Area Forecast Discussion
Issued by NWS Marquette, MI

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000
FXUS63 KMQT 230959
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
459 AM EST Fri Feb 23 2018

.SHORT TERM...(This evening through Friday)
Issued at 341 PM EST THU FEB 22 2018

Initial forecast problem for tonight will deal with anticipated
widespread snowfall that is expected to develop as the evening
progresses.  Isentropic ascent combined with increasing moisture
(precipitable water values above 0.50 inch) on the nose of an
increasing low level jet (and subsequent moisture convergence) and
some increasing divergence aloft ahead of a subtle short wave
trough.  Weak mid level static stability won`t hurt the cause either
(700-500mb lapse rates around 7C/km overnight into Friday morning)
Focus for heaviest QPF tonight into Friday morning looks to be
across the western half of Upper Michigan...with some enhancement
across the higher terrain along the spine of the Keweenaw Peninsula.
22/12z HREF snowfall probabilities show a decent shot of more than
8"/12h ending 12z Friday morning over parts of the Keweenaw...at
least fairly confident in a 4-6 inch swath of snow across Ontonagon/
Houghton/Keweenaw Counties through 12z Friday..with 3-5 inch amounts
west of an IMT-MQT line where best low level frontogenetic forcing
will be focused...and 1-3 inches east of that line where snow will
start latest (as low level dry air will take some time to overcome).
 Push of warmer air into the upper Great Lakes will have an impact
on snow ratios...with forecast soundings implying a deep layer (on
the order of 300-400mb) of temperatures warmer than -10C...which
support more riming/aggregation of snowflakes and tend toward a
wetter/high density type of snowfall (SLR in the 9-14:1 range???).
This type of snowfall...though perhaps not of "cement mixer"
consistency...will still be more difficult to drive through
especially with system timing (during morning drive time to
work/school).

Snow will still be ongoing at daybreak Friday morning...but is
expected to taper off from west to east as the morning wears on as
drying/subsidence take hold (though low level moisture/cloudiness
may persist).  Can`t rule out snow tapering off to drizzle/freezing
drizzle during the afternoon...some weak QPF signals especially in
the high-res guidance though usually prefer to see more of a low
level warm advection/convergence signal (though low level lapse
rates to steepen as cold advection kicks in).  Additional snowfall
for Friday expected to be in the 1 to 2 inch range (higher east)...
for event totals of 5 to 8 inches for Ontonagon County northeast
through the Keweenaw...3 to 6 inches west of IMT-MQT line and 2 to 4
inches east.  As for headlines...seems to make sense to expand
current advisory to encompass remaining central U.P. counties
(Alger/Schoolcraft/Luce) given similar snowfall amounts and concerns
about snow density (will simply shift the start/end times about 3h
forward).  This has been coordinated with our "friends" at APX
(since I`m staring at them from above my workstation).  Other
headlines look fine...including the Keweenaw warning.

.LONG TERM...(Saturday through Thursday)
Issued at 457 AM EST FRI FEB 23 2018

...More wintry precipitation expected Sat night into Sun...

Shortwave that will bring another wave of wintry precipitation to
our forecast area (now in a suddenly active pattern), is currently
over northern California digging toward Arizona and New Mexico later
today. As this shortwave emerges onto the central Plains Sat, expect
lee cyclogenesis to result in 995-1000mb sfc low over southern IA or
northern MO by Sat evening. Shortwave still looks to become
negatively tilted as it lifts toward Upper Great Lakes late Sat
night into Sun morning. Sfc low deepens to below 990mb by Sun
morning (GFS seems on the lower side, showing MSLP to around 980mb)
with a location somewhere over central or eastern Upper Michigan.
Last couple runs of NAM point to sfc low farther west, over western
Lk Superior, though that breaks with overall clustering from SREF so
think the NAM idea may be too far west.

Given the strengthening upper system and deepening sfc-H85 lows
with H7 mixing ratios of 3-3.5g/kg, a band of heavier qpf/snow
seems realistic. System moving through fast though so it should be
a quick hitter for when heaviest snow falls. 12 hour qpf either
from 00z-12z Sun or 06z-18z Sun over 1 inch to the west of sfc low
lifting through the region. Overall pretty good model consensus
and agreement from SREF and GEFS members that heaviest qpf/snow
occurs over far western U.P. and on to the west. This also lines
up well where GEFS indicates best link up of mid-level deformation
and moisture influx (sharp negative U component to H85 winds with
sharply positive V component to H85 winds). QPF and expected
dynamic forcing indicate far west sees best chance of seeing
warning criteria snow (6+ inches in 12 hours) Sat night into Sun
morning with moderate snow changing to a wintry mix of snow/sleet
and freezing rain/freezing drizzle over central and eastern forecast
area.

With strong/deepening sfc low, winds will be breezy ahead of the low
Sat night into Sun morning (east to southeast winds) and also in
cold air advection region behind the low (west winds late Sun
morning into Sun afternoon). Wind gusts could reach 40-45 mph later
Sun, especially on more exposed Keweenaw Peninsula. Snow will be
wetter type and temps will be around freezing or above later on Sun
so not sure extent of blowing snow. Kind of getting concerned about
possible power outages with lower SLRs and expected winds,
especially if end up seeing very heavy snow in parts of our forecast
area before the winds hit. Will be something to keep eye on. Overall
we will need a headline for this event unless something drastically
changes in the persistent guidance/ensembles. Not sure if all of
forecast area will need a headline but there is time to sort that
out. Impact will be lower compared to a weekday morning as heaviest
snow occurs late Sat night into Sun morning (though those attending
church services on Sun will certainly be affected). Stronger winds
and possible freezing rain/icing could be nudging factors though in
possible headline issuance.

Light LES possible in wake of the Sun system Sun night into Mon.
Marginal temps and decreasing low-level moisture as main system
quickly lifts into northern Quebec should limit extent of LES. Rest
of extended looks generally quiet at this time. No real cold air
intrusions with daytime highs most days in the 30s.
&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1138 PM EST THU FEB 22 2018

Overnight periods continue to be greatest concern for reduced CIGs
and VSBYs, possibly trending towards IFR conds for several hours
between 7-12z at TAF sites. Snow will potentially be moderate at
heavy at times especially at KCMX and KIWD. Exact timing of
snowfall is the greatest uncertainty, but confidence in the
forecast is otherwise good.
&&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 457 AM EST FRI FEB 23 2018

Next period of gale force winds arrives Sat night as strong
deepening low pressure system lifts across Upper Great Lakes
region into Sun. East gales expected Sat night, then west gales
(higher end possible) expected later Sun. Gale watches have been
issued. &&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  Winter Storm Warning until 10 AM EST this morning for MIZ001-003.

  Winter Weather Advisory until 10 AM EST /9 AM CST/ this morning
     for MIZ002-004-005-009>013-084.

  Winter Weather Advisory until 1 PM EST this afternoon for MIZ006-
     007-014-085.

Lake Superior...
  Gale Watch from late Saturday night through Sunday evening for
     LSZ264>267.

  Gale Watch from Sunday morning through Sunday evening for LSZ162-
     263.

Lake Michigan...
None.
&&

$$

SHORT TERM...JPB
LONG TERM...JLA
AVIATION...Beachler
MARINE...JLA



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