Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 160534

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1234 AM CDT Mon Apr 16 2018

.UPDATE for 06z taf discussion below...


.SHORT TERM...(This evening through Monday)
Issued at 329 PM CDT Sun Apr 15 2018

Light snow showers continue today as the storm system responsible
for yesterday`s blizzard was slow to exit the region. Areas to the
east can expect the most snow, while areas to the west will only see
light showers at times the rest of the afternoon. A few snow showers
will linger across western Wisconsin on Monday, but overall most of
the forecast area will by dry with some low stratus and northwest
winds of 10 to 20 mph. The warmest temperatures will be in western
Minnesota where lower 40s are possible, while the rest of the
forecast area will be in the 30s.

Early afternoon water vapor imagery was incredible in its depiction
of the synoptic cyclone across the central United States. The warm
conveyor belt was drawing up moisture from the warm waters of the
gulf. The areas of enhanced vorticity within the parent upper level
PV anomaly were the driving force behind the wrap-around snowfall
over Minnesota and Wisconsin. One area was responsible for the band
of heavy snow in eastern Minnesota through western Wisconsin. This
band could produce about 1 to 3 inches of snow over the rest of the
afternoon. Continued the current trend of the 03Z warning and
advisory expiration times for these locations, but trimmed off the
advisory to the southwest where the accumulating snow had ended.

.LONG TERM...(Monday night through Sunday)
Issued at 329 PM CDT Sun Apr 15 2018

The main time period of interest in the extended forecast is Tuesday
night into Wednesday. This storm has high-end potential to produce a
narrow band of a foot of snow on the northern side of the track.
However, uncertainties in the track are quite substantial considering
how compact the storm system is, especially in comparison to the
behemoth that just moved across the Upper Midwest this past weekend.
Pound for pound, this storm is equally as impressive.

A large reservoir of potential vorticity will quickly congeal into
a compact ball of positive vorticity as a an upper level wave takes
on a negative tile as it crosses the high plains and moves towards
the Upper Midwest. This will lead to cyclogenesis over the Cornbelt,
with an area of heavy, heavy precipitation northwest of the surface
low. The reason for the heavy precip is the tremendous forcing
associated with the positive vorticity advection by the thermal wind,
as well is the ageostrophic divergence from the sharp curvature of
the shortwave trough as it becomes amplified. Another impressive
feature of this storm is the fact the cold air can be traced to the
traced back to Lake Superior via the H850 temperature map and winds.

Models are in good agreement of at least an inch of QPF in the form
of snow, and the timing is favorable across the southwest with most
of it falling overnight. Meanwhile the snow will be falling during
the day as it approaches I-35 and heads east. The north/south
variability between the model is cause for concern. Do not have the
confidence to even consider a winter storm watch since the narrow
band of heavy snow could fall across northern Iowa, or it could fall
across central Minnesota. Once change made to forecast was to
significantly lower the potential for freezing rain. There should be
plenty of ice in areas where it is precipitating, so don`t
anticipate any issues with having snow. There could be some sleet
across southern MInnesota, but over all it looks like mainly a
rain/snow event.

Did not make any changes to the rest of the forecast as the blended
guidance seemed reasonable.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1235 AM CDT Mon Apr 16 2018

MVFR CIGS remain over parts of the eastern area in the wake of the
storm. Moisture seems to be finally cutoff from the circulation
but may linger a few hours to the east. Otherwise a period of VFR
possible before more MVFR cigs return for mainly the eastern cwa
today. KAXN/KRWF will be on the edge. N-NW winds will become a bit
gusty into the late morning again mainly to the east. Should see
CIGS lift into the late afternoon with becoming VFR after that.

Snow is ending. May yet see a brief burst through about 08z but
overall trend is improving. May see a period of VFR cigs into the
early morning before more MVFR cigs develop in the cyclonic flow.
Expect erosion of clouds with toward evening then VFR.

Tue...Mainly VFR. MVFR ceilings possible late. Wind E 5-10 kts.
Wed...MVFR conditions likely in -RA/-SN. Wind NE 5-10 kts becoming
Thu...MVFR with IFR possible. Wind NW at 10 kts.




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