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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
548 am CST Sat Feb 6 2016

Update...
issued at 527 am CST Sat Feb 6 2016

Updated to include 12z aviation discussion below.

&&

Short term...(today and tonight)
issued at 500 am CST Sat Feb 6 2016

The focus of this forecast was the increasing potential for a
ground blizzards Sunday afternoon through Monday. As is usually
the case...severity hinges on both snowfall and the snowpack...but
confidence is increasing that impacts will be high...in the form
of closed roads across much of western and southern Minnesota. For that
reason expanded the blizzard watch a few counties east.

Early morning water vapor imagery together with GFS 500mb heights
and winds showed low amplitude zonal flow upstream of the
Midwest through the Gulf of Alaska. Within this pattern was a
potent pv anomaly across the Idaho Panhandle. Over the next 24
hours...this open wave will take on a negative tilt and become a
cutoff upper level low. The surface reflection will track along
the international border toward northern Minnesota setting the
stage for cold air advection and strong winds across the Red River
and Minnesota river valleys. These types of setups favor boundary
layer mixing...strong winds...and blizzard conditions depending on
the snowpack.

The deterrents for achieving a high end blizzard with this event
are few. First of all...the warm sector ahead of the cold front
will raise temperatures into the middle 30s across the region. This will
slightly modify the snowpack...but often the degree of melting is
over estimated. For example...January 29 2008 surface temperatures warmed
near 40 with dewpoints in the middle 30s and rain across southern
Minnesota...yet after the cold front moved through with 40-50 miles per hour winds
and a band of light snow developed. Albert Lea was less than 1/4
Michigan for 7 consecutive hours and I-90 was closed. January 25 2010
temperatures started in the middle 30s across Iowa...but winds increased
causing blowing snow and whiteout conditions...closing most roads
across the state.

Another limiting factor is the fact that cold air advection is not
as strong as in other cases...and what that means is that the
forecast winds of 50+ kts off the BUFKIT soundings may not truly be
realized. Especially early on in the event when the higher winds
are somewhat inflated by the instability that develops in the
sounding. With that said...gusts of 40+ kts seem very realistic...
and the duration is also concerning. For the reason...expected the
blizzard watch eastward and extended the time to 18z Monday. In
reality...could see blizzard conditions another tier eastward of
counties...but will let the day shift handle this since the event
is still over a couple days out.

The takeaway is this...forecast soundings show the best utility
when trying to diagnose and predict a ground blizzard. The
soundings off both the NAM and GFS show over 50+ knots gusts...but
think that a more realistic value will be 40 to 45 knots gusts. This
is the upper end when compared to soundings from past ground
blizzards. There should be plenty of fresh snow from the winter
storm earlier this week. Its very unlikely that the snowpack will
melt enough to sustain itself with these expected winds...not to
mention additional falling snow of an inch or two as the thermal
profile saturates in the dendritic growth zone. If the numerical
models remains on track...a significant ground blizzard should
come to fruition Sunday into Monday.

Long term...(sunday through friday)
issued at 500 am CST Sat Feb 6 2016

The pattern of northwest flow aloft with a trough over the eastern Continental U.S. Coast
and a ridge over the western Continental U.S. Coast will remain steady throughout
the middle-to-end of next week. This will mitigate the potential
for any large systems to move through the area...thus a rather dry
several days is expected...along with promoting effective cold air
advection of Arctic air into the region through at least the end
of the week. At the surface...high pressure will drop in from central
Canada Tuesday- Thursday...making for generally cold but partly cloudy
conditions. A weak trough of low pressure...potentially evolving into a
cold front...will drop through the region Thursday night into Friday morning. This
looks to be the only chance for measurable precipitation throughout the
extended portion of the forecast...and chances are no higher than 20
percent at this time. Temperatures are expected to remain below normal
throughout the period with highs in the teens to lower 20s and lows
in the single digits above and below zero.

&&

Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 535 am CST Sat Feb 6 2016

Low clouds over eastern Minnesota into western WI will slowly erode off to the east
as a weak upper level ridge approaches the region...helping scour
out some of the low level moisture across the area. Middle-high level
ceilings will then prevail from late this afternoon through the
overnight hours. As the large storm system approaches from the
west...some -sn may try to make an appearance close to 12z but
chances are low /with chances increasing after daybreak Tuesday/.
Ceilings will gradually lower overnight through tmrw...potentially
into MVFR range for the northwestern Minnesota taf sites. The other main concern
will be strong winds developing late tonight through tmrw...
particularly in the lowest levels off the deck. While surface
sustained wind speeds linger close to 10 kts...but sometimes as
high as 15...strong low level winds will develop. Model soundings
and BUFKIT profiles indicate winds increasing to around 40 knots as well as
having a 40-60 degree directional difference from surface winds...so
have opted to include low level wind shear mentions at all taf sites.

Kmsp...once the MVFR clouds erode to the east later this morning... will
look for VFR conds to prevail through tmrw morning. No precipitation expected
within the first 24 hours...but then confidence drops after 12z tmrw
in keeping a dry forecast...particularly after 14z-15z. Have included
a prob30 mention in the 24-30 hour portion of this taf since chances do
increases late morning for having the -sn move into the area from the north.
Have also included the low level wind shear mention for kmsp as several MOS
guidance plus BUFKIT/model soundings support the thinking.

/Outlook for kmsp/
Monday...chance MVFR/IFR/-sn. Windy. Blsn possible. Wind northwest 20g35kt.
Tuesday...VFR. Wind northwest 10 kts.
Wednesday...VFR. Wind northwest 5-10 kts.

&&

Mpx watches/warnings/advisories...
WI...none.
Minnesota...blizzard watch from Sunday morning through late Sunday night for
mnz041-047-048-055.

Blizzard watch from Sunday afternoon through Monday morning for
mnz054-056>058-064>067-073>076-082>084-091>093.

&&

$$

Update...jpc
short term...jrb
long term...jpc
aviation...jpc

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