Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 211902

Area Forecast Discussion
National Weather Service Marquette MI
302 PM EDT Thu Sep 21 2017

.SHORT TERM...(This evening through Friday)
Issued at 301 PM EDT THU SEP 21 2017

19Z Visible satellite reveals that patches of fog still linger
over portions of Lake Superior this afternoon, especially eveident
over south central and eastern Lake Superior. Patchy fog/stratus
still lingers over portions of Luce County as well. Elsewhere
across Upper Mi, sunshine and associated mixing have scoured out
any lingering morning fog/stratus.

Tonight, the cold front which stalled out just east and south of the
fcst area this morning is poised to pivot and lift back northward
across the Western Great Lakes later tonight 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. Higher dew points with the warm
front combined with nocturnal cooling will also lead to areas of
fog, which could become widespread and possibly dense over southeast
portions of Upper Mi in a developing upslope sse flow. Given the
strong moisture transport and steepening mid-level lapse rates,
upwards of 1500-2000 J/kg should develop mainly over western
portions of Upper Michigan, and extending out across western and
central Lake Superior. While the effective shear is on the marginal
size around 20 knots, enough directional shear looks to be in place
that may sustain updrafts and aid 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.

Fri, could see some lingering scattered convection in the morning
mainly over the nw fcst area associated with the warm front and pool
of elevated cape lifting n of the area. If convection does linger
thru Fri morning, pcpn/cloud cover and disruption of sfc wind field
could hold back the rising of temps initially on Fri. However, still
would expect enough sunshine Fri afternoon as warm front lifts north
and core of 925 mb temps near 25c moves overhead that most of the
area should still realize highs in the mid to upper 80s with the
exception of 70s to lower 80s readings downwind of Lake Mi in a
southerly flow.

.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 PM Lake Superior forecast issuance)
Issued at 301 PM EDT THU SEP 21 2017

Latest visible satellite still reveals patches of fog over portions
of Lake Superior which could linger into tonight. With lower to mid
60s dewpoints moving across the area and lingering into early next
week, patchy fog could persist for several days across the lake.
Other than a brief increase this evening in winds to 20 to 25 knots
over the western part of the lake, expect winds to generally be
below 20 knots through the early part of next week under a weak pres
gradient. As a warm front lifts north across the area tonight,
scattered thunderstorms will be possible over mainly the western
half of the lake and a few could contain large hail.

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


LONG TERM...Rolfson
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