Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 261108
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
608 AM CDT Wed Apr 26 2017

.SHORT TERM...(Today and Tonight)
Issued at 402 AM CDT Wed Apr 26 2017

Upper level low pressure across northern South Dakota will continue
to wander east/northeast today, slowly intensifying as it moves into
northern Minnesota by Thursday morning. While a bulk of the deeper
forced precipitation has pulled east of the area other than across
parts of southwest MN, there remains an abundance of low level
moisture which continues to produce spotty drizzle across southeast
SD into northwest IA at 09z. Forecast concerns line up behind
precipitation type into the morning hours, specifically potential
for areas of light freezing drizzle toward highway 14. Because of
our location southeast of the upper wave in the larger scale mid-
level dry slot, most model thermal profiles indicate that saturation
depth does appear to be an issue with lack of ice production for
most areas, perhaps briefly reaching the threshold around KHON early
this morning. There would be the chance for some spotty very light
icing near highway 14 through around 14z as temps dance close to
freezing at the surface.

Does look as if there could be multiple attempts for precip in low-
to mid-level deformation band to wrap back into the Highway 14
corridor during the day, first and perhaps most aggressively through
mid morning, but larger scale subsidence will be working to erode
the southern end of the area as it pushes southward.  Another
attempt will be made around early afternoon, but by this time temps
near surface will be somewhat warmer and should see mainly liquid
precip.

Otherwise, the cyclonic environment promises to keep a lot of
cloudiness in place across the area, with perhaps a few breaks
working toward the western Missouri River later in the day. Temps
will moderate just a bit today, but generally warm no more than 5-10
degrees from current readings.

Despite the clouds and continued northerly winds, temperatures
tonight do appear likely to drop into the upper 20s to around 30 for
a good portion of the area, but uncertainty lies within two areas -
1) How many clouds will either linger or wraps southward into the
area overnight to impact temperature trends, and 2) impact of
continued winds overnight, which will become lightest near ridge
axis through central SD. As a result of the second, widespread frost
does not appear to be a viable concern for most areas, with
exception of perhaps toward lower Brule areas. However, if clouds
break with the light winds, an even stronger temperature fall will
make for a significant freeze as well.  A freeze watch would seem to
be the most appropriate choice at this point considering vegetation
state and expected temperature/wind. However, this would end up
being a CWA stand-alone and not prepared to do this given at least
some uncertainty on how long temps will be near the minimum. Will
highlight the sub-freezing temperatures in the HWO and social media,
for many areas, especially near and west of I-29, will have to be
very wary of overnight temps.

.LONG TERM...(Thursday through Tuesday)
Issued at 402 AM CDT Wed Apr 26 2017

Certainly the trends through the medium and extended portions of the
forecast continue to show cooler than normal temperatures into early
May.  The biggest story however could be a potential storm for the
upcoming weekend and it`s impact on temperatures and precipitation
types.

Thursday and Friday continue to trend dry with the CWA stuck
in between troughing over the northern Great Lakes and a
secondary open wave moving through the Central Rockies. High
pressure will be in control of the local area, with the best
chance for precipitation focused across Nebraska into southern
Iowa. Temperatures will remain on the cool side, with highs in the
lower 50s.

Focus then turns toward energy that is expected to dig into the
Rockies and pivot northeastward through the Plains and into the
Great Lakes Sunday and Monday. Models have trended deeper and
further northwest with this system. Which looks to deepen as it
moves by the forecast area on Sunday.  Obviously all kinds of mess
regarding the thermal profile and expected weather elements through
the event.  Precipitation may likely begin as rain Saturday night,
but as the system pivots northward, dynamical cooling could push
critical temperatures downward and more favorable for snow.  At that
point it becomes a matter of the warmth and depth of the shallow
warm layer at the boundary layer to determine if precipitation would
fall as snow, rain/snow, or rain in the afternoon. General cooling
of the lowest 1000 ft would favor a full transition to snow Sunday
night.  Still 4-5 days out for this system, but will need to keep a
close eye on critical elements such as temperatures. For now have
lowered temperatures on Sunday on average of 5 to 8 degrees and
blended in colder CONSRAW guidance.

Low consistency in guidance behind this system leads to few
modifications to the forecast.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 607 AM CDT Wed Apr 26 2017

Widespread IFR conditions will gradually lift to MVFR this morning,
with areas through the Missouri River valley reaching VFR during
the afternoon or early evening. Will likely get renewed area of
MVFR ceilings spreading southward through the night into KFSD and
KHON. Otherwise, a little light freezing drizzle will be possible
this morning for mainly KHON and eastward toward KBKX as
saturation into ice bearing levels will be difficult to come by,
except perhaps for a brief time around KHON. Additional MVFR
visibilities in drizzle will remain possible mainly along and east
of I-29 this morning.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...Chapman


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