Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 212337
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
637 PM CDT Sun May 21 2017

.SHORT TERM...(This evening through Monday)
Issued at 347 PM CDT Sun May 21 2017

Cloud coverage has been the main concern in the near term. Low to
mid clouds have slowly moved/eroded from west to east this
afternoon.  Light winds and widespread cloud cover across southwest
Minnesota may hold temperatures a bit cooler tonight. The
exception will be across southeast South Dakota and portions of
northwest Iowa where skies cleared out a bit today. Therefore,
lows temperatures will be in the lower 40s across southwest
Minnesota, to upper 50s along the Missouri River Valley.

As wind begins to shift to a west-southwest component late this
evening, a weak wave will start to move eastward from the Northern
Plains into the region. This disturbance will support another chance
for precipitation and isolated thunderstorms tonight into Monday
evening. Models are in good agreement in bringing higher
precipitation chances and isolated thunderstorms late Monday morning
and continuing into early Monday evening. Despite limited low-level
moisture, moderate instability will persist in the afternoon hours,
with CAPE values around 1700 J/Kg. This will support isolated
thunderstorms, with perhaps a few strong to severe storms across
northwest Iowa. If strong thunderstorms develop, large hail and
gusty winds will be the main risks.

Highs will be ranging in the mid to upper 60s, except across
northwest Iowa where less cloud cover are expected for a few hours.
These areas could see temperatures in the lower 70s.

.LONG TERM...(Monday night through Sunday)
Issued at 347 PM CDT Sun May 21 2017

Mid-level trough axis will work to clear northwest Iowa Monday
evening, and coverage and intensity of shower and isolated thunder
activity will quickly drop off. After some partial evening clearing
working across the area, more clouds will rotate southward into the
area, with a chance for showers to return toward east central SD and
southwest MN later in the night. Winds will also begin to increase
from the north/northwest late as 15 to 20 kts sits just off the
surface as low stacks east of the area.

As lobe wraps around the upper low on Tuesday, will find organized
area of showers cycling through the area, along with an enhancement
of coverage to clouds/precip with an assist of the diurnal cycle.
Little indication that depth of clouds will be sufficient to get
much in the way of efficient ice processes/charge separation, so
will keep thunder clear of the forecast.  Shallow MLCAPE profile and
cold temps at top of that layer bring potential for a little graupel
with stronger updrafts. With clouds and precip, temps likely to
remain somewhat cooler for areas near/east of I-29 and have shaved a
bit off the initial blend.

Wednesday one of the few days that looks to be dry at this point, as
upper ridge nudges upper-level low east of the area. Still will be
dealing with some residual low level moisture ahead of the ridge
axis and expect a healthy stratocumulus field from near I-29
eastward. Temps will moderate quite a bit from Tuesday, but still
remaining below normal.

One big change in the larger model set over yesterday is a much more
aggressive push to trough knocking down the upper ridge by Thursday.
As a result, influx of stronger EML is about 18-24 hours quicker and
suggests a chance for elevated showers/isolated thunder creeping
into the west by late Wednesday night and pushing slowly through the
CWA, mainly north/west on Thursday.  Increased southerly flow and
temps likely to push to near normal readings.

Should be another stronger wobble of trough into the northern plains
during the weekend, with frontal passage likely and threat for a few
thunderstorms.  So much discontinuity in solutions at this point,
have no confidence on changes to forecast from Friday onward to make
meaningful changes from initial blend. Temps at least should hover
around seasonable through the period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 634 PM CDT Sun May 21 2017

VFR conditions are expected through the TAF period. Scattered
showers and and isolate thunderstorms will develop after 18z, but
ceilings are expected to remain VFR. Could see some locally lower
visibility under stronger thunderstorms.

&&

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

$$

SHORT TERM...05
LONG TERM...Chapman
AVIATION...


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