Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 210752

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

.SHORT TERM...(This evening through Friday)
Issued at 335 PM EDT THU JUL 20 2017

Generally quiet weather expected through the short-term portion of
the forecast as high pressure and dry air lingers across the U.P.
tonight through Friday. High temperatures are expected to warm into
the 80s for most of the area Friday. There may be a few showers and
thunderstorms that try to work into the area late Friday afternoon
as WAA increases across the area and a shortwave slides across WI.
The better forcing and instability associated with this wave/850mb
frontal boundary is expected to remain over WI, but it may be just
far enough north to brush the southern tier of counties. Overall
wouldn`t expect too much precipitation out of this quick moving
shortwave late Friday afternoon.

.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 PM Lake Superior forecast issuance)
Issued at 335 PM EDT THU JUL 20 2017

Expect winds 15 kts or less late this week into Saturday. Next
chance for stronger winds from the north to northeast over 20 kts
will be on Sunday in the wake of a low pressure system crossing the
Upper Great Lakes. High pressure builds back over region by early
next week with winds mainly 15 kts or less.

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


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