Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 191226
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
626 AM CST Fri Jan 19 2018

.Updated for 12Z Aviation Discussion...
Issued at 622 AM CST Fri Jan 19 2018

&&

.SHORT TERM...(Today and tonight)
Issued at 415 AM CST Fri Jan 19 2018

Today will have high clouds with light winds and warm temperatures
well above the seasonal average for mid January. A thermal ridge
will move across the region but it will be difficult to tap into
the warm air aloft with light winds at the surface. A weak cold
front will move in from the northwest later today and tonight, but
that will stall out and become the thermal gradient for the
approaching storm system that will arrive Sunday evening.

.LONG TERM...(Saturday through Thursday)
Issued at 415 AM CST Fri Jan 19 2018

A strong cyclone will bring heavy snow somewhere across
central/southern Minnesota and central/northern Wisconsin. This snow
will come in two parts. The first will be with warm air advection
and low level frontogenesis late sunday night into early Monday
morning. Then there should be somewhat of a lull with another area
of heavy snow developing along a deformation band driven primarily
by the forcing of the upper level low later Monday morning. Expect
around 2 to 4 inches with that first wave of snow, and then another
3 to 5 inches with the second wave. However, this system appears to
develop a trough of warm air aloft as it occludes, so areas that are
under this TROWAL could easily pick up 6 to 8 inches with that
second round of snow. It is way to early to say where exactly these
features will develop, but based on the strength of this system and
CIPS analogs, should see a healthy band 50-70 miles wide of 5 to 8
inches, with a few bullseyes within this band of 12+ inches.

Forecast models have been very consistent with this system. There
has been some bouncing around with the track, but there is overall
agreement of a band of 6+ inches with 12+ potential along and north
of the surface low pressure track. There is plenty of moisture, a
strong jet, and a favorable track for this to come to fruition so
did issue a Winter Storm Watch for part of the forecast area. As for
wind potential, forecast soundings show around 30 to 35 kts atop the
channel, which by itself shouldn`t be enough to cause blizzard
conditions. In addition, the surface low is barely deepening off the
19.00Z GFS/ECMWF, while the GEM is a bit stronger with the surface
low, and hence the winds. All in all, this doesn`t appear to be a
blizzard event. Certainly some blowing snow and heavy falling snow,
but for some perspective, it doesn`t appear to have nearly the
strength of the February 20,21 2014 blizzard which had the surface
low deepening into the 970s.

As for the synoptic meteorology of this system, it all really starts
Saturday night as a northern stream jet rounding the base of an
upper level trough phases with the southern stream which leads to a
rapid increase in wind speed across northern Mexico as seen on the
H250 winds off the GFS/NAM/GEM/ECMWF. By Sunday afternoon at jet
streak of roughly 130 kts has developed, and this increase in wind
speed increases the positive vorticity within the upper level
trough, and therefore the positive vorticity advection downstream.
The end result is a surface low developing over the Texas and
Oklahoma panhandles. The atmospheric response to this surface low is
it lifts towards the midwest is an increasing northeasterly
component to the low level winds across Minnesota and Wisconsin,
with southerly winds strengthening across Missouri into southern
Iowa. By Sunday evening the low level thermal gradient in this wind
field will tighten leading to an southwest/northeast area of
frontogenesis, lift, and precipitation from Nebraska up towards
southern Minnesota.

As mentioned earlier, there will be a warm air advection band that
lifts up from the south early Monday, and then there should be a
break south of this line as a dry slot develops. Both the GFS and
ECMWF show a TROWAL developing which could easily produce snowfall
rates around 2 inches per hour. Areas that get snow from both of
these features could pick up over 12+, but this will be a relatively
small area compared to the overall snow band. A more general 5 to 8
inches from southwest Minnesota into northern Wisconsin seems
reasonable at this time.

One challenge with this system is trying to figure out where the
surface low will track, or better yet how the models runs will
change over the next couple of days. The aforementioned strong jet
streak has a fairly large west-to-east component with it, and there
is nothing downstream impeding its progress to the east and
dragging the surface low and heavy snow with it. However, all the
models develop a large area of convection across the southern
plains, so this latent heat release should help to build a ridge
downstream and keep the surface low tracking northeast as opposed to
east. If the models are underdoing this convection, then could see
the surface low and heavy snow track farther west. At this point,
don`t see that scenario really playing out. The GEM has shifted
southeast with the latest run, as has the ECMWF, and the GEFS probs
of 6+ inches of snowfall is along a line from Mankato MN to
Ladysmith WI. So at this point think a southeast shift would be more
likely than a northwest shift.

Once this system moves through should see cooler temperatures return
with highs in the lower 20s and lows near 10. Another similar
pattern develops late next week which could bring another storm
across the region but we`ll focus on that more after this one has
passed.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 622 AM CST Fri Jan 19 2018

VFR conditions throughout with light southwesterly winds becoming
more northwesterly this evening. Speeds will only be about 5 to 10
kts.

KMSP...
No additional aviation concerns.

/OUTLOOK FOR KMSP/
Sun...VFR early, IFR or lower with SN or wintry mix likely Sunday
afternoon changing over to snow late Sunday night. Wind NE 5-15
kts.
Mon...IFR or lower most of the day with SN/BLSN. Wind N 15-25 kts
gusting to 30 kts.
Tue...VFR with MVFR possible. Wind NW at 10-15 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from Sunday evening through Monday afternoon
     for WIZ014>016-023>025-027.

MN...Winter Storm Watch from Sunday evening through Monday afternoon
     for MNZ060>063-066>070-073>078-082>084-091.

&&

$$

SHORT TERM...JRB
LONG TERM...JRB
AVIATION...JRB


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