Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 131201
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
701 AM CDT Fri Apr 13 2018

...POWERFUL WINTER STORM TO IMPACT CENTRAL AND SOUTHERN MINNESOTA
THROUGH WESTERN WISCONSIN TODAY THROUGH SATURDAY NIGHT...

.UPDATE...
Issued at 619 AM CDT Fri Apr 13 2018

Updated to include 12z aviation discussion below.

&&

.SHORT TERM...(Today and tonight)
Issued at 415 AM CDT Fri Apr 13 2018

Surface analysis this morning shows a strengthening surface low
over central KS along a quasi-stationary frontal boundary
extending across nearly the entire lower 48, from central AZ
through KS across the Ohio Valley into New England. A dry line
extends south from the surface low through OK/TX. Meanwhile,
arctic high pressure is centered to the north of the front, over
southern Manitoba/Saskatchewan provinces. Aloft, a ridge axis
extends NW-SE from Alberta through the Dakotas to the Mid-
Mississippi River Valley. Immediately to the west of this axis, a
deep longwave trough is lumbering its way east across the Rockies
and as far south as the Four Corners region. All these features
will come together to form a powerful mid-latitude cyclone which
will produce significant winter weather across the WFO MPX
coverage area through the weekend.

While plenty of deep Pacific moisture has accompanied the deep
trough when it moved onshore earlier this week, the meridional
flow develop on the eastern fringe of the trough will allow an
efficient plume of Gulf of Mexico moisture to surge north into the
region, adding to the moisture in place available for the synoptic
features to utilize. The longwave trough will shift across the
Rockies today, with the main low offset to the west from the
surface low, allowing the surface low to deepen over the central
Plains and Mid-Mississippi River Valley through tomorrow morning.
With the low staying at relatively the same latitude due to the
surface frontal convergence feature along with the location of high
pressure to the north, this will temper the amount of convection
making it north of the IA/MN border. While the prospects of
appreciable severe weather is rather low, MUCAPE (even this
morning) is close to 1000 j/kg along the border. The lack of
additional instability will keep most storms tempered but some
storms could produce small hail (and potentially larger should
updrafts tap into the significantly colder air aloft). A Marginal
Risk of severe thunderstorms is placed into the far southern tier
of counties in MN for today into tonight. However, as alluded to
earlier, the larger concern will be the transition to wintry
precipitation later today into tonight.

Cold air advection will commence in earnest late this afternoon
into early this evening from north to south across the area. This
will occur while precipitation is falling over central MN so will
expect a transition to a rain/snow mix then all snow this evening
for much of central MN into northwest WI. The transition will take
on some icy precipitation for southern MN, especially southeastern
MN, where a slight warm layer aloft (potentially due to warm
frontogenesis in the 850-700mb later) will develop in advance of
the surface low. Minor to modest icing may well occur, mainly
southeast of an Eau Claire-Minneapolis-Mankato-Fairmont line.

As the low shifts east to near the KS/MO border by daybreak
Saturday morning, strong lift on multiple levels aloft will aid in
mod-heavy precipitation rates, most of which will be snow by
daybreak Saturday. Having snowfall rates of 1-2"/hour cannot be
ruled out by that time, especially with saturation highlighted in
the DGZ overnight and surface low temperatures in the middle to
upper 20s.

The other significant issue with this system: strong winds.
Sustained winds will increase to 20-30 mph with gusts upwards of
50 mph starting this afternoon and persisting through tonight over
much of western into southern MN. Winds this strong with the
significant snowfall rates expected tonight through Saturday
morning may easily create blizzard conditions in western through
southern MN. Have opted to add several counties in western MN into
the Blizzard Warning while maintaining the Winter Storm Warning
for much of central MN. The only questionable portion in central
MN the handful of counties from Douglas eastward to Kanabec. Snow
amounts will be much lower along with wind speeds. Have opted to
maintain those counties in the Winter Storm Watch due to continued
uncertainty. However, confidence has increased to the point where
all of the WFO MPX western WI counties have been upgraded to a
Winter Storm Warning.

The point cannot be stressed enough that this will be a
significant winter storm for mid-April with multiple winter
weather impacts, both from a travel standpoint and a health
standpoint, and will produce significant snow accumulations. The
snow will be of a high moisture content, meaning that it will be
quite heavy to shovel. The winds combining with the heavy snow
will make for blizzard conditions, particularly in western and
southern MN. The freezing rain expected in southeastern MN will
occur before a complete transition to snow, making for slippery
conditions. Residents and visitors are urged to pay attention to
winter weather watches, warnings and advisories.

.LONG TERM...(Saturday through Thursday)
Issued at 400 AM CDT Fri Apr 13 2018

The main concern in the long term is the continuing winter storm
affecting the area.

Latest model trends have been to slow the progression of the upper
circulation and therefore prolong the snow threat through much of
Sunday. Also, models trending a surge of heavier precipitation
moving through during the day Saturday as the upper circulation
pivots somewhat and main surge of moisture and enhanced forcing
lifts across the cwa. We have nearly 1 inch an hour snowfall rates
across the central CWA roughly from KRWF-KMSP-KCBG region during the
daylight hours. Also, some threat of elevated thunder with steep mid
level lapse rates and some elevated CAPE moving through mainly over
the southeast CWA Saturday morning. Will continue the slight chance
thunder that region. The mixed phase precipitation should be
transitioning to all snow across the southeast Saturday morning as
well as cooler air moves in and main upper trough exits to the east.
Strong northeast winds with gusts to 45 mph likely to the west and
south through the day Saturday continuing the blizzard threat to the
that region.

As the surface low exits to the southeast Saturday night and the
upper circulation moves east, we will likely see the continuation of
light snow across most of the eastern portion of the area. At the
moment, we have the warning`s extending through 06z Sunday. This may
have to be extended into at least Sunday morning across west central
Wisconsin.

Following this system cooler conditions will remain. Some warming
occurs ahead of the next event Tuesday night into Wednesday night.
Some wet snow accumulation is also possible with this system as it
moves through. The 00Z ECMWF is more amplified with the upper trough
and is colder and is producing more significant accumulation. Will
have to monitor trends into early next week.  Still temepratures
remain some 10 to 15 degrees below normal by the end of the period.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 619 AM CDT Fri Apr 13 2018

Significant winter storm will impact the area through Sunday.
Ceilings will steadily drop through the day as the precip shield
shifts north. MVFR ceilings are likely at all sites, mainly by
late this afternoon, with some thunderstorms in southern MN.
Precipitation will change over to -SN in central MN and the
changeover will then shift south tonight. Some sites, mainly in
eastern MN into western WI may be subject to FZRA before changing
to all snow. Winds will increase strongly from the NE, causing
areas of BLSN.

KMSP...Light to moderate rain expected through mid-afternoon with
an isolated thunderstorm possible. Precipitation will mix
with FZRA and SN late this afternoon before changing to all snow
this evening and continuing as all SN overnight through tomorrow.
Strong NE winds will persist throughout this TAF period.

/OUTLOOK FOR KMSP/
Sat...IFR or LIFR. SN, heavy at times. Significant accumulations
likely with rates in excess of 1"/hr possible. Wind NE 20G35 kts.
Sun...IFR/MVFR with lingering snow. Wind N at 15G20 kts.
Mon...VFR. Wind NW 5-10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning from 1 AM Saturday to 1 AM CDT Sunday for
     WIZ024-026-028.

     Winter Storm Warning from 7 PM this evening to 1 AM CDT Sunday
     for WIZ014>016.

     Winter Storm Warning from 10 PM this evening to 1 AM CDT Sunday
     for WIZ023-025-027.

MN...Winter Storm Warning from 1 AM Saturday to 1 AM CDT Sunday for
     MNZ076>078-083>085-092-093.

     Winter Storm Watch from 4 PM CDT this afternoon through late
     Saturday night for MNZ041>045.

     Winter Storm Warning from 7 PM this evening to 1 AM CDT Sunday
     for MNZ049>053-058-059-061.

     Blizzard Warning from 7 PM this evening to 1 AM CDT Sunday for
     MNZ047-048-054>057-064-065-073.

     Winter Storm Warning from 10 PM this evening to 1 AM CDT Sunday
     for MNZ060-062-063-066>070-075.

     Blizzard Warning from 10 PM this evening to 1 AM CDT Sunday for
     MNZ074-082-091.

&&

$$

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



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