Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211721
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1221 PM CDT Mon May 21 2018

.SHORT TERM...(Today and Tonight)
Issued at 319 AM CDT Mon May 21 2018

Continuous light showers continue to move slowly northward this
morning ahead of a well defined upper circulation spinning across
south central and southeastern South Dakota.  Despite the innocuous
appearance of this activity on radar, actual amounts have been a bit
more impressive.

For today, it`s looking like a dreary and damp day for many in the
Tri-State area as this wave very slowly slides eastward. The
majority of precipitation will be focused along or east of I-29
today, with pops now extended into the evening hours across the
Buffalo Ridge areas where wrap around precipitation may continue.
Temperatures will hold in the 60 over the eastern half of the
forecast area, with some hope for a rebound into the lower 70s west
of the James river, depending on cloud cover.

Tonight: Broad low level warm advection commences overnight,  and
light winds should hold temperatures in the 50s.  Will need to watch
the extreme eastern CWA for the potential of fog on the edge of the
eastward advancing stratus deck.

.LONG TERM...(Tuesday through Sunday)
Issued at 319 AM CDT Mon May 21 2018

Tuesday: Shortwave ridging slides over the area on Tuesday. With
low level temperatures quickly rising, and a breezy southeasterly
wind developing, we should see highs rise into the 80s. The stage
is then set for the development of late evening and continued
nocturnal convection. Guidance in good agreement showing strong
theta-e advection pushing northeast in the late afternoon/evening,
along with a very subtle shortwave. Moisture should pool along a
retreating elevated boundary bisecting the CWA by mid-evening, and
once LLJ begins to increase we should see convection begin to
blossom.

Soundings indicate MUCAPE values approaching 1000-1500 J/KG
advecting northward along with marginal mid-level lapse rates by
03Z.  Effective shear is also a bit marginal, but there certainly
will be the risk for severe hail into Wednesday morning.   The other
risk will be with locally heavy rainfall. Model PWAT values pushing
1.50" advect into the area Tuesday evening, well over the 90th
percentile for the 2nd half of May.  Rather slow storm movement
along the elevated boundary along with a slow moving shortwave
should add another ingredient to the potential of heavy rainfall.
Through coordination with neighboring offices, have bumped up
PoPs as well as introduced heavy rainfall wording for areas east
of I-29 into daybreak Wednesday.

Wednesday: A much more uncertain day on Wednesday.  The LLJ only
slowly backs away from the morning convection, which should then
focus well north. We should then recover into the 80s once again
in the afternoon. The biggest uncertainties fall on the mesoscale
side of the forecast later on in the day. Overall there may be
little focus for convection at the surface, but should a boundary
from morning convection linger then such focus will develop.
Temperatures aloft really don`t warm that significantly, to which
we`d be capped off from developing convection. So all it may take
is a subtle wave in the SW flow to develop afternoon and evening
activity in a still relatively weak sheared environment. Elevated
convection may continue through the night, despite the seasonally
weak low level jet.

Thursday: The upper troughing responsible for the early week rain
chances will finally begin to pivot eastward along the Canadian
border. Thursday will likely be a very warm and humid day.  The one
stark forecast element we`ll notice change this week are dew points.
 By Wednesday and Thursday mid to even upper 60 dewpoints will be in
the region.   We`ll begin to monitor a frontal boundary that will
slowly move southeast late on Thursday and Thursday night.  A
prefrontal trough ahead of the primary front may serve as a focal
point again for convection into the afternoon and through the
overnight hours. As we`ve seen in previous days, the overall
effective shear remains weak (AOA 20 knots), but instability and
other parameters are more than sufficient to justify a severe
weather concern.

Friday-Sunday: The upper trough drops into and through the Great
Lakes heading into next weekend.  This should push the more "humid"
air southward along with rain chances, but still keep temperatures
above normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1217 PM CDT Mon May 21 2018

A clearing line, currently extending from just west of KHON to
near KYKN South Dakota is pushing slowly eastward and northward at
this time. However, cumulus will likely develop on the western
edge of the cloud bank through the afternoon. Confident that KHON,
KFSD and KSUX will move solidly into the MVFR category, but its
possible that by mid and late afternoon, that the cumulus could
rise into the lower end VFR category and this scenario is
currently stated in the 18Z TAFs. Otherwise tonight should be VFR,
with a cloud deck near 800mb, or between 4,000 and 5,000 feet AGL
forming late tonight and affecting KSUX and possibly KFSD
associated with a warm front above the surface. Its possible that
KSUX may see an isolated TSRA threat from about 10Z to 15Z
Tuesday, but chances of this are too skittish to put in the KSUX
TAF site at this time.

&&

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

$$

SHORT TERM...Dux
LONG TERM...Dux
AVIATION...MJ


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