Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 180955

Area Forecast Discussion
National Weather Service Marquette MI
455 AM EST Sun Feb 18 2018

.SHORT TERM...(Today and tonight)
Issued at 246 AM EST SUN FEB 18 2018

Upper air pattern consists of a 500 mb trough in the Pacific NW,
zonal flow across the west half of the U.S. and a shortwave over the
lower Great Lakes this morning. The trough in the Pacific NW digs
southeast into the western U.S. by 12z Mon as a ridge retrogrades
back towards the southeastern U.S. Nam brings in some 850-500 mb q-
vector convergence this afternoon which remains through tonight
along with deeper moisture, but most of the moisture is to the north
of the area. This is the trend as it looks slightly drier than
before, so will make some slight adjustments downward on the
southward extent of pops and also lighter amounts of qpf. Still
looks like advisory amount of snow for the Keweenaw and will not
change timing at all. This would be the only spot that would need
to have an advisory out of this event.

.LONG TERM...(Monday through Saturday)
Issued at 455 AM EST SUN FEB 18 2018

Potential for light to moderate snow and possibly significant
freezing drizzle/rain accumulations Mon and Tue, but details are
still being worked out.

The challenges of precipitation type forecasting are made worse by
model disagreement stemming from differing handling of merging upper
jets over the area. Smaller differences in handling of these
features are having much greater impacts on differences in
precipitation amounts and timing, as well as moisture levels in the
mid levels. All this to say that uncertainty is limiting confidence
in precip type, amount and timing. May need headlines for the event,
but feel the magnitude of uncertainty in so many areas is too great
to issue any headlines at this time. However, will issue a Special
Weather Statement to highlight potential.

In general, here are some thoughts for the main two items of concern
(snow and ice):

Snow: The most snow will fall on Mon, with the greatest amounts over
the NW half of Upper Michigan. Currently have 3-4 inches in the
forecast for these areas on Mon, but could see up to 6 inches if
higher QPF models (CMC and GFS) verify, or less than forecast if NAM
and WRFs verify. Mon amount over the SW half are forecast to be
under 2 inches, especially near Lake Michigan, but could be a little
more or less. Then Mon night into Tue night, have a gradual 1-2
inches over the NW half and little to nothing over the SW.

Ice: This comes in a couple waves, with light accumulations possible
between them. The first is Mon and Mon evening and is primarily
caused by lack of moisture in the dendritic growth zone and should
be primarily confined to the SE half. Currently have amounts of 0.10-
0.15 inches over the far south-central and eastern U.P. with no ice
over the NW portions of the area and under 0.10 inches in between.
The second wave occurs Tue and Tue evening and is more of a classic
freezing rain/drizzle event where temperatures aloft rise above
freezing with sub-freezing temps at the surface. However, there is a
lack of moisture in the DGZ causing this as well. Have 0.10-0.15
inches of ice over roughly the E half of Upper Michigan, decreasing
to just a glaze over the far W. With both waves, there remains
considerable uncertainty in the amounts.

After a quiet period Wed into Thu, we move into a more active period
late in the week and weekend, but have no confidence in details at
this point.

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1202 AM EST SUN FEB 18 2018

Expect VFR conditions to prevail at at all sites into Sunday
morning. Light snow will move into the west at KIWD/KCMX by late
Sun morning with MVFR and possibly IFR vsby Sun afternoon. Best
chance for IFR vsbys will be at KCMX. Snow may stay far enough nw to
keep vsbys mostly VFR at KSAW. Cigs will lower to MVFR Sun
afternoon and should stay that way into Sun evening.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 246 AM EST SUN FEB 18 2018

South gales to 35 knots are expected over northern Lake Michigan
late this afternoon into this evening. Otherwise, winds are expected
to stay below gales through the rest of the forecast period.

Upper Michigan...
  Winter Weather Advisory from 9 AM this morning to 1 AM EST
     Monday for MIZ001-003.

Lake Superior...
Lake Michigan...
  Gale Warning from 4 PM this afternoon to 1 AM EST Monday for



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