Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231713

Area Forecast Discussion
National Weather Service Sioux Falls SD
1213 PM CDT Sun Sep 23 2018

.SHORT TERM...(Today and Tonight)
Issued at 339 AM CDT Sun Sep 23 2018

A subtle upper level ridge will move across the forecast area this
morning, with a southwest flow aloft developing this afternoon ahead
of an upper trough in the intermountain west. Minus our far
southeast zones and extreme west, the pressure gradient is tight
across the forecast area today with mixed layer winds increasing to
30 to 35 knots through the afternoon hours. Therefore expecting a
windy day for many locations with directions from the south helping
to provide a warm day. In fact highs will warm into the mid 80s
across our south central SD zones ahead of a cold front. Elsewhere
highs will mostly be in the mid 70s to around 80. The cold front
will not make a lot of easterly progress tonight due to a lack of a
short wave, leaving the forecast area under a southerly surface
flow. Therefore lows will be mild with readings of 55 to 60 degrees.
Generally used the warmer guidance values across the central
portions of the CWA where the gradient remains the strongest. Small
chances of high based showers or TSRA will exist late tonight in our
far western areas due to warm air advection. The surface cold front
will finally nudge eastward and move into our central SD zones by
late tonight.

.LONG TERM...(Monday through Saturday)
Issued at 339 AM CDT Sun Sep 23 2018

On Monday, the aforementioned cold front will move across the CWA.
Post frontal conditions do not look overly breezy at this time with
a lack of tight pressure gradient and brisk mixed layer winds. This
is partially due to the fact that the upper trough is still west of
this area and broadening in scope as it enters the front range of
the Rockies and western plains. Slight chance to chance pops are
warranted along and behind the front. Notably, there is quite a
difference in the elevated instability between the GFS and NAM in
northwest IA during the afternoon and evening hours. MU cape values
off the GFS in the 900-800mb layer are 500-1000 J/kg, with a much
more stable profile off of the NAM. The NAM has generally pseudo-
adiabatic lapse rates aloft with virtually no cape. Therefore
current thoughts are that some small hail is possible in northwest
IA across the marginal risk area. In addition, some wind gusts may
be strong with an average of 45 knots in the 3-6km layer. But if the
NAM pans out closer, northwest IA will not even see this.

The upper trough digs somewhat as it passes across the plains Monday
night through Tuesday night. A few showers are possible with this
trough behind the cold front Monday night and Tuesday morning,
especially since many of the models are showing post frontal cold
air advection stratus moving from west to east on Monday and Monday
night, possibly lingering into Tuesday morning. Highs on Tuesday
will be back down into the 60s with a large area of surface high
pressure moving across the plains Tuesday through Wednesday.

The various deterministic global models are in pretty good agreement
through Thursday next week, with some divergence in the low level
patterns after Thursday. Another cold front moves through Wednesday
night and early Thursday with upper troughing diving down from
Canada. This will reinforce the cool air through the remainder of
the week, although the upper flow does become more zonal this next
weekend. Timing and placement of light precipitation late next week
is very difficult due to the differences in short waves. Certainly
no major rain makers look to occur however.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1212 PM CDT Sun Sep 23 2018

VFR through most of the period. LLWS possible this evening and
overnight around KFSD and KSUX. MVFR ceilings possible at KHON
after about 8z.




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