Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202324
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
624 PM CDT FRI MAY 20 2016

.SHORT TERM...(This evening through Saturday)
Issued at 427 PM CDT Fri May 20 2016

The CWA remains in the middle of three disturbances rotating across
the northern US this afternoon. One wave continues to sink southward
out of North Dakota this afternoon, a second pushing away from the
area in the mid-Mississippi River valley and a third weak wave
ejecting out of the Central Rockies.

Generally speaking however, the area will remain quiet through
Saturday. A few sprinkles remain possible this afternoon and
evening, and we could also see a few very light showers or sprinkles
develop later tonight as ACCAS field over central SD drifts
southeast. Stratus will again build back into central SD, and hold
along or just west of the Missouri River on Saturday.  Further east,
skies should remain mostly clear, and temperatures will climb into
the lower to middle 70s.

.LONG TERM...(Saturday night through Friday)
Issued at 427 PM CDT Fri May 20 2016

Ridge will continue to have a hold on the area much of Saturday
night, especially in terms of keeping winds down a bit more early in
the night as well as a trajectory of near surface drier air into
parts of southwest MN and northwest IA. The gradient will increase,
keeping temperatures somewhat more mild in the 50s...and even near
60 in south central SD.  Will again maintain the push of moisture
into the SD CWA which should keep a fair amount of cloudiness
around, especially later in the night as the southerly low level jet
strengthens to 40 to 45 knots. Stratocumulus clouds will be very
prevalent into Sunday, slower to spread through parts of northwest
IA and southwest MN. Concern would be extensive clouds would impact
highs quite negatively, as strength of inversion suggests that
advection, perhaps some erosion on edge, will be the only way to
effectively reduce cloud cover.  Have trimmed, but perhaps as much
as 3-5F cooler than going forecast in the works through the heart of
the area, and lesser impact on the western and eastern CWA edges. In
terms of precipitation, models explicitly have produced quite a bit
of light precipitation on Sunday across southeast SD/northeast NE.
while there is a bit better mid level lapse rates, the moisture is
almost hopelessly trapped below the inversion. Not feeling strongly
about potential for ACCAS, and even less for any non-measurable type
of precip. At this point, will keep the morning dry, and only
mention a very small pop James Valley and west in the afternoon on
thermodynamic gradient.

Heading into Sunday evening, strong wave in the Rockies will begin
to push out into the western plains. Concern will be for convective
development and if severe storms. MLCAPE by early evening
concentrated mainly across central SD near and in a narrow axis just
ahead of frontal boundary. Shear is also more formidable west of the
CWA, with deep layer values on the order of 35-45 knots. Convective
initiation would certainly be favored west of the CWA around very
late afternoon to early evening, with progression gradually eastward
overnight. Problem to severe threat is that wave is shearing north
and east toward southern Manitoba overnight, with low level jet
gradually working to veer toward northeast and the net effective
deep layer shear largely failing to breach 15 to 25 knots later in
the night.  Elevated instability remains on order of 1000 J/kg as
line shifts eastward. However, shear is largely front parallel, so
eventually cold pools could be expected to outrun the inflow and
weaken the entire area heading eastward. Would seem severe threat
would be over a fairly small area, generally James Valley and
westward during the evening and early overnight hours. Not looking
for a large scale event, but perhaps a few severe storms with large
hail and damaging winds.   Progression of convergence boundary is
slower in 12z model runs, even to point where a couple solutions
would keep out of areas near highway 71 and east through 12z Monday.
Larger scale forcing to push boundary largely abandons the gradually
sinking boundary Monday, and will maintain a threat for showers and
thunderstorms with broad DIV Q lagging across the area along and
ahead of boundary. Temps warmest back through the James Valley and
west where clouds will likely break, while parts of NW IA and SW MN
stay in clouds and keep temps back toward lower to mid 70s.

For the extended range from Tuesday through Friday, very little of
consistent signal.  Larger scale pattern suggests potential for some
more active weather across the area, especially with wave around
Tuesday night and Wednesday, and again perhaps toward end of week as
more bodily portion of western trough ejects toward the region.
While guidance would suggest a nearly constant chance for
precipitation, there is likely to be a couple periods with lesser
chance, and if side toward a more reasonable looking ECMWF solution,
would perhaps end up contained in Monday night-Tuesday, and
Wednesday night-Thursday.  Tuesday is interesting in terms of a
severe weather threat, with instability of 1000-2000 J/kg hovering
near the southern CWA, and shear increasing in parts of NW IA and NE
NE as wave pushes up from the southwest. A more general shower and
thunder threat late week as main instability axis and jet pushed
south. Temps will generally be seasonally mild, and diurnal ranges
perhaps a bit lower. Mainly 70s for highs, and 50s for lows, but
clouds could further reduce this around precip favored times.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 622 PM CDT Fri May 20 2016

VFR conditions are expected through the TAF period.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Dux
LONG TERM...Chapman
AVIATION...JM5/08



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