Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 191944
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
344 PM EDT Tue Sep 19 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 potenital 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 through Monday)
Issued at 353 AM EDT TUE SEP 19 2017

Overall, there is not much of a change to the extended than what has
been advertised now for the last couple of days. We will see a
significant warming trend through the end of the work week, with
record heat looking likely on Friday as temperatures soar 15 to 25
degrees above normal for this time of year. This warm weather is
expected to persist through the weekend. Periodic chances for
showers and thunderstorms are expected into early next week,
especially out west.

Wednesday and Wednesday night: The main concern is the potential for
thunderstorms as a cold front arrives from the west during the
afternoon hours and into the overnight hours. Also, ahead of the
front, south-southwest winds are expected to increase and become
gusty and may impact mariners in the near-shore zones from Big Bay
down through Munising on Lake Superior. BUFKIT soundings show
momentum transport values approaching the 30 to 40 knots in these
downslope regions near Lake Superior during the afternoon hours.
Also ahead of the cold front, the medium range models are in fairly
good agreement with surface dew points climbing into the low to mid
60s; this combined with steepening mid-level lapse rates should
allow 1000-1500 J/kg of MUCAPE to develop through the afternoon
hours and sustain updrafts of any ongoing or newly developing
convection. With the deep-layer shear expected to be around 40 knots
and oriented quasi-perpendicular to the cold front, this will also
allow updrafts to remain organized. However, there are a few
concerns/caveat that will likely impact the coverage and/or
intensity of showers and thunderstorms. First, the main forcing is
expected to lift northeast into Canada as the main shortwave lifts
across the Arrowhead of Minnesota. Secondly, CAPE profiles do not
look terribly impressive and are very skinny given the amount of dry
mid-level air expected to be in place. That being said, do not
expect coverage to be widespread, rather thinking that convection
should remain more scattered along the front as it pushes east
through the day on Wednesday. A few storms may be strong to severe,
especially further south along the Wisconsin border where the nose
of better instability is progged to nudge northward. As the front
continues its eastward push, lingering showers and thunderstorms
will continue mainly across the central and east into the overnight
hours.

Thursday through Friday:  The above mentioned cold front will slow
its southward progress and retreat back northward as a warm front.
This should allow for small chances for showers and perhaps a few
rumbles of thunder late Thursday into Friday. Then the attention
turns towards the significant warm up expected as a highly amplified
upper-level pattern sets up across the central CONUS and ushers in
unseasonably warm air, with 925 to 850mb temperatures pushing
upwards of 19-22C on Friday. These warm temperatures within the
boundary layer will work in concert with downslope winds to allow
temperatures to climb into the 80s across much of the area.
Numerical guidance over the past couple of days has trended a bit
warmer each day; therefore, wouldn`t be surprised if a few locations
break the 90 degree mark. With the synoptic flow not expected to be
terribly strong, the lake breeze should develop and increase in
strengthen during the afternoon hours. A few models are showing the
potential for some convection along this lake breezy boundary across
the north central; however, capping looks stout given the warm air
aloft.

This weekend into early next week: The main baroclinic zone progged
to develop in response to the longwave troughing out west is
expected to remain west of the area across the Arrowhead of
Minnesota for the most part through the weekend and into early next
week. Therefore, we will remain on the warm side of this boundary
with temperatures expected to remain 10 to 15 degrees above normal
for this time of year. Confidence is not high in regards to how
precipitation chances will play out, but there definitely looks like
a chance for periodic showers and perhaps some thunder as the
baroclinic zone looks to wobble a bit west to east with subtle
shortwave energy lifting across the Upper Great Lakes region. The
main baroclinic zone is progged to finally push across Upper
Michigan early next week as the main trough axis finally begins to
eject out of the west and across the Upper Great Lakes region. This
will bring increased chances for showers and thunderstorms, along
with cooler more seasonable temperatures towards the middle/end of
next week.
&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 203 PM EDT TUE SEP 19 2017

VFR conditions will prevail into the evening hrs at KIWD/KCMX/KSAW
under a relatively dry low-level air mass over the area. An
approaching cold front/trough will allow the pressure gradient to
tighten tonight ahead of the system. A temperature inversion will be
in place tonight, allowing a low-level jet to form aloft. This will
create ideal conditions for LLWS for KIWD and KSAW tonight into
Wednesday morning. Upslope flow off Lake MI will likely lead to fog
(IFR vis) and potentially an LIFR cig late tonight at KSAW. Showers
and thunderstorms will be possible at KIWD in the morning; however,
KSAW and KCMX will have better chances during the afternoon.
Confidence in overall coverage is not high; therefore, have opted to
leave out of the TAF for this issuance, except for IWD early in
the day.
&&

.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.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...KEC
LONG TERM...Ritzman
AVIATION...KEC
MARINE...KEC



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