Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 211750
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
150 PM 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
front.

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...(Friday through Wednesday)
Issued at 445 AM EDT THU SEP 21 2017

Unseasonable warmth will dominate Fri-Sun with a number of locations
likely seeing record breaking late season heat. Not only that, some
locations may see max temps that have never been so high this late
in the year. Warmth will be a result of an amplified pattern,
featuring a western CONUS trof and downstream eastern N America
ridge. 500mb height anomalies of 250m will settle down into NV today
thru Sat in association with the western trof. Downstream response
will be building heights over eastern N America with a +230m anomaly
becoming centered s of James Bay during the weekend. Resulting deep
sw flow into the Upper Lakes will push 850mb temps to 10-14C above
long term late Sept averages and thus the unseasonable warmth. Heat
will be combined with uncomfortable mid summer type dwpts readings.
Next week, the western trof will weaken while progressing
downstream, reaching the Great Lakes midweek. This will obviously
bring cooler weather with temps falling back to seasonable levels
for late Sept. A little farther out, there have been some
suggestions in recent medium range model runs that heights may build
more so over western N America late next week. If so, troffing may
hold up/deepen a bit over the Great Lakes, resulting in temps
trending a little blo normal for a time late week. As for pcpn, a
nearly stationary sfc front will be setting up from the Plains to
northern Ontario between the western trof and eastern ridge. Pcpn
potential here will depend on how close to Upper MI this front is
located and whether any shortwaves ejecting from the trof pass close
to the area. At a minimum, proximity of the nearly stationary front
to western Upper MI will put that area at some risk of shra/tsra
thru the weekend. Best potential of pcpn across Upper MI will occur
early next week (at some point btwn Mon and early Wed) as frontal
boundary drifts across the Upper Lakes in response to progression of
the western trof. Given days of deep sw flow ahead of the trof
resulting in precipitable water of at least 1.5 inches (200-250pct
of normal) in the vcnty of the front, potential will be there for
mdt to hvy pcpn as front moves across the area.

Beginning Fri, will probably see some ongoing sct convection in the
morning associated with warm front and pool of elevated cape lifting
n of the area. There are some indications (NAM and the ARW and NMM
in particular) that a cluster of thunderstorms will form in the
vcnty of nw WI late tonight/early Fri morning and then spread across
at least parts of western Upper MI thru the morning. Only 2 of the
10 NCAR ensembles support that idea, however. If convection does
linger thru Fri morning, pcpn/cloud cover and disruption of sfc wind
field will hold back temps from what had been expected for Fri. At
this point, opted to nudge high temps down a degree or two given a
fair amount of guidance supporting possible morning convection
and/or more cloud cover. With the exception of downwind of Lake MI,
much of the area should see highs in the mid to upper 80s.

Unseasonable warmth will continue over the weekend with strong mid-
level ridge across the Great Lakes region (500mb heights topping
590dm). Sat should be just as warm as Fri under similar air mass,
maybe warmer since it appears there will be less cloud cover thru
the day. Temps aloft drop slightly on Sun, so probably not quite as
warm Sun as Sat. With dwpts into the 60s, nighttime min temps will
be equally as anomalous as the daytime highs. Expect some record
high mins to be set. Not out of the question that some locations
that see downsloping under southerly winds won`t fall much blo 70F
Fri night and Sat night. As for pcpn, much of the fcst area should
be capped thru the weekend with any shra/tsra confined closer to the
nearly stationary frontal boundary to the w and nw. Schc/low chc
pops will continue to be utilized over the far western fcst area
thru the weekend.

Given the strength of the eastern ridge, frontal progression across
Upper MI will likely be slow early next week. In fact, medium range
guidance has moved toward better agreement on a slower movement that
the ECMWF was first to begin showing. It`s quite possible that it
may not be until Tue that shra/tsra chances increase eastward across
the fcst area as front finally begins moving and clears the fcst
area by Wed. Although not as warm as the weekend, conditions will
probably remain on the warm side thru Tue with the more significant
cooling holding off until Wed.
&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 139 PM EDT THU SEP 21 2017

MVFR cigs at KSAW and KCMX burned off just before TAF issuance
with mixing from sunshine. MVFR to IFR ceilings will spread
northward across all terminals as a warm front lifts north across
the area. Upslope south to southeast flow could even lead to LIFR
conditions late tonight/early Friday at KSAW. Out west at
KIWD/KCMX, thunderstorms will be possible late tonight/early
Friday morning. Also all sites will experience low-level wind
shear late tonight/early Friday as a low-level jet max moves
across the region. &&

.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.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...Ritzman
LONG TERM...Rolfson
AVIATION...Voss
MARINE...Ritzman


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