Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 250541

Area Forecast Discussion
National Weather Service Grand Rapids MI
141 AM EDT Fri Sep 25 2020


Issued at 317 PM EDT Thu Sep 24 2020

- Warm with little precipitation through Saturday

- Cooling some Sunday but still little rain

- Breezy and much cooler with showers much of next week


.DISCUSSION...(This evening through next Thursday)
Issued at 317 PM EDT Thu Sep 24 2020

-- Warm with little precipitation through Saturday--

Through Saturday it for the most part will be warm and dry (little
if any rain). The frontal system tied to the northern stream jet
will lift north into Saturday. Mid-level ridging 850 to 300 mb
will cause a deep layer of dry air during most of this time frame
and prevent and upstream convection from getting into most of our

Our northwest and extreme northern CWA could see a few showers
tonight but any precipitation would be limited in amount and be
hit and miss.

Friday the lead upper shortwave that is associated with the upper
level system that will finally bring in cooler air to this region
is help to build mid to upper level heights and strengthen the
ridging. This will lead to a warm day Friday with only a few

Friday night the lead shortwave tracks well north of this area but
enough trailing energy remains that our northwest CWA may get some
showers and maybe a thunderstorm. Elsewhere it will just stay
mild and dry.

Saturday a 140 knot speed max digs into the western side of the
upper through over Southwest British Columbia, which starts to
deepen the trough. That leads to increasing winds here as surface
low develops on the front over the Central Plains and tracks
northeast. This will be our warmest day for the rest of the year
and likely the last day we will see highs in the 80s. My 1000/925
thickness tool gives GRR a high of 86 Saturday.

-- Cooling some Sunday but still little rain--

As that first speed max races through the bottom of the trough and
starts climbing out of the trough over MN/WI Saturday night,
significant mid level instability develops over western MI with
steep lapse rates. However there is way too much dry air at mid
levels to get much convection here. Our northwest CWA may see
thunderstorms in the late evening but the storms will struggle to
fight the dry air so they should not make it past GRR.

-- Breezy and much cooler with showers much of next week--

The surface front comes through Southwest Michigan during the day
on Sunday. We may see some jet entrance region (southwest quad of
departing jet) lift help to cause post frontal rain due to
isentropic lift later Sunday into Sunday night.

Meanwhile a strong jet core digs into the deepening trough and
closing off an upper low over the central Plains by Monday
morning. Then an even stronger jet core, near 180 knots digs into
the northern stream trough Monday afternoon over western Canada
and that causes the upper low and associated surface low to track
nearly due north toward Michigan. The systems merge on Tuesday
near Michigan. This causes an negative tilt upper trough and
deepening surface low. I do believe this will finally be enough
to bring rain to the entire area. There is enough mid level
instability for convection and thunderstorms Tuesday into
Wednesday. There are some significant differences between the
ECMWF and GFS but both give us rain and strong winds. It may take
until Friday of next week for this system to clear the Great
Lakes. Expect lake effect rain showers, water spouts, soft hail
and strong winds Wednesday into Thursday. Highs may not get out of
the 40s in some parts of our CWA Thursday.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 141 AM EDT Fri Sep 25 2020

Impacts to aviation for the terminals look to be limited, with a
lower chance of some fog at KJXN and KLAN being the biggest
possible impact. Skies are mostly clear with only some high clouds
near KGRR and KMKG, Some low stratus and fog is trying to move in
off of Saginaw toward the area, but this should stay north of I-96
and the forecast terminals as southerly winds should keep it from
getting too far south. We added some VCFG and even a tempo group
at KJXN with vsbys down a little bit around 8sm. KJXN is in a
lower spot that is usually quick to see fog if there is a chance.

Any patchy fog would mix out quick this morning with winds aloft
picking up, and nearly full sunshine expected. Some cumulus clouds
can be expected by afternoon. Southerly winds will likely pick up
toward 18z with good mixing. Wind gusts up to 20 knots will be
likely. These winds will fall off toward sunset. They do look to
increase again at KMKG late in the period.


Issued at 317 PM EDT Thu Sep 24 2020

We have two periods of strong winds in the short term, the first
is Friday night in warm advection part of the system. However the
stronger event is expected Saturday night as the cold front heads
into the area while a 992 mb low tracks northeast through
Wisconsin. The GFS model shows the mixed layer over the south Buoy
is mixing down 40 knots. The ECMWF surface wind speed is in the 25
to 30 knot range Saturday night with gusts over 45 knots. I would
have to believe we will have at least a small craft advisory for
tomorrow night into Sunday. We may need a gale warning north of

Of course when the system moves through the Great Lakes Tuesday
into Wednesday I would expect gales during that time frame too.




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