Scientific Forecaster Discussion
fxus63 kmpx 171747
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1147 am CST Sat Feb 17 2018
Short term...(today and tonight)
issued at 432 am CST Sat Feb 17 2018
Short term models continue to indicate a short duration light snow
event affecting the eastern area through about noon today.
Development is taking place to the southwest now and should
develop/fill into the southwest cwa through about 12z. The NAM
remains furtherest south with its forcing and snow potential. Most
of the hires models continue to indicate strong frontogenesis and
strong vertical motion in the -12c to -15c favorable dendritic
layer as far north as the Metro in the 11z-14z period. We could
see a quick 1/2 to 1 inch of snow moves through the Metro through
about midmorning. This forcing moves into west central WI after
14z and exits the area through 18z. Again, a quick 1 inch or so
accumulation possible where the stronger forcing sets up.
Clouds should gradually diminish from west to east during the
afternoon as the trough/surface front exits the area. Temperatures
will be tricky as best warming will take place during the morning
as the snow potential moves across. Will trim blended guidance a
degree or two.
Focus turns to the next short wave that will affect the western
cwa later tonight. Strong isentropic ascent moves into the
northwest quadrant of the cwa mainly after 06z. Will bring pops up
to near categorical over the far northwest late tonight.
Long term...(sunday through friday)
issued at 432 am CST Sat Feb 17 2018
Sunday through Tuesday...this period of the forecast continues to
be the focus as a potentially very impactful mixed wintry
precipitation event looks to unfold across the area.
A wavy quasi-stationary front will be draped over southern WI/Minnesota
into South Dakota and continuing westward through ID, with a conglomeration
of weak low pressure centers over western South Dakota/eastern Wyoming.
Meanwhile, a secondary cold front meandering over central Canada
will be dropping south. Aloft, a fairly zonal pattern (the
southern periphery of a large trough around a Hudson Bay low)
will be set up across the central Continental U.S. As a large longwave trough
moves onshore the Pacific northwest. As the day progresses Sunday, the
western Continental U.S. Trough will expand and play a significant role in
transitioning the upper level pattern to a more west-southwest-east-northeast flow,
capable of bringing in not only Pacific moisture into the region
but also allowing a pathway of some Gulf of Mexico moisture. The
other artifact will be an upward nudge in 700 mb-500 mb heights which will
play a key role in p-type Sunday through Monday.
The first main round of precipitation will be generally around and
north of the I-94 corridor early Sunday morning through Sunday
evening. With little in the way of an above-freezing layer
available for freezing rain, this batch of precip is expected to
come as nearly all light snow. With qpfs in the 0.1-0.2" range and
snow liquid ratios dropping from roughly 18:1 to 13:1, generally
about 1-2" of snow can be expected for central Minnesota into
There looks to be a lull in the action late Sunday night as the
first main low pressure center along the front moves off to the
east. However, with the western trough digging further into the
southwest Continental U.S. And an upper level wave dropping southeast in
conjunction with the aforementioned Canadian cold front,
precipitation looks to return in a more widespread fashion across
the coverage area early Monday morning. This is where the warm
layer aloft will be more of a player. Isentropic lift will be
heightened on the north side of the wavy front while moisture
deepens due a phasing of the moisture plumes nearly atop and east
of the weather forecast office mpx coverage area. Wet snow looks to accumulate for
much of the coverage area during the day Monday into Monday
evening, with amounts ranging 1-3" during this period. However, it
is during this period when icing has its greatest potential,
particularly along and south of a line from Mankato-New Richmond-
Ladysmith where several hundredths of an inch to around a tenth of
an inch can be expected Monday into early Tuesday morning.
The longwave western trough will then shift into the central Continental U.S.
Monday night through Tuesday which will enhance cold air advection
over the north-central Continental U.S.. effectively, this will gradually
strip out the above-freezing layer and force remaining
precipitation during the day Tuesday to come as all snow.
Overnight Monday through daybreak Tuesday, about an additional
inch of snowfall accumulation can be expected. Thus, all told for
this event, snowfall amounts look to range from 1-2" near the Iowa
border to 3-6" in central Minnesota. There is obviously room for some
fluctuations in these amounts due to frontal positioning and warm
air intrusions but due to fairly good continuity run-to-run over
the last 18-24 hours among the models, confidence is growing in
starting to advertise precipitation amounts. The precipitation
will wind down Tuesday evening as the surface front shifts off to
the east, the trough begins to wash out and surface high pressure
arrives from The Rockies.
Wednesday through Friday...a much more tranquil period is expected
for the latter half of next week as surface high pressure slowly
traverses east across the region Wednesday into Thursday. The cold
air advection which started on Monday will continue through
Wednesday as the base of the upper level trough moves atop the
region. By Thursday, with high pressure off to the east and a
small bubble ridge moving over the area, a warm-up is expected
with highs climbing back to around the freezing mark. The drying
out period looks to end Friday as another longwave trough
approaches from the west and a cold front drops in from the
northwest. Have included chance-range pops for the end of next
week into the weekend with these synoptic scale features.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1122 am CST Sat Feb 17 2018
This period begins with MVFR ceilings across south central Minnesota
through west central WI with light snow still occurring in Eau
Claire. Gradual clearing will allow for VFR conditions again across
all taf sites by the late afternoon. Winds will remain west-
northwesterly through the evening before becoming southeasterly
near daybreak. Cloud cover will again increase across from west
to east overnight. Winds will also pick-up with gusts near 20-25
kts through much of the late morning and early afternoon on Sunday
as another low pressure system approaches. This system will also
cause chances for snow and MVFR conditions, primarily for axn and
areas to the north for much of the day Sunday.
Kmsp...VFR conditions are forecast for this period. However,
chances for light snow will have to be monitored. At this time,
snowfall and MVFR conditions look to remain north of msp through
/Outlook for kmsp/
Sun night...becoming MVFR. Chc -sn late. Winds north-NE 10 kts.
Mon...IFR/-sn likely. Winds NE 10-20 kts.
Tue...chc MVFR/-sn. Winds north 10-15 kts.