Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 191211
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
611 AM CST Mon Feb 19 2018

.UPDATE...
Issued at 544 AM CST Mon Feb 19 2018

Updated to include 12z aviation discussion below.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 330 AM CST Mon Feb 19 2018

Complicated forecast over the next 36 hours with a wintry mix
precip event commencing across the area, already underway in
parts of the WFO MPX coverage area.

Surface analysis shows a main area of low pressure over SE
Colorado/SW Nebraska with a cold front turning northeast through
central Iowa, southeastern Minnesota and northwestern Wisconsin.
One weak low pressure center per 3mb pressure analysis is found
along the front in southeastern MN. To the north of this front,
surface high pressure is nudging southward from
Alberta/Saskatchewan provinces into the Northern Plains. South of
the front, deep southerly flow on the western periphery of broad
Eastern CONUS high pressure is bringing above-normal moisture
amounts from the Deep South into the Great Lakes. In addition,
southwesterly flow in advance of a Pacific Northwest upper trough
is bringing Pacific moisture into the region. The position of the
front is key to the precipitation type as model soundings indicate
a shallow subfreezing layer near the surface, indicated by
temperatures in the teens well north of the front and in the 20s
near and just south of the front. However, the pronounced
southerly/southwesterly flow is allowing for a sufficiently deep
warm layer aloft to keep liquid aloft rather than ice for much of
the southern and eastern portions of the area for much of the day
today through tonight. While the prevailing p- type for much of
the coverage area looks to be -SN, the portions that do see -FZRA
(mainly southeast of a line from Redwood Falls-St. Michael-Rice
Lake) may well see around a tenth of an inch of ice, and
potentially near a quarter inch (from Albert Lea thru Red Wing
thru Eau Claire). These increased QPF and hence ice amounts line
up well with the 00z progs of the HopWRF, HRRR, RAP and NAMNest
along with collab with other offices. So while warning criteria is
not expected, a larger swath of ice is expected so have nudged up
ice amounts. Going back further west, which actually is where -SN
is already occurring, the current wave of -SN will continue for
much of the day and diminish slightly late this afternoon into
this evening before picking up again overnight through daybreak
Tuesday. In fact, in general, the main waves of precipitation look
to come this morning through mid-afternoon, then become less
widespread/intense late this afternoon through this evening before
picking up across much of the coverage area during the early
morning hours Tuesday. It is in this second batch that the greater
snow amounts will come for western and northern parts of the
coverage area (3-4" tonight- tomorrow compared to 1-2" today).
However, the greater ice amounts look to come today with slightly
lesser amounts tonight-tomorrow.

With the front having little movement until tomorrow, the
transition to all -SN for the entire coverage area will not
occur until mid-to-late day tomorrow. As such, have maintained
the mention of -FZRA for much of the southern and eastern portions
of the coverage area, including throughout the Twin Cities metro,
through tonight then shifting further east tomorrow. As such,
have maintained the winter weather advisory throughout the
coverage area. Main change was to move up the start time of the
advisory in western/central MN due to that part of the storm total
snow accumulation is already being realized this morning. The
precipitation will gradually wind down from west to east tomorrow
as the surface front shifts further away into the Great Lakes and
mid-Mississippi River Valley while high pressure slides south and
east from the Northern Plains.

Highs today and tomorrow will run quite similar to one another,
with mid-upper teens in western MN to lower-middle teens in
eastern MN into western WI. Lows tonight will also run quite
similar to lows this morning, ranging from around 10 in western MN
to the lower 20s in eastern MN and western WI.

.LONG TERM...(Tuesday through Sunday)
Issued at 310 AM CST Mon Feb 19 2018

Long term concerns remain timing of any trough energy ejecting from
the southwest during the period. Also, some warming expected.

The Tuesday system will exit the area leaving decreasing clouds
Tuesday night along with diminishing winds as high pressure builds
over the area. Then we expect cool and dry conditions to continue
through at least the first half of Thursday.

The next snow threat arrives later Thursday afternoon into the
southern cwa. The short wave ejects northeast across the area
through at least early Friday. The ECMWF is faster with this system
and drives a more northern stream trough extension through then. The
slower GFS holds back the trough and the upper flow isn`t as
split. It lifts a weak short waves northeast with a continuing
threat of small chance PoPs into Friday night. We will continue
the blended guidance trend at this time with the likely PoPs
spreading through Thursday night and trail them off through Friday
night. Snow amounts dont appears significant at this time,
perhaps another 1 to 3 inch type event.

The deterministic models diverge then into the weekend. Both models
drive more short wave energy northeast over the central CONUS with
cyclogenesis over the mid Mississippi River valley region Saturday.
It appears the anomalous Bermuda Ridge will flatten some as this
energy moves east and the western CONUS trough reloads. This will
mean at least a chance of precipitation moving through the area
Saturday/Saturday night.

Temperatures will remain below normal through much of the week but
should trend closer to normal again by late in the week and into
next weekend.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 544 AM CST Mon Feb 19 2018

Complex scenario to unfold over the next 30 hours, particularly
over southern MN into western WI where a mixture of -FZRA and -SN
may occur. Prevailing p-type throughout this event will be -SN but
timing of the -FZRA looks best late this morning through early
afternoon and then potentially late this evening before becoming
all -SN once again. Terminals to be impacted by -FZRA look to be
KMKT-KMSP-KRNH-KEAU. Confidence is a little higher in the earlier
timing so have included mention but -FZRA may easily extend
further in time so frequent AMDs may be necessary. Precipitation
does not look to wind down until midday tomorrow so MVFR-IFR
conditions will be prevalent by late this morning through tomorrow
morning for all sites.

KMSP...Precip will commence at KMSP around 14z, starting as mostly
if not all -SN. As the morning progresses, chances increase of
having -FZRA mix in. While this may improve ceiling/visibility,
obviously the freezing rain will present surface icing concerns.
Precipitation is likely to diminish later this afternoon into the
evening hours before resuming overnight, with greater confidence
of all -SN. Precipitation will steadily diminish during the day
tomorrow, possibly bringing a return of VFR conditions during the
afternoon hours tomorrow.

/OUTLOOK FOR KMSP/
Wed...VFR. Wind W 5 kts.
Thu...Mainly VFR. Chc MVFR/-SN late. Wind SE 5 kts.
Fri...Chc MVFR/-SN. Wind variable less than 5 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 6 PM CST Tuesday for WIZ014>016-
     023>028.

MN...Winter Weather Advisory until 6 PM CST Tuesday 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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