Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241745
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1145 AM CST Sat Feb 24 2018

.SHORT TERM...(Today and Tonight)
Issued at 403 AM CST Sat Feb 24 2018

Current upper air analysis shows the strong upper wave moving across
the Rocky Mountains at this time. At the surface, a large area of
high pressure based over the Great Lakes region will move eastward
and out of the area through the day. The upper wave remains open
through the day, and moves toward the I 29 corridor by 00Z, lifting
out in a negative tilt east of our area tonight into the MS valley.
The PV is still fairly strong albeit not quite as strong as previous
runs, with the 1.5 PVU surface lowering to about 550mb south of the
forecast area from south central NE to western IA.

Initially this morning, a pretty strong area of low to mid level
thetae advection lifts northward through the area, coupled with the
CWA placed in a right entrance region of a jet streak in central MN
and Great Lakes region. These features will begin the enhancement of
lift and saturation before the short wave arrives in the midday and
afternoon hours. The surface low is forecast to be not real strong
in the central plains. However when the short wave arrives, we still
have very strong mid level frontogenesis placed under instability
aloft, with very unstable EPV* from 700 to 600mb in a saturated
layer. The one exception to this is in far southeast zones around
midday, where mid level saturation is a bit weak, near the Storm
Lake vicinity. Therefore would not rule out a bit of midday freezing
rain in that area as ice crystal production could be a little
limited. But overall, the various models were too low with their ice
crystal production so raised them to get more mention of snow in the
forecast in our far eastern zones. One other interesting item
besides the strong mid level frontogenesis, is our eastern two tiers
of counties is showing enough instability aloft to produce cape
values of around 70 j/kg in the 750-700mb layer. The soundings do
not look quite unstable enough to add a mention of thunder, but it
could be enough to start a convective process with the snowfall
adding to intensity of rates this afternoon. Therefore did not take
the section of southwest MN out of the winter storm warning due to a
potential to still get 6 or more inches of snow in that location in
a short amount of time. A little further south of there in northwest
IA, slightly drier air in the mid levels should keep their
accumulations in advisory level. Initially through mid morning, a
fetch of drier air aloft will feed into some of our eastern zones
which should limit snowfall for a while. However the various cams
then begin to produce precipitation rapidly as saturation aloft
strengthens by late morning. But do not be surprised if the radar
echoes are pretty light in parts of our east this morning. So
overall, we are looking at a widespread 3 to 4 inch snow event,
perhaps a bit less in some MO River valley counties, and the timing
still looks good on the headline. The cams are not really picking up
on any particular heavy banding anywhere, except toward the area
where we already have the winter storm warning where a few of them
are showing some heavier QPF likely due to convective processes
mentioned above.

Lastly, extended the dense fog advisory to 15Z. The dense fog will
probably get a bit less widespread, but it may not totally subside
until the precipitation begins to move into the area scouring it
out.

The wave exits rapidly overnight tonight. The QG forcing moves right
over our forecast area but becomes well east by 09Z Sunday.
Therefore the precip will shutoff pretty fast in our eastern zones
after midnight tonight. As the surface low strengthens to our east,
wind speeds will become rather breezy east of I 29. Cannot rule out
some patchy blowing snow especially in southwest MN after midnight
but to keep things simple with everything going on right now, did
not include that in the forecast yet.

.LONG TERM...(Sunday through Friday)
Issued at 403 AM CST Sat Feb 24 2018

Did not spend a long time perusing over the longer range portion
of the forecast. In general though, temperatures look near to a
bit below normal. There is a lot of model spread with the mid week
system next week. The GFS and GEM global still show a southern
stream wave impacting our forecast area with primarily snowfall.
However the ECMWF cuts off this area with the northern stream. But
certainly low end pops as a heads up are warranted at this time
in the Wednesday and Thursday time frame.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1135 AM CST Sat Feb 24 2018

Messy start to the aviation period that will improve over time. The
quick moving batch of snow that is impacting the TAF sites as of
late morning will exit just as quickly. While light snow and IFR
and lower ceilings could hang around thru the afternoon hours, the
reduced visibilities from falling snow should quickly improve at
KSUX and KFSD over the next hour or two, with KHON seeing
visibility improvement by mid afternoon. Ceiling restrictions will
remain until this evening before clearing in the stratus is
expected from west to east with VFR conditions expected
thereafter.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 9 PM CST this evening for SDZ039-
     040-054>056-061-062-066-067-070-071.

     Winter Weather Advisory until 7 PM CST this evening for SDZ038-
     050-052-053-057>060-063>065-068-069.

MN...Winter Weather Advisory until 9 PM CST this evening for MNZ071-
     097-098.

     Winter Weather Advisory until midnight CST tonight for MNZ072-
     080-081-089-090.

IA...Winter Weather Advisory until 9 PM CST this evening for
     IAZ001>003-012>014-020>022-031-032.

NE...Winter Weather Advisory until 9 PM CST this evening for NEZ013-
     014.

&&

$$

SHORT TERM...MJ
LONG TERM...MJ
AVIATION...Kalin



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