Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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851
FXUS63 KGRR 222356
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
756 PM EDT Sat Jul 22 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 328 PM EDT Sat Jul 22 2017

Generally fair weather will continue the next few days with the
exception of a few showers and thunderstorms tonight and Sunday
north and east of Grand Rapids. A cold front moving through Sunday
afternoon could drive some stronger thunderstorms towards Lansing
and Jackson.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 328 PM EDT Sat Jul 22 2017

Forecast challenge in the near term is for coverage of showers
tonight and potential for strong to severe storms Sunday mostly
east of Grand Rapids.

For tonight...expect showers and isolated thunderstorms after
midnight across the northern zones as vigorous short wave trough
with closed upper low approaches in fast northwest flow regime.
This trough will also be driving a cold front through Sunday
evening. Sfc convergence ahead of the front combined with
afternoon instability and about 30 to 35 knots of 0 too 6 km shear
should be enough to generate a line of thunderstorms during the
afternoon.

The strong west to northwest sfc flow in the afternoon indicates
that lake modified air will be driven well inland leading to the
convective lake shadow effect and this is also indicated by the
NCAR HiResW ARW which has a line of strong convection forming
across the eastern forecast area around 18Z to 20Z and quickly
moving east. So the severe weather threat does appear limited to
the eastern zones on Sunday afternoon.

Once the storms go past there will be a few instability showers
remaining through Sunday night in cyclonic flow which could be
diurnally enhanced on Monday before the upper trough axis moves
east and sfc ridging builds in for Monday night.

.LONG TERM...(Tuesday through Saturday)
Issued at 328 PM EDT Sat Jul 22 2017

Tuesday will start off with high pressure overhead.  This fair
weather system is predicted to track eastward through the day.
Near seasonable temperatures and dry weather are expected.

Southerly flow increases Tuesday night into Wednesday ahead of an
approaching cold front.  Warmer temperatures will make a return to
the region with highs approaching the mid 80s for many locations.
This will also lead to building instability during the day. Any
morning showers could turn into thunderstorms for the afternoon.
Models do vary on the amount of instability that develops with the
High Res Euro showing much less than the GFS.  Either way...the
combination of synoptic lift...instability and a surface cold front
approaching will feature a risk for storms in the forecast until the
cold front pushes through.

A positively tilted mid to upper level ridge is shown to move in for
the beginning of the weekend.  Will go with dry weather and close to
seasonable temperatures.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 756 PM EDT Sat Jul 22 2017

It would seem considerable low clouds (IFR/MVFR) ceiling) are
indicated overnight into midday Sunday before the cold front
clears this all out by early to mid afternoon. There could be
showers and thunderstorms with the cold front Sunday but if that
happened it would be brief (less than 30 minutes).

This is a rather complex situation tonight into Sunday afternoon
fro our TAF sites. We have an upper low dropping southeast into
Michigan for Sunday. Meanwhile we have a lingering surface low
stalled near Lake Huron/Western Lake Erie this evening (it is
waiting for the upper low to pick it up and move it along).
The surface winds are mostly easterly near and east of I-75 and
northerly between US-131 and I-75, then westerly west of US-131.
That creates a convergence band clearly seen on the GOES 16
variable image loops from near MKG to GRR. That has MVFR cigs
assoicated with it. North of that it becomes VFR. Over areas near
and east of I-75 we have IFR conditions.

Early tonight the convergence band clouds should dissipates so
most of the TAF sites should be VFR for at least a few hours. Then
overnight the east winds north of that low bring the IFR
conditions westward into most of the TAF sites. With LAN much
closer to Saginaw Bay, I could see them going solid LIFR in dense
fog and low clouds by 09z or so.

Meanwhile the cold front continues to march eastward and that
currently has thunderstorms assoicated with it. That band of lift
and instability crosses our western TAF sites in the 09z to 15z
time frame and eastern TAF sites in the 15z to 21z time frame. It
seems to me there is not enough instablitly for thunderstorms over
the western TAF sites so I put VCSH there. For the eastern TAF
sites instabilty is higher so I did put VCTS there. All TAF sites
should become VFR by 21z or so.

&&

.MARINE...
Issued at 328 PM EDT Sat Jul 22 2017

Winds and waves will remain relatively light through the weekend.
There could be areas of fog tonight and a low chance of
thunderstorms late tonight and through SUnday afternoon.


&&

.HYDROLOGY...
Issued at 1123 AM EDT Sat Jul 22 2017

We again escaped the heavy to excessive rain that fell in
southern Wisconsin and northern Illinois. While that area floods,
most of our area continues to run drier than normal these past 1
to 2 weeks. With high atmospheric water vapor content, the showers
Saturday morning efficiently put down over a half inch of rain but
only in a few isolated spots. Scattered showers or storms Sunday
will provide only spotty relief to the dry weather. After that, the
next potential for a storm will be Wednesday.

&&

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

$$

SYNOPSIS...Ostuno
SHORT TERM...Ostuno
LONG TERM...MJS
AVIATION...WDM
HYDROLOGY...CAS
MARINE...Ostuno



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