Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 201133

Area Forecast Discussion
National Weather Service Marquette MI
733 AM EDT Tue Aug 20 2019

.SHORT TERM...(Today and tonight)
Issued at 515 AM EDT TUE AUG 20 2019

Water vapor imagery and RAP analysis show an elongated mid-level low
extending from northern Manitoba across southern Hudson Bay with
ridging upstream over western N America. A shortwave is moving
across Saskatchewan/Manitoba. This wave is generating increasing
coverage of shra/tsra across eastern ND/se Manitoba into nw MN early
this morning on the nose of instability axis of 2000-4000 j/kg
MUCAPE. Closer to home, it`s been a clear night across the fcst
area. With calm winds in the interior, temps have slipped just blo
50F at some of the traditional cold spots. Temps range up into the
60s at most locations along the Great Lakes, but it`s still in the
lower 70s at some locations in the Keweenaw where winds are still in
the 10-20mph range.

Main concern in the short term is fate of upstream convection. Model
guidance shows this convection continuing to organize over the next
several hrs as it moves across MN. Then, as it moves out ahead/away
from the instability axis, all models show this convection fading
away. How quickly that occurs as it moves across western Lake
Superior is uncertain. General trends from the HRRR during the night
is for this convection to weaken and mostly dissipate before
reaching western Upper MI. This matches well with the 00z ARW, NMM
and NSSL WRF and 06z high res NAM window. For now, opted to carry
schc/low chc pops into the w early this aftn with dry conditions
persisting farther e. Remnant clouds will probably limit mixed layer
instability development during the day prior to cold front arrival
late aftn/early evening, and fcst soundings generally suggest air
mass will be capped. Thus, will likely be difficult for any
additional convection to develop along the cold front as it moves
across Upper MI late aftn thru the evening. For now, only included
schc and some low chc pops across the w into central Upper MI late
aftn/evening. Should any sfc based convection take off, deep layer
shear in excess of 35kt would support storm organization and a
possible svr risk. Expect high temps today in the mid 70s to lwr
80s, warmest east away from Lake MI. Temps tonight will fall back to
mostly btwn 50F and 60F.

.LONG TERM...(Wednesday through Monday)
Issued at 358 AM EDT TUE AUG 20 2019

Not much has changed in the forecast compared to what has been
advertised for days now, with below-normal temperatures and another
dry stretch of weather expected through the middle/end of the week.
The main weather hazards through the middle part of the week look to
be wave-related, with waves building along the Lake Superior
shoreline and causing a high swim risk east of Marquette. A gradual
warming trend back to near-normal temperatures for the weekend is
expected as upper-level ridging slides east across the region. The
next best chance for widespread precipitation doesn`t look to arrive
until early next week.

Fall-like weather is on the way Wednesday through Friday morning as
a cutoff low north of Lake Superior slowly rotates east of the
region, sending multiple shortwave troughs south across the region,
along with strong cold air advection. During this time period,
daytime highs will struggle to climb into the mid to upper 60s. The
cold air advection will start to move in on Wednesday, bringing with
it sharply cooler temperatures, but 850mb temperatures down to 3-5C
overspreading the region on Thursday will easily keep daytime highs
in the 60s, if not cooler in some spots where cloud cover is more
prevalent. Cloud cover is expected to be more widespread on
Thursday. A few models are hinting at the potential for some light
shower activity, but have kept silent PoPs for now. Overnight lows
will be chilly especially inland from the Great Lakes during this
stretch of cold air advection, and may lead to the potential for
some patchy frost early Friday morning. Subsequent forecasts will
need to monitor cloud cover, as that looks to be the only thing that
would inhibit temperatures from dropping in the low to mid 30s.

The onset of the cold air advection on Wednesday will bring with it
gusty northwest winds, especially over the Keweenaw Peninsula early
in the day and then the east half of Upper Michigan throughout much
of the day. The unstable conditions over Lake Superior will create
ideal conditions for waves building along the shoreline, especially
east of Marquette where fetch over the lake will be maximized. These
building waves will create a high swim risk for Lake Superior
beaches in Alger County on Wednesday. Winds slacken quite a bit on
Thursday/Friday as high pressure traverses the region, but we could
see lingering swim risk issues on Thursday.

Friday through the weekend, we will get back into a gradual warming
trend with more seasonable temperatures expected. As previously
noted yesterday, while high pressure shifts east of the region this
weekend, dry air looks to win out for much of the weekend.
Therefore, as expected models have slowed down quite a bit with the
arrival of the next shortwave towards the end of the weekend, and
thus precipitation chances. There is a small chance a few
showers/storms could work into far western Upper Michigan by Sunday
morning as the shortwave lifts through, but if this wave continues
to slow down chances for precipitation could be pushed back even
further. The next best chance for widespread precipitation now looks
to be early next week, especially Monday/Tuesday, as a slow moving
front pushes east across the Upper Great Lakes.

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 732 AM EDT TUE AUG 20 2019

VFR conditions will likely prevail for most, if not all, of this
fcst period at KIWD/KCMX/KSAW. Complex of shra/tsra over northern MN
will likely weaken/dissipate as it crosses western Lake Superior
later this morning. However, a few shra may survive into western
Upper MI, so VCSH was included in fcst at KIWD/KCMX. No restrictions
out of VFR are anticipated even if a shra occurs. Isold/sct
shra/tsra may redevelop around KIWD late aftn/early evening, but
confidence is low. Cold front will pass across west and central
Upper MI late aftn/evening. Upslope postfrontal winds off Lake
Superior could lead to a period of MVFR cigs at KIWD this evening.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 515 AM EDT TUE AUG 20 2019

Breezy w to sw winds gusting to 20-30kt at high obs platforms across
much of Lake Superior during the night, especially across the w
half, will diminish this morning as pres gradient weakens. Winds
will mostly be under 15kt during the day. However, showers and
thunderstorms currently across northern MN will weaken/dissipate as
they move out across western Lake Superior later this morning.
Although dissipating, these showers may result in a brief period of
locally stronger/gusty winds upwards of 30kt for a time, mainly over
western Lake Superior. Passage of a cold front this aftn/evening
will usher in a cool air mass for Wed into Thu. With unstable
conditions developing btwn the warmer water and cooler air moving
over the lake, winds will likely reach the 15-25kt range at times,
and perhaps gusting up to 30kt occasionally, primarily over the east
half of the lake. High pres will arrive later Thu into Fri to bring
diminishing winds.

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


SHORT TERM...Rolfson
LONG TERM...Ritzman
MARINE...Rolfson is the U.S. government's official web portal to all federal, state and local government web resources and services.