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Area Forecast Discussion (AFD)

Issued by Marquette, MI (MQT)

FXUS63 KMQT 200916

Area Forecast Discussion
National Weather Service Marquette MI
516 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 06Z TAFS through 06Z Tuesday night)
Issued at 137 AM EDT TUE AUG 20 2019

Under a dry air mass, VFR conditions will continue through most of 
the fcst period. This afternoon, increasing mid clouds are 
expected at KIWD and KCMX but it will remain well above MVFR 
thresholds. Shower chances are expected this afternoon mainly at KIWD 
and KCMX where VCSH was mentioned. Shra are also possible at KSAW 
toward evening. TS may also develop but confidence was not high 
enough to mention. Behind the cold front, enough low level moisture 
will move in to drop cigs near or below MVFR at KIWD. Any lower cigs 
at KSAW should hold off until after 06z.

.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