Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 251144
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
744 AM EDT Fri Sep 25 2020

.SHORT TERM...(Today and tonight)
Issued at 333 AM EDT FRI SEP 25 2020

As GOES imagery and sfc obs suggest, it`s a little soupy out there
this morning. Seems to be more in the way of lower clouds than
visibility restrictions, though a few observations coming in
suggesting more fog than I can find anywhere on webcams. Model
soundings and upstream observations suggest this trend should
continue through the rest of the overnight period, improving from
west to east through the morning. With the low clouds socked right
in, not expecting temperatures to go much of anywhere through the
rest of the night.

Interesting little setup today with lingering low-level moisture in
the morning bringing widespread lower clouds and patchy dense fog,
to a chance at a few severe storms late this afternoon into the
evening. A shortwave trof, currently seen well on GOES along the
MT/Dakotas border, will approach western Upper Michigan this
evening. Potent WAA along with a stronger LLJ between 40 and 50
knots will move in ahead of this wave. Depending on the model,
anywhere from 1000 to 2000 J/kg of MLCAPE will nudge its way into
western Upper Michigan around 00Z tonight. NAM is a little bullish
on the shear, but also has the LLJ reaching up to 60kts this
evening, but helicity values on the conservative side around
300m2/s2 and up to around 450m2/s2 and bulk shear between 50 and 60
knots is conducive for severe storm development. The caveat to all
this, is whether or not storms can tap into sfc-based environment.
The shortwave and associated cold front should be able to kick off a
line of thunderstorms tonight, as good agreement in CAMs suggest,
but as a warm front and WAA push northward, it could serve as
elevated cap. Daytime trends will need to be monitored as add`l
mixing than soundings suggest with daytime heating could tip the
atmosphere`s scale to a more severe storm threat. The best chance of
this severe threat would be along the warm frontal axis and along
the triple point mainly in Gogebic, Ontonagon, and perhaps Iron
counties. CAMs also suggest the potential for storms to merge more
into a cluster, which could prolong a wind threat further east. All
in all, it seems the convection outruns its ability to tap into sfc-
based environment and severe/convective potential diminishes as the
line pushes east through the overnight hours.

Winds today into tonight look to get gusty, and depending on depth
of mixing, could get quite gusty with 40+kt winds only a couple
thousand feet off the sfc. For now, have gusts up to 30 mph across
much of the western and central portions of Upper Michigan, with the
potential for some gustier winds ahead of the front...in addition to
any svr wind chances. Winds calm down and with residual moisture
from precipitation, looks like a good bet for patchy to areas of
dense fog to develop across the west and central portions of Upper
Michigan.

.LONG TERM...(Saturday through Thursday)
Issued at 512 AM EDT FRI SEP 25 2020

Pattern will transition from nearly zonal flow early this weekend to
becoming highly amplified across the CONUS for much of next week as
strongly positive height anomalies and amplified ridging developing
from the Pacific NW into the Northwest Territories induces a broad,
amplified trough and abnormally negative height anomalies across the
central CONUS and Great Lakes region. The main result will be a
transition to above normal temps this weekend to much below normal
temps for much of next week.

Weak shortwave ridging will briefly build in to the area Saturday
morning before the next storm system moves into the area later
Saturday into Saturday night. While lots of sunshine is doubtful, a
break or two can`t be ruled out.  Regardless, it will be a fairly
mild day with highs most locations in the lower 70s...perhaps the
last one for awhile. Shortwave and associated low and warm front
approaches late afternoon/evening which is when the majority of
models show convection initiating. Track of shortwave and associated
sfc low across the western cwa would suggest better (numerous)
coverage over the nw half of the cwa with scattered or perhaps
isolated coverage southeast half. Depending on amount of elevated
instability that develops (NAM indicating more CAPE than the GFS),
effective shear values at or above 50 knots could maybe briefly
support a marginal hail threat into early evening from a stronger
storm.

As shortwave and surface trough/cold front move through the Upper
Lakes late Saturday evening/early overnight expect convection to end
as dry slot and sfc ridging move in behind the front. Post frontal
day on Sunday will be breezy with seasonable temperatures.

While most models have trended away from a wrapped up fall storm for
early next week, the 00z GFS now shows a rapidly deepening sfc low
developing near the Mackinac Straits on Tue which lifts quickly
north through Ontario to Hudson Bay on Wed. The GFS ensemble does
show some support for a deepening sfc low northeast of the cwa late
Tue/Tue night but not to the degree of the 00Z GFS. Nonetheless, a
fairly sharp baroclinic zone developing from the southern Plains
through the Ohio Valley/Lower Great Lakes due to the increasingly
amplified trough developing over the central CONUS could certainly
lead to a stronger storm spinning up during this time frame so this
will be something to watch. Model trends continue to favor a cooling
trend with the upper troughing increasing across the Great Lakes.
Models indicate the coolest air will arrive mid to late in the week.
The 00Z GFS shows H85 temps falling into the -4 to -7C range late
Wed night into Fri. Early week 85H temps will be more on the order
of 0-3C range which may be enough to generate some lake effect rain
showers from time to time especially if any passing shortwaves aid
in enhancement. For the highlands inland from the lake...the first
snowflakes or at least graupel showers look possible for later in
the week especially late Wed night into Friday with thermal
troughing at its maximum. All in all...anticipate a very fall-like
week next week.
&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 744 AM EDT FRI SEP 25 2020

Lingering low-level moisture and morning fog will continue to lift
over the next couple hours, and has already lifted at IWD and CMX.
Expecting IWD and CMX to scatter out to VFR by this afternoon, while
SAW takes a little while longer with continued southerly, upslope
winds. Ahead of a cold front this evening, gusty winds will develop
at IWD and SAW as a LLJ approaches the UP. This front passes through
late this evening, bringing some thunderstorms to IWD and CMX. These
storms are expected to diminish before reaching SAW, leaving just
SHRA, but there is a chance a thunderstorm reaches SAW and will let
next TAF reassess. As the LLJ moves overhead tonight, LLWS is
expected at CMX, but gusty winds at SAW will limit the LLWS chances.
Behind the front and precipitation tonight, low-end MVFR to IFR cigs
and vsbys will move back into all TAF sites. There is a chance of
some brief LIFR conditions late at CMX and SAW.
&&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 342 AM EDT FRI SEP 25 2020

Southerly winds increase this afternoon ahead of an approaching cold
front. As is usually the case for southerly winds, the stronger
winds will be over the e half of the lake where gusts to 20-30kt are
expected by late aftn and on into the evening, particularly at high
obs platforms and probably toward and into the nearshore waters e of
Marquette. Wouldn`t be surprised if we see a few gale force gusts to
35 kts at the highest observation platforms, with a LLJ of 40 to 45
knots a few hundred feet above the lake sfc. With the passing front
weakening, post frontal winds will fall off to under 20kt Fri night
and to under 15kt across the lake for Sat. A fast moving
developing/strengthening low pres will lift ne across western Lake
Superior Sat night. Winds will ramp up with gusts probably to 25-
30kt. Depending on how quickly the low deepens, a brief period of
gales is still possible; however, the potential for a strong fall
storm to affect Lake Superior early next week continues to diminish.
With colder air flowing into the area, winds will likely be into the
15-25kt range at times through the first half of next week.
&&

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

$$

SHORT TERM...JAW
LONG TERM...Voss
AVIATION...JAW
MARINE...Rolfson/JAW


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