Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180009

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
609 PM CST Sat Feb 17 2018

.UPDATE...For 00Z Aviation discussion below


.SHORT TERM...(This evening through Sunday)
Issued at 337 PM CST Sat Feb 17 2018

Skies continue to clear this afternoon and temperatures have
responded accordingly, rising into the mid/upper 30s. Gusty
northwest winds have also developed with the sunshine, especially
over areas with no snow pack. Winds will ease this evening quickly
with a ridge of high pressure approaching. This axis will shift
east across the area tonight, with return flow already developing
over western Minnesota after midnight. A warm front will form
across central Minnesota in response to the potent low pressure
center over South Dakota early Sunday morning. Widespread snow
north of the low center will build east to the northern half of
Minnesota early Sunday morning and northern Wisconsin by late

Interestingly, on an overall basis, this resembles a typical
summertime MCS pattern with a dry warm sector and a very strong
LLJ helping forcing along and north of the boundary. Not expecting
much, if any, precipitation in the warm sector, and have reduced
PoPs for much of the CWA except along the border with FGF and
DLH. Perhaps an inch could still fall there, but the trend has
been to keep shifting north which isn`t surprising in this setup.

Speaking of the LLJ, the wind field in the warm sector will be
quite impressive. A narrow 70-85 kt 850 mb jet will shift east
across the area mid morning to early afternoon Sunday. Forecast
soundings even suggest 50+ kt winds at 2kft. NAM is probably
overdone with its wind field due to its cold bias in the boundary
layer over snow pack which is resulting in a strong, shallower
inversion. Nonetheless, there is potential for 40+ kt surface
gusts across southern and eastern Minnesota. A short duration Wind
Advisory may be needed in these areas. The wind and thermal ridge
should also make for a mild day with highs in the 40s. Could
approach 50 in areas without snow cover.

.LONG TERM...(Sunday night through Saturday)
Issued at 337 PM CST Sat Feb 17 2018

The cold front should be pushing through early Sunday evening. The
thermal gradient will dive south into Kansas, northwestern
Missouri, and southeastern Iowa by early Monday. The next low
pressure center will track along it, focusing an area of heavy
rain near the front. Lower level moisture will be abundant north
of the boundary, but the upper level pattern will steer higher
level moisture around the large east coast ridge to the east of
the region. Therefore, not expecting heavy precipitation Monday
into Tuesday, but it does bring quite a few complications to

Models do not handle these types of events well. Forecast
soundings clearly show low RH in the DGZ region between 500-600
mb across southern and eastern MN into western WI for most of the
event. This leaves a lot of supercooled water in the low levels.
P-type on most models suggest snow given the column remaining
below freezing, so many manual adjustments were required for this
forecast. The thinking is precipitation will break out very late
Sunday night across western and central Minnesota, spreading east
Monday. Drier air aloft will work in Monday evening, likely
resulting in a lull of the steadier precip, before another round
reaches western Minnesota late Monday night.

Areas generally northwest of a line from Redwood Falls to
Hutchinson and Taylors Falls should remain mostly snow due to a
cooler column, and thus a lower and more moist DGZ. Ice introduced
to the supercooled water below should maintain snow for these
areas. To the southeast of that line, the DGZ is placed a bit
higher in the drier air which results in almost exclusively
supercooled water, except during the onset of the heavier precip
Monday morning. The moisture may be deepest during that time
allowing for a period of mostly snow. The loss of some moisture
depth Monday evening will leave freezing drizzle behind. Sleet or
graupel is expected occasionally if some modest amounts of ice
can be introduced. If the freezing drizzle does continue into
Monday night, it won`t have trouble sticking efficiently with
temperatures dropping into the lower and middle teens. Given the
relatively light QPF with this event, warnings probably won`t be
needed and therefore no watch is being issued at this time.

It looks dry into late week, but there are some indications of
another system by next weekend that could bring the next chance
of snow.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 609 PM CST Sat Feb 17 2018

The period starts of quiet and then gets impactful on Sunday night
and Monday. Overnight we see an increase in mid level clouds,
which lower and thicken on Sunday. Current indications are that
the bulk of the snow will stay north of the TAF sites on Sunday.
The one in question is KAXN toward daybreak, where we could see a
brief round of snow and MVFR vsbys/cigs. Otherwise, we see
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
abate by early afternoon. Surface gusts to 25 knots are possible,
with a shift to more south/southwesterly during the afternoon.

Could have sct MVFR level clouds on Sunday, but MVFR is expected
through the period. 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.




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