Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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628
FXUS63 KFSD 211126
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
526 AM CST Wed Feb 21 2018

.SHORT TERM...(Today and Tonight)
Issued at 338 AM CST Wed Feb 21 2018

Today and tonight are marked by a digging upper trough over the
western United States, with surface high pressure moving slowly
eastward across the plains. A very strong jet streak of 200 to 210
knots exist in a subtle confluent area over southeast Canada.
Interestingly, a southwest to northeast orientated tail of this jet
streak forms tonight from central SD to northeast MN which places
are forecast area in a right entrance region late tonight and early
Thursday. In the near term, the day period today will be quiet with
sunshine and light winds, but seasonably cool highs mainly hovering
a few degrees either side of 20. However tonight as the
aforementioned upper trough continues to dig in the west, broad
ascent forms with the jet streak aided in the lower levels by very
strong mid level frontogenesis and thetae advection moving northward
late tonight and Thursday morning. This should generate an area of
precip to form which lifts northward Thursday morning, especially
affecting the southern half of our area with light snow as the 850-
700mb layer becomes saturated.

What happens Thursday afternoon and evening is a strong question
mark. The various models are in pretty good agreement with the large
scale details. However, the NAM really dries out the mid levels
throughout northwest IA and very immediate adjacent locations. The
GFS remains saturated through the ice production layer. Therefore
the NAM is pointing at a freezing drizzle, perhaps light freezing
rain event throughout northwest IA, whereas the deeply saturated GFS
would be all snow. The ECMWF and German standard have no continuity
with each other either in terms of strongest precip location,
therefore confidence is low in the Thursday afternoon and evening
forecast. In general, it still looks like a fairly prolonged precip
event as the aforementioned dynamics Thursday morning, are replaced
by the short wave passage and accompanying PV late Thursday
afternoon and evening. So there are two different dynamical
mechanisms going on. It all hinges on what happens to the saturation
between 700mb and 500mb. Chances are if the NAM pans out closer,
real freezing rain may be tough to find in our southeast zones,
likely closer to freezing drizzle. Therefore despite the QPF amounts
being just over five hundredths of an inch, hedged northwest IA and
adjacent locations toward freezing drizzle mixed with the snowfall
Thursday evening, but opted for very light freezing rain and
snowfall Thursday afternoon. It should be primarily snow
elsewhere. Will just have to clean this up later when future runs
come in. Still looks overall that snowfall amounts will be three
inches or less spread out over a 30 to 36 hour period as snowfall
rates are rather benign. Where the heaviest snowfall occurs is still
quite a question mark due to the factors listed above.

Temperatures tonight will begin to rise some as mid and high level
cloud cover move in after evening. Thursdays highs will be moderated
compared to today but still a fair amount below normal. The short
wave will exit after midnight Thursday night, leaving another chilly
night in store.

.LONG TERM...(Thursday through Tuesday)
Issued at 338 AM CST Wed Feb 21 2018

The weather looks quiet on Friday but with another below normal
day temperature wise. However things will begin to change very
late Friday night, lasting through Saturday. The GFS has become
very strong with an upper wave passage during this time frame,
with a negative tilt on the southern branch of the jet stream
lifting through Saturday afternoon. The NAM and GEM Global are
more benign, cutting this area off more with the drier northern
split flow. But a quick glance at the ECMWF and German Standard on
the web also shows a very strong band of precip only slightly east
of where the GFS has it. The GFS paints a lot of instability where
the heavy band of precip resides from the Yankton and Tyndall SD
areas, northeastward through Sioux Falls and Marshall MN Saturday
morning. This band then shifts eastward into extreme northwest IA
and into southwest MN Saturday afternoon before exiting Saturday
evening. The EPV* is actually negative above the maximum
frontogenesis layer in the 700-600mb layer, and that layer is
saturated. Following suite, soundings on the GFS have lapse rates
more unstable then pseudo adiabatic in this band above 850mb. The
model consensus was kicking out too much freezing rain in the
heaviest QPF areas so backed that off as ice production will not
be a problem. This is due to the blend that occurs with the drier
NAM and GEM influencing the precip type. As mentioned above, we do
not have the dynamical details of the ECMWF and German standard
models, but their 500mb patterns are very much closer to the GFS
then what the GEM and NAM are advertising. Thus their QPF patterns
are closer to the GFS but slightly eastward right along very
strong 850mb baroclinicity, which strongly suggests instability in
northeast NE, northwest IA, and into southwest MN and extreme
southeast SD. If the saturated layer, amount of PV, 700mb
frontogenesis and instability all pan out, this system will need
watched closely. Do not want to hit it real hard yet though
because there is still a chance that the GEM and NAM could pan out
closer, making this system a northern stream dominated dud.
Hopefully future model runs will shed some light for Saturday.

After Saturday, overall temperatures still look a bit below normal
into mid week next week. There could be some light precip around
on Wednesday due to isentropic lift but that is a long ways out.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 521 AM CST Wed Feb 21 2018

VFR conditions with light winds expected through most of the
period. Clear conditions are expected today at HON and FSD, with
perhaps a stray low level cumulus cloud but not enough for any
restricted ceilings. The next light snow event begins to move into
the region near the end of the TAF period, leading to MVFR
conditions at SUX after 09z Thursday.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Wind Chill Advisory until 10 AM CST this morning for SDZ038>040-
     050-052>061-063>065.

MN...Wind Chill Advisory until 10 AM CST this morning for MNZ071.

IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...MJ
AVIATION...Ferguson



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