Area Forecast Discussion
Issued by NWS Marquette, MI

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124
FXUS63 KMQT 210825
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
425 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
forecast.

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...(Sunday through Friday)
Issued at 425 AM EDT SAT OCT 21 2017

So far, this autumn has be unseasonably warm. For just the last 30
days, temps are running roughly 5-8F above normal across Upper MI.
The upcoming week will see a pattern change that will send temps
down to near normal to modestly blo normal for late Oct. Given the
prevalence of unseasonable warmth this autumn, the change will seem
fairly dramatic. This pattern change will feature 2 periods of sharp
trof amplification into the central and eastern CONUS downstream of
a mean ridge along/near the w coast of N America that will undergo 2
distinct/strong amplifications. The second amplification late in the
upcoming week will be due in part to current typhoon Lan in the
western Pacific which will go on to become a powerful extratropical
Bering Sea storm. As for pcpn, a lead weakening trof reaching the
area on Sun will bring a period of shra across the area tonight/Sun.
Energy in the southern portion of this lead trof will briefly
separate out of the main flow and drop into the Lwr Mississippi
Valley late Sun/Mon. The first sharp trof amplification next week
will then pick up this energy. Depending on how sharply the trof
amplifies will determine whether this southern energy phases with
the upstream trof, spinning up a strong storm system that could
affect the Great Lakes region Mon night/Tue. Whatever happens, the
larger scale trof amplification will lead to colder weather and at a
minimum lake effect pcpn late Mon thru Tue. The next trof will
amplify into the central CONUS Thu/Fri, sending another cold front
across the area late week. Should see another period of unsettled
weather followed by an influx of colder air and lake effect pcpn to
round out the week.

Beginning Sun, cold front associated with aforementioned lead trof
will be over western Upper MI at 12z and will steadily move e thru
the day, exiting the eastern fcst area around 00z Mon. Rather
vigorous deep layer forcing and abundant moisture with pwat upwards
of 1.3 inches/250pct of normal should support a band of shra in the
vcnty of the front. Very little instability is fcst along/ahead of
the front by 12z Sun and thru the day Sun (MUCAPE is mostly less
than 100j/kg), so no mention of thunder is warranted. Shra should be
out of the w half by late aftn and exit the e by mid evening. Skies
should clear fairly quickly once the pcpn ends. Sun will be cooler
under clouds/shra and passing cold front. Expect highs generally in
the upper 50s to mid 60s.

Mon into Tue, attention turns to the trof that will be amplifying
from the Northern Plains to the Mississippi Valley, Ohio Valley, and
Great Lakes region. The ECMWF had been very consistent from run-to-
run, phasing the amplifying trof with the energy over the Lwr
Mississippi Valley to generate a strengthening low pres system that
becomes a strong low as it lifts from the Gulf Coast States to ne
Lower MI/Lake Huron. The last 2 CMC runs have joined the ECMWF. The
00z ECMWF broke continuity, and now shows no phasing, leading to a
somewhat weaker, but much less organized low pres system. The GFS
has always offered an unphased solution, and the latest run shows a
low pres wave lifting from the Gulf Coast to Lake Ontario. The UKMET
supported this idea, but the 00z run has trended notably toward the
stronger CMC/previous ECMWF solutions. At this point, fcst will lean
a bit toward the stronger solutions given the support from the 00Z
CMC/UKMET and some support from the CMC ensembles for what had been
a very consistent ECMWF. Always plenty of uncertainty with phasing,
so stay tuned. In leaning toward the stronger solution, potential
for fcst area to be impacted by strong winds and possibly mdt/hvy
rainfall is a little higher for this fcst cycle than the previous.
While CAA to ensure efficient mixing will be largely missing during
the period of strongest winds aloft, there will still be the risk
for at least advy criteria winds, especially near the Lake Superior
shore on Tue. Stronger winds and resulting higher waves would also
lead to potential beach erosion/flooding issues on Lake Superior
beaches. Rain is likely to be the ptype given the lack of a strong
connection to colder air. However, the deepening of the mid-level
low/falling heights, especially if system ends up on the stronger
side of the spectrum, offers at least a small potential that there
could be a changeover to hvy wet snow in the higher terrain of the n
central and also w if the heavier synoptic pcpn spreads that far w.

As trof shifts e on Wed, shortwave ridging will build toward the
Upper Great Lakes ahead of the next shortwave trof dropping thru sw
Canada toward the northern Rockies. This will lead to a trend to dry
weather on Wed. Upstream trof will then amplify sharply into the
central CONUS late week. Uncertainty in how this next trof evolves
is high with the ECMWF showing a notable westward trend in the trof
over the last 2 runs. The westward shift with the 00z run is
significant and results in much warmer weather again late week into
the weekend as deep southerly flow redevelops into the Great Lakes.
00z CMC ensembles show no support for such a significant westward
shift of the next trof amplification into the CONUS. For now, fcst
will utilize a consensus of a more central CONUS trof axis,
resulting in cool/unsettled weather late week with pcpn largely lake
effect rain/snow dominated.
&&

.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.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...KEC
LONG TERM...Rolfson
AVIATION...JLB
MARINE...KEC



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