Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 210818

Area Forecast Discussion
National Weather Service Marquette MI
418 AM EDT Thu Sep 21 2017

.SHORT TERM...(Today and tonight)
Issued at 416 AM EDT THU SEP 21 2017

Showers and thunderstorms have continued to exit eastern portions of
the area this morning, with the weakening cold front trailing just
behind. Across the west, drier air has begun to advect into the
region as dew points have dropped into the 40s and low to mid 50s in
spots this morning. With surface high pressure building in behind
the cold front, skies have cleared across the west. Clear skies and
light winds have given way to ample radiational cooling and the
development of shallow ground fog. Additional fog has developed out
east, where lingering moisture continues to pool ahead of the cold

The patchy, shallow ground fog will continue through the morning
hours across the west and portions of central Upper Michigan. With
visibilities bouncing around this morning and drier air slowly
working eastward do not expect widespread dense fog to develop;
however, as the sunrises visibilities could briefly drop to around
1/2 mile or less. Locations that see fog early this morning across
the west should see a rapid improvement after sunrise, given the
shallow nature of the fog, as mixing begins to increase. Fog
across the east this morning will likely linger for a while as the
cold fronts southward push as slowed and dew points remain in the
60s. Otherwise, a quiet weather day is expected across the area.
Temperatures will climb into the 70s today; however, expect
temperatures to remain a bit cooler near the shores of Lake
Superior as onshore flow should keep temperatures in the 60s. With
fog showing up on satellite imagery this morning across Lake
Superior, it is possible that the marine layer could advect inland
in some locations along the shoreline around and east of Big Bay
given the north-northeast flow expected throughout the day.

Tonight, the above mentioned cold front is progged to pivot across
the Western Great Lakes and retreat back northward as a warm front.
As this occurs, strong warm air advection and moisture transport
will lift north-northeast across the area bringing back chances for
showers and thunderstorms overnight. Given the strong moisture
transport and steepening mid-level lapse rates, upwards of 1500-2000
J/kg should develop mainly across the western portions of Upper
Michigan, and extending out across western and central Lake
Superior. While the effective shear is on the marginal size with
around 20 knots, enough direction shear looks like it will be in
place and may help sustain updrafts and storm organization at times.
Therefore, a few strong to marginally severe storms will be
possible. Given the elevated nature of the convection, hail should
be the primary concern with any stronger cells.

.LONG TERM...(Thursday night through Wednesday)
Issued at 311 PM EDT WED SEP 20 2017

High amplitude trough/ridge pattern setting up over the CONUS by the
end of the week will position the western Great Lakes in a south-
southwest flow which will likely result in record warmth across the
area from Fri through Sun. Best potential of pcpn across Upper MI
will occur early next week (at some point between Mon and early Wed)
as a slow-moving frontal boundary drifts across the Upper Lakes in
response to the eastward progression of the western CONUS mid-level
trough. Given the days of deep moist sw flow ahead of the trof and
fcst precipitable water values exceeding 1.5 inches or more than 200
pct of normal for this time of year, the potential will be there for
moderate to heavy rainfall as the front moves across the area.

Beginning Thu night...The front moving through the fcst area today
and tonight will lift back north Thu night/Fri morning as mid-level
trough deepens over the western CONUS. Could be some showers/t-
storms on the elevated cape gradient as it lifts nne in conjunction
with low-level jet. Nose of low-level jet suggests the western and
northern fcst area has a better chance of seeing some convection

Friday through Sunday look to be unseasonably warm to record warm
days across the fcst area. As the warm front lifts well north of the
area Fri afternoon, warm advection in a ssw flow will bring 925-
850mb temperatures up to 20C or greater across the area. Mixing this
warmer air to the sfc will result in max temps 20 to 25 degrees
above normal for this time of year across much of the area on Friday
and possibly Saturday into Sunday as well with expected readings
in the mid to upper 80s over much of the western two thirds of
Upper Mi. Would not be surprised to see a few downsloping areas
along Lake Superior reaching 90F where compressional warming will
occur under southwest winds of 15 to 25 mph. Cooler 70s to lower
80s readings can be expected downwind of Lake Michigan. Southerly
flow in warm sector ahead of frontal boundary over the Plains will
also tap into Gulf moisture and bring low to mid 60s dewpoints
northward through the central CONUS into the Great Lakes region.
As a result, expect the increase in humidity to make for rather
muggy and uncomfortable conditions over the weekend. However, the
cap of warm air aloft will likely inhibit formation of convection
for the most part, despite the increase in moisture and
instability. By Sunday, model uncertainty increases regarding
progression of Plains frontal boundary eastward into Upper Mi. The
GFS is still the most aggressive pushing boundary eastward toward
western Upper Mi, while the ECMWF and GEM keep the stronger mid-
upper ridge intact with the frontal boundary farther west into MN.
Still possible there could be a few storms popping up over the
western cwa Sat night into Sun as frontal boundary draws closer
from the Plains. While there may be some increase in clouds to
keep max temps down a bit over the nw portion of Upper Mi on
Sunday, much of the cwa should still see plenty of sun to allow
for low to mid 80s readings away from the moderating influence of
Lake Mi.

Given strength of the eastern ridge in place, the slower frontal
progression of the ECMWF and Canadian models will likely be favored
over the quicker GFS solution. Slower solutions would argue that it
might not be until late Tue or early Wed that the front finally
clears Upper MI. As a result, Monday may trend drier across much of
the cwa while the better chance for showers and thunderstorms and
potential for locally heavy rain may be delayed until Tue with a
drying trend still expected Wed afternoon. Temps by Wed are expected
to cool back down to more seasonable late Sept readings.

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 121 AM EDT THU SEP 21 2017

Patchy fog will play havoc at times at KIWD overnight, with periods
of IFR/LIFR conditions. There is also a potential for shallow fog at
KCMX as well, especially once the winds shift back to the E late
tonight. Light downslope flow at KSAW will keep the fog at bay for a
time, but as these winds shift and weaken further, fog will return
to that area. Expect periods of IFR conditions there until the fog
lifts in the morning as the boundary layer mixes out. VFR conditions
to prevail through much of the rest of the forecast, except at KSAW,
where developing moist SE flow to bring in stratus late Thursday

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 416 AM EDT THU SEP 21 2017

Based on satellite trends and observations across the central and
eastern portions of the lake, fog developed overnight and should
linger through a good portion of the day today mainly across the
central and east. Winds are expected to remain around or under 15
knots through the day, but will increase this evening through early
Friday to around 15 to 25 knots as a warm front pushes north across
the Upper Great Lakes. Thunderstorms are possible tonight, mainly
across the west and central, with a few strong storms are possible.
Friday into early next week, winds will remain below 20 knots.

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


SHORT TERM...Ritzman
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