Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 230948

Area Forecast Discussion
National Weather Service Marquette MI
548 AM EDT Mon Oct 23 2017

.SHORT TERM...(Today and tonight)
Issued at 548 AM EDT MON OCT 23 2017

...Powerful fall storm to begin impacting the area tonight...

Water vapor imagery and RAP analysis show a trof beginning to dig
sharply into the central CONUS. Just downstream is a vigorous
shortwave over the lower Mississippi Valley. This shortwave will
phase with the amplifying trof, leading to a rapidly deepening,
powerful fall storm over the Great Lakes region late tonight and
Tuesday. Zone of waa ahead of the amplifying trof is producing a
band of mid/high clouds which are spreading into western Upper MI.
Radars show some returns within the cloud deck, and a few sprinkles
have been noted on obs. Farther w, sct shra are over western MN in
the vcnty of a vigorous shortwave. Closer to home, lingering low-
level moisture in the wake of yesterday`s shra has led to some
fog/low clouds over portions of central Upper MI.

Models have moved into excellent agreement with the phasing of the
lower Mississippi Valley shortwave with the amplifying central CONUS
trof. The GFS was the last holdout and was very late to finally come
around to the strong phased solution. Sfc low pres will begin to
organize over the Ohio Valley as it lifts n today, and then it will
rapidly deepen as lifts to far northern Lake MI by 12z Tue. Pres
will bottom out around 29.0 inches. Prior to that system arriving,
falling heights and approaching shortwave from the w may lead to
some -shra today over the far w. Late this aftn, developing hvy rain
associated with deepening storm to the s may begin to back nw into
the eastern fcst area. This area of mdt/hvy rain will back to the w
tonight, spreading across all of the area by morning. Strengthening
usplope northerly flow off Lake Superior with air mass cold enough
for lake enhancement will lead to heaviest rainfall over the higher
terrain. Wind fields rapidly increase in response to the rapidly
deepening storm system with all models showing a core of 60-70kt
850mb winds developing nw of the sfc low. While caa is largely
missing to ensure very efficient mixing, still expect winds to ramp
up significantly from e to w overnight. As a result, headlines for
strong winds will begin late tonight and extend thru Tue. The water
level of Lake Superior is currently running about 10 inches above
the long term Oct avg, and this will exacerbate lakeshore
flooding/erosiong problems that will occur with the very large waves
that will build on Lake Superior. Lakeshore flood
warnings/advisories have been issued for most areas along Lake
Superior, beginning late tonight.

.LONG TERM...(Tuesday through Sunday)
Issued at 450 AM EDT MON OCT 23 2017

Focus remains on Tue into Tue night with a potentially significant
low pressure system and associated potential threats. Potential
threats include: Damaging winds, lakeshore flooding/beach erosion
from very large Lake Superior waves, and maybe some snow over
portions of the interior W.

Forecast period starts at 12Z Tue. Models are in much better
agreement that a 980-985mb surface low will be somewhere in the
vicinity of eastern Upper Michigan (models vary between northern
Lake Michigan and just north of the SOO). Models show the low
meandering around some but staying in the same general vicinity for
the following 12-18 hours through 00-06Z Wed, then weakening while
shifting to the E. The CMC is a little different in that it starts
around the same area at 12Z, but shifts the low farther south over
the following 12 hours. At this point, there is still uncertainty
given how potent the system will be and how much of an impact the
position/strength of the low will be on winds. Due to this, will not
go with the absolute strongest guidance, but will show a significant
increase in winds/waves from the previous forecast.

Winds: Using a consensus of guidance for the winds, most shoreline
areas and the Keweenaw should see gusts to around 50mph, with 60-
70mph gusts between between Big Bay and Munising or Grand Marais and
also over the tip of the Keweenaw. The exceptions would be over far
western Upper Michigan where shoreline gusts may be a lower and
around and E of Grand Marais where, depending on the track of the
low, winds may be more out of the SE. If the low tracks are
currently shown, strong gusts of possibly 45-55mph would extend
inland over central Upper MI, including as far south as the Escanaba
area. These peak winds would be strongest between 12 and 21Z Tue,
particularly in the 15-18Z time frame. Weaker, but still significant
winds will continue until after 06Z Wed near central and eastern
Lake Superior. The combination of the extent and duration of these
winds would quite likely lead to many trees coming down in the most
impacted areas, likely leading to widespread power outages.

Waves: Waves will increase to 25ft  over central Lake Superior Tue
morning through the afternoon, resulting in lakeshore flooding and
significant beach erosion on the N side of the Keweenaw and
Marquette and Alger Counties. Elsewhere along Lake Superior, peak
waves will range from 14-20ft, leading to lakeshore flooding and
beach erosion.

Snow: With the system now being modeled as more wrapped up, colder
air may allow for some snow accumulations over the interior higher
terrain of the W half Tue evening into Tue night. Currently have the
highest amounts over the higher terrain of Baraga and northern Iron
Counties with 2-4 inches. However, there is a lot of uncertainty in
that since some parameters will be near borderline values for snow
and track/strength will play an important role. Some guidance
suggested higher amounts up to 6 inches, even as far E as our office
in Negaunee Township, but did not feel comfortable going with those
high of values at this time since some models show little to no
snow. Will not issue any headlines for snow at this point given the
uncertainty and borderline amounts in the forecast. Will let the
next couple shifts take a close look.

There is potential for another significant storm system late in the
week, but given model variability/disagreement, did not make any
changes to the forecast during that time frame. Left the blended
initialization untouched for time periods after this upcoming storm.

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 130 AM EDT MON OCT 23 2017

Although there was strong mid-level drying in the wake of a cold
front, persistent low-level moisture has allowed low clouds to
develop over cntrl Upper Michigan which should persist overnight
before dissipating this morning with daytime mixing. Another
disturance moving in later today could support isold/sct
-shra along with cigs dropping to near MVFR over the west half.
 Cigs will drop off to MVFR this evening as upslope nrly winds
 develop with a strengthening area of low pressure.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 548 AM EDT MON OCT 23 2017

A powerful fall storm system will lift n across Lower MI tonight
with sfc low pres probably ending up over far northern Lower MI
Tuesday morning. High end storm force winds will develop over
central and eastern Lake Superior late tonight/Tue morning with high
end gales over far western Lake Superior. Hurricane force wind gusts
will likely occur as well late tonight/Tue morning. Winds will
diminish w to e Tue aftn and Tue night. A short period of light
winds under 20kt will occur on Wed as weak high pres ridge arrives.
Another strong low pres system will impact the Upper Great Lakes
late this week, bringing another round of gales late Thu/Fri.

Upper Michigan...
  High Wind Warning from 5 AM to 11 PM EDT Tuesday for MIZ001-003.

  Lakeshore Flood Warning from 6 AM to 11 PM EDT Tuesday for

  Wind Advisory from 7 AM EDT /6 AM CDT/ to 11 PM EDT /10 PM CDT/
     Tuesday for MIZ002-009.

  Lakeshore Flood Advisory from 6 AM EDT /5 AM CDT/ to 11 PM EDT
     /10 PM CDT/ Tuesday for MIZ002-009.

  High Wind Warning from 4 AM to 11 PM EDT Tuesday for MIZ005-006.

  Lakeshore Flood Warning from 5 AM Tuesday to 2 AM EDT Wednesday
     for MIZ005-006.

  Lakeshore Flood Advisory from 5 AM to 2 PM EDT Tuesday for

  High Wind Warning from 7 AM to 5 PM EDT Tuesday for MIZ004.

  Wind Advisory from 7 AM EDT /6 AM CDT/ to 5 PM EDT /4 PM CDT/
     Tuesday for MIZ011>013.

Lake Superior...
  Storm Warning from 4 AM to 2 PM EDT Tuesday for LSZ250-251-267.

  Storm Warning from 4 AM to 11 PM EDT Tuesday for LSZ249-266.

  Storm Warning from 5 AM to 11 PM EDT Tuesday for LSZ243>248-264-

  Storm Warning from 6 AM EDT /5 AM CDT/ to 8 PM EDT /7 PM CDT/
     Tuesday for LSZ241-242-263.

  Gale Warning from 2 AM EDT /1 AM CDT/ Tuesday to 2 AM EDT /1 AM
     CDT/ Wednesday for LSZ162-240.

Lake Michigan...
  Gale Warning from 11 PM this evening to 10 AM EDT Wednesday for



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