Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202150

Area Forecast Discussion
National Weather Service Sioux Falls SD
350 PM CST Fri Jan 20 2017

.SHORT TERM...(This evening through Saturday)
Issued at 350 PM CST Fri Jan 20 2017

The biggest concern continues to be the persistent fog/stratus
threat across the region. Low visibility and ceilings have gradually
improved for areas west of the James River as clouds eroded
northward; but still they are struggling east of the I-29 corridor.
A thick cloud deck layer persists along and east of the James River;
therefore keeping mild temperatures, and a wealth of low level
moisture (fog) in the area. With this low level moisture, light
winds, and no changes in the airmass, dense fog is likely to
redevelop tonight into early Saturday morning. Models differ in some
in terms of fog coverage. HRRR and NAM suggest widespread low
visibilities to less than a quarter mile this evening and over the
nighttime hours. While the RAP only shows patchy fog across this
time frame. Given that discrepancies our confidence in low
clouds/visibility is not high for areas along and west of the I-29
corridor. At this point decided to extend the dense fog advisory for
areas along the Buffalo Ridge and portions of northwest Iowa, where
stratus on the ground will continue. A new advisory may be needed
for portions of the I-29 corridor later this evening.

The other issue is a shortwave lifting northeast tonight into
Saturday morning bringing a chance for light rain-snow. Profiles
support deeper saturation in the low-level combined with decent PVA
forcing at 500mb. There is a warm layer of air near the surface that
will keep most of the precipitation as rain. This layer is less
pronounced near the Highway 14 corridor, and may result in a little
bit of snow mixed in with the rain.  Any snow accumulations are
expected to be very minor.  The heaviest QPF with this system is
expected closer to the center of the upper level disturbance, down
in northwest Iowa.

On the backside of this system, there is a trough that sweeps
through the area resulting in a wind shift near the surface. While
the winds become northwest, moisture has a hard time scouring out,
and have therefore kept skies cloudy with small diurnal range.

.LONG TERM...(Saturday night through Friday)
Issued at 350 PM CST Fri Jan 20 2017

Not too much going on the second half of the weekend. Residual low
level moisture will linger over many areas. The moisture is deepest
through the lowest kilometer in southwest MN where a bit of freezing
drizzle or drizzle could linger. This area is aided by some speed
convergence just off the surface and a subtle surface wind shift
aiding in ascent. But no accumulations of freezing drizzle are
expected late Saturday night or early Sunday in that zone.

With a stout northwest flow, would not be surprised to see the low
clouds scour out Sunday afternoon as suggested by the various
models, only to be replaced by fairly thick mid and upper level
clouds. Still, it will be seasonably mild Sunday and Monday.

After that, all eyes are focused on the next storm system. The
various deterministic models between the 12Z GFS, ECMWF and GEM
global have actually come around in pretty good agreement with its
track in both space and time. The low is rather positively tilted and
sheared, therefore the event looks like a prolonged precipitation
event but one that lacks a lot of intensity. It looks like the type
of event of a typical 3 to 6 inch snow event on average through the
forecast area, but will monitor of course over the coming days.
Precipitation type is a bit tricky on Tuesday from near Vermillion
SD, to Sioux City, to Storm Lake IA. Temperatures off the surface
are near zero celsius in these areas which could result in a rain or
snow mix. The superblend highs on Tuesday looked a bit warm however
in our southern zones south of I 90 due to precip anticipated and a
lot of cloud cover. Therefore blended in some raw model values
especially from the ECMWF which cooled highs down a category.
Followed this same trend on Wednesday as once again, superblend
highs looked a bit too warm through our entire forecast area. It
will be quite windy throughout the forecast area after the surface
lows moves east Tuesday night and Wednesday and bumped up the all
blend winds.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1148 AM CST Fri Jan 20 2017

IFR/LIFR conditions ceilings and visibility will continue through
the TAF period, mainly east of the James River. There will be a
slight improvement late this afternoon, but ceilings/visibility
could drop to 1 mile or less late tonight due to a short wave
lifting northeast producing drizzle/light rain across the TAFs
sites. The light precipitation may saturate the near surface
later, resulting in widespread fog once again late tonight.


SD...Dense Fog Advisory until 9 AM CST Saturday for SDZ040-056.

MN...Dense Fog Advisory until 9 AM CST Saturday for MNZ071-072-080-

IA...Dense Fog Advisory until 9 AM CST Saturday for IAZ001>003-013-



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