Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 181959

Area Forecast Discussion
National Weather Service Marquette MI
359 PM EDT Mon Sep 18 2017

.SHORT TERM...(Today and tonight)
Issued at 519 AM EDT MON SEP 18 2017

Water vapor imagery and RAP analysis show sw flow into the Upper
Great Lakes early this morning btwn a mid-level low over far
northern Ontario and ridging extending from the western Gulf of
Mexico to southern Quebec. Upstream, a weak shortwave trof over the
Plains will be the next feature of interest as it lifts ne. With sfc
high pres building over the area, it`s been a quiet, cool night.
Patchy/shallow fog has developed in the interior. Temps have fallen
to the low to mid 30s at traditional interior cold spots, so there
will be some patchy frost early this morning as well.

Shortwave trof over the Plains will lift ne, reaching the western
Great Lakes this evening. Radar imagery early this morning shows
shra/tsra across the Plains associated with the wave, and this pcpn
appears to be tied closely to the right entrance upper divergence
zone associated with upper jet extending from western Nebraska
toward southern Hudson Bay. Following progression of the upper jet,
the upper divergence should support a band of shra streaking ne
across western Upper MI as vast majority of model guidance shows.
Fcst will thus show chc pops spreading across western Upper MI this
aftn with some likely pops toward the MI/WI state line. Recent
hourly RAP/HRRR runs are insistent on the spread of pcpn ne into the
area. Will continue to monitor those runs in addition to 06z runs of
CMC/NAM/GFS to see if an update to higher pops is warranted. Shra
will be moving into a drier air mass over the Upper Lakes, so shra
should tend to diminish as they spread ne into the area. Expect high
temps in the 60s today. A few spots in the e where there will be
less cloudiness may touch 70F.

Shra will dissipate tonight, possibly without ever reaching the
eastern fcst area. Lingering cloud cover will keep min temps higher
than observed early this morning. Mid 40 to mid 50s should be the

.LONG TERM...(Tuesday night through Monday)
Issued at 358 PM EDT MON SEP 18 2017

High amplitude trough/ridge pattern setting up over the CONUS
particularly by the end of the week will keep the western Great
Lakes in a warm south-southwest flow which will result in well above
normal temperatures throughout much of the extended period. The best
chances for showers and thunderstorms will be Wednesday into early
Thursday as a cold front slowly pushes across the area.

Continued large scale anticyclonic flow will maintain dry conditions
Tue night. The warming trend will continue Wednesday and Thursday,
with high temperatures expected to climb into the 70s. Precipitation
chances will increase late on Wednesday through Thursday as a cold
front slides southeast across Upper Michigan. With MLCAPE values
upwards of 1000-1500 j/kg and deep layer shear of 30-40 knots there
will be a risk for strong to possibly severe thunderstorms Wed
afternoon into Wed night over the western two thirds of the fcst
area. Not surprisingly, this is the area where SPC has a marginal
risk for severe storms on the day3 convective outlook. During the
day on Thursday, models indicate the cold front slowing its
eastward/southward progress and then pushing back northward into
Upper Mi as warm front Thu night. This could bring isolated
showers and t-storms back into the area.

Friday through Sunday look to be the warmest days of the week. As
the warm front lifts well north of the area strong warm air
continues to stream northeast into the Upper Great Lakes region.
Models indicate 925-850mb temperatures at or above 20 C. These warm
temperatures coupled with southerly winds of 10 to 20 mph to enhance
mixing should easily allow temperatures to climb 10 to 20 degrees
above normal for this time of year, especially for downsloping areas
along Lake Superior. Southerly flow in warm sector ahead of frontal
boundary over the Plains will also tap into Gulf moisture and bring
it northward through the central CONUS into the Great Lakes region.
As a result, expect the humidity to increase as well making for
rather warm and muggy conditions over the weekend. However, cap of
warm air aloft will likely inhibit formation of convection for the
most part, despite increase in moisture and instability. By Sunday,
model uncertainty increases regarding progression of frontal
boundary eastward into Upper Mi. The operational GFS is definitely
quicker than the GFS ensemble mean or the 12z ECMWF/GEM-NH but would
anticipate there could be a few storms popping up at least over the
western cwa as frontal boundary draws closer from the Plains.

Would expect showers and thunderstorms to increase in coverage
heading into next Monday as there is better model agreement that the
front will move eastward into central Upper Mi.

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 142 PM EDT MON SEP 18 2017

A relatively dry low-level air mass should allow VFR conditions to
prevail thru the fcst period at KIWD/KCMX/KSAW. A disturbance
approaching the area will spread some -shra into w and central Upper
MI this aftn/evening. If -shra occur at any of the terminals,
conditions won`t fall out of VFR given that the pcpn will be light.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 519 AM EDT MON SEP 18 2017

High pres over the Upper Lakes will lead to winds mostly under 15kt
across Lake Superior today and tonight. As the high shifts e on Tue
and a low pres trof moves out over the Plains, winds will begin to
increase a bit on Tue, but much more so Tue night and Wed as the
trof moves closer. SE to S winds will increase to 20-25kt across
much of the lake Tue night and will increase up to 30kt over eastern
Lake Superior on Wed. With passage of the trof, winds will drop off
quickly to under 15kt from w to e Wednesday night with these lighter
winds lingering thru Thu.

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


SHORT TERM...Rolfson
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