Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 201131
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
531 AM CST Tue Feb 20 2018

.UPDATE...
Issued at 514 AM CST Tue Feb 20 2018

Updated to include 12z aviation discussion below.

&&

.SHORT TERM...(Today and tonight)
Issued at 330 AM CST Tue Feb 20 2018

While the complex storm system is expected to shift off to the
east and bring an end to all precipitation across the WFO MPX
coverage area by this afternoon, still a tricky morning forecast
before all is said and done.

Surface analysis shows a low pressure center over the small
portion of the IA/WI border, with a warm front extending eastward
and a cold front extending southwestward. The center of high
pressure behind it has now shifted into Montana. Aloft, a deep
longwave trough continues to dig into the Four Corners region with
pronounced southwest flow atop the region. Surface flow continues
to come out of the north with RAP/NAM soundings still showing
enough of a warm layer aloft to keep precipitation in far
southeastern portions of the coverage area as -FZRA/PL while the
rest of the coverage area is prevailing -SN. For some locales,
roughly along a corridor of Rice Lake through the Twin Cities to
near Mankato, there looks to be some icy mixture in amongst the
precipitation but it is also mainly -SN per surface reports.

The longwave trough is expected to flatten out as the day
progresses today, helping to nudge the surface low and the frontal
system as a whole off to the east and northeast. This will bring
an eventual end to the precipitation, but not before a weak
shortwave trough axis aloft moves northeast through the area
within the longwave flow. This is helping keep efficiently
precipitation going across the area. QPFs are even impressive for
this 6-9 hour period, in the 0.10"-0.15" range, and are helping
to be realized by a small PVA max in advance of the trough.
Nevertheless, cooling of the overall column, particularly in the
DGZ with plenty of moisture available, is making for a transition
to mainly wet snow across the area. Where snow is most persistent
(western and northern portions), up to an additional 3 inches can
be expected. Further south and east, closer to an inch of
accumulation with the added ice of up to a tenth.

The precipitation will wind down late morning with no additional
precipitation expected from this afternoon on through tonight. In
fact, skies will gradually clear out this evening on the way to
mostly clear skies by sunrise Wednesday morning. The only issue by
then will be very cold wind chills in the -15F to -25F as a winds
increase due to a tightened pressure gradient on the eastern
fringes of the incoming high pressure center. Along with the
clearing due to the high pressure, cold air advection will also
accompany the high. Max temps today will only range from the mid-
teens to mid-20s, which will be followed by lows tonight between
nearly -10F and +5F.

.LONG TERM...(Wednesday through Monday)
Issued at 330 AM CST Tue Feb 20 2018

The long term concerns are the snow possibilities returning
Thu/Thu night and again Sat/Sat night.

We begin with a quiet and cool weather day Wednesday with high
pressure moving across the area. Temperatures remain 10 to 15
degrees below normal.

Timing of the next short wave with the 00Z models looks to be on
track for the Thu/Thu night period. We should see clouds increase
into Thursday with snow develop over at least the southwest
portion of the cwa through Thursday afternoon. This spread rapidly
northeast with the trough Thursday night. There was some threat of
loss of ice over the far southeast portion of the area Thursday
night as the trough lifts through. However, the GEFS probabilities
and overall site plumes for P-type indicate mainly snow over the
area. Did trail ice off a bit to the southeast to include a slight
chance ZR- mainly late Thu night to the far southeast. Snowfall
amounts currently range in the one to three inch category. This
may change if the system slows more.

Following the Thursday system, the models drive more energy
northeast toward the western Great Lakes Saturday/Saturday night.
The 00Z GFS has trended back to is highly amplified trough
developing a significant Colorado/Oklahoma panhandle cyclone as it
lifts toward eastern Iowa late Saturday night. This provides a
rather heavy band of snow to the southeast portion of the cwa
during this period. The ECMWF is less amplified with this trough
but still provides generous forcing and QPF potential to the
region.

These two system will have to be monitored for possible headlines.
Temperatures appear to trend back toward at least normal values
by late in the week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 514 AM CST Tue Feb 20 2018

Will still be dealing with mainly -SN (mixture at KEAU) for
roughly the first couple hours of the 20/12z TAF period.
Visibilities may run between 2SM-5SM during the times of precip.
By 20/15z, all precipitation is expected to be done and
visibilities up over 6SM. Main issue is then ceilings, which are
likely to run in the MVFR range through mid-afternoon. Going into
the evening hours, as high pressure from the west starts to really
exert its influence, skies will gradually clear out, resulting in
just high cirrus overnight through Wednesday morning.

KMSP...A few additional rounds of -SN, possibly mixed with -PL,
will move across the MSP area through 20/14z. Little to no
additional accumulation is expected. Low stratus will remain in
place through much of the day, including the start of the evening
push. There is some variability as to whether ceilings will be
above or below 1700ft. Conditions expected to rise to VFR around
or after 21/00z then remain as such through the rest of the TAF
period.

/OUTLOOK FOR KMSP/
Thu...VFR in mrng, MVFR or lower in aftn. Chc IFR/-SN late. Wind
      SE 5-10 kts.
Fri...Mainly VFR. MVFR ceilings possible. Wind W 5 kts.
Sat...VFR in mrng, MVFR or lower in aftn. Chc IFR/-SN late. Wind
variable 5 knots or less.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until noon CST today for WIZ014>016-
     023>028.

MN...Winter Weather Advisory until noon CST today for MNZ041>045-
     047>070-073>078-082>085-091>093.

&&

$$

UPDATE...JPC
SHORT TERM...JPC
LONG TERM...DWE
AVIATION...JPC



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