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Area Forecast Discussion (AFD)

Issued by Sioux Falls, SD (FSD)

FXUS63 KFSD 182326

Area Forecast Discussion
National Weather Service Sioux Falls SD
626 PM CDT Tue Sep 18 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 330 PM CDT Tue Sep 18 2018

Concerns in the short term remain focused on thunderstorm potential 
overnight into Wednesday, along with continued and growing risks of 
heavy rainfall and impacts from heavy rainfall Wednesday into 

This afternoon, a series of shortwaves continues to focus convection 
well south of the area. Stratus is plentiful across the area and 
will continue to be so into the evening. Where the sun has broken 
through in central South Dakota, temperatures have risen into the 

This evening:  The one thing to watch for this evening will be 
drizzle development with the onset of light isentropic lift. There 
is a corridor of low stratus already in place, and it may not take 
much to produce drizzle. 

Tonight: Models project the next shortwave to enter the Plains later 
this evening and in return, the LLJ is expected to back and 
intensify quickly after 00Z.  Strong theta-e advection and 
increasing mid-level lapse rates brought northward by another 
shortwave entering the Western Plains, should initiate convection 
along the 800 mb elevated front very late this evening. Model 
soundings suggest upwards of 1500 J/KG MUCAPE in the initiation 
zone, with effective speed shear approaching 45 knots.  This could 
support the occasional hailer in the initial development stages 
before transitioning to heavy rain. While not the greatest risk, 
elevated DCAPE does approach thresholds that could support some 
strong wind gusts, especially given neutral surface lapse rates. 

The greatest concerns overnight will be with heavy rainfall.  The 
combination of strong perpendicular advection overtop the 
elevated boundary, combined with slightly zonal flow producing 
west to east storm motions, along with near record PWAT values for
this time of the year, could increase the flood and flash flood 
risks into Wednesday morning. The challenging aspect of the 
forecast is pinpointing where convection will form and where the 
elevated boundary will setup later tonight. Through coordination, 
will hold off on a flash flood watch, but continue to monitor this
evening for the potential of a short fused watch if confidence 
increases. It's possible that some areas could see excessive 
rainfall by Wednesday morning. Leaning on the RAP/HRRR/HREF a bit 
heavier in the 12 hour forecast.

Wednesday: Much like today, a progressive shortwave will move 
eastward across the Dakotas early. This wave should spread the 
chance for rain eastward in the morning, along with reinforcing 
the cool stratus layer across the CWA. Have lowered temperatures a
bit in many areas, with the warmest air in the far southeastern 
CWA on the edge of the effective boundary. Morning convection and 
associated shortwave moving east may produce a bit of a lull in 
convection Wednesday afternoon. 

.LONG TERM...(Wednesday night through Tuesday)
Issued at 330 PM CDT Tue Sep 18 2018

Wednesday night into Thursday: While there remains a high degree 
of uncertainty on the mesoscale details by Wednesday night, models
continue to suggest a potent setup for heavy rainfall Wednesday 
evening and into Thursday morning. The large upper trough begins 
to eject into the Northern Plains, deepening return flow into the 
region by late Wednesday and producing an elongated west to east
elevated frontal zone in the process. While there will be a 
continuing risk for marginal severe weather along the elevated 
boundary, heavy rain and potential for both flash flood and 
flooding will be a growing concern. Latest projections indicate 
the potential for several inches of rainfall in the region, but 
again, the specific areas of impact are still undetermined due to 
the uncertainty in the mesoscale evolutions tonight. Still a 
period to keep a close eye on. 

The frontal boundary will move through the area during the day on
Thursday, with a sharp advection of cooler air and very windy 
conditions into Thursday night. Stratus will rotate around the 
upper system, and could produce lingering drizzle or sprinkles 
into Friday morning.

Friday-Saturday:  Cool Canadian high pressure moves into the region 
to start the weekend. Have lowered temperatures on Friday 
significantly, with highs in a few areas struggling to reach 60s.
Morning lows Saturday morning will be dependent on how quickly 
the surface high moves away and return flow develops, but could 
see lows approach 40 degrees. 

Sunday: Building mid-level heights cause a return to slightly warmer 
conditions and dry weather for Sunday. 

Monday-Tuesday:  Latest projections bring another system through the 
central US to start next week, with rain chances returning by 


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 618 PM CDT Tue Sep 18 2018

Another complex and eventful aviation forecast over the next 24
hours complete with both thunderstorms and ceiling/visibility
restrictions. To start, MVFR stratus currently runs in between 
KHON and KSUX but is impacting KFSD. Expect ceiling restrictions 
to expand and lower into the IFR category later tonight and may 
even be accompanied by periods of drizzle. Around/after 05z, 
thunderstorms should begin to develop across the region and have 
tried to pinpoint a more narrow window for each TAF site. With 
this said, actual timing and length may vary in reality. By mid 
morning Wednesday, activity may become more hit or miss thru the
afternoon but is expected to again expand by late in the TAF 
period with a very active Wednesday evening/overnight.