Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 210017
AFDMPX

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

.UPDATE...
Issued at 539 PM CST Sat Jan 20 2018

Updated to include 00z aviation discussion below.

&&

.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 30s.

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

...MAJOR WINTER STORM NOW HIGHLY LIKELY MONDAY ACROSS SOUTHERN/EASTERN
MINNESOTA INTO WESTERN WISCONSIN...

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 changes.

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
reassess.

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
evening.

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 00Z TAFS through 00Z Sunday evening)
Issued at 539 PM CST Sat Jan 20 2018

High ceilings look to remain fairly stoic through the overnight
period into much of tomorrow in advance of the potent winter storm
expected to strongly impact the region Sunday night through
Monday. The consequence of the high clouds tonight will be to
prevent fog development to a more widespread degree, despite winds
going calm and warm air resident over residual snowpack.
Additional moisture will begin to stream in from the Deep South
ahead of the storm system but this is taking the form of the high
clouds so have opted to keep a generally optimistic set of TAFs
going and keep fog potential in far southern MN (MKT) and into the
WI TAF sites. The high clouds will then steadily lower during the
day tomorrow, resulting in MVFR to IFR ceilings tomorrow evening.
Still too early to mention precipitation at this point as chances
won`t increase until after dark tomorrow. Winds will be fairly
light out of the north, if not calm, overnight before becoming
northeast 5-10 kt tomorrow.

KMSP...Will maintain VFR conditions through tomorrow afternoon as
high clouds lower to mid-levels. Cannot rule out MVFR-IFR fog
close to daybreak Sunday morning but confidence is fairly low in
this scenario so will run with only OVC skies. Ceilings will lower
to MVFR then IFR levels tomorrow evening in advance of the strong
winter storm system. Conditions will deteriorate further beyond
06z tomorrow evening, making for a difficult Monday morning push.

/OUTLOOK FOR KMSP/
Mon...IFR/LIFR with +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...Mainly MVFR with a chance of IFR cigs. Winds 5 kts or less.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
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 MNZ069-070-077-078.

     Winter Storm Watch from late Sunday night through Monday evening
     for MNZ053-059>063-066-068.

     Winter Storm Watch from Sunday evening through Monday evening
     for MNZ064-065-067-073-093.

     Winter Storm Warning from 9 PM Sunday to midnight CST Monday
     night for MNZ074>076-082>085-091-092.

&&

$$

UPDATE...JPC
SHORT TERM...MPG
LONG TERM...BORGHOFF
AVIATION...JPC



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