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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Wednesday
• 
• High: 81 °
• Low: 46 °
• T-Storms

• Thursday
• 
• High: 68 °
• Low: 50 °
• T-Storms

• Friday
• 
• High: 68 °
• Low: 57 °
• Chance of T-Storms

• Saturday
• 
• High: 81 °
• Low: 64 °
• Chance of T-Storms

• Sunday
• 
• High: 79 °
• Low: 59 °
• Chance of T-Storms

Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
649 am CDT Tuesday may 22 2012 


Discussion.../issued at 520 am CDT/ 
busy weather day across the area today into tonight...with a cold 
front and a few upper level waves expected to traverse the region. 
Currently seeing an area of showers and thunderstorms between 
Mitchell and Sioux Falls associated with the cold front and an 
initial weak wave. Expect these storms to continue to slowly push 
off to the east through the morning hours. Elevated instability is 
rather weak...so thinking is that these storms will not be 
severe...with just some small hail and briefly heavy rain. This 
initial thunderstorm activity should diminish as we head into late 
morning and the upper level support dissipates. High temperatures 
today in the 70s behind the front...and well into the 80s out ahead 
of it in northwest Iowa and portions of southwest Minnesota. 


The 2nd upper level wave is expected to move in from the west 
between 21z and 0z this evening. This will likely set off another 
round of showers and thunderstorms. While the 3rd wave moves in 
around 6z enhancing the thunderstorm activity even more. As all this 
is going on...a cold front will be pushing southeast across the County Warning Area 
bringing in drier air into the low levels. So areas too far west of 
the front...generally along and west of the James River...will see 
the least amount of rain through tonight. While areas further east 
get a substantial amount. 


Severe weather threat today will be focused out ahead of the surface 
cold front where instability will be maximized. Will initially be 
capped in this warm sector...but falling heights associated with the 
2nd wave...combined with increasing convergence along the 
front..should eventually break this cap...probably after 21z. Model 
consensus puts the front just east of Sioux Falls at this 
time...thus severe threat should be focused across northwest Iowa 
and portions of southwest Minnesota. Instability expected to be 
around 2000 j/kg...with deep layer shear of 35 to 45 kts. Thus 
thinking is that may initially see supercells form just ahead of the 
front with large hail possible. However storm motion is expected to 
parallel the front. Thus any storms that form should end up merging 
and form more into a line. So the severe threat should quickly 
transition to strong winds across northwest Iowa. Low level shear is 
strong enough that an isolated tornado will initially be possible as 
well. Although somewhat limited moisture resulting in higher local 
heights and the expected transition to a linear state should limit 
the tornado threat somewhat. The severe threat should end once the 
surface front exits the County Warning Area by 3z. 


The other concern later today into tonight is flash flooding. Have 
issued a Flash Flood Watch for the most likely areas to see heavy 
rain. The initial convection ahead of the front will produce one 
batch of heavy rain. Then as the 3rd wave approaches by 
midnight...the thinking is that heavy showers and thunderstorms 
continue on the cool side of the surface front....to around the 
850mb front. Storm motion parallel to the middle level front will allow 
training of storms through the night into early Thursday. And with 
precipitable waters  around or just over 1.5 inches...about 2.5 Standard deviations 
above normal...these storms will likely be putting down heavy rain. 
Storm total rain of 1 to 3 inches is likely in the watch...with 
isolated higher amounts possible. Areas just west of the 
watch...including Sioux Falls...should see up to around an 
inch...with areas further west seeing considerably less. 


On Thursday...should see the heavy rains in the east come to an end. 
However the the upper level low will continue to spin in southern 
Canada...and yet another lobe of energy will swing through on 
Thursday afternoon. Moisture will be limited...but with the cold 
temperatures aloft...should see scattered showers and thunderstorms 
form during the afternoon. Given the limited instability severe 
weather is not anticipated with this convection. Highs should 
generally be in the upper 60s and lower 70s. 


The front will lift back north as a warm front Friday into Saturday 
bringing more active weather to the area. Looks like the frontal 
passage is sometime Saturday...thus Friday looks like another cooler 
day with highs in the 60s and lower 70s. As the front approaches 
late Friday night ito Saturday elevated instability increases over 
the area. Expect elevated thunderstorms to form on the cool side of 
the front with low level thetae advection. Will have to watch for 
possible severe weather...mainly large hail...with any convection 
Friday night and early Saturday. 


As the front lifts north Saturday...much of the County Warning Area should enter the 
warm sector. Thus Saturday will be warm..especially south of Highway 
34...where 80s or even possibly low 90s are expected. This warm 
sector will be very unstable with impressive deep and low level 
shear. However it looks like a very strong cap will also be in 
place...thus not expecting any storms in the warm sector Saturday. 


By Sunday...the approach of an upper level wave will result in 
falling heights and the approach of a surface cold front. These 
factors should help break the cap Sunday...with more severe weather 
possible. 


We finally should get a more calm day Monday behind this 
front...with highs in the 70s. However by Tuesday yet another wave 
brings the chance of more showers and storms. /Chenard 




&& 


Aviation.../for the 12z taf issuance/ 
a cold front will be moving from northwest to southeast across our 
forecast area today...and should be on the far southeast edge of our 
area near Storm Lake Iowa by early evening tonight. So the main 
issue is thunderstorms...where they will develop and when...and 
exactly how strong they will be. In the large scale...numerous 
thunderstorms will likely develop in the late afternoon and early 
evening hours along and behind the frontal boundary. The best 
chances for storms will be along and east of a Yankton South Dakota...to Sioux 
Falls to Marshall Minnesota line. One short wave is overriding the surface 
front late this afternoon and early evening...with a second short 
wave well behind the frontal boundary overnight tonight. In the late 
afternoon and early evening...some storms may be severe with heavy 
rainfall in our eastern zones. Then overnight and late tonight with 
the passage of the second short wave...more showers and thundertorms 
are possible in southwest Minnesota...northwest Iowa...and extreme southeast 
South Dakota with heavy rain again a concern...but with much less of a severe 
threat. The problem is exact timing of storm activity with the devil 
in the details. There is very low confidence in the timing of the 
storm activity in the kfsd and ksux taf sites...and low confidence 
in how low the visibility and ceilings may go. In heavy rainfall... 
certainly IFR and possibly even brief LIFR conditions are 
possible...again mainly along and east of I 29. Outside of 
thunderstorms...VFR to MVFR will likely prevail. For areas of west 
of I 29...conditions become increasingly more stable and dry and it 
is anticipated that the khon taf site will remain VFR today and 
tonight. /Mjf 


&& 


Fsd watches/warnings/advisories... 
Iowa...Flash Flood Watch from this evening through Thursday afternoon 
for iaz001>003-012>014-020>022-031-032. 


Minnesota...Flash Flood Watch from this evening through Thursday afternoon 
for mnz080-081-089-090. 


NE...Flash Flood Watch from this evening through Thursday afternoon 
for nez013-014. 


South Dakota...Flash Flood Watch from this evening through Thursday afternoon 
for sdz071. 




&& 


$$