Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180953

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
353 AM CST Sun Feb 18 2018

.SHORT TERM...(Today through Tuesday)
Issued at 345 AM CST Sun Feb 18 2018

Complicated scenario expected to unfold across the WFO MPX
coverage area, including a combination of accumulating snow and
ice accretion.

To start, surface analysis shows a wavy cold front sagging
southwest from Hudson Bay through central WI into northern IA
which then transitions to a warm front extending westward through
the central Plains and northern Rockies. A secondary cold front is
slowly dropping south through south-central Canada. Aloft, a large
upper low continues to rotate atop northern Hudson Bay while a
potent upper level trough moves onshore the Pacific Northwest.
Atop the north-central CONUS, nearly zonal flow exists but with
strong low level jetting out of the south. Winds in the lower
levels are in the 50-60kt range. Although mixing profiles are not
optimal, there is enough subsidence present to allow for some
mixing of these winds down to the surface. This will create
windy/gusty conditions over far southern MN, such that winds of
30-45mph will be achieved. Therefore, have opted to issue a wind
advisory for far southern MN today.

Also aiding in the strong winds will be the aforementioned wavy
front lifting back north as a warm front. This will allow strong
warm air advection over the southern 2/3 of the coverage area.
Temperatures are expected to surge to the lower-middle 40s south
of the front. However, north of the front, where isentropic lift
will be heightened within incoming Pacific moisture, light snow
will drift eastward across the area, potentially producing between
0.5-1.0" of snow accumulation mainly north of a Morris-St. Cloud-
Cambridge-Rice Lake line today through tonight.

Going into tonight, the western trough will dig into the western
CONUS while the upper level pattern atop the Upper Mississippi
River Valley becomes more southwesterly, including a slight
nudging upwards of H7-H5 heights. This will introduce a key warm
layer which will factor into accumulating ice over mainly southern
and eastern portions of the coverage area late tonight through
midday Tuesday. A deep plume of Gulf of Mexico moisture will phase
with the incoming stream of Pacific moisture nearly atop and just
east of the coverage area. This deepening of moisture will then
remain over the area, being tapped into by the slow departure of
the prominent surface front and the weaker incoming northern one
plus the large western trough. These features will produce
multiple waves of precipitation over the coverage area late
tonight through Tuesday. With the bulk of the warmer air mainly
south of a line from Redwood Falls to Oak Grove to Rice Lake, it
is south and east of that line where a combination of freezing
drizzle/rain along with light snow will occur. North and west of
that line, precipitation is expected to remain nearly all snow.
With precipitation expected to persist through Tuesday, the most
accumulating snow will occur over western and west-central MN into
northwest WI, ranging from 4-7 inches over the roughly 48-hour
period. This will come out of QPF in the range mainly 0.3-0.4"
liquid equivalent.

In southern MN into western WI, QPF will be slightly less, more in
the 0.25-0.35" liquid equivalent range. However, because of the
expected warmer air lower level intrusion, this will come as a
combination of ice and snow. Snowfall over the Monday-Tuesday
period will generally range 2-4", possibly up to 5". But, the
bigger issue will be the ice accretion. The freezing rain will
come in waves early Monday morning through Tuesday afternoon,
resulting in 0.01"-0.10" ice from Redwood/Brown counties
northeast through Barron County, including west/north/east
portions of the Twin Cities metro. For the remainder of southern
MN into western WI, including southern portions of the TC metro,
ice accretion will range from 0.10"-0.25", making for hazardous
travel conditions. As such, have opted to issue a Winter Weather
Advisory at this point.

The precipitation will wind down during the day Tuesday, at which
point the vast majority of what is left by that point will come
as light snow since an incoming trough axis will force upper level
height falls and incoming high pressure will result in surface
cold air advection.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM CST Sun Feb 18 2018

Long term concerns remain timing of any trough energy ejecting from
the southwest during the period. Also, some warming expected.

The Tuesday system will exit the area leaving decreasing clouds
Tuesday night along with diminishing winds as high pressure builds
over the area. Cool and dry conditions will ensue through Thursday.

The next snow threat arrives later Thursday night as a short wave
ejects from the southwest. The ECMWF is a bit faster on driving this
through Friday with the GFS slower holding the trough to the west
through Saturday morning. We will hold onto the blended guidance
PoPs which gave us medium PoPs for Friday and tapering off Friday
night into Saturday.

The deterministic models diverge greatly then into the weekend with
the ECMWF more progressive with any trough energy ejecting eastward
from the western CONUS trough. The GFS much slower and is more
amplified with its energy into Monday and develops a closed
circulation and attendant snow storm for much of the area. It
will all depend on how quickly the highly anomalous Bermuda ridge
breaks down to the east. At least we should see temperatures
trend toward at least back to normal readings by late in the


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 955 PM CST Sat Feb 17 2018

Rather benign weather through most of the period, at least until
Sunday night. Overnight we see an increase in mid level clouds,
which lower and thicken on Sunday. Still expecting the main area
of snow on Sunday to stay north of the TAF sites. The one in
question is KAXN around/after daybreak, where we could see a
round of light snow and MVFR vsbys/cigs. Otherwise, we see
south/southeast winds increasing ahead of the trough tomorrow,
with an impressive low level jet of 50-60 knots developing. Have
included a mention of LLWS at most sites after daybreak, which
does look to diminish by early to mid afternoon. Surface gusts to
25 knots are possible, with a shift to more south/southwesterly
during the afternoon, then north/northwest Sunday night. More
widespread MVFR conditions look to build in Sunday night as well.

KMSP...Could have sct clouds around 2500 ft develop Sunday, but
VFR is expected until Sunday night when a higher possibility of
MVFR arrives. Other concern is low level wind shear, which looks
probable between 14z and 18z.

Mon...IFR/-FZRASN likely. Winds NE 10-20 kts.
Tue...Chc MVFR/-SN. Winds N 10-15 kts.
Wed...VFR. Winds WSW 5 kts.


WI...Winter Weather Advisory from midnight tonight to 6 PM CST
     Tuesday for WIZ014>016-023>028.

MN...Winter Weather Advisory from midnight tonight to 6 PM CST
     Tuesday for MNZ053-060>063-065>070-073>078-082>085-091>093.

     Wind Advisory from 9 AM this morning to 3 PM CST this afternoon
     for MNZ082>085-091>093.



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