Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 161136
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
736 AM EDT Sat Sep 16 2017

.SHORT TERM...(Today and tonight)
Issued at 450 AM EDT SAT SEP 16 2017

Water vapor imagery shows the main trough axis associated with the
developing surface low across the Upper Mississippi Valley
continuing to gradually lift across the central and northern Plains
this morning. Ahead of this trough, broad warm air advection has
allowed a fairly narrow band of showers and thunderstorms to develop
across central and eastern portions of the area this morning.
Overnight lows have remained very mild as a stalled out, stationary
front remains anchored across northern portions of Upper Michigan.
Today`s forecast highlights include a high swim risk for Lake
Michigan beaches along the shores of southern Schoolcraft County,
another unseasonably warm day with increasing humidity, and then the
arrival of additional showers and thunderstorms later this
afternoon/evening across the west, a few of which could be strong to
severe.

In response to the above mentioned wave lifting into the Upper
Mississippi Valley, the stationary front will become reinforced
across far northern portions of the area as a secondary area of low
pressure develops across western Wisconsin. This elongated surface
trough/cold front will be the primary focus for convective
initiation later this afternoon across western Lake Superior and
Upper Michigan. It looks like a few storms could be strong to
severe, especially closer to the Wisconsin border where the nose of
1500-2000 J/kg of MUCAPE is progged to develop as mid-level lapse
rates steepen and moisture transport increases. The deep-layer shear
is also progged to increase to around 30 to 40 knots later this
afternoon and evening as jet streak energy lifts northeast across
the Upper Mississippi Valley. With the deep-layer shear vectors
oriented fairly perpendicular to the surface trough, wouldn`t be
surprised if a few stronger updrafts develop and remain better
organized. The main hazards from stronger storms will be hail and
strong downdraft winds. Otherwise, expect the unseasonably warm
temperatures to linger, with many locations likely climbing into the
low to upper 80s once again today. The humidity will be on the
increase, so it may feel rather uncomfortable at times outside.
Breezy south winds may offer some relief, but these breezy winds
will create a high swim risk for the Lake Michigan beaches along the
shoreline of southern Schoolcraft County. Therefore, a beach hazard
statement has been issued as dangerous currents are expected to
develop.

Tonight, ongoing convection across the west is expected to lift
northeast across Lake Superior, and central and eastern portions of
Upper Michigan as the surface trough/cold front continues it`s
eastward track. Storms should weaken in intensity through the
overnight hours, but with lingering elevated instability expect
thunder to remain prevalent. Temperature wise, we`re looking at
another mild night across much of the area and these temperatures
will be highly dependent on the arrival of the cold front. Across
the west, temperatures should drop into the mid 50s as the front is
expected to move through during the overnight hours. Further to the
east where the front will be slow to arrive, expect low temperatures
to only drop into the 60s.

.LONG TERM...(Sunday through Friday)
Issued at 505 AM EDT SAT SEP 16 2017

With regard to the big picture, last 24hrs of medium range model
runs remain in good agreement with recent days runs. For roughly the
next 7 days, a mean trof will be set up over the western CONUS into
adjacent western Canada with downstream mean ridging over eastern N
America. Pattern will become quite amplified with 500mb height
anomalies in the trof and ridge reaching upwards of 250m in the mid
and late part of the upcoming week. Under sw flow btwn the trof and
ridge, warm conditions/above normal temps will dominate the Upper
Great Lakes on most days with coolest conditions (around normal)
Mon. One or two days will likely be unseasonably warm. Next weekend
into the early part of the following week (7-10 day period),
guidance points toward progression of the pattern, but given
strength of the eastern ridge, progression may be slow. In any
event, western trof should weaken considerably as it shifts e. So,
while cooler weather will return in 7-10 days, conditions will be
seasonable for what will be late Sept. As for pcpn, a more
significant shortwave lifting out of the mean western trof toward
Hudson Bay will push cold front across the area later tonight and
Sunday, bringing at least sct shra/some thunder. Following fropa,
dry weather will return Sun night into at least Mon morning. Under
wsw flow, another weaker shortwave may affect the area Mon
night/Tue, providing some potential of pcpn. Then, with trof
becoming amplified over the western CONUS, pcpn mid and late week
will depend on whether any of the shortwaves lifting out of the trof
track close enough to Upper MI to spread pcpn across the area. Given
the deep sw flow, the Upper Lakes at some point should have the
potential for decent rainfall as the trof eventually drifts e,
especially if the associated cold front is slow moving as it passes
across the area.

Beginning Sunday, vigorous shortwave over northern MN early in the
day will lift into northern Ontario. Associated cold front should be
extending across central Lake Superior/central Upper MI at 12z. With
best height falls and strongest deep layer forcing per q-vectors
lifting ne well to the nw of here, coverage of shra and some thunder
along/just ahead of front will likely diminish thru the morning hrs
as the front continues eastward. If there is sufficient heating
before front clears the eastern fcst area, there may be an uptick in
convection. Behind the front, skies will become mostly sunny.
Despite fropa, temps will still be above normal over the central and
eastern fcst area. Max temps should range from low 60s far w to the
mid 70s s central. Will be a breezy day on the Keweenaw under
favorable post-frontal westerly winds and decent mixing. Expect
gusts to 30-35mph there.

With sfc high pres building over the Upper Lakes on Sunday night, it
will be a quiet/cool night. May see some frost in the interior as
temps fall into the mid 30s for at least the traditional cold spots.

While weakening sfc high pres lingers over the Upper Lakes on Mon,
models still show a shortwave approaching in continued sw flow
aloft. While the last 2 days of model runs have showed this
shortwave, area impacted by associated shra and timing of pcpn
(whether arriving on Mon or Tue or affecting both days) is still not
well agreed upon. At this point, will utilize a consensus of recent
model runs to construct fcst, resulting in schc/low chc pops late
Mon into early Tue.

Mid and late week, deep trof will set up over the western CONUS.
Model trends show the ECMWF trending toward the CMC and especially
GFS idea of a lead shortwave swinging out of the deepening trof and
across MN into northern Ontario Wed/Wed night. As a result, fcst
will reflect a chc of shra late Tue night into Wed night. Once
timing becomes more certain, period of potential shra will be
narrowed into a shorter time frame. Shra chances from Thu thru next
weekend will deepend on the uncertain timing/track of shortwaves
lifting out of the western trof and on the eventual progression of
the trof. Deep sw flow and the likelihood of a slow moving sfc
frontal boundary associated with the trof are suggestive of the
potential of mdt/hvy pcpn at some point. This pcpn could be enhanced
by remnant moisture from current Hurricane Norma off the w coast of
Mexico if it gets captured by the western trof.
&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 736 AM EDT SAT SEP 16 2017

Winds will increase and become gusty through the late morning and
afternoon hours across all TAF sites. With broad lift overrunning
the area today, we could see some scattered showers at KIWD and KSAW
through the morning hours. The attention then turns to the arrival
of a cold front from the west, which will bring showers and
thunderstorms to much of the area later this afternoon/evening and
through the overnight hours. Eventually all TAF sites should see
thunderstorm activity, but KIWD was the only terminal where
confidence was high enough to include prevailing mentions of
thunderstorm activity. Elsewhere, have just included mentions of
VCTS for now.
&&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 450 AM EDT SAT SEP 16 2017

Winds of 20 to 30 knots are expected this morning, but they will
weaken to around or less than 20 knots through the day today. By
Sunday morning, expect the winds to pick back up to around 20 to 30
knots as a cold front pushes east across the lake. Winds will relax
to around or less than 15 knots by early Monday morning through
Tuesday. By Wednesday winds will increase to 20 to 25 knots briefly,
and then will be around 20 knots through the end of next week.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  Beach Hazards Statement through this evening for MIZ014.

Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...Ritzman
LONG TERM...Rolfson
AVIATION...Ritzman
MARINE...Ritzman



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