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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
530 PM CST Sat Jan 24 2015

Short term...(this evening through sunday)
issued at 328 PM CST Sat Jan 24 2015

Current radar shows band of rain and snow extending from central
North Dakota into northeast South Dakota. This is associated with a
decent area of middle level frontogenesis and upper level wave moving
across the area. The middle and lower levels of the atmosphere will
initially be quite expect mainly virga this afternoon.
Although by 0z expect the profile to saturate and light rain to
begin. Most of the initial light rain will likely be along and east
of a Huron to Sioux Falls to Storm Lake line...closer to the
frontogenesis axis. As the atmosphere continues to saturate...and
forcing associated with the upper wave moves overhead...would expect
periods of light precipitation to impact much of the region by later
this evening into the overnight. Would expect precipitation
intensity to decrease through the night frontogenesis
weakens and we see just a broad area of weak lift.

Precipitation type will initially be rain given surface temperatures
from the upper 30s to low 40s. However colder air will work in from
north to south this evening as the wave pushes across and winds turn
northerly. Decent model agreement in this temperature
followed a blend of the raw models. Then derived precipitation type
from these surface any warm layer is expected to
be surface based. Thus thinking we begin to see snow mix in with the
rain from Brookings into southwest Minnesota between 7 and 9 PM.
This mix and eventual changeover should make it to Sioux Falls
around midnight and Sioux City by 6 am. Any light snow should come
to an end by around 7 am...although flurries could linger through
the morning. As far as amounts go...thinking we see 1 to 2 inches
from Brookings into most of southwest Minnesota. Not out of the
question we see an isolated 3 inch amount...but given temperature
concerns and dry air...did not go that high in the forecast.
Thinking Sioux Falls sees between a half inch to an inch...with less
than a half inch to the south.

Winds will also be quite blustery tonight out of the northwest.
Winds gusting 25 to 35 miles per hour seem likely...strongest near the Missouri
River. Given the wet nature of the snow...not thinking we see much
in the way of drifting. Although could be some pockets of lower
visibility when it is snowing...but given the light nature of the
snow...not thinking we see anything too extreme. Model trends
continue to show a cooler day Sunday...and trended highs down...with
30s expected.

Long term...(sunday night through saturday)
issued at 328 PM CST Sat Jan 24 2015

On Sunday night...very strong warm air advection begins in earnest
shortly after the evening strong low pressure moves
eastward across southern Canada. So therefore the eternal question
is how far temperatures will least east of the James River
Valley...during the evening before rebounding as the surface winds
turn from south to west overnight. Blended the European model (ecmwf) and Gem global
raw values as they looked to handle the potential cooling east of
I 29 during the evening the best...possibly down into the lower
20s. Conversely...they kept our southwest zones in the lower to
middle 30s. Many other guidance values kind of mashed the temperature
regime altogether and did not give enough gradient which will
likely occur from west to east. Temperatures still look extremely
mild on Monday...and continued to go mainly with bias corrected
European model (ecmwf)/Gem values. On Tuesday although still very mild...there may
be a bit of a cool down due to a southeast surface flow.

On Wednesday...the deterministic solutions of the Gem global...European model (ecmwf)
and GFS now all have the well advertised cold frontal passage
totally clear of our forecast area by the end of the day Wednesday.
The Gem global is the fastest and most aggressive with the frontal
passage. Blending the bias corrected values of the three models
yields highs generally from the middle 40s across our northern near 50 along I 29 and middle 50s over Sioux City. This
stands to reason given 925-900mb temperatures and a good mixing
northwest wind behind the front. But there will likely be enough
cool air advection to keep US away from record highs. The day period
on Wednesday looks dry but some of the models continue to show
skittish probability of precipitation Wednesday night of what would likely be a Post
frontal...very light rain and snow mix. The eastern zones have been
the most favored by this scenario recently so kept the probability of precipitation confined
to east of I 29.

Surface high pressure then dominates this area on Thursday...with
departing high pressure and a southeast surface flow of air on
Friday. There is quite a bit of model discrepancy on Friday as the
European model (ecmwf) is a lot warmer than the GFS. Taken literally the European model (ecmwf) would
easily give widespread highs in the 40s...but with a south southeast
wind... not great for mixing this time of year...tended to keep
highs in the 30s to lower 40s. Then the Arctic plunge moves in
Friday night and Saturday which will dramatically cool temperatures
off this next weekend. Would not be surprised to see highs in the
teens and 20s Saturday and Sunday. The European model (ecmwf) is even more aggressive
than the GFS with the cold air and would be even colder. But for
now...the message is temperatures back to reality for next weekend.


Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 530 PM CST Sat Jan 24 2015

From 25/00z-05z ceilings 1-3k feet will replace VFR conditions
from the northwest with areas of visibilities 3-5sm -rasn developing
from the northwest. These conditions will continue through 25/12z with
local visibilities below 3sm in snow. 25/12z to 25/20z conditions
will improve from the northwest to VFR with VFR conditions
prevailing 25/20z-26/00z. Occasional surface gusts above 25 knots
from the north/northwest 25/03z-12z then winds will decrease from the northwest.


Fsd watches/warnings/advisories...
South Dakota...none.


Short term...chenard
long term...mj

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