Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 211756

Area Forecast Discussion
National Weather Service Marquette MI
156 PM EDT Sat Oct 21 2017

.SHORT TERM...(Today and tonight)
Issued at 459 AM EDT SAT OCT 21 2017

No significantly impactful weather expected in the short term.

Weak shortwave energy may bring a couple rounds of isolated to
scattered showers and possibly some thunder to western Upper MI
through this afternoon. There isn`t expected to be enough
instability for strong storms, just general thunder. S winds will
gust to 20-35mph today, strongest near northern Lake Michigan and in
downslope areas along Lake Superior. Temps will again be nearly 20
degrees above normal with highs in the mid 60s to mid 70s.

A cold front shifts into the W tonight, which will likely bring
showers and maybe a thunderstorm. Chances should be confined to
mainly the W half through 12Z Sun. Lows tonight will be 20-25
degrees above normal with lows in the 50s.

.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 18Z TAFS through 18Z Sunday afternoon)
Issued at 152 PM EDT SAT OCT 21 2017

Showers and maybe some thunderstorms are possible near KIWD today,
but with limited confidence in coverage just used VCSH in the
forecast. A cold front will move through the area late tonight and
Sunday, bringing rain and reduced CIGs from west to east across
the TAF sites. LLWS is expected tonight at KSAW. Improvement to
VFR is expected at KIWD and KCMX by early Sunday afternoon with
drying behind the front. &&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 459 AM EDT SAT OCT 21 2017

South winds will gust 20-30 kts through tonight, strongest over the
eastern half Lake Superior. For Sun into Mon, southeast winds will
gust 20-30 kts, strongest over the western half of Lake Superior.
North gales are possible Tue into Tue night depending on the
strength of a low pressure system that some models show moving
through. 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...


LONG TERM...Rolfson
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