Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 130444

Area Forecast Discussion
National Weather Service Grand Rapids MI
1244 AM EDT Fri Oct 13 2017


Issued at 317 PM EDT Thu Oct 12 2017

A cloudy night with pockets of light drizzle will give way to
partial sunshine and milder temperatures on Friday. A frontal zone
will waver over the area Friday night through Saturday night as a
wave of low pressure tracks through the northern Great Lakes.
Occasional rain and thunderstorms will develop in the vicinity of
this front. Appreciable rain appears likely in areas north and
west of Jackson, with a soaking rainfall over 2 inches possible in
Central Lower Michigan. Cooler and drier weather will be in place
early next week followed by moderating temperatures mid-week.


Issued at 1012 PM EDT Thu Oct 12 2017

Have issued a quick update to expand the mention of patchy
drizzle to include the entire area. Several ob sites have carried
drizzle this evening or have had rain began/end times in the
remarks indicative that some light precip has occurred. This
should continue on and off overnight as RAP bufkit overviews
show ocnl weak omega in the cloud layer.


.SHORT TERM...(This evening through Saturday night)
Issued at 317 PM EDT Thu Oct 12 2017

A moisture-laden thermal trough at 850 mb will advect through the
area tonight. Steep lapse rates within the stratus layer (between
about 1 to 5 kft AGL) may encourage enough upward vertical motion
to grow droplets large enough to precipitate. Low-level moisture
in areas north of Holland and Grand Rapids will be augmented by
flow off Lake Michigan. As such, the greater potential for precip
tonight will be in west-central Lower Michigan. Dry air in layers
colder than +3 C and a lack of cloud ice in the column will limit
the potential for measurable precipitation, as cloud droplet size
may only grow large enough for drizzle.

Warmer low-level air (around 10C at 850mb) will flow into the area
on Friday. With breaks in the clouds, could see highs reaching 70
near/south of Holland to Lansing.

Light rain will overspread portions of the area Friday night under
mid-level moisture transport and isentropic ascent. There may be
just enough elevated instability for convection to reach the -10 C
layer to introduce thunder chances early Saturday morning.

Occasional thunderstorms will develop Saturday in the vicinity of
the surface warm front, which should lift north late Saturday as
southerly flow strengthens ahead of the deepening surface low.
Temperatures could reach the mid 70s in southern Michigan if rain
stays north.

Stronger storms with locally heavy rainfall are possible Saturday
evening and overnight as low level winds a couple thousand feet
off the surface exceed 50 knots, MUCAPE values climb above 500
J/kg, and PW values climb to around 1.5 inches. If the surface
stays warm enough overnight to maintain DCAPE where elevated
convection tracks, isolated severe wind gusts would not out of the
question. Rainfall potential is discussed further in the Hydrology

.LONG TERM...(Sunday through Thursday)
Issued at 317 PM EDT Thu Oct 12 2017

Strong storm system will be exiting the region Sunday...leaving
significantly colder air and a strong and gusty west to northwest
wind in its wake.  Strong cold advection will likely result in
falling temperatures Sunday afternoon...indicated in the grids.
Sunday night frost could be more widespread than currently
indicated if the wind can ease faster than expected.

Cold air will linger Monday but warmer air will return quickly in
the progressive mid level flow.  Strong warm advection pattern
develops later Tuesday into Wednesday...which should provide a
return to unseasonably warm temperatures.  Dry frontal passage on
Thursday will be followed by another episode of warm and windy
weather into next weekend.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1244 AM EDT Fri Oct 13 2017

Conditions continue to range from VFR to IFR across the region,
making for a difficult forecast. In general expect much of the
overnight to remain MVFR, but occasional periods of IFR will be
mixed in. So overall little change until late morning when more
wind/mixing it expected and the lower ceilings and vsbys will

A VFR period is expect from late morning through the afternoon.
Then the I-96 TAF sites should trend back to MVFR into the
evening, while the VFR should be maintained closer to I-94. Some
showers should move into the I-96 TAFs toward the end of this
forecast period, 06Z.

As mentioned above, the winds will increase by late morning and
continue through the afternoon, with SSW wind around 10 knots.
This will lessen again toward evening.


Issued at 317 PM EDT Thu Oct 12 2017

Winds veering southwest tonight will build waves to 2-3 feet north
of South Haven on Friday. Winds will shift southeast on Saturday
but thunderstorms are a possibility. Gales appear increasingly
likely Saturday night into Sunday.


Issued at 1236 PM EDT Thu Oct 12 2017

We are monitoring the potential for heavier rains and possible
flooding for this weekend. A frontal system settling over the area
beginning Friday night and lasting into Sunday will bring multiple
periods of rain.

Precipitable water values will be about as high as possible by
mid-October standards with values over 1.5 inches expected. This
will bring the potential to see locally heavier downpours, and
some narrow bands of heavier accumulations through the period.
The axis of the heaviest rain for now looks to occur generally
north of a line from South Haven to Saginaw Bay. This is still
subject to change depending on where exactly the front sets up.
Right now, heaviest amounts looks to be over 2 inches, with some
areas possibly seeing locally up to 4 inches.

Thankfully this is displaced a little north from the heavier
rains earlier this week. We still could be looking at some minor




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