Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 290910
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
510 AM EDT Fri Mar 29 2024

.KEY MESSAGES...

- Snow overspreads the area late tonight and Saturday with
  accumulations mostly 1-3 inches. Precipitation maybe mixed
  with light freezing rain or sleet (10-30 percent chance of
  freezing rain) over southern portions of the west half. Expect
  some hazardous travel conditions late tonight/Saturday
  morning.

- Developing system in the Great Lakes late Monday to early
  Wednesday is still worth monitoring for a snow event.

- Temperatures around normal to below normal through at least
  the middle of next week trend above normal by the weekend.

&&

.SHORT TERM /THROUGH TODAY/...
Issued at 422 AM EDT Fri Mar 29 2024

Current radar mosaic at this time is generally quiet with the
exception of a few lingering snow showers over the far east.  Latest
water vapor imagery though shows strong subsidence spilling over an
approaching 500 mb ridge and into western Upper Michigan. As a
result, any remaining snow showers to the east are expected to end
by 12Z this morning.  After that, it will be a dry day with mostly
sunny skies.  In addition, it will be warmer than the last several
days with widespread highs in the 40s. The exception will be across
the Keweenaw, which will only top off in the mid to upper 30s.
Currently, temperatures across the forecast area are ranging from
the teens across the west where clearing has commenced to low 30s
over the far east.  Central portions of the UP are generally in the
20s.

&&

.LONG TERM /TONIGHT THROUGH THURSDAY/...
Issued at 507 AM EDT Fri Mar 29 2024

Models remain consistent in depicting blocky high latitude flow into
and thru next week with the AO and NAO both in the negative phases,
though the AO only weakly negative and trending neutral. The
magnitude of the -NAO, weakening late next week, will be a player in
a Great Lakes to E Coast system during the first half of next week.
Positive height anomalies that have been across northern and eastern
Canada are now in the process of consolidating into what will become
a strong anomaly centered over the Labrador Sea/Davis Strait region
during the first half of next week. Meanwhile, ridge currently
building over the N Pacific will shift e and tilt ne into sw Canada
and the nw U.S. early next week. For Upper MI, these changes in the
flow with the shifting/evolving anomalies will result in temps
around normal to slightly blo normal thru the middle of next week.
Late next week, the anomaly over the Davis Strait/Labrador Sea will
weaken, but shift sw and expand significantly across eastern Canada
to the Great Lakes. Meanwhile to the w, troffing will set up over
the western U.S late week as well. This change in the flow will
cause temps to swing to above normal here late next week. As for
pcpn, a trof is currently amplifying along the W Coast. A couple of
shortwaves ejecting from the amplifying trof will track to the Great
Lakes region by Sat, bringing widespread pcpn late tonight/Sat.
Early next week, the combination of ridging into sw Canada and
consolidating/strengthening positive height anomaly over the
Labrador Sea/Davis Strait will force troffing into the eastern U.S.
Meanwhile, the trof over the w will begin shifting downstream. This
brings the potential for interaction of that trof with a shortwave
forced s and se due to the sw Canada ridge and Labrador Sea anomaly.
As has been and continues to be the case, there are some
EPS/GEFS/GEPS ensemble members showing sufficient interaction for a
snow event for Upper MI in the late Mon to early Wed timeframe. The
overall number of ensembles indicating such an event remains small.
So, it remains a very low probability, but there have been some
intriguing changes with the northern stream wave that supports the
potentially significant low pres system. More below.

Beginning tonight, pair of shortwaves moving across the Plains and
vcnty of the International Border will reach the Great Lakes region
Sat morning. Reflection at the sfc will be a weak sfc low pres
tracking from the central Plains to the southern Great Lakes with an
inverted trof extending nw from the low in response to what is the
stronger shortwave moving along the vcnty of the International
Border. The northern wave will provide respectable forcing with 12hr
500mb height falls of 70-80m. Responding WAA/isentropic ascent will
be the main player in pcpn development. Spotty light -sn may develop
early this evening into western Upper MI with a lead arc of moist
ascent as shown on the 295k sfc, roughly 700-750mb, but better push
of moisture/ascent overspreads the area overnight/Sat morning. NAM
is slowest, not showing pcpn arriving into the w until after 09z.
Once the main isentropic ascent arrives, there is good moisture avbl
with mixing ratios of 3-4g/kg avbl per 295k sfc. This would support
potential of up to 3-4 inches of snow per 6hrs for relatively strong
ascent. Much of the guidance shows 25-30kt of wind perpendicular to
fairly tight isobars for 3-6hrs, so that would support a high end
potential of 3-4 inches of snow for areas that are under the longest
period of ascent. However, in general, should be looking at a 1-3
inch snowfall with localized up to 4 inch amounts. The snow will be
on the denser side with low SLRs. The waa does introduce ptype
issues with temps possibly pushing sufficiently above freezing
somewhere in the 2-5kft agl layer early on for partial or complete
melting aloft of the falling snow. The HREF mean suggests a 10-30pct
chc of fzra from w into s central Upper MI before height
falls/cooling column transition pcpn to all snow. Mean probability
of sleet is just a little higher than that of fzra. Probability of
fzra increases considerably across the border in northern WI, so
anyone with travel planned in that direction tonight should take
that into consideration. Snow will diminish/end from w to e during
the day on Sat.

Dry weather follows Sat night thru Sun with seasonable temps for the
end of Mar. Then, as previously mentioned, fcst early next week
hinges on potential interaction of a progressing western trof with a
shortwave dropping around a sw Canada/nw U.S. ridge. There are still
a relatively small number of EPS/GEFS/GEPS ensemble members that
bring a significant snow event to portions of Upper MI with some of
the low pressures still dropping into the 980s mb even a couple of
970s mb. The shortwave that will interact with the trof exiting the
western U.S. is currently over the N Pacific near the Dateline. That
wave has to travel around the ridge shifting into sw Canada, and
then the amplitude of that ridge will determine just where and how
sharply that shortwave dives s and se into the U.S. It is noteworthy
that much of the recent deterministic guidance, beginning with the
12z/Mar28 run, has trended toward dropping this wave much more
sharply s into the Mississippi Valley on Tue. Then, question is when
does it phase with the trof progressing out of the western U.S., and
where does the sfc low pres response occur. Also in the mix is the
-NAO. It typically favors negative height anomalies more toward the
E Coast than in the Great Lakes. So, there are a lot of features at
play influencing the resulting weather across the Great Lakes and
eastern U.S. early next week. At a minimum, the evolving pattern is
likely to produce a deep mid-level low somewhere in the eastern U.S.
Will it be expansive enough or close enough to bring pcpn to Upper
MI? That remains uncertain, but the -NAO argues for something too
far e and se of here to bring much pcpn to Upper MI, and the
positive tilt of the trof progressing out of the w would suggest
initial sfc low response is more likely to be progressive rather
than quickly winding up poleward in the Great Lakes region. The 00z
EPS shows the response of the sharper northern stream wave in that
it now has a cluster of deep lows in the se Lwr MI/southern Lake
Huron area Wed as the initial low gets somewhat wrapped to the n in
response to the deepening mid-level low that develops from the
northern stream wave dropping into the Mississippi Valley. EPS 24hr
4 inch snow probabilities have blossomed to the nw of that cluster
thru Lwr MI, including into Upper MI, but the probabilities are very
low at under 20pct. Secondary sfc low development which quickly
becomes the primary then occurs vcnty of the Mid-Atlantic Coast, a
typical scenario for a -NAO. So, the northern stream wave will be
the feature to watch in the coming days. If it trends deeper/closes
off a mid-level low more quickly as it reaches the U.S., that will
increase the potential of the initial low wrapping around it and
close enough to produce accumulating snow in Upper MI. It likely
won`t be cold enough for pure LES as winds become northerly around
the deepening system to the e and se, but if system snow does wrap
back into Upper MI, lake enhancement would certainly enter the
picture to boost snowfall. Expect drying on Wed unless Upper MI
falls under the influence of an expansive mid-level low. Dry weather
should then round out Thu/Fri.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 112 AM EDT Fri Mar 29 2024

VFR TAFs are expected throughout the period as the ridging moves
through the area. There could be some high-end MVFR cigs seen over
KCMX and KSAW this morning, but I`m thinking chances are around 30
to maybe 40%. Cloud cover moves over KIWD late this evening as a
shortwave low pressure moves from the Northern Plains into the Upper
Midwest. While the chance is small (about 20%), it is possible that
we could see some marginal LLWS at KIWD near the end of the TAF
period.

&&

.MARINE...
Issued at 507 AM EDT Fri Mar 29 2024

WNW winds still gusting a bit above 20kt across the e half of Lake
Superior will fall back some today as sfc high pres ridge shifts e
to Lake Superior. An approaching sfc trof will then bring an
increase in E to NE winds over western Lake Superior tonight. Expect
gusts up to 25kt there tonight, possibly extending into early Sat.
Otherwise, with no significant systems in the area during the
weekend, expect winds under 20kt Sat/Sun. Low pres will be
developing into the Great Lakes late Mon to early Wed. NE winds will
begin to increase on Mon, but more so Tue when n to ne winds will be
at 20-30kt. Northerly gales may develop across the e half of Lake
Superior for Wed. Current probabilities of gales are in the 30-50pct
chc at that time.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
  Small Craft Advisory until 6 AM EDT early this morning for
     LSZ244.

  Small Craft Advisory until 2 PM EDT this afternoon for LSZ250-
     251.

Lake Michigan...
None.

&&

$$

SHORT TERM...TDUD
LONG TERM...Rolfson
AVIATION...TAP
MARINE...Rolfson


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