Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 161715

Area Forecast Discussion
National Weather Service Sioux Falls SD
1215 PM CDT Wed May 16 2018

.SHORT TERM...(Today and Tonight)
Issued at 313 AM CDT Wed May 16 2018

Overall, little changes made in the short term.  Clear skies will
yield to a field a cumulus developing as southerly winds draw
moisture into the region.  With daytime heating, could see a few
showers/thunderstorms develop in the afternoon hours mainly west of
the James. Thunderstorms are expected to remain below severe limits
with environmental laps rates nearly moist adiabatic only leading to
a few hundred J/kg of surface based CAPE.  With relatively dry
profiles, however, could get outflow and gusty winds from storms
that do develop, but with winds through the entire depth of the
troposphere generally below 15 knots, do not expect anything to
reach severe limits.

Expect storms to die away through the evening hours with the loss of
heating, but could see a resurgence towards 06Z and after as 850 hpa
warm air advection increases across central South Dakota.  Have
shifts pops through the overnight hours a bit west, with the warm
air advection mainly remaining west of the forecast area through 12Z

Increasing dew points along with lingering clouds will keep
temperatures in the mild category Wednesday night with lows around
10 degrees above normal.

.LONG TERM...(Thursday through Tuesday)
Issued at 313 AM CDT Wed May 16 2018

Thursday looks be another warm day across the plains ahead of a
disorganized upper level trough moving into the intermountain west
on Thursday.  The leading short wave with this system moves across
North Dakota during the day on Thursday, but looks to be too far to
the north to have a significant impact on the forecast area.  What
will be more noticeable will be the increasing southerly wind
ahead of this system, drawing moisture into the region. Gulf of
Mexico is wide open ahead of this system, drawing fairly rich
moisture northward into the plains. This southerly flow leads to
PWAT values increasing to around 1.2 inches on Friday, and around
1.3 inches on Saturday. Both of these values lie well above the
median value of around 0.8 inches for this time of year. The 90th
percentile is around 1.2 inches, so this is a fairly significant
surge of moisture. As this occurs, a short wave train is expected
to break off the parent low lifting northeast through the region.
With this feature, much of the dynamics remain across the western
half of the forecast area as upper waves train northeast through
the region.

Friday night into Saturday the remaining pieces of the upper low are
expected to move through the region. Near the surface, a slow moving
cold front is expected to slowly work from west to east across the
region. What is somewhat concerning is the orientation of corfidi
vectors are similar to the orientation of the front. This would
suggest the potential for training storms, however, there is some
question as to how much instability will be available in the
environment with moist adiabatic lapse rate from just above the
surface through the depth of the troposphere. In this very
efficient rain set up, blended the higher QPF model output (ECMWF)
with the superblend to increase rain amounts.

Both the ECMWF and GFS suggest the remaining upper wave will migrate
towards the western Great Lakes region on Sunday.  This will result
in drier air returning to the region, though both 00z solutions were
slightly slower with the exit than their 12z counterparts.

Not a lot of model agreement as we head into next week, but both the
GFS and ECMWF hint at the potential for additional disturbances
moving into the plains Tuesday into Wednesday. At this point,
instability appears to be quite limited, so just have showers with a
mention or two of thunder.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1214 PM CDT Wed May 16 2018

VFR is expected through Thursday.




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