Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 091304

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
704 AM CST Mon Jan 9 2017

Issued at 552 AM CST Mon Jan 9 2017

Updated to include 12z aviation discussion below.


.SHORT TERM...(Today through Tuesday)
Issued at 428 AM CST Mon Jan 9 2017

Certainly an interesting next 36 hours is expected with one area
of low pressure departing and a second stronger system expected to
slowly move through the area, bringing periods of light snow along
with the potential for mixed wintry precipitation over southern
portions of Minnesota.

The first low pressure area, currently over eastern MN, will shift
into western WI around daybreak and continue moving eastward. A
trailing cold front will transition into a quasi-stationary front
over western WI into southeastern MN today into tonight, the western
portion of which will drop from southeastern MN over the central
Plains and become associated with a stronger low pressure system
developing over the Rockies. The upper level pattern is taking on a
more zonal flow with little influence from any longwave trough or
ridge axes. That said, plenty of Pacific-origin moisture will stream
across the western and central CONUS today through Tuesday due to
the progressive flow aloft, setting the stage for a prolonged light
snow event for the entire WFO MPX coverage area along with the
potential for a wintry mix in southern MN tonight into tomorrow. The
first wave of light snow will come late this morning into this
evening as isentropic lift is heightened in advance of the
developing warm front (what will have been the stalled out
stationary front south of the WFO MPX coverage area to the low
pressure center out west). This wave of -SN will be light producing
generally around an inch of snow through this afternoon,
particularly north of the I-94 corridor. The precipitation will wane
a bit this evening, both in terms of intensity and coverage.
However, warmer air from the south as the warm front lifts north and
the low pressure center approaches will mix into the area, mainly
from northern IA through the I-90 corridor to near the southernmost
portion of the Minnesota River. A deeper swath of warmer air in the
lower levels, approaching 2 deg C, will contribute to potentially
some of the precipitation transitioning to or mixing with sleet
overnight which could produce a light glazing. Confidence is only
low-medium in this thinking at this point since the models have been
rapidly changing where the warm air layer will reside. Since this is
not expected until tonight and confidence is relatively low, will
hold off issuing an advisory at this point and mention the
possibility in the HWO.

The low pressure center will then shift from southern SD to over the
western MN/IA border by daybreak Tuesday, at which point a shortwave
trough looks to develop aloft which will help kick the low pressure
center eastward but also enhance omega lift within a continually
moist atmosphere. The proximity of the low will also start to impact
the depth of the warm air which will impact p-type for the far
southern portions. The wintry mix looks to transition away from -IP
and more to -ZR, but again confidence is low-medium due to how far
north the warm air will shift with the existing snowpack and the
depth along with coverage of precipitation. Confidence is higher on
the resumption of -SN for much the coverage area during the first
half of Tuesday, likely producing an additional 1-3 inches of snow
accumulation, highest over northern and eastern portions of the
coverage area. Thus, over the next 36 hours, the potential is there
for 3-5 inches over northern and eastern portions of the coverage
area with lesser amounts in southern and western portions. Since
this event is over a prolonged duration with only up to 2 inches
expected over any 12-hour block, we have opted to hold off issuing
any winter weather headlines and continue using the HWO for this
event. The low pressure center will then shift northeast during the
day Tuesday, moving to a position over Lake Superior by late in the
day and dragging its cold front through the coverage area.
Precipitation is expected to wind down by the second half of the day
on Tuesday with any and all precipitation having ceased by dusk
Tuesday evening.

Temperatures will be noticeably milder over the next 36 hours in
response to the passage of the organized low pressure system through
the region and the end of the cold air advection experienced over
the previous several days. Zonal flow aloft with more southerly low
level flow will be the main driving factors but existing snow pack
will help keep temperatures from warming more than currently
advertised. Highs will climb to the middle teens to middle 20s
today, followed by highs ranging from the mid-to-upper teens in
western MN to the upper 20s to lower 30s in western WI tomorrow.

.LONG TERM...(Tuesday Night through Sunday)
Issued at 428 AM CST Mon Jan 9 2017

Tuesday night through Sunday...the low pressure system will be off
to our northeast by Tuesday night but the next disturbance in this
active weather pattern will be on the way.  There are still timing
issues with the Tuesday night through Wednesday forecast with the
GFS and GEM being quicker than the ECMWF and NAM.  It`s a
complicated pattern the next several days and these temporal
differences are partially responsible for the Pops in each period
being dampened out.  Even so, light snow with this disturbance will
move through the region sometime from late Tuesday night into
Wednesday evening.

The with the high pressure beginning to build in behind this
activity, the GFS is quite a bit weaker than the ECMWF  with the
strength of it initially, and hence wants to spread broad QPF
into our area Thursday owing to a system to our south. It may be
overdoing it, especially if the high strengthens like the ECMWF
solution and keeps the QPF at bay to our south. So, the GFS
indicates the potential for a couple inches of snow while the
ECMWF is essentially zero. The Canadian sides with the GFS and the
NAM is with the ECMWF. Continued to advertise a chance of snow
with this period and remained on the conservative end of QPF until
higher we have better agreement. The better amounts could be just
southeast of here.

The guidance comes into better agreement actually by Friday
morning with a very strong surface high of 1046mb building in.
Lowered the min T forecast a couple degrees below guidance but
will have to watch our sky cover because even a little cloud cover
could hinder the cold potential. Widespread -20s are likely in
western MN and central MN. It`s possible this high could
strengthen as it drifts east toward western WI. The GFS indicates
to potential for 1050mb by Friday night.

A strengthening northern stream jet across Canada looks likely early
next week which would allow more mild temperatures for us locally
and keep the arctic air locked to our north.  The CPC is advertising
the potential for above normal temperatures in the 8-14 day outlook
which agrees with what the deterministic solutions are indicating at
this point.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 700 AM CST Mon Jan 9 2017

Tricky forecast over the next 24-30 hours trying to time when the
heaviest bursts of -SN will impact each terminal. Model timing
shows one band mid-to-late afternoon into the early evening hours
then another close to daybreak tomorrow morning. Reductions to IFR
are certainly possible and could be slightly earlier and longer
than currently advertised. Not looking for much accumulations but
visibilities could be affected more than expected should wind
speeds climb up to and over the 10 knot mark.

KMSP...MVFR conditions to start with ceilings to drop below 1700ft
by midday. Visibilities may also drop below 4SM around the same
time so use of both parallels may well be in jeopardy from that
point on thru the rest of the TAF period, mainly due to ceilings.

Wed...VFR. MVFR with -SN possible late. Wind W 5-10 kt becoming
Thu...VFR. Wind NW 5-10 kt.
Fri...VFR. Wind variable 5 kt or less.




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