Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 210808

Area Forecast Discussion
National Weather Service Marquette MI
408 AM EDT Fri Jul 21 2017

.SHORT TERM...(Today and tonight)
Issued at 408 AM EDT FRI JUL 21 2017

This morning, IR satellite imagery shows multiple complexes of
ongoing convection from the northern Plains down across the Corn
Belt. A rather complex surface field was present across the Plains
as two areas of low pressure have tried to organize, one coming out
of the northern Rockies and the other the central Rockies. At the
upper-levels, broad ridging was observed across the central CONUS
with a progressive upper-level low beginning to work its way east
across the Canadian Rockies. Ahead of this upper-level low, a
shortwave began digging south-southeast this morning and will play a
role in how things evolve later today/tonight. Across Upper Michigan
we`ve remained under surface ridging with mostly clear skies and
light winds. This has allowed for patchy ground fog to develop
across much of the area with surface observations coming in and out
of the shallow fog. Expect the fog to diminish as mid- and upper-
level clouds stream in from the west.

The shortwave digging south into the northern Plains this morning
will allow the northern most area of low pressure to eject out
across the northern Plains and into portions of the Great Lakes
later tonight. The warm front is progged to near portions of Upper
Michigan later this afternoon, but the better low-level moisture
looks like it will only make it into northern portions of Wisconsin
at best. Therefore, things will become more humid as the day
progresses, but expect the oppressive dew points to remain south of
the area. Convection is expected to develop along the warm front and
eventually grow upscale into a MCS as low pressure rolls across
Wisconsin late this afternoon and through the overnight hours. Right
now it looks like the best warm, moist air advection will arrive
after convection has begun to grow upscale in Wisconsin, so this
should keep the threat of any severe storms and heavy rains off to
our south. However, cannot rule out a few areas of showers and
perhaps a rumble of thunder along converging lake breezes this
afternoon and as the main shortwave moves across the area tonight.
The SPC has included far southern portions of Upper Michigan within
a marginal risk for severe storms; however, with convection expected
to develop across the warm sector and grow upscale south of the area
think that the main threat for severe storms should remain south of
the Wisconsin/Michigan border.

.LONG TERM...(Saturday through Thursday)
Issued at 348 AM EDT FRI JUL 21 2017

West northwest flow aloft persists into Sat then strong shortwave
trough crosses Upper Great Lakes Sat night into Sun. Associated sfc
low will provide focus for showers and thunderstorms late Fri night
into Sat morning. Looks like main stationary front and edge of
greater instability stays from central MN into southern WI which
should keep most of the concentrated shra/tsra complexes south of
here as well. However, with stronger shortwaves tracking farther
north across Upper Michigan and some elevated instability could
still see scattered convection over Upper Michigan especially as the
sfc low begins to influence the weather over Upper Great Lakes.

Some concern that there will be enough of a lull later Sat aftn into
Sat evening for instability to build up ahead of main shortwave
trough that moves in later Sat night. Decent consensus of at least
1000 j/kg of CAPE with effective shear more than sufficient at 40+
kts. Main question is whether the instability will be realized as
gradient east winds off Great Lakes to the north of sfc low is
not typically a setup that allows for a lot of warmth and models
are indicating there would be lingering low clouds through the
day. Even if convection does not develop on Sat aftn there would
also be potential for it to move in out of MN or northern WI Sat
evening. Given the shear forecast will have to keep monitoring
this time frame. SPC day 2 outlook does show western cwa within a
slight risk for severe and with the shear that seems reasonable.
Right moving supercell motion from NW-SE would keep stronger
storms along the WI border based on where max CAPE is located, but
SW-NE storm motion for left movers could lead to some hail
potential for more of the cwa. Freezing levels/wbzero heights
13-14kft/11-12kft are a bit on the high side to support larger
hail though strong shear (greater chance of rotating updrafts)
will offset that some.

As sfc low slides to the central Great Lakes on Sat night, inverted
sfc trough with axis of elevated instability will linger across the
Upper Great Lakes. Could be at least isold-scattered shra/tsra
through the night as this focus interacts with approaching strong
shortwave trough and associated height falls/resulting lift. If this
larger scale trough is slower to arrive there will be greater chance
of seeing diurnal driven shra/tsra developing on Sun, especially
over the south central where the forcing would interact with sfc
based CAPE of 500-1000j/kg. At this point, appears NAM 60-72hr out
is most aggressive with that idea while most other guidance is
quicker with overall trough and less emphatic with diurnal
development. No matter how the precip chances work out this weekend,
certainly looking at a cooling trend over the summer warmth and
humidity seen the last couple days as the area will be on the north
side of the sfc low sliding across to the central Great Lakes. ESE
winds on northern cyclonic side of the low should lead to higher
potential for low clouds for Sat and then north winds should
increase to at least 15 to 25 mph on Sun as sfc low moves farther
east and high pressure expands from scntrl Canada to northern

Should dry out on Mon as high pressure over Canada arrives from
northern Plains to the Upper Great Lakes. Dwpnts less than 50F in
the aftn on Mon over the interior will feel very refreshing. NE
winds in between departing low and sfc ridge moving in will lead to
coolest temps in the 60s near Lk Superior. Temps inland and near Lk
Michigan will be able to push back into the 70s though. Mostly dry
weather should hold into Tue with temps continuing to warm toward
the lower 80s, especially inland west and north central near Lk
Superior. Next front in NW flow pattern aloft is due in on Wed.
Front could spark another round of shra/tsra. Sfc based CAPEs could
reach 1000j/kg and effective shear may be over 30 kts. If both of
these factors occur could not rule out some strong to severe storms
on Wed aftn as the front crosses the area. Generally dry and
slightly cooler behind the front to end the last full week of July.

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 121 AM EDT FRI JUL 21 2017

Under a relatively dry air mass, VFR conditions will prevail thru
the fcst period at KIWD/KCMX/KSAW. Winds will also be light.
Shra/tstms will be tracking ese to the s of the area this evening,
but there may be some shra/tstms farther n in Upper MI. However,
confidence in any development is too low at this point to include
a mention in the fcst.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 408 AM EDT FRI JUL 21 2017

Winds will start off less than 15 knots across the lake today, then
towards the end of the day and into the overnight hours winds across
the western half of the lake will increase to around 20 to 25 knots.
By Saturday morning, expect winds of 15 to 20 knots to spread across
much of the lake before diminishing to around 15 knots by the
evening hours. Sunday afternoon expect the winds to increase across
the lake to around 20 to 25 knots. These winds will diminish to
under 15 knots by mid-morning on Monday. Through the rest of the
week, winds will remain under 20 knots.

Upper Michigan...
Lake Superior...
Lake Michigan...


SHORT TERM...Ritzman
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