Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
557 am CST Friday Nov 21 2014
Short term...(today and tonight)
issued at 325 am CST Friday Nov 21 2014
With high pressure to the east...and a trough of low pressure to the
west...southerly winds have increased recently allowing temperatures
to remain steady...and even slowly rise as expected. The winds off
the surface in the mixed layer today are averaging 30 to 40 knots
east of the James River Valley. Therefore wind speeds will become
quite breezy. Because of the strong warm air advection...and despite
a fair amount of middle and high level clouds moving into this area...
temperatures will finally warm near or above freezing for the
forecast area. It has been a while since this has occurred.
Now the next question is how quickly the stratus in the Southern
Plains will move into our zones. At this time...the stratus has
nudged into extreme southeast Kansas. There could be a little erosion on
the northern side of the stratus this afternoon due to mixing.
However it appears that it will begin affecting our far southern
zones around Sioux City by late afternoon...and certainly by this
evening as it moves northward along and east of the I 29 corridor.
This will be something that future shifts will have to watch...is
how far westward the stratus will extend. Currently do not have it
moving as far west as the James River Valley...but especially with a
southerly flow...it is not out of the realm of impossibilities that
the clouds could do so this evening. At any rate...with a southerly
flow of air...lows will be mild tonight despite decreasing wind
speeds after midnight. The last item to consider is freezing
drizzle. Not overly thrilled about it...but there could be some
patchy to areas of freezing drizzle in our far southeast zones where
the stratus will have to greatest depth...and surface dew points
will be mild. So included the mention of some freezing drizzle for
eastern Woodbury County and locations around Storm Lake late tonight
and very early Saturday.
Long term...(saturday through thursday)
issued at 325 am CST Friday Nov 21 2014
Models agree that with the low level flow becoming temporarily
west southwesterly...any drizzle of freezing drizzle in
northwest Iowa should end with clouds slowly decreasing
Saturday. The rest of the area is expected to be mostly sunny and
mild. Temperatures should get well up into the 30s or lower
40s...middle 40s far southwest...as a fairly light southwesterly flow
gradually becomes southeasterly. Of course temperatures would be
balmier without the current snow cover.
Clouds will likely increase again in northwest Iowa by Saturday
night as a weak system to the southeast passes across and the low
level flow continues to back. Will go with a little chance of
light rain as temperatures should hold just above freezing
under the warmer air aloft. For the rest of the area...clouds
will increase more slowly.
Sunday will bring the cold front crossing the area. A few areas of
light rain or snow will be possible...mainly east...with the upper
wave pushing the front through. Temperatures will poke up a
little into the 30s to lower 40s before starting to cool. Sunday
night and Monday will then bring windy and colder weather with areas
of light snow or flurries. Will keep the probability of precipitation at low chance while
also keeping patchy blowing snow in case light accumulations develop
with the very strong winds. The winds...light snow chance...and
possible blowing snow will continue Monday evening then decrease
later Monday night. Meanwhile...temperatures Monday will hold below
freezing with wind chills going down further.
Tuesday through Thursday will bring a small attempt at warming
before another cooldown...with high temperatures in the teens to
lower 20s by Thursday. There will be small chances of light snow as
this midweek cold front travels across the area.
The operational GFS and ec present a major difference in the Post
forecast period with the GFS keeping it fairly cold...but the ec
bringing much more ridging into The Rockies and plains...with a
significant warmup. Not sure but would favor the persistence of the
cold pattern if I had to choose.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 555 am CST Friday Nov 21 2014
The biggest aviation concern in the 12z taf set is the onset of
stratus for kfsd and ksux. Plenty of it exists from southeast Kansas
and points southward into the Southern Plains...and it will
rapidly advect northward today along a brisk southerly flow of
air. Used the rap13 for timing of the stratus to edge into ksux
and kfsd. At this time...it is difficult to tell whether the low
clouds will be low end MVFR or IFR. Currently...even LIFR exists
in some locations in central Oklahoma. For now...stayed on the
optimistic side and went lower end MVFR. But fully realize that
stratus conditions could erode into the IFR category as tonight