Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 262021

Area Forecast Discussion
National Weather Service Sioux Falls SD
321 PM CDT Wed Oct 26 2016

.SHORT TERM...(This evening through Thursday)
Issued at 319 PM CDT Wed Oct 26 2016

Weak surface ridge nosing from northeast Minnesota to central South
Dakota at mid afternoon. However, a widespread stratus field
continues in shallow cold push behind exiting wave, at mid afternoon
with moisture capped by warmer layer aloft. Tendency for slow
erosion along areas west of the James valley, but really is all to
be counted on with inversion. Problem tonight will be a quick
reversal during the evening to an east and southeast flow, which
will slow the erosion to the east where ceilings are a bit lower.
Clouds likely to hang tough east of the I-29 corridor overnight.
The challenge will be potential for fog development. Most hi-res
models forecasting some degree of fog development, although areas
are a bit more difficult to pin down between solutions. Ensemble
visibility probabilities strong enough to consider fog as well.
While some of the stratus to the east could lower enough to bring
some fog potential to parts of southwest MN and northwest IA, the
greater fog threat will occur toward the edge of the clearing where
mixing today has been very limited. Patches of fog likely to start
developing toward 06z in lower elevation locations...with fog
becoming more widespread later in the night and early morning. Some
of the fog may become fairly dense up and down the I-29 corridor.
Further west toward the James valley, a bit stronger low level
southerly flow could keep fog from becoming too widespread or dense.

After some morning fog/stratus, will get increasing south flow which
should more effectively push out of the way. With pressures falling
through the western plains, any tendency to mix toward westerlies
aloft will be impeded, and should help to limit mixing. The far
western CWA should make a stronger run toward the upper 70s, while
temps remain closer to 60 degrees with delayed clearing across SW MN.

.LONG TERM...(Thursday night through Wednesday)
Issued at 319 PM CDT Wed Oct 26 2016

A very pleasant end of the work week is in store for the region,
thanks to a large thermal ridge and deep southerly flow. By the
afternoon, weak surface troughing will enter the CWA, bringing a
lull to what is shaping up to be a rather breezy first half of the
day.  Temperatures, given anomalous 925:850 temperatures will top
out in the 70s in many areas.  Still concerned about the mix-down of
a rather large dry layer aloft in the afternoon and an impact on RH
and fire danger.  GFS is perhaps overmixing, with the NAM/ECMWF
showing slightly higher boundary layer moisture levels.  The end
result will be RH in the middle to upper 20s, and with a slight
weakening of the surface winds due to the trough, we`ll likely avoid
any very high or extreme fire danger in central SD.

Mid-level flow will flatten for Saturday, allowing a minor shortwave
to move through the region.  Canadian high pressure will settle
southward through the day, with an increased risk for a few
afternoon/evening sprinkles or showers.  Temperatures will be
considerably cooler through the day, falling shy of seasonal

The cooldown will be shortlived as mid-level ridging amplifies once
again due to a deep upper trough arriving on the West Coast.
Southerly winds will push temperatures back above normal on Sunday,
with halloween temperatures well into the 60s, and perhaps 70s.

Speaking of an early look at halloween forecasts, a slight
disagreement in timing between the GFS/ECMWF, leads to a lower than
normal precipitation forecast.  Both models indicate a trough
entering the area later in the day, with the best chance for any
light rain holding off until the evening or overnight.

Quiet and warm conditions continue into the middle of next week as a
rather stagnant pattern persists.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1253 PM CDT Wed Oct 26 2016

Widespread MVFR and IFR ceilings only slowly eroding through
the afternoon. Will struggle to erode near/east of I-29, and
those locations near the edge of the stratus will be most
vulnerable to redevelopment of fog overnight. There is potential
for some spots of dense fog, especially up and down the I-29
corridor, as will be in the sweet spot between the slightly
stronger low level gradient west and the higher probability of
stratus lingering to the east. Have taken visibilities down to
around 1SM for the time, with uncertainty in the exact erosion
potential through the evening on stratus.




SHORT TERM...Chapman
AVIATION...Chapman is the U.S. government's official web portal to all federal, state and local government web resources and services.