Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231120

Area Forecast Discussion
National Weather Service Sioux Falls SD
620 AM CDT Sun Apr 23 2017

.SHORT TERM...(Today and Tonight)
Issued at 313 AM CDT Sun Apr 23 2017

Concerns for today are depth of mixing in increasing south-southwest
flow, effects on temperature/dew point/RH, and resultant impacts on
fire weather.

Even with fairly light wind flow within surface ridge Saturday, we
were able to attain highs in the mid-upper 60s throughout the area.
Forecast soundings indicate potential to mix above 800mb, and low
level (925-850mb) temperatures progged to be a couple of degrees
warmer today. Going forecast already near the top end of available
guidance for highs today, but given RAP tendency to excel on well-
mixed days, have nudged highs even a little higher, especially over
the western CWA. Deep mixing into dry air mass should also result in
dew points mixing lower throughout the mid-late afternoon, and this
idea supported by looking at afternoon dew points in the 20s over
most of western and central Nebraksa yesterday afternoon. RAP and
ARW soundings supportive, and have lowered afternoon dew points from
previous forecast as a result. RAP forecast actually even lower, but
will remain slightly conservative for now and day shift can monitor.

These changes to temperature/dew point result in afternoon minimum
humidity levels dropping to 15-25 percent, lowest over the southwest
half of the forecast area. With anticipated south-southwest winds of
20-30 mph, will certainly have to monitor fire weather conditions
today. However, our fire partners have indicated that cool season
grasses are greening enough that fire starts will be low, and these
grasses dominate in the area of stronger winds/lower humidity. The
warm season grasses which dominate across the east are still cured
enough to carry fire if something should start. However, with wind
and humidity levels more marginal in these areas for fire weather
headlines, will hold off issuance for now and highlight higher fire
danger in the HWO.

For tonight, breezy south winds continue, especially in our higher
elevations as southerly 925mb winds persist at 30-35kt and low level
jet increases to 40-50kt. This will result in a mild night with lows
in the upper 40s to lower 50s. Mid-high clouds increase ahead of an
approaching wave, but soundings remain quite dry below 700mb, so see
little if any precipitation potential prior to 12Z.

.LONG TERM...(Monday through Saturday)
Issued at 313 AM CDT Sun Apr 23 2017

An upper level shortwave tracks across the region on Monday, with
the better upper level dynamics remaining to the north of our CWA.
At the surface, low pressure slides from central SD in the morning
into south central MN by early evening, pulling a frontal boundary
across our area. With the approach of the front, may see some
scattered shower development through south central SD and the
western James River Valley in the morning, then mainly north of
Interstate 90 in the afternoon, closer to the better upper level
dynamics/slightly deeper moisture, as the boundary continues to
slide to the east. Cannot rule out a couple rumbles of thunder in
the afternoon if anything develops, with weak instability across the
area. There will be strong southerly winds out ahead of the front,
and with good mixing stayed on the high side of guidance with
respect to temperatures, with lower to mid 70s over the mid and
lower MO River Valley/northwestern IA to mid 60s back to the north.

There will be decent cold air advection behind the front, and Monday
night is looking cooler with lows mid 30s north and west to mid 40s
through our lower IA zones. Late on Monday night another shortwave
begins to lift through eastern Nebraska with upper level support
provided by an upper level jet streaming northeastward out of
Colorado. With that, may see showers develop over south central SD
toward Tuesday morning, and cannot rule out some snow mixing in from
Chamberlain to Huron in the morning along the northern extent of the
precipitation shield where temperatures aloft would support snow.

Models in good agreement on likely rain chances on Tuesday as the
shortwave tracks through the region and midlevel frontogenesis sets
up across the area. 850 mb temperatures continue to cool through the
day, and with clouds and showers went on the cool side of guidance
with highs mid to upper 40s, with a few lower 50s in our far

The theme for the remainder of the week looks to be very cool
temperatures for this time of year with highs 40s/50s and lows in
the 30s, with possible freezing temperatures over parts of the area
on Tuesday and Wednesday night. Model agreement becomes more chaotic
toward the end of the week, so confidence in precipitation chances
is low for that time period, though the higher probability of rain
chances would exist some time in the Saturday time frame.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 620 AM CDT Sun Apr 23 2017

Gusty south-southwest winds will develop by late morning/early
afternoon today, with frequent gusts near to above 25kt expected
through sunset. Gusts will ease slightly overnight, although an
increasing low level jet reaching 40-50kt will result in low
level wind shear across the region after 03z-05z.

VFR conditions will prevail through this TAF period.




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