Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201809

Area Forecast Discussion
National Weather Service Sioux Falls SD
109 PM CDT Sun May 20 2018

.SHORT TERM...(Today and Tonight)
Issued at 316 AM CDT Sun May 20 2018

Strung out vorticity maxima continues to pivot across the Tri-State
area, producing very light rainfall across the area.  The persistent
clouds and very light rain will be an issue through Sunday and
Sunday night, but overall any rainfall amounts will be on the very
light side.

Today: Light showers or sprinkles may wane through the morning
hours, focusing themselves across portions of NW Iowa, SW Minnesota,
and adjacent areas of South Dakota and Nebraska.  Temperatures will
again be a struggle, with better chances of some limited sunshine
near Huron where temperatures could reach into the 60s.

Tonight: Short term guidance suggests that a secondary piece of
energy over north central Nebraska will pivot eastward by this
evening and slowly slide east overnight.  Models continuing to show
broad and weak isentropic lift through the night focused mostly
along or east of I-29. QPF will be very light, and will include a
low PoP into Monday morning.

.LONG TERM...(Monday through Saturday)
Issued at 316 AM CDT Sun May 20 2018

As we look out into the medium and extended portions of the
forecast, it`s clear to see that it`s going to be a rather unsettled
week ahead. Disorganized troughing over the Rockies along with a
trough over the northeastern US will allow shortwave ridging to
increase slightly over the Central US.

With no really strong cold front to push moisture southward this
weekend, return flow should quickly become setup by Monday and push
low level moisture into the Plains. At this point however, the
forecast details become a little less certain.

Most guidance is showing the potential for some nocturnal convection
already by Monday night across Nebraska on the nose of a LLJ
increasing over the Plains. Perhaps more confidence in the
development of convection locally arrives by late Tuesday and
Tuesday night, as there appears to be a more focused area of
strong theta-e advection aloft coincident with a very subtle
shortwave moving through the region. Mid-level lapse rates are
increasing, and this could push nocturnal MUCAPE values over 1000
J/KG by Tuesday evening/night. Once the LLJ increases, convective
development along a retreating 850mb boundary should take place.
Shear remains somewhat paltry, but could see an isolated severe
weather risk increase Tuesday night into Wednesday.

Upper troughing over the central and southern Rockies pivots
northward on Wednesday, moving along the Canadian border by
Thursday. This setup should allow for a very warm Wednesday and
Thursday, with a surface front jumping northward. Convection chances
Wednesday and Wednesday night are very uncertain and based more on
mesoscale features from morning convection that just can`t be
defined at this point.

With the aforementioned upper trough slowly moving east Thursday,
this will certainly increase our rain chances once again Thursday
afternoon through Thursday night. Once again, given the synoptic
pattern, strong to severe storms again appear possible.

The upper wave pivots once again and will begin to drop southeast
into the Western Great lakes by Friday and Saturday. A sweeping cool
front should move through late Friday, which should dry out the
boundary layer and cool temperatures slightly into next weekend.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1250 PM CDT Sun May 20 2018

MVFR conditions will continue to impact much of the region
through the TAF period. While VFR conditions will continue in south
central SD into the highway 14 corridor of east central SD and
southwest MN this afternoon, the stratus is expected to spread
northward late this evening and tonight. After 15z Monday morning,
stratus will begin to clear gradually in southeast SD, but the
KFSD and KSUX will likely see MVFR or IFR ceilings throughout the
TAF period. A few sprinkles or spotty light rain showers will
also be possible today and tonight, but probability is too low for
mention in TAF.




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