Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 200738

Area Forecast Discussion
National Weather Service Marquette MI
338 AM EDT Wed Sep 20 2017

.SHORT TERM...(Today and tonight)
Issued at 329 AM EDT WED SEP 20 2017

Upper-level clouds continued to stream eastward across Upper
Michigan ahead of the MCS tracking east-northeast across Minnesota
this morning. The cold front that originally initiated this
convection has now just begun to push into western portions of
Minnesota this morning. Further to the east, across central and
eastern portions of Upper Michigan stratus has begun to develop
and/or lift northwest this morning. A few areas may see some patchy
fog this morning, especially in areas that see light winds and
upslope conditions.

Today there are a few forecast concerns, including the potential for
multiple rounds of showers and storms across the west and central
(one this morning and then another round later in the afternoon) and
strong and gusty winds ahead of the cold front expected to arrive
later today. A few storms this morning and later this afternoon
could be strong to severe, with the main hazard being strong
downburst winds.

Through the morning hours, winds will increase ahead of the
advancing cold front and become quite gusty across the east. Ongoing
convection in Minnesota looks like it will make its way into western
portions of Upper Michigan. Given the enhanced low-level winds ahead
of the convection wouldn`t be surprised if a few storms produce
strong to severe wind gusts. However, as the axis of better MUCAPE
values continue to drop south of the area with the 850mb flow
veering out and the main shortwave lifts north across the Arrowhead
of Minnesota, convection is expected to gradually weak across the
west, and possible making it into central portions of Upper Michigan
through the morning hours.

Through the rest of the day, ahead of the cold front the primary
concern will be the potential for strong and gusty south-southwest
winds. With gusty gradient winds already present ahead of the cold
front this morning, expect this trend to continue as it pushes
through the region later today. Overall, models are in good
agreement with 925mb flow of around 40 knots just ahead of the cold
front. Therefore, with the front expected to push eastward across
Upper Michigan during the daytime hours, the combination of mixing
and gradient flow should allow wind gusts to approach at least the
30 to 40 mph at times. These winds gusts will be possible through
the morning hours across the west, and then central and eastern
during the afternoon and into the evening hours. Thankfully it does
look like this corridor of strong and gusty winds will only linger
for a few hours as it remains fairly transient. The only condition
that may limit how strong wind gusts get is the cloud cover
expected, which will hinder diurnal heating slightly and therefore
the depth of turbulent mixing.

As the cold front pushes eastward across Upper Michigan strong to
severe storms remain possible, primarily across the west and central
portions of the area. There is some uncertainty in the coverage and
overall intensity of convection given the ample cloud cover expected
ahead of the front; however, as a subtle shortwave is progged to
lift across the area this should be enough support to initiate
scattered storms along the cold front later this afternoon and
evening. CAPE profiles are not terribly impressive, but with a
plethora of dry air and enhanced low-level flow the main hazard
looks to be strong to severe downburst winds. Given the amount of
deep-layer shear and its quasi-perpendicular orientation to the
front, if storms can remain organized/rotate a hail threat will also

Tonight, 850mb flow veers our rather rapidly so expect ongoing
thunderstorms to diminish in coverage and intensity across the east.
As a post frontal, cooler and more stable airmass moves in
immediately behind the front, light winds and clearing conditions
should give way to radiational fog across the west and central.
Patchy areas of dense fog will be possible, especially in areas that
see rainfall later today.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 346 PM EDT TUE SEP 19 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 from lingering
convection along the cold fropa Wednesday night over mainly the
eastern two thirds of the cwa and then early next week as the warm
weekend ridge finally breaks down allowing another cold front to
move in from the Plains.

Wednesday night into Thursday night...With MUCAPE values upwards of
1000-1500 j/kg and deep layer shear of 40 knots or more there is a
concern for an isolated strong to possibly severe thunderstorm into
Wed evening over the eastern two thirds of the fcst area where SPC
has a marginal risk for day2 convective outlook. Negatives
working against coverage of strong to severe storms is that best
dynamics along the front lift well north of Lake Superior and the
atmosphere looks pretty well capped at low-levels. It appears from
model soundings that parcels must be lifted from near 850 mb to
realize higher CAPE values. During the day on Thursday, models
indicate the cold front slowing its eastward/southward progress
and then pushing back northward into Upper Mi as a warm front Thu
night. This could bring some isolated showers and t-storms back
into the area, mainly west half.

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.
Model soundings indicate 925-850mb temperatures 20-22C. These warm
temperatures coupled with southerly winds of 15 to 25 mph with
possible higher gusts to enhance mixing should easily allow this
warmer air to reach the sfc resulting in max temps 15 to 25 degrees
above normal for this time of year across much of the area on Friday
and possibly Saturday too, especially for downsloping areas along
Lake Superior. Would not be surprised to see temperature records
broken at many locations Friday into Saturday. Southerly flow in
warm sector ahead of frontal boundary over the Plains will also
tap into Gulf moisture and bring low to mid 60s dewpoints
northward through the central CONUS into the Great Lakes region.
As a result, expect the increase in humidity to make for rather
muggy and uncomfortable conditions over the weekend. However, the
cap of warm air aloft will likely inhibit formation of convection
for the most part, despite the increase in moisture and
instability. By Sunday, model uncertainty increases regarding
progression of Plains frontal boundary eastward into Upper Mi. A
few of the models, particularly the 12z GFS and ECMWF, indicate
that the boundary could push slightly east toward the western cwa
in response to a shortwave rounding the ridge through MN. As a
result there could be a few storms popping up at least over the
western cwa Sat night into Sun as frontal boundary draws closer
from the Plains. While the expected increase in clouds could keep
temps down a bit over the nw half of Upper Mi on Sunday, the
southeast half should still see plenty of sun to allow for low to
mid 80s readings away from the moderating influence of Lake Mi.

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

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 123 AM EDT WED SEP 20 2017

Much to consider this forecast package. First concern is the
development of LLWS over KIWD and KCMX later tonight, with this
lasting into the daytime hours. Winds have already increased ahead
of approaching front/trough as shown by latest upper air sounding to
the west of the area, and these winds will move in overnight.
Some Bufkit data suggests KSAW may see the development of LLWS by
18Z, but have left out for now as there is a good degree of
uncertainty at this time. There remains the potential for
stratus/fog development at KSAW in moist, upslope SE flow off of
Lake Michigan, which has the potential to produce IFR/LIFR
conditions for a time late overnight. There remains a possibility
for some shower and thunderstorm activity to move into the KIWD area
this morning, however the better chance exists later in the morning
as the front/trough moves in. Still some uncertainty in the areal
coverage early on. Winds will be gusty at times during the day but
then will settle down once the boundary passes through.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 338 AM EDT WED SEP 20 2017

Early this morning thunderstorms are expected to move across western
portions of the lake, a few could be strong to severe with winds
being the primary hazard, along with dangerous lightning. Winds will
remain around 20 to 30 knots ahead of a cold front approaching from
the west today. A few locations across the central and eastern
portions of the lake could see gale force gusts to 35 knots at times
this afternoon. However, confidence is low in regards to how
persistent these gusts will be. As the cold front pushes east across
the area this afternoon and evening, winds will decrease to around
10 to 20 knots. Behind the front, we may see fog develop tonight and
linger into Thursday. Friday through early next week, winds look to
remain between 15 and 20 knots.

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


SHORT TERM...Ritzman
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