Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 300920
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
420 AM CDT Tue May 30 2017

.SHORT TERM...(Today and Tonight)
Issued at 420 AM CDT Tue May 30 2017

The cold air advection regime continues today, with cool northwest
flow aloft and breezy surface winds. Skies will generally be mostly
sunny to partly cloudy, however greater cloud cover will likely
persist in portions of southwest Minnesota and surrounding counties.
A vort max lobe is expected to drop through southern Minnesota
midday, and could spark a few isolated showers or sprinkles near the
Buffalo Ridge area. Highs today will be similar to yesterday, with
the 60s to lower 70s expected - warmest near the Missouri Valley.
Breezy northwest winds will increase to 15 to 25 mph this
morning with some higher gusts to 35 mph possible.

Winds quickly become this evening, with surface high pressure moving
overhead. Given the light winds, clear skies and cold airmass, we
should cool rapidly. Dropped lows further and degree or two. Lows
will reach the lower to mid 40s.

.LONG TERM...(Wednesday through Monday)
Issued at 420 AM CDT Tue May 30 2017

After a cool start, Wednesday will be a pleasant day as the core of
high pressure drifts into Minnesota and Iowa. Southerly flow will
strengthen across lower Brule areas, otherwise winds should be
fairly light and sunshine in good quantity. By late day, will start
to see impact of elevated push of slightly more instability toward
south central SD with development of mid-level clouds, but have kept
any precipitation chances out through the day with lower levels
remaining quite dry below just the very early start of steepening
lapse rates. Highs mainly lower to mid 70s are suggested by mixing
to 900-800 hPa.

Subtle wave should help to enhance at least some scattered
precipitation along the advancing mid-level boundary Wednesday
night. Flow toward mid-levels does remain northwest, and development
should generally remain through the Missouri valley at most
populous, and perhaps as far north as around I-90.  About 300-700
J/kg instability based above 800 hPa around the Missouri River, less
to north. With little MCS possible to the south early Thursday,
there could be some modified air/cloudiness streaming northward
across eastern Nebraska and western Iowa which could impact
temperatures to some degree even if rainfall is not long lasting or
significant earlier in the day. For this reason, have coordinated a
slight drop in parts of northwest IA into the mid to upper 70s, but
some of the raw data suggests a greater potential for a low cloud
field and much cooler temperatures.  Further west, impacts of any
precipitation on lesser potential cloud trajectory yields temps well
into the lower 80s from the James valley westward.

Likely that any weak capping could be overcome on Thursday evening
near and north of advancing warm frontal boundary with increase in
low-level jet. Best chance for scattered thunderstorms looks to be
from northwest Iowa into southwest Minnesota, and given the
increased instability and strong directional shear (making up some
for the overall weak or negative speed shear) have no qualms about
some marginal risk for a severe storm wandering into the area as we
get closer.  Also, potential QPF likely a bit higher for spotty and
likely slow moving storms with increasing moisture in profiles.

Ridging continues to build through Thursday and into Friday, also
signifying that warmer temps aloft will be expanding across the
area. With warm frontal boundary moving northward toward southwest
MN, any activity looks more likely than not to begin to cap off
south of the boundary through the day.  Friday certainly has
potential to feel quite summery with dewpoints climbing well into the
60s and temps well into the 80s.

Convection likely be become more significant feature on Friday night
as models depict (in one form or another) a wave moving through the
mean ridge to break down the core of warmest temperatures aloft.
Location of the boundary will be critical to largest threat, but at
this time would tend of favor east central SD across southwest MN
and perhaps toward the Iowa Great Lakes. Degree of instability and
the locally enhanced shear parameters near the boundary would
certainly bring in a threat for a few evening to overnight severe
storms as well.

Models continue to exhibit some fairly decent differences over the
weekend, but now seem to be more with timing as the general
evolution of a deepening trough digging southeast into
Minnesota/Wisconsin over the weekend.  The main difference in
evolution would appear to be how the energy is distributed between
the lead wave Friday night/Saturday, and that which follows later
Saturday.  ECMWF solutions favors a more balanced distribution,
while GFS is a bit more back-weighted with energy and digs a slower
and deeper wave not reaching MN until early Sunday, a full 18 hours
slower.  In general, will continue with some PoPs through Saturday
favoring the north/eastern sections of the forecast area as boundary
begins to sink southward behind leading wave and allows cooler
northerly flow to develop through the day.  Temps at least 10-15
degrees cooler for most with clouds/precip, but some mid to upper
70s should remain draped through the Missouri River corridor on
Saturday, but a full ECMWF solution could allow some 80s to return.
Not much help to support changes to Sunday/Monday grids given
diversity of solutions. In general, lesser chances for precip on
Sunday, only lingering in parts of southwest MN and northwest IA
closer to upper wave, with ridge expanding again toward the region
on Monday along with next push of mid-level warm advective
precipitation chances. Cooler air will remain on shallow easterly
flow, with highs below normal in the upper 60s to lower 70s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1015 PM CDT Mon May 29 2017

VFR conditions expected through the period. Another day with
gusty northwest winds up to 25 kts and cumulus at or around 6000
feet is expected Tuesday.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...
LONG TERM...Chapman
AVIATION...Ferguson/JM


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