t get a drop of rain while some may get occasionally
heavy downpours. Storm chances persist tonight as the LLJ holds
up over the area but again not expecting widespread activity at
this point.

The prospect for widespread showers and thunderstorms goes up
considerably Saturday, particularly late in the day. Surface based
CAPE across southern Lower MI will likely reach 1000 J/Kg or more,
coincident with increasing deep layer shear to 40 kts by 00z and
possibly 50 kts by 06z Sunday. The GFS and GEM have pretty vigorous
H1000-H700 convergence overhead during the evening and late night
hours Saturday. This occurs as H1000-H500 RH increases to 90%. With
a 40 kt LLJ moving in and solid upper level divergence, convection
may very well blossom over the region and it could be quite heavy.
PW values off the ECMWF are around 2.00" as this occurs.

The question becomes where will the convection fire and then where
will it propagate? The best instability may end up being near and
south of I-96. This is where the convection may be strongest and
it may tend to sink a bit south as it heads eastward with the mean
wind. The exact region is subject to change as there will be
areas of convection preceding the main show Saturday night, which
will influence mesoscale boundaries. Both strong wind gusts and
hail would be the primary hazards. While we certainly need some
rain and the ground will soak up quite a bit of what is coming,
there may be localized flooding (ie. nuisance street flooding) in
the heaviest storms.

.LONG TERM...(Sunday night through Thursday)
Issued at 320 AM EDT Fri Jun 16 2017

Chances for rain are not very high during the long term. By Sunday
night the cold front will have already moved through the cwa and
cooler and drier air will be advecting south from Canada. We should
see 2-3 days of highs in the 70s and lows in the 50s. We`ll see some
sunshine too, but the upper trough that will be moving across the
Great Lakes will generate moderately steep lapse rates that will
result in quite a bit of cu and perhaps scattered showers early in
the week.

A stronger wave rotating through the flow Tuesday may produce a
thunderstorm; Tuesday looks like the best chance of rain. High
pressure then builds back into the cwa. As the upper flow
transitions to zonal, we`ll see heights and temperatures rise toward
the end of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 208 PM EDT Fri Jun 16 2017

Outside of any shower/tstm activity, VFR weather is anticipated
over the next 24 hours.

Minimal thunder threat the rest of the afternoon as thick cloud
deck with bases around 10,000 ft is holding down instability. Next
decent chance of convection is after 06z tonight and kept VCSH in
the TAFs. Thunder is possible as well late tonight into Saturday
morning but was not confidence enough to put in the TAFs yet. The
better chance of thunder holds off until after 18z Sat.

&&

.MARINE...
Issued at 320 AM EDT Fri Jun 16 2017

Few concerns outside of any thunderstorm activity through
Saturday. Expect conditions to get a bit rougher on Sunday after
the cold front moves through and winds at the surface and aloft
pick up. Could certainly see some 2-4 footers with west winds.


&&

.HYDROLOGY...
Issued at 1150 AM EDT Fri Jun 16 2017

A small area of showers and thunderstorms moved through West
Michigan prior to sunrise...producing isolated half inch
accumulations across Ottawa/Kent/Ionia counties. No impacts are
expected on area rivers as a result of this rainfall.

An unsettled weather pattern will continue through Sunday morning...
with multiple episodes of showers and thunderstorms. Precipitable
water values of 1.5 to 2.0 inches are expected through early
Sunday...which would support areas of heavy rainfall.  Model QPF
forecasts differ with the location of the heaviest rainfall...but
locations south and east of Grand Rapids appear to have the
highest likelihood of accumulations of one to two inches by
Sunday morning when the cooler and drier airmass moves into the
region.

Area rivers are running well below bankfull stage. Significant
rises will be possible on area rivers and streams...especially in
the Grand and Kalamazoo river basins...but do not anticipate
rises above bankfull at this time.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

UPDATE...JK
SYNOPSIS...Hoving
SHORT TERM...Hoving
LONG TERM...04
AVIATION...Meade
HYDROLOGY...MWS
MARINE...Hoving



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