FXUS63 KGRR 161753

Area Forecast Discussion
National Weather Service Grand Rapids MI
153 PM EDT Thu Aug 16 2018


Issued at 323 AM EDT Thu Aug 16 2018

Expect mostly cloudy skies with scattered showers and a few
thunderstorms from late morning into mid evening. It will remain
fairly humid with highs near 80 degrees. The low pressure system
causing the clouds and showers will slowly move across Lower
Michigan today and across Lake Huron tonight. Weak high pressure
will build into the area Friday and remain in control over
Southwest Michigan's weather through this weekend. There will be
enough instability around yet Friday for afternoon showers and
thunderstorms to still be possible. However Saturday and Sunday
should be warm and dry. Early next week when another storm system
will again bring the threat of showers and thunderstorms. Cooler
weather is expected by mid week behind that system.


Issued at 1115 AM EDT Thu Aug 16 2018

Latest trend in the short range guidance is to have the highest
concentration of convection north of I-96 - especially near the
U.S. 131 corridor north of GR - late this afternoon and early
evening. This is where the best sfc convergence is progged to be,
related to the lake breeze front. Will feature highest pops (still
50/scattered though) for places like Big Rapids, Baldwin, Reed
City and Newaygo. These storms are not expected to move much at
all, so a localized threat of heavy rainfall may develop. As
cells collapse look for new cu development on the outflows.
Elsewhere, generally only random isolated showers/storms
anticipated. Convection peaking between 4 pm and 7 pm,
dissipating with sunset.


.SHORT TERM...(Today through Saturday)
Issued at 323 AM EDT Thu Aug 16 2018

The main question for today is will there be significant rainfall
in any part of Southwest Michigan? Also, will there be enough
instability and mid level dynamics around Friday for afternoon
showers and thunderstorms?

There has been an overall trend for the significant rainfall
(more than a half inch in 12 hours) to farther to the south and
east of the Grand Rapids forecast areas, for several days now.
That trend continues in the latest models. Also the latest radar
and satellite data confirm the trend will turn out to be valid.
Dynamically speaking, the low level jet is and will continue to
be south and east of this area today. The best 1000/850 moisture
transport will be toward Lake Erie today into tonight. Thus the
heavier and more persistent rainfall will be well east of our
area today. Radar trends confirm this idea.

Even so we do have that shearing out upper wave moving over the
area today, ahead of the much stronger closed upper low currently
centered over southern South Dakota. That will rotate southeast
and miss us to the south Friday. As the shearing out upper wave
moves through there will be just enough instability and deep
moisture for some convection by late morning into the evening.
Nothing severe or even close (no significant deep layer shear) to
that but enough that if you had one of these showers pass
overhead you'd get a downpour as precipitable water values are
near 2 inches.

Friday afternoon instability combined with the upper level trough
passing over the area should be enough of showers and isolated
thunderstorms, mostly east of US-131.

Surface high pressure and weak upper level short wave ridge should
provide a warm and dry Saturday.

.LONG TERM...(Saturday night through Wednesday)
Issued at 323 AM EDT Thu Aug 16 2018

High pressure over the Great Lakes will produce warm and dry weather
during the weekend. Highs will be in the mid 80s Sunday. After
Sunday, the models have very different solutions. The ecmwf has been
consistent in showing a deepening surface low over Missouri moving
northeast across Michigan Monday night and Tuesday. The gfs has a
much weaker and farther south low. These differences can be traced to
the evolution of the supporting wave. The ecwmf is a bit deeper and
phase northern and southern stream waves to produce a deeper low
over the state. The gfs just has a southern stream wave and as a
result is weaker and farther south. Much of the precipitation shown
on the gfs is from showers produced as a cold front moves south over
Lower Michigan Monday night. For now, we'll lean toward the gfs as
it's always tricky forecasting the phasing of northern/southern
stream systems.

Cooler temperatures and lower dewpoints will move south across the
cwa Tuesday/Wednesday behind the departing system. Highs will drop
into the upper 70s and dewpoints into the mid 50s leading to less
humid conditions next week.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 153 PM EDT Thu Aug 16 2018

MVFR to VFR cigs of 2500-4000 ft are expected this afternoon.
Widely scattered showers/tstms could produce IFR or lower vsbys
if they pass over a terminal. Have only mentioned thunder in the
GRR TAF from 22Z to 02Z as latest HRRR guidance shows convection
most focused along U.S. 131 at peak heating. Thunder can't be
completely ruled out at the other terminals and AMDs can be
issued later if on radar it looks like they'll be impacted.

Showers and tstms will dissipate by midnight, then cigs lowering
again in most places late tonight. IFR or lower stratus and fog
is expected mainly 10Z-14Z, particularly south and east of GRR.
Cigs gradually lifting Friday from mid morning on, with more
widely scattered showers and storms developing the afternoon.


Issued at 323 AM EDT Thu Aug 16 2018

With weak surface systems around and not much in the way of
significant cold air to follow this system Friday into Saturday
winds and waves should remain below small craft criteria.


Issued at 153 PM EDT Thu Aug 16 2018

Since Wednesday morning...isolated rainfall amounts of one half to
one inch occurred south and west of Kalamazoo...between Ludington
and Big Rapids...and in the Lansing area. Lesser amounts were noted
in other showers. The isolated nature of the rainfall did not
produce significant rises on area rivers.

Areal coverage of precipitation today is also expected to be
limited. While isolated heavy rainfall amounts will be possible...
significant runoff will be localized and impacts to river stages
will be minimal.




Back to Main Forecast Discussion Page