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 Sun Oct 25 2020

.SHORT TERM...(Today and tonight)
Issued at 516 AM EDT SUN OCT 25 2020

Radar this morning showers scattered lake-effect snow showers over
mainly eastern Lake Superior and moving onshore into Baraga,
Marquette, and Alger counties. Expect these to persist through the
morning with occasional localized moderate to heavy rates possible.
Weak winds will keep the snow showers from penetrating too far
inland, with accumulation expected to stay confined to areas within
about 10-20 miles of the Lake Superior shoreline.

For the rest of today, as a northern stream short wave digs into the
northern Rockies this morning, a weak mid-level wave analyzed over
southern Minnesota this morning will eject northeastward along the
jet into the Upper Great Lakes. This wave will help usher in deeper
moisture, which with cold air already in place, will further enhance
the lake-effect snow potential this afternoon and evening. By this
afternoon, model soundings show lake sfc-850 mb delta T`s of around
17 C, inversion heights around 7 kft, and lake-based equilibrium
heights around 9 kft. So the environment will support moderate to
occasionally heavy LES.

The question is where. As has been advertised for several days, it
still looks like convergence along an inverted trough will be the
focus for LES showers today, with that band oriented north-south
through eastern Lake Superior. The consensus among the CAMs seems to
be that LES will intensify in a band along this trough after about
18z and orient itself into eastern Marquette County and far western
Alger County. QPF for this period is around the 75th percentile of
guidance given that CAMs are more likely to resolve the localized
heavy LES. In short, there is a medium to high confidence for a
plowable snowfall in the area bound by Big Bay to Marquette to
Melstrand. Given the background flow remains weak through tonight,
still not expecting the band to penetrate as far inland as it
otherwise might early in the season. Came up with about 3-8" of snow
in these areas given SLRs around 15;1 to 20:1 thanks to a DGZ
spanning 800-600 mb and very weak winds below the DGZ keeping the
dendrites from shattering. Given it`s likely snow will reach
plowable depth along stretches of US-41, M-28, and M-94, opted to
issue a Winter Weather Advisory for Marquette and Alger counties.
Could have waited until around 18z to start it but given LES showers
are already ongoing, opted to start it immediately.

The other area to watch will be around Ironwood and the Porkies.
CAMs are more split here with some advertising significant snowfall
and others coming up empty. Shot down the middle of the CAMs for QPF
and came up with around 3-6" of snow around Ironwood. Given
confidence is lower here and amounts just barely touch advisory
criteria, opted for just an SPS and HWO mention here instead.

Outside of the lake-effect areas, synoptically-forced rain and snow
showers will graze the southern and eastern U.P. this afternoon and
evening, amounting to around a half inch to an inch outside of the
aforementioned lake-effect areas. Highs will be stuck in the 30s
today across the U.P. With cloud cover around everywhere tonight,
opted for lows on the warmer side of guidance, which is still around
the upper teens interior west to mid 20s south & east.

.LONG TERM...(Monday through Saturday)
Issued at 310 AM EDT SUN OCT 25 2020

A mostly quiet long term pattern with a warm up back to near normal
by the end of the period. With a cut-off low establishing itself
over the Desert Southwest with an amplified trough northward from
there on Monday, the upstream ridge along the Gulf of
Mexico/southeast Atlantic coast will help produce near zonal flow
interrupted by a few shortwaves across the Upper Great Lakes. The
cutoff low will migrate across the Plains into the Mississippi
Valley toward the end of the week, reaching the mid-Atlantic coast
by Friday. 0z guidance suggests below normal temps will continue
this week until Saturday, when 850mb temps rise back up near
+10C.

Beginning Monday, a weak trough will pass through the region. This
will result in continued lake effect snow showers in the northwest
wind belts, mainly in the east though. High pressure and dry air
building into the west will help limit any LES development. As this
ridge builds in, LES showers should come to an end by evening in the
east. With 850mb temps still hovering near -10C across the region,
daytime highs should only top out in the low 30s in the west and
high 30s to near 40 in the east and south.

With that dry air and winds going light, expecting Monday night to
be on the chilly side. Overnight lows in the teens with maybe even
some single digits will be possible in the west. In the east and by
the lakeshores, expecting lows to bottom out in the low 20s to high
teens. As the sunrises, the dry air should be enough for some
welcomed sunshine! Daytime highs though will still be on the chilly
side with widespread 30s being expected. Tuesday night, a quick
moving cold front looks to move through the region that`ll bring the
next shot for precip. At the moment though, precip looks to struggle
given the dry air ahead of the system. Among the guidance, the NAM
appears to be the most aggressive with the snow showers along a
sharp 850-700mb f-gen axis.

The parent low for this quick moving system will pivot through
Ontario during the day on Wednesday, which will send another
secondary cold front through the Upper Great Lakes Wednesday
evening. Following the frontal passage, low pressure will move up
the Appalachians/into the Mid-Atlantic. This will allow winds to
become northerly, which will help set up the next brief lake effect
period Thursday. Friday, the next high pressure will build in and
help maintain dry conditions. Accompanying this will be warm temps.
Right now guidance suggests a return to normal temps for the
weekend.
&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 744 AM EDT SUN OCT 25 2020

Concern today will be lake-effect snow showers and associated lower
cigs and vis. Expect CMX to stay VFR with only flurries or isolated
light snow showers. IWD will stay mostly MVFR today with period of
IFR vis when the heavier lake-effect snow showers move through. Most
likely window for that right now looks to be late morning through
mid-afternoon. Pretty much same story at SAW with long duration of
MVFR cig and occasional IFR vis, except most likely window starts
later (early afternoon) and runs longer - into this evening. Snow
showers should shift east of the airfield this evening but MVFR cigs
will persist through the end of the TAF period.
&&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 516 AM EDT SUN OCT 25 2020

Light winds are expected across the lake through Monday night. There
will be a band of lake-effect snow showers from about the eastern
tip of the Keweenaw southward to about Munising and Marquette today
and tonight that could briefly reduce visibilities.

Low pressure moving east through Manitoba and Ontario Tuesday night
early Wednesday will result in increasing southwesterly winds on
Tuesday. Still looks like SWerly gales are likely on the west half
Tuesday evening. Have increased winds a few kts this forecast
package to note that a few 40 kt winds are possible, especially in
the favored channeling area between the Keweenaw and Isle Royale.

These gales will subside Wednesday morning, but winds will remain
around 20-30 kts through Wednesday night once colder air moves over
the lake behind this system. Winds are expected to remain below 20
kts across the lake on Thursday.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  Winter Weather Advisory until 6 AM EDT Monday for MIZ005-006.

Lake Superior...
None.
Lake Michigan...
None.
&&

$$

SHORT TERM...RJC
LONG TERM...JP
AVIATION...RJC
MARINE...RJC



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