Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 160828
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
328 AM CDT Tue May 16 2017

.SHORT TERM...(Today and Tonight)
Issued at 328 AM CDT Tue May 16 2017

Focus today and tonight:

- Isolated severe threat this morning with some heavy rain possible,
especially in central SD.
- A higher threat for severe thunderstorms from about 23z through 6z.
- Heavy rain an increasing focus from about midnight tonight into
Wednesday.

A pair of MCS`s ongoing early this morning. One was a weakening area
of convection over western IA and western MN. Cloud tops were
warming and convection diminishing. However along the nose of the
LLJ over parts of east central SD and southwest MN scattered
thunderstorms were quickly lifting northeast. The other MCS was
closer to the upper level wave over northwest Neb and was heading
towards central SD. This area will need to be watched for a little
heavy rain as well as isolated severe storms through mid morning.

Through mid morning showers and thunderstorms will continue across
the area and will need to keep an eye out for southern parts of SD
towards east central SD. The wave and mid level LLJ that is likely
aiding development of MCS in northwest Neb will aim towards central
and parts of se SD this morning before lifting northeast. Not
expecting a lot of severe with this feature but it does have a good
environment going to help maintain convection through the morning
with the potential for some heavy rain and quite a bit of
lightning.

During the afternoon shower and thunderstorm activity should be at a
minimum with weak shortwave ridging ahead of the next piece of
energy. Not exactly a strong ridge so will see some potential for
thunderstorm development late in the afternoon south an east of a
line from about Yankton to Sioux Falls to Windom, which will be the
approximate location of the surface boundary. Capping looks to play
a role so will keep POPs fairly low until after about 2z to 3z. The
flow along and ahead of the front is also not very convergent which
might keep thunderstorm chances low until later.

By early evening a strong upper level trough will dig into the
western High Plains which will back low level flow and should set up
a very busy evening and overnight period. If the surface to near
surface flow can back soon enough an isolated tornado would be
possible over parts of far southeast SD and northwest IA before
about 3z. There is the potential for about 2500 J/kg surface based
CAPE with strong, balanced, unidirectional shear through about
150mb. After this the 925mb to 850mb layer will likely become the
main focus with some of the strongest instability and weakest
capping. The threat for severe weather will likely continue through
about 6z with the attention then turning towards heavy rain. The
backing of the low level winds will turn the advection of the deeper
moisture and instability towards south central SD which will likely
allow for some back building and heavy rainfall. Looks like a decent
potential for some locations to get a couple of inches of rain so a
flash flood watch may be needed for late tonight into Wednesday as
confidence on location and amounts increases.


.LONG TERM...(Wednesday through Monday)
Issued at 328 AM CDT Tue May 16 2017

... Due to ongoing weather, did not make any major changes to the
extended....

Increasing rain chances and a cooling trend through the
weekend are the main focus over the next following days.

Widespread showers and thunderstorms continue on Wednesday as the
lead negatively tilted trough lifts northeast, and finally makes its
entrance into the area. At this time, the severe weather risk is low
due to extended cloud cover and limited diurnal heating. A few
general thunderstorms east of I-29 cannot be ruled out, due to weak
instability persisting in the afternoon and evening hours. Soundings
continue to suggest a deep, and rich boundary layer moisture across
the area; therefore, widespread moderate to heavy rain will be the
main concern. Models continue to show high QPF values with storm
total ranging from 1 to 2 inches of rain by Thursday morning; higher
amounts possible.

Thursday looks cooler and breezy as cooler air begins to filter in
behind the exiting trough. An upper ridge across the Northern Plains
and the International Border will result in dry conditions for most
of the day. Highs will be from the mid 50s to lower 60s. Rain
chances return on Friday as another shortwave from the west Pacific
moves into the High Plains. Severe weather potential is limited.
Most of the instability will remain further south, thus only rain
chances are anticipated. The same pattern of periodic rain showers
and isolated thunder persists on Saturday.

Unseasonably temperatures are the main impact for the remainder of
the week with highs 15 to 20 degrees below normal. Highs will
generally be in 50s to mid 60s through this weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1254 AM CDT Tue May 16 2017

Area of showers and scattered thunderstorms will move across the
area through the early morning hours. Isolated strong wind gusts
in excess of 40kt will be possible as the showers approach.
Ceilings will largely remain VFR with the showers/storms, but very
brief visibility reductions to MVFR will be possible.

Additional showers/storms are possible late in this TAF period,
mainly along/east of I-29.


&&

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

$$

SHORT TERM...08
LONG TERM...05
AVIATION...JH



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