Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 212123

Area Forecast Discussion
National Weather Service Marquette MI
523 PM EDT Wed Oct 21 2020

.SHORT TERM...(This evening through Thursday)
Issued at 506 PM EDT WED OCT 21 2020

Water vapor imagery and RAP analysis show a mid-level trof
dominating Canada from coast to coast with the trof anchored by a
mid-level low centered just n of Hudson Bay. The southern extent of
this trof leaves broad troffing across the northern tier of the
Lower 48. Within this broad cyclonic flow, shortwave that generated
the rain/snow across the fcst area last night is currently in the
vcnty of James Bay. Upstream, the next shortwave of interest is
dropping along the BC coast into the Pacific NW. This wave will
impact the weather here Thu into Thu night. Closer to home, in the
wake last night`s wave, clouds have been slow to clear out today.
Some spotty light pcpn has been noted on the Keweenaw under a brisk
westerly low-level flow and under 850mb temps that have fallen to
around -6C. These w winds have been gusting to around 30mph at KCMX.

Sfc high pres ridge currently extending from the Dakotas to the
Lower Ohio Valley will shift ne tonight, reaching Upper MI late
tonight/Thu morning. So, it will be a quiet night. Under chilly
westerly flow in advance of the high, 850mb temps around -6C, at
least this evening, will be marginally cold enough to support some
light lake effect pcpn across the Keweenaw. Moisture does not extend
to the -10C isotherm, so ptype would likely be sprinkles or
-shra if anything does occur. Mid/high clouds area already spreading
into the area, so even where lower clouds have cleared out or will
clear out during the evening, not expecting any notable period of
radiational cooling tonight. Mins should range from the mid 20s to
the mid 30s.

On Thu, attn turns to the aforementioned shortwave currently over
the Pacific NW. This wave will move se into the northern Rockies
tonight and then shift out across the northern Plains on Thu. Fcst
is full of challenges, making for low confidence in what the outcome
will be on Thu. Backing winds ahead of the wave will spread a decent
push of isentropic ascent into the area on Thu. Mixing ratios in the
700-750mb layer of at least 4g/kg will support the potential of
4in/6hr snowfall rates where pcpn is all snow. There are
indications, especially in the GFS, of -EPV situated above the
sloping FGEN, which supports the potential of banded heavier pcpn as
well. As for ptype, models over the last day or so have been
trending s and colder, bringing a more wintry scenario into the
picture. While the GFS offers a more solid snow event over the nw
third, a significant one at that as it is high qpf outlier, the NAM
is more aggressive in surging a well elevated warm nose above 0C n
into the area, roughly in the 7-9kft layer. Deep subfreezing layer
blo would indicate sleet depending on whether the snowflakes
completely melt or not in the warm layer. Better chc of that
occurring would be later in the aftn from Iron/Dickison counties
northeastward. Given much uncertainty in the thermal profiles and
the pcpn amounts due to model spread, fcst leans away from the
extremes. Best chc of reaching advy level snow during the day on Thu
(2-4in for low SLR snow) should be across Gogebic/Ontonagon counties
e to western Marquette County and Iron County. If the GFS verifies,
those amounts will be way underdone with the Keweenaw getting in on
hvy snow as well. Ptype should be predominantly rain from roughly
Marquette-Iron Mtn e. Some instability well aloft may lead to a few
rumbles of thunder s central and e later in the day.

.LONG TERM...(Thursday night through Wednesday)
Issued at 446 PM EDT WED OCT 21 2020

Thursday evening the complex and messy winter storm will be
continuing over the entire U.P. With models continuing to trend
colder last night and today, and come more in line with the GFS
(which has had the colder and snowier solution all along).
Regardless of choice of model, still looks like a period of
impressive forcing in the 00z-06z time window. NAM shows strong fgen
throughout the column, maximized in the 800-700 mb layer, as well as
-EPV in the 700-600 mb layer. NAM cross sections further illustrate
CSI in the mid levels with geostrophic momentum surfaces crossing
theta-e* surfaces, and even a few spots of elevated CAPE (where the
vertical derivative of theta-e* goes negative). All of this points
to a band of heavy precip setting up over the area, or rather,
probably two bands: one over the western half of the U.P. and one
over the south-central and east. Went bullish with QPF during this
period given the impressive forcing. Still uncertain what the exact
ptype will be, and given such heavy rates in a relatively short
window, the ptype is crucial. Therefore, have to stress that
certainty is low, even less than 36 hours out. Think the most likely
outcome is predominantly snow north of a line from the Hurons to
about Watersmeet, and then a 50/50 mix of sleet and snow south of
that line and north of a line from about Munising to southern
Dickinson County. have hoisted a Winter Weather Advisory for the
west and central, in two segments: one for mainly snow and one for a
mix of snow and sleet.

Late Thursday night (after 06z) as the synoptic theta-e advection
forcing for ascent pulls out and CAA wraps in, there will be a
transition to lake-enhanced upslope precip. Model soundings show a
lack of cloud ice as the cyclone develops further and the mid-level
dryslot develops overhead. This means there will probably be a mix
of snow and drizzle in the upslope/north wind lake-effect regions.
Surface temps during this period should stay above freezing for the
most part, so not expecting much if any FZDZ though can`t completely
rule some out over the highest elevations of the interior west. This
could continue through late morning Friday until temps aloft cool
enough to reintroduce cloud ice, at which point precip will
transition to pure lake-effect rain/snow showers with additional
snow amounts around an inch possible in the N to NW wind LES belts
of the west half (it`ll be too warm for much if any snow in the
eastern half LES belts).

Friday night and Saturday look quiet expect for some lingering light
lake-effect snow showers Friday night in the NW wind LES belts as
high pressure builds in to the southwest.

The next chance for more widespread precip comes Sunday into Monday
as a frontal wave passes by to our south across Lower Michigan. The
EC continues to be more robust with its QPF, but even that`s only
around 0.1-0.2". Other models are drier. Thus, while rain and snow
showers are likely across the area, significant accumulations are
not expected. One possible exception to that would be Sunday night
into Monday morning. During this time period, the GFS and EC both
indicated an inverted trough extending northward from the frontal
wave into Lake Superior. This trough could serve as the focus for
enhanced convergence and generate a more significant LES band given
sufficient lake-based instability (850 mb temps around -7 to -10 C)
and deep moisture/high inversion heights. Definitely something to
watch there.

By the middle of next week the models and their ensembles are split
on whether a warm front brings a brief respite to our period of
below normal temps, or a reinforcing shot of cold air prolongs
below normal temps and lake-effect chances. Stay tuned.

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 136 PM EDT WED OCT 21 2020

As somewhat drier air moves into the area this aftn, expect MVFR
conditions at KIWD/KSAW to improve to VFR over the next few hrs if
not there already to start the fcst period. At KCMX, upslope
westerly winds off Lake Superior will likely result in persistent
MVFR cigs thru the aftn, and probably thru tonight. Ocnl
-shrasn/sprinkles will be possible for the next few hrs. Winds at
KCMX should gust to around 30kt thru the aftn. As the next low pres
begins to take shape over the central Plains, winds swing around to
the ne and pcpn spreads ne toward Upper MI, expect cigs to fall back
to MVFR at KIWD/KSAW Thu morning. Near the end of the fcst period,
-snra should begin to develop at KIWD/KCMX with cigs dropping to
IFR. Pcpn will begin at KSAW just beyond this fcst period.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 522 PM EDT WED OCT 21 2020

Relatively tight pres gradient btwn low pres trof to the ne of Lake
Superior and high pres ridge from the Dakotas to the Lower Ohio
Valley is currently supporting a few westerly gale gusts over
central and eastern Lake Superior. As the high pres ridge lifts ne
tonight, reaching Lake Superior late tonight/Thu morning, winds will
diminish to under 20kt. On Thu, as the ridge continues shifting ne
and low pres develops across the central Plains, ne winds will
increase. Could be a few gale force gusts down toward the Apostles
and Duluth late in the aftn. NE winds to 30kt and possibly some gale
gusts to 35kt will then be the rule across most of Lake Superior Thu
night. As the low lifts across Lower MI Thu night and toward Quebec
on Fri, winds will back n to nw. While there may be some gale force
gusts over the e half of the lake on Fri, winds will diminish to
under 20kt over the w by late aftn. With high pres ridge building
across the Upper Lakes on Sat, expect winds under 20kt. These
lighter winds will linger thru Sun.


Upper Michigan...
  Winter Weather Advisory from 2 PM EDT /1 PM CDT/ Thursday to 8
     AM EDT /7 AM CDT/ Friday for MIZ005-006-010-011.

  Winter Weather Advisory from noon EDT /11 AM CDT/ Thursday to 8
     AM EDT /7 AM CDT/ Friday for MIZ002>004-009-084.

Lake Superior...
  Gale Warning until 8 PM EDT this evening for LSZ242>244.

Lake Michigan...


SHORT TERM...Rolfson
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