Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 202128

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
328 PM CST Sat Jan 20 2018

.SHORT TERM...(This evening through Sunday)
Issued at 328 PM CST Sat Jan 20 2018

The short term period is really the calm before the storm. Really
the main question mark during the short term is what happens with
mid/upper clouds tonight. The answer to that will determine whether
or not we have widespread dense fog tonight into Sunday morning or
not. Removing the cloud variable, we have an absolutely primed
environment for dense fog, with calm winds expected and ample low
level moisture from now our 3rd day of melting. However, RH time
heights in BUFKIT show a lot of mid/upper level moisture hanging
around through the night and you can see lots of moisture on water
vapor heading our direction from the trough digging over the 4-
corners region. As a result played the cloud as opposed to fog
route. Looking at layered RH plan views through from the NAM/GFS, if
we have one area that could clear out overnight it`s down along the
I-90 corridor. If skies end up being more clear than what we are
currently thinking, then dense fog will be a serious issue tonight.

Biggest change to the forecast was to go drier. Everything now
pretty much keeps our area dry through 00z Monday, so reduced pops
quite a bit across the board. The two areas to watch though for
precip will be in west central MN and along the I-90 corridor. For
west central MN, there will be an warm frontal FGEN band across SD
Sunday, with the question being how far east does it extend, most
guidance says not into MN, but did leave some small chances in the
west just in case it ends up a bit farther east. For south central
MN, NAM soundings along with hi-res reflectivity forecasts show a
threat of drizzle coming up out of IA and into far southern MN
during the afternoon. Again, doesn`t look like anything big, so just
left a small PoP down along I-90.  Everywhere else, it will be dry
and we should have no problems once again warming into the mid/upper

.LONG TERM...(Sunday night through Saturday)
Issued at 328 PM CST Sat Jan 20 2018


12Z models have come into very good agreement Sunday night
through Monday night, including ensemble members. Confidence has
increased past 80 percent, therefore Winter Storm Warnings are
being posted for a chunk of the watch area.

The NAM shifted southeast, GFS remained steady, and ECMWF
shifted northwest resulting in very good model consistency today.
This matches very well with GEFS members and its 50-75% chance
for greater than 12 inches from south central MN to west central
WI. Still some minor tweaks are expected, primarily near the
edges of the swath of heavy snow, but confidence is very high for
the general area of heaviest snow stretching from St.
James/Mankato northeast across the southern and eastern Twin
Cities metro into most of west central MN. This area is being
upgraded to a warning, with a watch continuing on the edges where
a slight shift one way or the other may result in large forecast

The biggest uncertainty with this forecast is where the very
tight snowfall gradient will set up. It could be another scenario
where the same county has an inch or two on the northwest side, to
a foot on the southeast side. This issue is not clear and
probably won`t be for another day or even until the event begins
to unfold. The 18Z NAM illustrates this point by continuing the
shift to the south and is the sole reason the Twin Cities metro
is not being upgraded at this time. It may be a fluke due to the
upper low temporarily heading due east Sunday night while the
other guidance continues an east northeast track. Essentially all
other guidance brings at a minimum 2/3 inch of QPF to the metro,
with the GEFS mean at one inch (about a foot.) Future shifts will

Precipitation will break out over Iowa Sunday evening and slowly
advance northward across southern Minnesota during the late
evening and into western WI/central MN late Sunday night or early
Monday morning. Strong frontogenesis will focus a band of intense
snowfall with rates of an inch to inch and a half per hour from
south central MN to west central WI Monday. We could see 8 to 12
inches on Monday alone in this area. There is still a possibility
of a wintry mix from near Albert Lea to Eau Claire, but the column
has cooled a bit from the previous forecast resulting in more
snow. The snow will wind down from southwest to northeast Monday

The final concern is whether wind will be strong enough for
blizzard conditions being met. Forecast soundings don`t appear
mixed enough for excessive wind gusts, but this could be due to
the model precipitating. Visibility will likely be at or below 1/4
mile due to heavy snow for much of the day in the warning area,
so if we can get 35 mph sustained winds or gusts an upgrade to a
Blizzard Warning will be necessary. The best chance of this
happening is in the open areas of south central MN.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1212 PM CST Sat Jan 20 2018

Things will break one of two ways this period and it`s all
dependent on the mid/upper clouds we have right now. Route 1 is we
maintain a bkn-ovc mid/upper cloud deck through the night, with
little change in weather conditions over the next 24 hours. Route
2 is we clear out at least some this cloud cover, which would
result in very dense fog forming. Satellite shows plenty of
moisture streaming northeast ahead of the western trough, so
played route 1 in these TAFs for now. Looks like best potential
for clearing is down along the I-90 corridor and depending on how
far north it ends up, could bet MKT, EAU, and maybe MSP into the
dense fog game tonight.

KMSP...If it weren`t for the expected cloud cover, this looks like
one of the rare setups where MSP is in line for dense fog. So as
you can imagine, we will have to watch cloud trends closely.
Otherwise no weather issues until Monday, when MSP will see high
impacts from heavy snow late morning into the evening.

MON...IFR/LIFR +SN/BLSN. Several inches of snow likely. Winds N
      15-25 kts gusting to 30 kts.
TUE...Mainly VFR. Winds NW 5-10 kts.
WED...MVFR chc IFR cigs. Winds 5 kts or less.


WI...Winter Storm Warning from 3 AM Monday to midnight CST Monday
     night for WIZ015-016-023>027.

     Winter Storm Watch from late Sunday night through Monday evening
     for WIZ014-028.

MN...Winter Storm Warning from 3 AM Monday to midnight CST Monday
     night for MNZ060-062-063-068>070-077-078.

     Winter Storm Watch from late Sunday night through Monday evening
     for MNZ053-059-061-066.

     Winter Storm Warning from 9 PM Sunday to midnight CST Monday
     night for MNZ067-074>076-082>085-091-092.

     Winter Storm Watch from Sunday evening through Monday evening
     for MNZ064-065-073-093.



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