Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 181821
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1221 PM CST Sun Feb 18 2018

.UPDATE...
Issued at 559 AM CST Sun Feb 18 2018

Updated to include 12z aviation discussion below.

&&

.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
week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1154 AM CST Sun Feb 18 2018

A band of snow remains to the north of the TAF area this
afternoon. Meanwhile, skies remain mostly cloudy with high clouds
overhead and southeasterly winds sustained near 15-20 kts,
gusting to near 25-30 kts. A frontal boundary will cause winds to
become west-northwesterly this evening with breezy conditions
continuing overnight. Chances for snowfall increase overnight with
the best chance being in West Central MN beginning early Monday
morning and arriving in West Central WI during the mid to late
morning. Regardless, ceiling heights are expected to decrease with
MVFR conditions expected for much of Monday morning.

KMSP...LLWS near 55kts through 2kft is expected through the
remainder of the afternoon with sustained surface winds near
15-20kt, gusting to near 25 kts. MVFR ceiling conditions are
expected by early Monday morning. The best chances for snow and
reduced visibilities at MSP are expected begin during the mid
morning.


/OUTLOOK FOR KMSP/
Tue...MVFR/-SN. Winds N 10-15 kts.
Wed...VFR. Winds NW becoming SW 5 kts.
Thu...VFR. Winds S 5 kts.

.MPX WATCHES/WARNINGS/ADVISORIES...
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 until 3 PM CST this afternoon for MNZ082>085-
     091>093.

&&

$$

UPDATE...JPC
SHORT TERM...JPC
LONG TERM...DWE
AVIATION...AMK



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