Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 132048 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Marquette MI
348 PM EST Sat Jan 13 2018

.SHORT TERM...(This evening through Sunday)
Issued at 258 PM EST SAT JAN 13 2018

...Bitter temps expected for parts of Upper Michigan tonight...

Generally quiet in the short term, but that changes just beyond the
short term. Continue to see shallow lake effect with inversions no
higher than 3kft affecting northwest wind snow belts. The light lake
effect (mainly leading to brief reduction to vsby) will diminish
from west to east by the early overnight hours. Attn then turns to
min temps away from the lake effect areas. Setup is quite favorable
for temps this evening to quickly plummet with light winds, clear
skies and very dry airmass tied to high pressure ridge moving from
the Upper Mississippi River valley to the Ohio Valley. Shortwave
over northern Alberta and Saskatchewan moving toward Upper Great
Lakes on Sunday will spread high clouds over western U.P late
tonight, but don`t think it will impact low temps too much.

For min temps tonight favored blend of already cold official grids,
guidance that performed well with last couple of widespread cold
nights with similar expected lows (1 and 6 January) and the usually
superior GEM-bias corrected guidance. Overall, expect favored cold
spots over interior west to drop to at least 20 below and maybe even
near 30 below as occurred this morning over parts of Minnesota.
Favored cold spots interior east should drop to 10-20 below.
Light winds late tonight into Sunday morning should limit wind
chill issues, though as winds kick up on Sunday morning ahead of
approaching shortwave/low pressure system, could see wind chills
near criteria for a few hours over west. With the light winds
though did not do wind chill advisory attm. Soundings indicate a
lot of low level dry air to overcome initially on Sunday, but will
see thickening mid clouds through the day. Should be enough
saturation with incoming shortwave and moisture advection to allow
for some light snow to develop late day over the west. Also,
winds backing southwest ahead of the system will result in lake
effect currently affecting northern lower Michigan to quickly
expand into far eastern forecast area to east of Manistique by
sunset. This will just be the beginnings of possible moderate to
heavy snow off Lk Michigan for parts of south and east forecast
area Sunday night into Monday.

.LONG TERM...(Sunday night through Saturday)
Issued at 336 PM EST SAT JAN 13 2018

Pattern has switched back to cold weather/blo normal temps across
the Upper Lakes under renewed eastern troffing downstream of a
building ridge along the W Coast. Ridge will reach peak amplitude on
Mon as it drifts across western Canada (500mb heights peak at
upwards of 400m above normal). This will force another shortwave to
dive se and reinforce the cold over the area thru Tue. Advertised
warmup for the last half of next week is still on track as the ridge
breaks down and shifts e in response to a trof moving across the ne
Pacific to the W Coast. The western trof and its forcing of rising
heights downstream across the central and eastern CONUS will drive
Pacific air across the CONUS as the upcoming week progresses. This
is consistent with the signal from the current active MJO over the
Indian Ocean, in phase 3, which favors western N America troffing
and eastern N America ridging. Unless the MJO weakens, it will be a
forcing mechanism for mild conditions/above normal temps over the
Upper Lakes for the next few weeks, suggesting cold spells thru the
end of Jan will be short-lived, and pattern will likely be more
variable btwn above and blo normal temps. As for pcpn, pcpn will be
largely lake effect driven thru the first half of next week under
cold air mass. That said, shortwave reinforcing the cold into early
next week will provide a period of widespread light snow for the
fcst area late Sun into Mon. Mdt to hvy lake enhanced snow off Lake
MI and off Lake Superior will also occur. LES will wind down/end
midweek as warmer air arrives, then there may be some light mixed
pcpn late week. With Pacific energy moving across the CONUS in the
overall mild pattern during the last half of the month, there should
be more frequent opportunities for storm systems/pcpn to affect the
area, but unfortunately, that may include mixed ptypes.

Active lake enhanced snow period will occur in the Sun night thru
Tue time frame. Localized significant impacts seem likely. Shortwave
diving into the Northern Plains Sun aftn will go on to close off a
broad mid-level low over the western Great Lakes during the day Mon.
Ahead of this wave, winds will become southerly up Lake MI Sun night
into Mon morning. With 850mb temps around -15C and deep
moisture/forcing overspreading the area, stage should be set for hvy
lake enhanced snowfall to develop over the lake and stream into
Schoolcraft county and vcnty. Models are a bit hesitant to drive
heaviest of the lake enhanced snow off Lake MI into Upper MI,
surprising given 25-30kt of 950mb wind, to as much as 40k at times.
Maybe the models are on to something with some indication of the
long fetch up the lake tending to converge more directly at sfc
troffing situated just off the Door Peninsula toward the Garden
Peninsula. Until we get closer to the event, where the main
convergence sets up (very critical for the hvy snowfall) will be
uncertain. In any event, this has the makings of a lake enhanced
event that could produce very intense/very significant snowfall
(several inch/hr snowfall rates) given the deep moisture, fairly
vigorous deep layer forcing and the DGZ placed pretty much ideally in
the lake induced convective layer. For now, there is sufficient
signal to place southern Schoolcraft and Delta counties in a watch
for Sun night/Mon. Emphasis in Delta county will be over the eastern
part. Snow amounts will diminish n and w of there. Away from the lake
enhancement, waa/isentropic ascent will spread light snow across the
area Sun night/Mon. Mixing ratios up to 1g/kg are avbl around 750mb,
suggesting a general/widespread snowfall of 1-3in for the Sun night
thru Mon morning period of ascent.

Heading thru Mon into Tue morning, attention will shift to lake
enhancement off Lake Superior as low-level winds back around to the
ne and n. As the winds back around, diminishing lake enhanced snow
off Lake MI will swing across the rest of Delta county and thru
Menominee county. During the day Mon, there is good model agreement
for convergent northerly winds to develop into far western Upper MI
with sfc troffing aligned from Lake MI to vcnty of the Keweenaw.
With lingering, but waning deep layer forcing, this will be a good
set up for hvy snow to develop into Gogebic/Ontonagon counties
during the day into Mon night. Deep DGZ layer coinciding with max
upward motion will raise snow-to-water ratios, increasing
accumulations further. Keweenaw is much more uncertain than the far
w as placement of sfc trof and resulting wind direction is critical.
If trof is over the Keweenaw and there is easterly flow across the
lake converging there, it will be a big snow day for the Keweenaw as
well. Looks like n central Upper MI won`t get into mdt/hvy lake
enhanced snow until later Mon night/Tue morning when winds finally
back to the ne to n. With deep layer forcing diminishing and
slipping e, lake enhanced snow into the n central will be shorter-
lived and shouldn`t be as significant as it will be over the far w.
Still, there should be several hrs of hvy snow late Mon night/Tue
morning. Synoptic forcing will depart from w to e Tue, followed by
departure of deep moisture from w to e Tue night. As a result, lake
enhanced snow off Lake Superior will diminish and transition to
lighter pure LES from w to e during this time. Backing wind from n
to w will also shift the lingering snow showers.

During the day Wed, strong waa will raise 850mb temps sufficiently
to end LES. The strong waa/isentropic ascent could generate some -sn
on Wed, but right now, it looks like better shot of moistening the
antecedant low-levels will be farther n and ne, deeper into
retreating cold air where it`s easier to achieve. Warming will push
temps above normal Thu-Sat. May see a little mixed pcpn at times Thu
night thru Sat under continued waa pattern with the potential of a
couple of shortwaves to pass across the area. Any pcpn would be
light if it occurs.

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1242 PM EST SAT JAN 13 2018

Lake effect will continue at KCMX into this evening with IFR to MVFR
conditions. Late tonight conditions will become VFR as lake effect
moves north of the terminal. VFR flying conditions at KIWD and KSAW
through the period with increasing mid clouds late tonight and
Sunday morning. Conditions deteriorate late Sunday into Sunday night
at all terminals as low pressure system with snow crosses the region.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 253 PM EST SAT JAN 13 2018

Closing out the 2017 shipping season with strong winds to 30 kts
tonight through Monday. Could see a few south gale gusts to 35 kts
over eastern portions of Lk Superior late Sunday afternoon into
Sunday evening. Heavy freezing spray should be limited to early this
evening over east half, but otherwise expect only moderate freezing
spray through late Monday evening when the Sault locks close for
annual winter maintenance. Could be more heavy freezing spray
later Monday night into Tuesday and again on Wednesday. Expect
winds of 20 to 30 kts for rest of the week with potential for
west gales on Wed.

Upper Michigan...
  Winter Storm Watch from late Sunday night through Monday evening
     for MIZ013.

  Winter Storm Watch from Sunday evening through Monday afternoon
     for MIZ014.

Lake Superior...
  Heavy Freezing Spray Warning until 7 PM EST this evening for

Lake Michigan...


LONG TERM...Rolfson
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