Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
543 am CST Friday Dec 13 2013
12z aviation discussion added below.
Short term...(today and tonight)
issued at 329 am CST Friday Dec 13 2013
Split flow is in place this morning across the Continental U.S.. northern stream
jet stretches from NE nodak over to southern New England...with a
strengthening southern stream jet down along the Arizona/mex border and
working into West Texas. At the moment...a band of snow is being
helped along by fgen forcing in the 700 mb-h6 within the right
entrance region of the northern stream jet. Short term guidance all
similar in showing this band of light snow/flurries diminishing
through the morning as the fgen forcing falls apart as the upper
jet continues to work off to the east...taking the right entrance
forcing with it. As for that southern wave...the importance of it is
that it is in the process of generating an area of low pressure
over southeast Colorado. At 3am this was a 1008 mb low...but by 18z...the
hrrr shows a 1004 mb low moving into SW Kansas. As this is going
on...another Arctic high will be approaching northern Minnesota...with
pressure gradient and attendant NE winds expect to increasing
during the morning. This will allow the lower ceilings that have
been more or less stuck along I-94 much of the night to expand
southwest...eventually encompassing the entire area in stratus. As
this happens...have slight chance probability of precipitation coming south with
clouds...but think this may be a bit overdone as middle levels look
to dry out in the wake of the upper jet...which may result in very
little snow being seen today...as is seen with the hrrr.
Instead...looks better for light snow to expand across the area
tonight as a shortwave currently coming onshore over British
Columbia dives east-southeast across the Dakotas and Minnesota. The atmospheric
response to this is to develop an inverted trough in the low
levels...which when combined with the return of deeper moisture
should result in the blossoming of light snow after 00z out in western
Minnesota that will slowly work east across the rest of the area into
Sunday morning. Like the other snow makers we have seen in the
last few of days...this will only produce 0.5-1.5 inches of fine
With cloud cover returning for this period...we will see a
considerable damper put on diurnal trends. For today...the
combination of said cloud cover along with weak cold air advection on those east-northeast
winds will keep temperature rises during the day today to within about 5
degrees of current temperatures. The only exception will be down along
the I-90 corridor...where some sun this morning and warmer low
level thermal profiles will allow places from Fairmont over to
Albert Lea to take another run at 20. For tonight...all models
have come in a bit slower with the return of the Arctic
airmass...and combine that with the expected continuation of
cloudy skies...did bump up lows for tonight a few degrees.
Long term...(saturday through thursday)
issued at 329 am CST Friday Dec 13 2013
The vast majority of the long-term period will consist of cold northwest
flow aloft with several weak systems drifting through the
region... producing minor amounts of snowfall. Attention is still
drawn to the end of the period and beyond where models still
indicate a significant system will impact much of the central
An upper level shortwave axis will swing through the region on
Saturday...on the heels of a potent storm system that will impact
much of the eastern Continental U.S.. weak warm air advection in advance of this trough...which
acted to mainly prevent Arctic air from moving in than a true warm
up per southeast...will cease as surface winds swing to northwest underneath northwest flow
aloft...allowing cold air advection to commence late day Sat and will continue
through sun. In conjunction with this trough pushing through...flurries to
light snow can be expected Sat through Sat night with no more than a
couple inches of accumulation expected. Weak high pressure extending
from the deep south will move through...allowing for a precipitation-free
period late Sat night through the first half of sun. With this dry
period will come much colder temperatures as h925 temperatures drop from
the low-middle teens below zero c to nearly -20 degree c and coming from
the northwest. Highs will drop from the 10-20 degree f range on Sat to the
5-10 degree f range on sun.
The next system will cross the region Sun night through late Monday...
again producing flurries to light snow with very little snow
accumulations. However...associated with this system will be a
noticeable warm-up that will remain in place through the first
half of the work week. H850 temperatures rise to just a few degrees below
zero c...but with significant snow pack in place...surface temperatures will
remain below freezing but highs will rise into the 20s through Wednesday.
Starting on Thursday is where the forecast becomes quite interesting.
Models are still indicating the development of a potent Colorado-
type low on the backside of an upper level ridge shifting from the
central Continental U.S. To the eastern Seaboard. This will allow the system to
become entrained within strong southern stream jet energy...producing a
powerful storm that will track northeastward across much of the country.
There is still much uncertainty in its exact track which would
certainly affect if and how much snow...and it would be snow as
opposed to any other wintry precipitation...would fall across the mpx
County warning forecast area. Regardless of its exact track...the other significant impact
would be another shot of frigid Arctic air on the backside.
Indications already are such that by next weekend...high
temperatures will be sub-zero. So...at this point...confidence is
growing that there will be a significant winter storm affecting
the region next weekend with a strong punch of Arctic air. It is
just a matter of narrowing down the details of the snowfall
potential. Stay tuned.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 543 am CST Friday Dec 13 2013
Lower confidence with ceiling forecast in tafs this morning. Band of
2k-4k feet ceilings stretched along I-94 now will move southwest as NE
winds increase this morning. Those northeast winds will also be
bringing down drier air...so may see MVFR ceilings get scoured out at
axn/stc/rnh for a time this afternoon as indicated by the gfslamp.
Followed the idea of the rap/hrrr with -sn just north of mpx
terminals at the start remaining there as it weakens this morning.
Confidence increasing in light snow moving back into the area
tonight as upper wave currently moving through British Columbia
heads for Minnesota. This looks to be yet another prolonged -sn type
event...with about 12 hour of 2-4sm -sn possible once it sets in.
Kept timing similar to existing tafs...keeping it close to the
higher sref precipitation probs...which picks up well from where the hrrr
leaves off with -sn over nodak this afternoon.
Kmsp...by no means are the MVFR ceilings currently over the field a
part of a solid shield of MVFR ceilings...so if anything would say
current taf is a bit pessimistic when it comes to ceilings this
afternoon. Confidence is becoming high in -sn moving into the
field tonight...with the main question being when. Looking at
forcing and the arrival of deep moisture...the GFS would bring in
-sn close to 06z...while the NAM brings it in closer to 3z. Split
the difference for now...but some refinement in timing will be
needed as we work through the day. Once the snow sets in...msp
looks to in for a prolonged period of 2-4sm -sn until between 18z
and 21z on Saturday.
/Outlook for kmsp/
Sat...MVFR with chance of IFR with -sn. Winds becoming west 5 kts.
Sun...MVFR ceilings possible. Chance of -sn overnight. Winds north 5kts.
Monday...MVFR ceilings early...otherwise VFR. Winds SW 5-10kts.