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Area Forecast Discussion (AFD)

Issued by Marquette, MI (MQT)

FXUS63 KMQT 170857

Area Forecast Discussion
National Weather Service Marquette MI
457 AM EDT Sat Aug 17 2019

.SHORT TERM...(Today and tonight)
Issued at 405 AM EDT SAT AUG 17 2019

...Patchy dense fog this morning...

Patchy fog has been dense at times across the west half of Upper 
Michigan early this morning, especially so across the Keweenaw 
Peninsula and central parts of the area. While surface winds 
remained light, fog has been intermittently dense likely due to the 
residual turbulent boundary layer winds. Once the sun comes up later 
this morning, visibilities could locally worsen before starting to 
improve through the mid-morning hours. Did opt to hang onto the 
inherited Special Weather Statement to draw attention to the fog and 
its associated hazards. 

Today, a mid-level shortwave will lift northeast across the Upper 
Great Lakes. The combination of increased sunshine today and warm 
air advection lifting into the region ahead of shortwave energy, 
will favor warmer temperatures UP-wide. Gradient winds will 
predominantly be out of the south-southwest today, between 5 and 10 
mph. These weaker gradient winds and increased sunshine will allow 
lake breezes to develop on both Lakes Michigan and Superior. Expect 
the south-southwest flow to keep the Lake Superior breeze confined 
close to the shoreline across the north-central. Over across the 
north end of Lake Michigan, the lake breeze boundaries will initially 
start off along the shoreline and then will gradually lift north 
throughout the day. Forecast soundings do show some weak capping 
aloft, but a few high-res models do show some isolated to scattered 
showers developing along the lake breeze boundaries, especially area 
with enhanced convergence due to terrain or interacting boundaries. 
Therefore, did introduce a few areas of isolated showers today. 

Tonight, once we lose isolated any ongoing showers or diurnally 
driven cumulus will dissipate. Tonight should remain quiet across 
the UP, with mild conditions as south winds persist overnight ahead 
of another approaching front from the west. Models have trended 
slower with this next wave coming in tonight, so did back off on the 
PoPs quite a bit. Some patchy fog is possible north of Lake Michigan 
tonight, but given the turbulent boundary layer winds expected do 
not think we will see and dense fog.

.LONG TERM...(Sunday through Friday)
Issued at 456 AM EDT SAT AUG 17 2019

Strong positive height anomaly over the AK Peninsula/Aleutians will 
shift nw and n to far eastern Siberia/adjacent Arctic Ocean over the 
next few days. This will initially support a deep trof over western 
Canada and then over the Gulf of AK Tue/Wed as the positive height 
anomaly continues to retrograde. A portion of the initial trof will 
shift e during the week underneath a positive height anomaly n and 
nw of Hudson Bay. As this occurs, the deep trof developing over the 
Gulf of AK Tue/Wed will result in a building ridge over the Rockies 
that will link up with the positive height anomaly n and nw of 
Hudson Bay. This will force the eastward moving trof to amplify thru 
the Great Lakes Wed/early Thu and then New England Fri. Under these 
changes, above normal temps Mon/Tue will fall back blo normal 
midweek behind the aforementioned amplifying trof. Given the growing 
model agreement on the trof amplification, it will turn quite cool 
midweek, providing a preview of fall which is just around the 
corner. Temps will rise back above normal by next weekend as ridging 
over the Rockies into central Canada shifts e. Farther down the line 
into the following week, ensemble trends over the last 24hrs are 
toward an increase in western N America ridging, resulting in a 
little more northerly component to the flow downstream into the 
Great Lakes. Thus, temps will probably be close to normal or leaning 
somewhat blo normal at times during the last week of Aug. As for 
pcpn, as the aforementioned trof moves across southern Canada during 
the week, a shortwave will swing thru the western Great Lakes on 
Sun, providing the next chc of pcpn. There are still indications of 
a fairly well-defined lead wave that will provide a focus for more 
significant pcpn which may impact the fcst area. Another shortwave 
leading the trof amplification into the Great Lakes will arrive Tue 
night. Associated cold front will probably produce isold/sct shra 
Tue aftn/evening. While there may be a few instability 
-shra/sprinkles Wed and/or Thu under a cool air mass/cyclonic flow, 
dry weather will generally prevail, continuing into Fri as mid-level 
ridging approaches and then moves across the area. Shra may return 
over the weekend. 

Beginning Sun, models are in good agreement on the mid-level trof to 
move across the Upper Great Lakes. One area of focus will be on a 
shortwave over the Plains that lifts into the Great Lakes on the 
front side of the trof. ECMWF has been most consistent in showing 
this feature as the main player for pcpn and would suggest the se 
half of the fcst area has best opportunity for widespread decent 
rainfall. Whether or not that works out, decent forcing, theta-e 
advection and moisture transport should support sct-nmrs shra and 
some embedded thunder spreading across the area Sun morning thru 
early aftn, then ending w to e late aftn thru the evening. Deep 
layer shear of 30-40kt would support storm organization and a svr 
risk on Sun. However, it seems quite likely that abundant cloud 
cover to start the day or arriving during the morning will limit 
instability so that svr storms will not be a concern. After the 
rainfall on Sun, clearing skies/diminishing winds Sun night may 
allow for patchy fog to develop. Temps in the interior, especially w 
half will fall into the 40s Sun night.  

With a dry air mass dominating on Mon, expect sunny skies. There may 
not even be any cu given the dry columm. Opted to lower dwpts to at 
least the NAM/GFS mos guidance (mid 40s F at the low end). Mixing 
potential on fcst soundings suggest dwpts could fall off into the 
30s F over the interior w half. With high temps mostly in the upper 
70s/lower 80s, the lower dwpts will make for a nice mid Aug day.  

Shortwave leading the trof amplification into the Great Lakes will 
arrive Tue night. Associated cold front will move across the area 
Tue aftn/evening. Isold to sct shra, maybe a few tsra, will 
accompany the front. 850mb temps will likely fall into the 4-6C 
range at some point Wed into Thu. Cool air/cyclonic flow will result 
in abundant cloud cover with daytime heating and may support a few
-shra/sprinkles either day, depending on when the core of the 
coolest air passes. Whichever day has the lowest 850mb temps, high 
temps will be in the 60s to lower 70s. If timing of the sfc high 
pres into the area coincides with nighttime, frost will be a concern 
in the interior. Wed night may be the coldest night over the w and 
Thu night the coldest night over the e. A warming trend will occur 
Fri/Sat as mid-level ridging passes and w to sw flow takes hold.

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 810 PM EDT FRI AUG 16 2019

Partial clearing is expected at KSAW this evening behind the band of 
rain showers while clear to partly cloudy skies prevail at KIWD and 
KCMX. Fog development is likely overnight at all three sites but 
there is uncertainty with how dense it will become. IFR vsby was 
included at all three sites by late tonight. Some brief periods of 
LIFR are possible but was not confident enough to include. Vsby will 
lift quickly as sun comes up Saturday.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 405 AM EDT SAT AUG 17 2019

Satellite trends show some area of stratus and perhaps some fog over 
the lake early this morning. However, no ship observations appear to 
have reported any fog this morning. Expect fog to persist across 
parts of the lake, but it should be on a the move throughout the 
day. Not totally out of the question there could be some locally 
dense fog out there this morning. 

Winds will remain between 15 and 20 knots for the most part of the 
weekend. A brief area of high pressure will track east across the 
region, reducing the winds somewhat Monday/Tuesday. Then, a cold 
front, followed be a sharply cooler air mass look to arrive late 
Tuesday and linger across the region into Thursday. This cooler air 
mass working over the relatively warmer waters of the lake will 
create an unstable conditions and the potential for northwest winds 
20 to 25 knots, and increased wave heights especially funneling into 
Whitefish Bay.

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


SHORT TERM...Ritzman
LONG TERM...Rolfson