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Area Forecast Discussion (AFD)

Issued by Marquette, MI (MQT)

FXUS63 KMQT 202051

Area Forecast Discussion
National Weather Service Marquette MI
451 PM EDT Tue Aug 20 2019

.SHORT TERM...(This evening through Wednesday)
Issued at 227 PM EDT TUE AUG 20 2019

As expected, early morning thunderstorms over northern MN fell apart 
as they crossed Lake Superior this morning, but rain showers and 
some briefly gusty winds (even as high as 35 mph at KIWD) did make 
it into western Upper Michigan today. Early this afternoon the last 
remnants of the rain showers continue to diminish as they cross the 
Keweenaw Peninsula. The earlier rain and cloud cover has kept 
temperatures in the 60s over the far west but they should be able to 
climb back into the lower 70s this afternoon with brief clearing 

The front remains a little behind schedule today. 18Z obs indicate 
that it is located from just west of Thunder Bay, ON to near Ely, MN 
to St. Cloud, MN. There is still a chance for some isolated shra in 
the Upper Peninsula late this afternoon through the evening as the 
front crosses, but in general it is just producing some post-frontal 
low cloudiness over Canada and MN this afternoon and that is 
expected to persist tonight. As drier air moves in behind the front, 
portions of the interior west will cool of pretty efficiently and 
with the nights getting continuously longer a few spots will likely 
fall into the mid to upper 40s. Lows east and lakeside will be in 
the 50s and lower 60s. Could be a little patchy fog tonight but it's 
a little questionable whether the boundary layer winds will calm 
enough for anything widespread.

Wednesday, we'll be looking at cooler highs in the upper 60s to mid 
70s, warmest near the Bay of Green Bay where downsloping 
northwesterlies will keep things a tad warmer than elsewhere. By 
midday it looks to get a little breezy as the mixed layer deepens to 
5-7 kft. Gusts to 20 mph are possible across much of the region, 
lower near MNM and higher over the east where a few 25-30 mph gusts 
seem a good bet. Expecting a high swim risk for Alger County by the 
mid to late afternoon and Small Craft Advisories may be needed for 
the Pictured Rocks region and areas east as well.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 449 PM EDT TUE AUG 20 2019

Fall-like weather is expected to impact Upper Michigan for the 
latter half of the work week as a closed low over central Canada 
rotates slowly se across James Bay into Quebec, sending multiple 
shortwave troughs across the Upper Great Lakes region along with 
associated strong cold advection.  

Wednesday night into Friday, initial strong shortwave and blast of 
cold advection will move in during the day on Wednesday but 
northwest wind gusts up to 25 mph will likely continue into Wed 
evening with continued cold advection, especially near Lake Superior 
and across the east half of the cwa. The unstable conditions over 
Lake Superior will create ideal conditions for building waves 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 Wednesday into early Thursday. 
Northerly winds should slacken quite a bit Thursday into Friday as 
sfc high pressure over northern Manitoba builds southward toward the 
Upper Great Lakes region. 

With 850 mb temps forecast to drop to 4C or colder late Wed night 
into early Fri, temps will be well below normal through much of the 
period. Expect inland min temps Wed night to drop into the lower to 
mid 40s with perhaps some mid to upper 30s readings at the typical 
spots, especially interior west. Daytime highs Thursday will 
struggle to climb into the mid to upper 60s and may be even colder 
where cloud cover is more prevalent. Model soundings suggest there 
could be a potential for some diurnal or lake induced instability 
showers at times through the period Wed night into Thu night as 
periodic shortwaves move across the area. This is not surprising 
given Lake Superior water temps in the lower 60s and 850 mb temps 
around 4C or colder. Large scale anticyclonic flow with the building 
sfc high should limit coverage of showers to isolated, or at best 
scattered, for the most part. Overnight lows Thu night will likely 
be coldest temps we've seen since at least early June, especially 
for locations inland from the Great Lakes, as radiational cooling 
conditions should be near ideal under the center of sfc high. If 
skies clear out, readings could easily dip near freezing, especially 
for the typical inland cold spots. As a consequence, would not be 
surprised if frost headlines may be needed Thu night into early Fri. 

Friday afternoon into early next week, a gradual warming trend is 
expected ahead of a slowly approaching frontal boundary over the 
Plains. A series of shortwaves moving through the northern Plains 
into south central Canada will gradually push the frontal boundary 
toward the Upper Great Lakes early next week. Gradual warm advection 
ahead of the frontal boundary and on the backside of the sfc ridge 
axis will yield more seasonable temperatures. With models trending 
slower with the arrival of the frontal boundary, persistent high 
pressure should keep conditions mostly dry through the weekend.  The 
next best chance for widespread precipitation now looks to be early 
next week, especially Monday/Mon night, as a slow moving front 
pushes east across the Upper Great Lakes. A few of the models even 
hint at a post-frontal shortwave moving through the area on Tue 
which could sustain lingering isolated showers.

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 117 PM EDT TUE AUG 20 2019

An area of showers accompanied by gusty winds recently moved through 
KIWD and is now approaching KCMX. Ceilings will remain well above 
MVFR thresholds with this activity, which is not expected to hold 
together much longer. Cold front will pass across west and central 
Upper MI late aftn/evening. Isold/sct shra/tsra may redevelop around 
KIWD as it passes, but confidence is relatively low, and even lower 
in shra reaching KSAW. Upslope postfrontal winds off Lake Superior 
could lead to a period of MVFR cigs at KIWD late this evening. Skies 
will begin to clear out again late tonight with NW winds becoming 
gusty towards the end of this TAF period, mainly at KCMX.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 236 PM EDT TUE AUG 20 2019

Passage of a cold front late this afternoon through just after 
midnight will usher in a cool air mass for Wed into Thur. 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. On Thursday winds should die down over 
much of the lake but some 15-25 kt gusts will remain, mainly east of 
a line from Michipicoten Island to Grand Marais, MI. High pressure will 
arrive later Thu into Fri to bring diminishing winds that are 
expected to remain below 20 kts until late Saturday night.

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