Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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597
FXUS63 KGRR 301727
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
127 PM EDT SAT JUL 30 2016

LATEST UPDATE...
Aviation

.SYNOPSIS...
Corrected at 346 AM EDT Sat Jul 30 2016

Periods of showers and isolated thunderstorms are expected today
into Sunday. Some of the showers will be locally very heavy with the
potential for localized flooding. It will be cooler too with highs
only in the upper 70s to lower 80s both today and Sunday. The slow
moving storm that is causing all of this rainfall will finally move
out of the area by Sunday night. This will allow a large high
pressure system to move over Michigan, clearing the skies and
bringing warmer afternoon temperatures.

A warm front will come through the area on Wednesday putting
Southwest Michigan back into the hot and humid air. A cold front
will move into the area late Thursday with the threat of
thunderstorms, but not before highs reach around 90 degrees.

&&

.UPDATE...
Issued at 1034 AM EDT Sat Jul 30 2016

We have freshened up the forecast a bit to account for the latest
trends in the rainfall this morning. Band of rain and embedded
thunder has been focused mainly south of a Holland to Lansing
line. This rainfall, as has been the case the past two days, has
been very efficient with a very moist atmosphere in place and due
to the slow movement of the rain.

We expect that this band of rain will diminish through early
afternoon. Additional more cellular showers and a few storms will
then develop this afternoon as another short wave moves in from
the W/SW. Fortunately, instability will be limited down south due
to the clouds and rain this morning. Some showers/storms could develop
just inland of the lakeshore where a diurnal trough/convergence
zone develops. The lack of instability and deep layer shear will
keep severe potential quite low. Heavy downpours will be the main
threat, especially with the slow movement of cells.

UPDATE Issued at 709 AM EDT Sat Jul 30 2016

I have updated the zones to have the more significant rainfall
farther north through the day today. The deformation axis at mid
levels stays from near Holland to Grand Rapids to Saginaw but the
strength of the deformation increases significantly this
afternoon as the upper wave from NE moves into the western Great
Lakes. This also increased the upper divergence and given our
precipitable water is forecast to be nearly 2 standard deviations
from normal, I have to believe locally heavy rainfall is a real
threat. So I have that in our forecast wording too. Once this NE
shortwave reaches lower Michigan the lift should diminish so all
of this should quiet down this evening.

&&

.SHORT TERM...(Today through Monday)
Corrected at 346 AM EDT Sat Jul 30 2016

The main issue today is the threat of localized flooding from slow
moving heavy rain showers and thunderstorms. This threat really goes
into Sunday as the system creating all of this will likely not move
out of the area until Sunday night.

We have precipitable water values near 1.75 inches this afternoon
over a good part of our CWA. While that is happening, we have the
deformation axis in the 500 to 700 mb layer from near MBS to GRR and
not too surprisingly, we have decent upper divergence over this same
area. All of this is caused by the next upstream shortwave
(currently near Nebraska and Kansas) moving into the trough over the
Great Lakes. That significantly increases the upper divergence and,
in turn, lift. So I see bands of showers rotating from south to
north through the CWA then stalling near the deformation axis into
this evening. Afternoon heating will help increase the lift and
strength of the showers. I do not at this time plan to issue a Flood
Watch as it would seem any heavy rain will be rather localized.
Still we will have to watch this.

It will take until Sunday evening to finally start lifting this
system out of the area. So that means I have extended the showers
into Sunday. Sunday will feature decent instability, so
thunderstorms are once again expected. Locally heavy rainfall is
also possible.

Ridging builds in Sunday night into Monday, so skies should clear
and allow for some warming of the afternoon highs by Monday.

.LONG TERM...(Monday night through Friday)
Issued at 330 AM EDT Sat Jul 30 2016

The trend in the forecast for next week is warmer and drier with a
fair amount of sunshine. In general, the upper level pattern will be
defined by ridging through much of next week. 1000-500 mb
thicknesses and 850mb temps will gradually climb Monday night
through Thursday. Highs in the mid 80s should be common Tuesday and
Wednesday, with upper 80s to around 90 degrees more likely for
Thursday.

There are a couple shots at precip next week. The first will be
Tuesday into Tuesday night as a weak shortwave dives southeast
embedded in broad northwest flow aloft. As the shortwave moves
through Chicagoland into northern Indiana, a warm frontal boundary
will be draped across Indiana and Ohio. Will feature highest PoPs
(30 pct) across Southwest Lower.

A better chance for precip will arrive Thursday night and Friday. A
cold front is forecast to move through the state on Friday. This
feature is associated with a closed upper low over Hudson Bay. A
moisture-rich, humid airmass will be in place out ahead of the cold
front as evidenced by a plume of higher theta e extending from the
Gulf Coast states into Michigan.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 121 PM EDT Sat Jul 30 2016

Three areas of concern for the 18z TAFs.  First and foremost is
the low cloud cover across west Michigan in the wake of morning
showers and thunderstorms. Most observations in southwest Michigan
and adjacent areas of Indiana are MVFR with occasional IFR
ceilings. While there may be breaks that develop as the initial
wave moves east...will continue ceilings for the rest of this
afternoon...but will bring ceilings up solidly into the MVFR
range.

Second forecast conundrum is precipitation chances. Plenty of
moisture is available for convection and upper level system is
approaching from the west. Even so...short range convection
allowing models do not develop widespread precipitation this
afternoon and evening...likely due to limited destabilization from
cloud cover. Will follow the HRRR/HRRREXP output...which
emphasizes development in central Michigan (KLAN/KJXN)...with a
secondary area near KMKG. Forecast confidence is low.

Final issue is low cloud and fog development overnight. Previous
forecast brought in IFR conditions...and see no reason to deviate
from that...especially given the abundance of low level moisture
from precipitation today.

&&

.MARINE...
Corrected at 346 AM EDT Sat Jul 30 2016

Northeast winds of 5 to 15 knots will not cause much in the way of
beach hazard conditions. However, the threat of locally heavy
showers and thunderstorms will be an issue through Sunday.

&&

.HYDROLOGY...
Issued at 328 PM EDT Fri Jul 29 2016

Thunderstorms the past two days have primarily been heavy rain
threats, and this will continue to be the case into Saturday. A slow
moving trough with weak vertical shear will continue to promote
isolated to scattered slow-moving storms. No substantial dry air can
be found in the deep convective layer, and precipitable water values
are still around 1.5 inches which is near the 90th percentile for
late July climatology. Slow moving storms Thursday into Friday
produced a number of isolated reports of rainfall greater than 2.50
inches, along with street and parking lot flooding several inches
deep. We will continue to stay alert for the isolated flooding
threat through Saturday.

&&

.GRR Watches/Warnings/Advisories...
MI...None.
LM...None.
&&

$$

UPDATE...NJJ
SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...EBW
AVIATION...MWS
HYDROLOGY...CAS
MARINE...WDM



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