Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 171851

Area Forecast Discussion
National Weather Service Marquette MI
251 PM EDT Wed May 17 2017

.SHORT TERM...(Today and tonight)
Issued at 340 AM EDT WED MAY 17 2017

...Flash Flood Concerns for NW Half of U.P. into early Thu...

Radar trends have shown convection weakening over south central
Upper Mi early this morning and thus severe t-storm watch for MNM
county has been cancelled. Satellite imagery shows a vigorous
shortwave rapidly ejecting ne from mid-upper trough over the
Western Plains and pushing the next wave of convection north thru
into north central WI. Models don`t appear to have a real good
handle on the timing of this next round of convection based on
regional radar trends. Looks like it will be moving into Upper Mi
quicker than any of the models suggest...NAM may be handling it
the best but even it is too slow. Based on tracking tools leading
edge of convection should reach into sw Upper Mi just aft 08z
which is an hour or two quicker than the NAM. So long story short
there won`t be as much of the break in shra/tsra early this
morning as previously thought. Convection over northern Wi
supported by MUCAPE gradient of 500-1000 j/kg just to the south,
h85 jet max of 40 kts and strong 850 mb moisture transport.

Today...RAP and NAM shows the MUCAPES in 500-1000 j/kg range along
the sfc trough/cold front into central Upper Mi through much of the
day today while MUCAPE values taper off to 200-400 j/kg over western
counties. Therefore, the best chance for thunderstorm activity will
be over the central portions of Upper Michigan. Best effective shear
will remain farther to the west where instability is weaker which
should limit chances for severe during the day especially with
expected cloudy skies.

Tonight...vigorous, but weakening shortwave, will be approaching
Upper MI this evening and passing across the area Thu morning/early
aftn. Associated sfc low pres will track along a sw-ne oriented
frontal boundary extending somewhere across the se half of Upper MI.
This still points toward the nw half of Upper MI for the area to see
widespread moderate to heavy rainfall. Combination of fgen, vigorous
deep layer forcing per q-vectors, diffluent flow aloft with some
coupling of upper jets, and precipitable water upwards of 1.5 inches
(250+pct of normal) feeding across frontal boundary will support a
good potential of moderate to heavy rainfall. Certainly could see
widespread 1-2 inches of rainfall with some local 3 inch amounts
across roughly the nw half of Upper MI. Given model trends of
heavier pcpn shifting slightly e have decided to include Iron,
Baraga and southern Houghton counties into flash flood watch which
extends into Thu morning. Likely in response to vigorous shortwave
dynamics models show MUCAPEs ramping up a bit along frontal boundary
into 1000-1300 j/kg range this evening as deep layer shear increases
to least 40kt, which will support the potential of isold svr storms
near the front at least during the evening hrs over s central Upper

.LONG TERM...(Thursday night through Wednesday)
Issued at 251 PM EDT WED MAY 17 2017

Nam shows a 500 mb trough over the upper Great Lakes 00z Fri with a
closed low over the central Rockies and a ridge over the northern
plains. This ridge moves into the upper Great Lakes 00z Sat. The
closed low moves into the northern plains 00z Sun. Nam shows 850-500
mb q-vector convergence with deeper moisture moving in on Sat. Looks
to be quiet through Fri night and then have chance pops Sat morning
over the west half and have likely pops over the west half in the
afternoon and a chance of pops over the east half in the afternoon.
Overall, did not make too many changes to the going forecast.

In the extended, GFS and ECMWF show a closed 500 mb low and trough
near the upper Great Lakes 12z Sun. This trough remains over the
area through Wed with some colder air returning and a cold front
that moves through on Tue. Temperatures look to remain below normal
for this forecast period.

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 138 PM EDT WED MAY 17 2017

Challenging aviation forecast this afternoon as periodic rain
showers continue to lift across the area allowing visibilities to
fluctuation from LIFR to MVFR across the west. Expect KIWD/KCMX to
remain fairly socked in the cloud cover this afternoon, visibilities
will likely improve as daytime heating continues and winds begin to
back slightly. However, expect the LIFR/IFR ceilings to persist.
Additional rain chances will arrive later this afternoon and
overnight. Heavy rain may limit visibilities at time. Conditions
will gradually improve through the morning/afternoon hours on
Thursday as drier air begins to move into the region, but winds will
become rather gusty.

Fog came onshore much earlier than anticipated at KSAW; therefore,
KSAW has been socked in LIFR conditions through much of the day so
far. Current satellite trends show a pocket of clearing just to the
south of KSAW. Difficult to say if the advection off the lake will
hold strong enough to keep the lower ceilings and visibilities. If
the clearing does start to lift north across KSAW, visibilities very
well may drop down to VLIFR briefly. Multiple rounds of showers and
possibly a few embedded thunderstorms are expected to move across
KSAW bringing heavy rain and possibly gusty winds.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 325 AM EDT WED MAY 17 2017

Relatively light winds this morning will become nne this afternoon
following a cold frontal passage and then increase to gale force
tonight over the w half of the lake as the gradient tightens between
hi pres over scentral Canada and a lo pres moving northeast along
the stalling front over se Upper MI. These gales will overspread the
e half of the lake on Thu morning as the sharper pres gradient
follows the lo pres shifting to the e. The closer approach of the
trailing hi pres will then cause diminishing winds w to e on Thu,
with winds diminishing under 20 kts on Thu night even over the
eastern portion. These lighter winds under 20 kts will then persist
through Sat as hi pres/flat pres gradient dominates. The arrival of
a tighter pres gradient with lo pres moving into the Upper Lakes
might allow for winds to increase back up to 25 kts on Sun.

Combination of showers and increasingly humid air arriving over the
cold lake waters will result in fog over the entire lake into this
evening. This fog may become dense at times. The fog will dissipate
late Wed night/Thu morning as the strong n winds behind the lo
pres passing to the s draw drier air over the area.

Upper Michigan...
  Flash Flood Watch through Thursday afternoon for MIZ002-004-009-

Lake Superior...
  Gale Warning from 8 AM to 8 PM EDT Thursday for LSZ267.

  Gale Warning from 4 AM to 4 PM EDT Thursday for LSZ265-266.

  Gale Warning from 11 PM EDT /10 PM CDT/ this evening to 11 AM
     EDT /10 AM CDT/ Thursday for LSZ162.

  Gale Warning from 11 PM this evening to 2 PM EDT Thursday for

  Gale Warning from 1 AM EDT /midnight CDT/ to 10 AM EDT /9 AM
     CDT/ Thursday for LSZ240-241.

  Gale Warning from 1 AM to 11 AM EDT Thursday for LSZ242-243.

Lake Michigan...


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