Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 200524

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

.SHORT TERM...(This evening through Wednesday)
Issued at 344 PM EDT TUE SEP 19 2017

Tonight: Upper-level ridging in place this evening will drift off to
the east of the U.P. overnight. At the surface, high pressure will
also slowly shift to the east of the area, allowing a warm front to
push through late tonight into early Wednesday morning. This will
allow WAA/moisture to increase into the Upper Great Lakes region.
This will be ahead of a surface low and upper-level low progged to
drift very slowly from the Northern Plains into Canada. A north-
south oriented cold front is expected to shift from the Plains,
eastward to MN and IA by 12Z Wednesday morning. The main impacts
that these features will bring to the U.P. will be steadily
increasing clouds from the west, along with an increasing potential
for some WAA showers and thunderstorms along the warm front late
this evening and possibly a more concentrated line of showers and
thunderstorms along the cold front. Most of the east half of the
area will be mainly dry; however, chances will be higher over the
far west with the approach of the aforementioned features. While
severe weather is expected this evening over the Northern Plains,
the loss of daytime heating will allow the showers and thunderstorms
to weaken as they approach the western U.P. late tonight. This is
indicated by a majority of the hi-res model solutions.

Wednesday: The aforementioned cold front will slide from the western
U.P. early in the day to the central and eastern U.P. by 00Z/21.
Most of the morning hours will remain dry over the central and east
as fairly strong capping remains in place well ahead of the cold
front. Along and immediately ahead of the cold front, there will be
enough forcing and instability in place to keep at least scattered
showers and thunderstorms ongoing or slowly increasing with
afternoon heating. The larger scale system forcing is progged to
lift across the Arrowhead of MN and into Ontario, which will keep
the more widespread showers and thunderstorms over the far
northwestern portions of the area and most likely over Lake
Superior. In fact, the hi-res models are in good agreement that this
will be the case throughout the day. As far as the severe weather
potential, a few strong storms may approach the western U.P. early
in the morning; however, the better chances will be during the
afternoon hours as daytime heating increases. MUCAPE values in the
afternoon are progged to be around 500-1000 J/kg along and ahead of
the front for most of the central and eastern U.P.; however, values
may approach 1500 J/kg over the south-central portions of the area.
Deep-layer shear values are progged to be up to 40 knots along and
ahead of the front, with a small portion of that being perpendicular
to the forcing, leading to an some potential for better storm
organization/structure for any stronger storms that form. The
combination of the two will lead to an increased risk of severe
thunderstorms during the afternoon hours. As previously mentioned,
the displacement of the broader/stronger forcing will likely keep
the coverage limited, which continues to support the marginal risk
issued by SPC. At this point, the main threats from the strong to
severe thunderstorms would be large hail and damaging winds. DCAPE
values are progged to be around 500-1000 J/kg during the afternoon
along with storm motions expected to be from the southwest to the
northeast at or around 35 to 45 knots. The quick motion along with
the potential for colder downdrafts (drier air aloft) will help
support stronger wind gust potential in the stronger
thunderstorms. In addition to thunderstorm wind potential,
stronger winds aloft may also mix down during the afternoon,
especially in the favored south wind downslope areas along Lake
Superior. This will likely cause wind gusts (outside of
thunderstorms) to be in the 25 to 35 mph range. In fact, 925mb
winds are progged to be around 40 to 50 mph, so it wouldn`t take
much to mix down some higher wind gusts with afternoon mixing or
with strong to severe thunderstorms.

.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 PM Lake Superior forecast issuance)
Issued at 344 PM EDT TUE SEP 19 2017

Winds will continue to steadily increase more so tonight into
Wednesday as the trough moves closer. Southeast to south winds will
increase to 20-25kt across much of Lake Superior tonight and will
increase up to 30kt over north central and eastern Lake Superior on
Wednesday. Not out of the question that there could be some gale
force gusts to 35 knots. With passage of the trough/cold front,
winds will drop off quickly to under 15kt from west to east
Wednesday night with the lighter winds lingering through Thursday.
Wind should be mostly under 20kt Friday and Saturday as Lake
Superior becomes situated between a low pressure trough to the west
and high pressure to the east. Could be some stronger winds at
times, depending on whether any low pressure waves track northeast
along the trough.

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


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