Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 130359
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1059 PM CDT Thu Apr 12 2018

.SHORT TERM...(This evening through Friday)
Issued at 358 PM CDT Thu Apr 12 2018

Very active forecast headed into the weekend as an intense storm
system impacts the region. Storm system is currently across the
intermountain west, and is expected to move into the western high
plains by daybreak Friday morning.  Ahead of the system, pressure
gradient is expected to increase leading to increasing easterly
winds overnight.  Mid level boundary is expected to work north
through the area overnight.  This could lead to bands of showers and
thunderstorms.  With steep lapse rates in the 500-700 mb layer, have
elevated CAPE values up to 2000 J/kg in Northwest Iowa.  With
increasing shear aloft, some storms in northwest Iowa could produce
severe hail. Am much less concerned with wind gust potential given
the deep saturation in the near surface layer and inversion. Once
you get north of highway 18, the lapse rates become much less steep
in the 700-500 mb layer, minimizing the severe threat further to the
north.

Expect band of showers/thunderstorms associated with warm air/theta-
e advection at 850 mb to lift north through the forecast area on
Friday morning. As the inverted trough approaches from the west,
expect precipitation to transition from rain to snow across
central SD. This transition is expected to gradually collapse
south and east through Friday evening. Could see another round or
two of elevated convection across northwest Iowa mainly in the
afternoon hours as surface warm front and dry slot work north in
the late afternoon hours.

With the winter weather transition occurring mid-day, have resolved
the western portion of the winter storm watch into a Blizzard
Warning beginning at 18Z.

.LONG TERM...(Friday night through Thursday)
Issued at 358 PM CDT Thu Apr 12 2018

On Friday night, transition toward a fierce winter storm will
continue across the area from west and north toward the southeast.
Model trends continue a subtle southward shift to things, but
overall the availability of moisture and persistence of dynamical
forcing northwest of the track of the low will suggest potential for
a significant to historic storm for many areas.

Elevated thunderstorms will continue to be a concern during the
early evening hours in parts of northwest IA and potentially even
toward I-90 riding above the low-level stable air pool.  Elevated
shear Friday evening as jet continues to punch northward across IA
should not be too much to overwhelm the up to 500-1000 J/kg
instability based around 800 hPa. Continued upscale growth into a
more linear structure likely as storms lift northeast across
northwest Iowa early evening.  This convection will lift northward
and feed into the deformation band developing across southwest MN.
At the same time, lower level temps will be working to cool off. NAM
continues to have a cold bias on surface/near-surface temps, with
other solutions slightly warmer.  Appears the temps aloft are more
reasonable in other solutions being not quite as cool in the 950-900
hPa layer, which seem to cool off too quickly and sharply given the
saturated conditions off-surface with persistent east to northeast
flow. Given likely liquid form to precip well into the evening, this
leads to probably the most questionable element of this system - the
degree to which ice accumulation will be possible.  Surface
temperatures could remain just above freezing for several hours with
impact from warming from above, especially toward the Iowa Great
Lakes, but every degree will certainly count with readings right
around freezing. Probably will be a meld of freezing rain/sleet for
a few hours. All things considered, the threat would largely be
limited to areas across parts of southwest MN into the IA Great
Lakes area as we transition through the evening and early overnight
hours. At this point, expect ice accumulation less than a quarter
inch (less than a tenth toward NW IA) given marginal surface temp
parameters, but any ice with the winds expected could have impact on
powerlines and trees.

Speaking of winds, northeast to northerly winds will likely maintain
a gusty 30 to 45 mph most locations overnight, a bit less in baggier
gradient over southern portions of northwest Iowa. It will not take
long for visibility to drop to a quarter mile or less once an inch
or two snow accumulates to initiate blowing snow.  Convective
instability will continue to feed elements into the greater
frontogenetic zone arcing from southwest MN back toward south
central SD. Some negative EPV along band suggests it may tighten up
and further some intense snowfall rates which should reach an inch
or two per hour in the main banding. Even some thundersnow is
possible. Travel will become extremely difficult if not
impossible heading through Friday night.

To summarize important elements Friday night: 1) A few evening
thunderstorms possible across northwest IA and far southern MN
capable of up to 1" hail, 2) Some light ice accumulations mainly
southwest MN and toward IA Great Lakes, but larger impact will
likely be the snow later in the night coupled with wind, and most
importantly 3) BLIZZARD conditions spreading from the James Valley
across eastern SD into southwest MN, northwest IA and far northeast
NE going through the night.

Saturday will be downright ugly for April, with blizzard conditions
likely over much of the area. There is still a bit less certainty as
to snowfall amounts over northwest Iowa from Sioux City toward Storm
Lake, but trends in later runs has increased amounts somewhat by
collapsing/reorganizing frontal dynamics into a secondary heavier
band.  Conditions will begin to improve - gradually - from west to
east during the afternoon and evening, as both winds start to
decrease and snow winds down.

As a result of the expected snowfall and wind, have issued a
blizzard warning in all areas that previously were covered by the
winter storm watch.  With the expected trends to increase snowfall
in the remainder of northwest Iowa along with the strong winds, have
issued a winter storm watch for potential blizzard conditions for
area from Sioux City to Storm Lake starting 09z Sat, although worst
of the potential conditions would certainly be morning through at
least mid afternoon on Saturday.

Rest of the forecast, very little time to quibble about details,
other than to continue to nudge down daytime temperatures a bit for
a couple days following this snowfall.  More precipitation midweek,
but appears more likely than not that would remain as rain, at worst
mix with some light snowfall at night Tuesday night.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1054 PM CDT Thu Apr 12 2018

Showers and isolated thunderstorms are expected to develop
overnight with increasing northeasterly winds. Rain may begin to
mix with snow at KHON by Friday afternoon with decreasing
visibilities. MVFR/IFR ceilings will dominate through the TAF
period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Blizzard Warning from 7 PM Friday to 7 PM CDT Saturday for
     SDZ039-040-054>056-060>062-065>070.

     Blizzard Warning from 1 PM Friday to 4 PM CDT Saturday for
     SDZ038-050-052-053-057>059-063-064.

     Blizzard Warning from 1 AM to 7 PM CDT Saturday for SDZ071.

MN...Blizzard Warning from 7 PM Friday to 7 PM CDT Saturday for
     MNZ071-072-080-097-098.

     Blizzard Warning from 10 PM Friday to 10 PM CDT Saturday for
     MNZ081-089-090.

IA...Blizzard Warning from 1 AM to 10 PM CDT Saturday for IAZ002-003-
     013-014.

     Blizzard Warning from 1 AM to 7 PM CDT Saturday for IAZ001-012-
     020.

     Winter Storm Watch from late Friday night through Saturday
     evening for IAZ021-022-031-032.

NE...Blizzard Warning from 1 AM to 7 PM CDT Saturday for NEZ013-014.

&&

$$

SHORT TERM...BT
LONG TERM...Chapman
AVIATION...JM



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