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643
FXUS63 KGRR 160543
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1243 AM EST Mon Dec 16 2019

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 235 PM EST Sun Dec 15 2019

- Storm system passes by to the south on Monday

- Lake effect snow showers late Tuesday into early Wednesday

- Cold for mid week

- Not as cold next weekend

&&

.UPDATE...
Issued at 918 PM EST Sun Dec 15 2019

The inherited forecast is on track with no changes needed.

More technical discussion:

A large band of wintry precipitation extends from West Virginia
into west central Illinois associated with a baggy surface low
pressure trough across the southern US. A separately-forced area
of light radar returns extends from central Wisconsin to eastern
Iowa, which is associated with a subtle and weak shortwave trough
embedded in otherwise zonal mid-level flow. Surface observations
indicate nothing is making it to the ground roughly north of I-80
and indeed with the latter feature, with the 00Z DTX RAOB as well
as recent AMDAR aircraft soundings from the Milwaukee and Chicago
areas showing an incriminating pocket of very dry air centered at
about 3000 ft. So, most of the radar returns upstream of our area
are indicative of virga. As a broad trough continues digging into
the Four Corners region through the overnight hours, the
aforementioned shortwave responsible for virga upstream will
dampen considerably leading to weakening support for continued
mid-level precipitation development. Coupled with the pocket of
dry air at 3000 ft, it seems unlikely anything more than a flurry
or two will be able to make it down across western Lower Michigan
overnight. As such, we favor a continued dry forecast.

With continued cloud cover and nearly no wind in the lowest few
thousand feet of the atmosphere (e.g. little in the way of any
sort of advection), temperatures overnight will remain steady
through daybreak.

&&

.DISCUSSION...(This evening through next Sunday)
Issued at 235 PM EST Sun Dec 15 2019

The main challenges in the period deal with timing and amount of
any impacts. To start there is an area of light snow in eastern
Nebraska and western Iowa. This area of snow was tracking
east...in our direction. Models show the lift weakening with this
feature as it moves into Southern Lower Michigan late tonight into
Monday morning. This will support a diminishing trend the light
snow through the night into Monday morning. Still a small risk for
a period of flurries or perhaps light snow exists for Southern
Lower Michigan. Bufkit overviews do show a dry layer from 2000 to
roughly 5000 ft. This likely is not enough to prevent some of the
snow from reaching the ground. We will feature low POPs for
measurable precipitation after midnight tonight for the
Interstate 94 corridor.

Next up will be the lake effect snow later Tuesday into Wednesday.
Deeper cyclonic flow and the arrival of an arctic airmass will
support some impacts for parts of the region. Right now the models
are showing the best lift arriving Tuesday evening after 00z as a
700 mb wave drops in from the northwest. This is when we will
feature the highest POPs. With a Lake Superior connection...the
bands in northwest parts of our CWA will likely produce the most
impacts...which could linger into Wednesday morning. Guidance
shows a low level ridge building in for Wednesday afternoon and
evening which should support decreasing and less organized snow
shower activity. It is worth noting that the DGZ lowers close to
the ground...under the cloud bases late Tuesday night into
Wednesday which would likely alter the accumulation rate.

Deeper southwest flow develops for late in the week and through
the weekend which will support a moderating trend to the
temperatures.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1243 AM EST Mon Dec 16 2019

Ceilings have begun to lift towards VFR for most sites apart from
immediate lakeshore areas such as MKG and BIV where MVFR ceilings
persist. Virga is still expected at all TAF sites through the
morning although a few flurries will be possible, especially for
southern TAF sites such as AZO, BTL, and JXN. Winds will remain
light and variable through the entire TAF period.

&&

.MARINE...
Issued at 235 PM EST Sun Dec 15 2019

With surface high pressure forecasted to ridge into the region
tonight into Monday...the winds and waves are forecasted to remain
relatively low. That will change for Tuesday. The wind is
forecasted to strengthen out of the northwest as an arctic
airmass spreads in through Wednesday. Thus...hazardous small craft
conditions are likely to redevelop. Freezing spray may be possible
as well.  High pressure ridges in for later Wednesday into
Thursday. At this time it looks like conditions will improve
somewhat then.


&&

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

$$

UPDATE...Borchardt
SYNOPSIS...MJS
DISCUSSION...MJS
AVIATION...Honor
MARINE...MJS