Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 210522

Area Forecast Discussion
National Weather Service Marquette MI
122 AM EDT Sat Oct 21 2017

.SHORT TERM...(This evening through Saturday)
Issued at 354 PM EDT FRI OCT 20 2017

No major impacts expected  during the short-term portion of the

Tonight through Saturday: Surface high pressure, in place across the
Upper Great Lakes, will slowly begin to slip off to the east of the
U.P. through this time period. Aloft, an upper-level ridge axis
running through the Upper Great Lakes through late this evening,
will shift eastward to the Lower Great Lakes by Saturday afternoon.
This along with fairly dry air in place will keep most of the U.P.
dry tonight into early Saturday. Sky cover will begin to increase
over the west half of the area late tonight as return moist air flow
kicks in on the back side of the departing surface ridge and out
ahead of an approaching cold front. While models are hinting at some
precipitation pushing into the far western U.P. late tonight into
early Saturday morning, the dry air in place will likely keep the
rain showers west of the area into Saturday morning. There are
indications that a shortwave may lift notheastward into the far
western portion of Lake Superior and far wester Upper Michigan
Saturday morning, which may bring some isolated rain showers early
in the day. By Saturday afternoon, a cold front, extending southward
from a strong surface low over northwestern Ontario. Along and
directly ahead of the cold front, forcing and moisture will push
into the far western portions of the U.P. by very late Saturday
afternoon. This will allow for a few scattered rain showers to slide
into far western Lake Superior and possibly even into the Ironwood
area; however, much of the rest of the U.P. should remain dry
through Saturday afternoon. MUCAPE values are progged to be around
250-500 J/kg between 21Z/21 and 00Z/22, which will also introduce
the chance for thunderstorms over the far west.

.LONG TERM...(Saturday night through Friday)
Issued at 354 PM EDT FRI OCT 20 2017

Main weather impacts in the long-term portion of the forecast is the
thunderstorm potential Saturday and Saturday night, then cooler
temperatures and possibly some snow mixing in with the rain next

Saturday night into Sunday: The aforementioned cold front is
expected to slide eastward into and then through the Upper Great
Lakes region through this time period. At the same time, upper level
troughing will slide in behind the cold front along with drier air.
Increased moisture and forcing ahead of the front will allow for
widespread rain showers to spread across the U.P. Models continue to
show roughly a quarter to half inch of precip across the area by the
time the precipitation ends from west to east mid to late Sunday
afternoon. MUCAPE values remaining in the 250-500 J/kg through the
late evening/early overnight hours.  Severe potential looks to be
fairly limited as CAPE is fairly skinny and not overly impressive;
however, deep layer shear values are progged to be around 30 knots
parallel to the front, which may allow for a couple stronger
thunderstorms early Saturday evening. The best chance of seeing a
stronger storm or two will be along the WI border over the west
half.  Temperatures will be about 10 to 20 degrees above normal
through this time period.

Sunday night through Tuesday: There will likely be a brief break in
the precipitation Sunday night as a brief ridge builds into the area
along with drier air; however, a broad surface low is progged to
slide across the Upper Great Lakes region through this time period.
Aloft, a much deeper trough will steadily dig into the area. The
combination of the aforementioned features along with a decent shot
of moisture sliding into the area will allow for unsettled rainy and
cooler weather through Tuesday. As the trough axis slides across the
U.P., 850mb temperatures are progged to drop into the -5C to -7C
range Monday night into Tuesday, creating delta-T values in the 17C
to 19C range. The EC is a bit warmer and a little slower to bring in
the colder air aloft. These surface to 850mb temperature differences
will be primed for lake enhancement. The favored areas will have to
be pinned down as models come into better agreement, but it looks
like the best chances will be for northwest wind lake effect prone
areas. There may even be enough cold air to slide into the region to
cause some snow to mix in with the rain over the higher terrain of
the west as the trough axis slides overhead. The ground is still
warm and overall QPF is not expected to be that impressive, so not
expecting too much in the way of accumulation. It may be breezy over
the east half Tuesday afternoon into Tuesday night depending on the
intensity of the next low pressure system.

Wednesday through the extended: A shortwave ridge is expected to
slide through the Upper Great Lakes Wednesday, which will likely
lead to another break in the precipitation. Wednesday night through
Friday, models begin to diverge on solutions for this time period;
however, there is at least some agreement that a cold front will
slide into the area at some point Thursday into Friday. The issue is
with the timing. The GFS is more progressive, which is not uncommon
for this time of year, while the EC is a bit slower, bringing the
cold front through Thursday night into Friday morning. Depending on
how quickly the front moves through and the moisture exits the
region, there is a possibility that rain may mix with snow again
over the west half. Behind the front, both models show a trend
toward a deeper trough pattern over the central portion of the U.S.
toward next weekend. A consensus of the model solutions was used for
this time period due to lack of confidence in a single solution.

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 121 AM EDT SAT OCT 21 2017

VFR conditions will continue. LLWS is expected at all sites tonight.
A cold front will approach the western TAF sites which will bring in
ceilings and possibly a light shower later Saturday. Rain chances
will gradually increase Saturday evening with cigs dropping into the
MVFR range at KIWD.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 354 PM EDT FRI OCT 20 2017

Tonight through Monday, winds will remain mostly between 20 and 25
knots. There is potential for gusts up to 30 knots late Saturday
ahead of an approaching cold front. North gusts to 30 knots (and
maybe some gales) are possible late Tuesday afternoon into Tuesday
night after the cold front passes and another area of low pressure
and cold advection moves into the Upper Great Lakes. The tightest
pressure gradient would likely be over the eastern portions of Lake
Superior. There are still considerable differences in the track of
the low, which will have an impact on where or if the gales develop.

Upper Michigan...
Lake Superior...
Lake Michigan...


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