Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1125 am CST Tuesday Nov 25 2014
issued at 1054 am CST Tuesday Nov 25 2014
Just a quick update for expectation on snow and wind for later
today and tonight. GFS has shifted heavier snowfall into SW Minnesota and
northwest Iowa similar to NAM although it remains faster. Looking at the
water vapor image showing a real strong wave in northeast Montana...do
like the the slower and more amplified NAM/rap solution over the
GFS and Canadian regional. That would keep at least a chance of
snow into portions of SW Minnesota and northwest Iowa into Wednesday morning. Does look
a little less windy on Wednesday as mixed layer is rather shallow
so that winds at the top of the mixed layer are 25 to 30 kts
instead of 30 to 35 kts. Expect this will limit blowing snow
outside of areas where snow is falling. There are some hints that
there could be some light snow or flurries in SW Minnesota into Wednesday
afternoon with a secondary front which may produced localized
lower visibilities due to the combination of falling and blowing
No significant updates to forecast at this time.
Short term...(today and tonight)
issued at 328 am CST Tuesday Nov 25 2014
Band of warm air advection clouds are heading southeastward...
currently in our western zones...and will overtake our eastern zones
from 12z through the morning hours. Went ahead and added an isolated
flurry with the mention of these clouds for many locations through
15z this morning...as the cloud layer is on the edge of dendritic
temperatures near -10 to -12c. Then once these clouds exit...middle and
high level clouds will fill in from west to east which are currently
in the Western Plains.
The next vigorous short wave dives rapidly southeastward right
across our forecast area tonight. Its an Alberta clipper and the
track of the surface low is ever so important with these systems...
as the strong dynamics will exist to the north and east of the
surface low track. First of all today...ahead of this low
temperatures will be close to normal. Moisture depth suggests that
precipitation chances really will not start in our western zones until middle
afternoon...and with surface temperatures being above freezing...
the precipitation in those areas will either be light rain or light rain
and snow mix. Then for tonight...as of right now...model consensus
including the European model (ecmwf) has the surface low tracking roughly from near
Huron...to just west of Sioux Falls...and southeastward to close to
Storm Lake Iowa by 12z Wednesday. Therefore currently...much of the expected
snowfall of about 1.5 to close to 3.0 inches is expected along and
east of that line...cutting off rapidly to the west. What our
western zones will have however is wind. After midnight tonight...
very strong pressure rises and cold air advection pour into those
locations as their threat of light snowfall moves to the east.
Therefore did not issue headlines yet. This is because at this time
a Wind Advisory will probably be needed late tonight and Wednesday
morning in our western zones...given the fact that the snow should
be falling east of there. Where most of the snow does fall tonight
generally east of the James River...the track of the low is leading
to winds which are not very strong. So therefore it does not make a
lot of sense to issue a Winter Weather Advisory for our eastern
zones when snow amounts by itself is not reaching criteria.
So we will need wind and blowing snow to warrant a Winter Weather
Advisory...and by the time this scenario gets juxtaposed with
falling snow...we are nudging into the third period of Wednesday
morning. In addition is still a bit of uncertainty with regards to
the exact track of the surface low. If this system continues to
strengthen...the track of the low will likely adjust westward...
thus thrusting more of our forecast area into a decent snow band.
Will mention all of this in the forthcoming severe weather potential statement.
Long term...(wednesday through monday)
issued at 328 am CST Tuesday Nov 25 2014
The nature of weather problems Wednesday...including what highlights
are needed...will depend how the approaching system behaves today.
The bottom line is that Wind Advisory level winds seem to be a given
for the southwest half of the area...and not far from that to the
northeast. The key will be the timing...amount...and location of the
best snow. The worst case scenario would be the upper system digging
a little more...slowing down some...and wrapping up more as it moves
southeast. This could not only bring better snow amounts...but some
overlap between the best snow falling and strong winds Wednesday
morning...which we do not have much of as it stands now. This could
produce blizzard conditions for a little while. For now it seems
likely that we will need a Wind Advisory and/or Winter Weather
Advisory for snow and blowing snow. Currently southwest Minnesota seems
most likely to have winter weather problems for the better snow
potential...but that could change with a system slowdown and/or a
track of the best snow into a little higher winds. The forecast and
especially the hazardous weather outlook will reveal the potential
for winter weather problems and the strong winds but details seem
too uncertain at this time to issue specific advisories.
Very strong winds should be the rule for the area Wednesday with
light snow lingering in the morning east. Will mention blowing snow
where accumulations are expected...to linger in patchy form into the
afternoon when snow no longer will be falling. Temperatures will be
falling Wednesday...and of course Wednesday night as winds decrease.
There will be some decrease in clouds Wednesday...then it loos like
clouds at middle and possibly low levels will be moving across the area
Wednesday night and Thanksgiving day as warm advection develops
aloft. At this time with little or no upper support other than the
warm advection...will leave precipitation out of the forecast for
that time period. Temperatures will drop to the single digits either
side of zero Wednesday night...except some teens lows southwest
corner. Thanksgiving day will bring highs in the tens northeast to
the 30s southwest as southeast winds develop at the surface.
Friday will bring a decent warmup...then a cooldown will take place
overt he weekend...with warming trying to start back up again early
next week. Will keep the warming Friday to 30s highs northeast half
with expected snow cover...to the lower 50s southwest. It has the
potential to be warmer still. However...aside from the possible snow
cover effect...the sun angle is getting low and allowing less punch
to daytime heating. It is a close call on adding a little light snow
around Saturday behind the cold front but with little upper support
will leave out for now. Warm advection starting up by Monday seems
to weak through that time to produce precipitation that day...but
there will be further chances to revisit the extended time period.
In any event...the biggest weather event will be early in this
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1124 am CST Tuesday Nov 25 2014
Main focus for aviation will be developing IFR conditions with
snow and low clouds from this evening through Wednesday afternoon. Radar
shows a large area of light snow over ND and moving into
northwestern South Dakota. This area of snow should move into Hon around
00z...fsd around 03z and sux around 04z. For sux...surface
temeperatures are warm enough that precipitation may start as a
mix of rain and snow before changing to all snow. Visibilities with snow
are expected be from 1 to 3 miles overnight with the lowest
visibilites around kfsd and then east into SW Minnesota and northwest Iowa. Ceilings
will fall below 2000 feet and likely will fall below 1000 feet around
kfsd toward midnight. As the snow ends later tonight into early
Wednesday morning...visibilities will improve back to 6 miles but ceilings will
remain around 1000 feet through 18z. One concern on Wednesday morning is
winds gusting to near 30 kts. Without falling snow...expect that
this will mainly produce drifting snow and not impact visibilities
but will need to monitor in case winds are stronger and blowing
snow does develop.