Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241835

Area Forecast Discussion
National Weather Service Sioux Falls SD
135 PM CDT Thu May 24 2018

.SHORT TERM...(Today and Tonight)
Issued at 340 AM CDT Thu May 24 2018

Another day of convective uncertainty, as we deal with interactions
of weak forcing with convective remnants throughout the day. Early
this morning, hi-res guidance similar in depicting development
through the pre-dawn hours as low level jet interacts with outflow
from dying complex in south central South Dakota/central Nebraska.
This scattered activity expected to move toward the I-29 corridor in
time for the morning commute, then weaken after 15Z as the low level
jet diminishes.

Questions then become how persistent lingering cloud debris will be,
and what effect this will have on heating/destabilization potential
later today as subtle waves slide northeast across the forecast
area. Modest consensus points to mid-late afternoon re-development
in our eastern CWA, but run-to-run consistency in HRRR is severely
lacking, so confidence in favored timing/location is low. Somewhat
higher confidence in scattered development in our northwest CWA
toward evening, as tail end of a wave moving across the Dakotas
interacts with pre-frontal trough/dryline west of the James River.

Despite the uncertainty in heating, should be looking at decent
instability this afternoon, with even weakest models showing mixed
layer CAPE values in 1-2kJ/kg in our eastern CWA. Problem is deep
layer shear is quite weak at less than 20kts, so may be tough to
sustain organized storms. Stronger instability in our western areas
thanks to steeper mid-level lapse rates, though even there shear is
limited, so still looking for pulsy, multi-cell clusters tracking
across the CWA through the evening/overnight hours. Given the lack
of good shear, cannot argue with Marginal outlook from SPC for our
area today, keeping Slight Risk just north of our CWA where shear is
slightly better.

.LONG TERM...(Friday through Wednesday)
Issued at 340 AM CDT Thu May 24 2018

Stretch of calmer weather expected to dominate much of the upcoming
holiday weekend, as upper ridge builds into the region. This will
bring well above normal temperatures Friday into early next week,
with Saturday still looking to be the warmest of the bunch with
highs in the 90s most areas. Dew points currently expected to be in
the upper 50s-lower 60s Friday/Saturday, possibly creeping a little
higher Sunday as low level flow turns more southerly again, so may
have to watch for some areas reaching heat advisory criteria, though
readings remaining a few degrees shy at this time. Will continue to
highlight the upcoming warmth online, and will add a mention to the
HWO as well. While cannot rule out a stray shower or storm at some
point through Sunday, precipitation chances are minimal with the
warm ridge in control, so will maintain the dry forecast for now.

Sunday night/Monday, the upper ridge axis shifts east as a piece of
energy lifts northward through the central High Plains. This could
bring spotty showers/storms to the area for the Memorial Day holiday
itself, but precipitation chances should grow through midweek as the
broad western trough shifts east into the northern Plains. Fairly
good consensus in the 00Z models for a wet period late Tuesday into
Tuesday night, but confidence not quite high enough to go with a
likely pop that far out, so will stick to just high chance range for
now. This should bring some relief from the weekend heat, though at
this point still looking like temperatures will remain a bit above
normal as May comes to a close.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 134 PM CDT Thu May 24 2018

Isolated to scattered thunderstorms will be possible from mid
afternoon through about 3z. The chance in any one location is
fairly low so no mention of thunderstorms will be included in the
TAF. Outside of any thunderstorms VFR conditions are expected.




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