Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182115
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
415 PM CDT MON JUL 18 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 414 PM CDT Mon Jul 18 2016

A quiet late afternoon and evening ahead, but thunderstorms look
fairly likely after about 4z to 5z. The 19z surface analysis and
satellite show a pretty good indication of where the deeper moisture
and better instability reside, which was from about Storm Lake Iowa
west along the Missouri River Valley into south central SD. Water
vapor imagery showing a wave rounding the ridge into southern
Wyoming and northern Colorado, which will be the likely culprit in
getting low to mid level winds to veer overnight. These veering
winds will aid in advecting the more unstable air to the south
northward. Will initially fight a dry mid level cap but somewhere
close to 850mb begins to saturate shortly before 6z tonight.
Instability likely to climb up to about 2500 j/kg but shear may be a
bit lacking as the stronger winds lag to the south. But decent
directional shear will still support isolated severe storms with
hail to ping pong balls possible. These storms will gradually shift
east in the morning, but could linger through the afternoon over
parts of southwest Minnesota and northwest Iowa. As this wave passes
winds will become more southerly which will bring the heat and
especially the humidity. Looking at highs in the mid 80s in
northwest Iowa to near 100 in central SD.

As for the excessive heat watch will leave that in place for areas
west of Interstate 29. The main reason for not going with a warning
or advisory is that any time heat advisory criteria are met for 4
straight days a warning is usually warranted. There is still a
chance that for central SD this could happen. If confidence is high
enough overnight with the new model information a decision will be
made.

.LONG TERM...(Tuesday night through Monday)
Issued at 414 PM CDT Mon Jul 18 2016

Lingering gradient in elevated mixed layer along with last remaining
lift forcing with mid-level wave should interact with residual
boundaries for at least a low threat for convection mainly during
the evening hours east of I-29. Warming temps aloft overnight should
effectively close down the convective threat with heights in broad
ridge across the northern plains holding strong through Thursday.
After perhaps a little disruption to the near surface moisture in
wake of Tuesday daytime convection, surface dewpoints will
aggressively rise with the input from climatologically high
evapotranspiration into the upper 60s to mid 70s across much of the
area through Friday. Exception will be afternoons in some areas west
of the James river where mixing could knock afternoon values into
the upper 50s to lower 60s, but in that case the temps would be much
warmer than areas to the east for more or less a push to expected
heat indices from 100 to 110. With the likelihood of either a
Tuesday to Friday or Wednesday to Friday period of dangerous warmth
across the CWA, and very limited improvement on overnight readings
in the mid 70s to around 80, have maintained the Excessive Heat
Watch at this time.  A few solutions suggest a potential for a
backdoor front to leak southward by Friday which would lower temps,
but perhaps also keep moisture levels a bit higher.  GFS repeatedly
attempts to push surface temps to 112-117 degrees for south central
South Dakota toward the lower James valley on Wednesday and
Thursday, and while this seems a bit aggressive, some readings near
110 would not be impossible.

Wave on Friday night and early Saturday looks to break down warmest
temps aloft and bring along attempt for storms from northern areas
to push east/southeast through mainly areas near and north of I-90.
As boundary pushes southward through the weekend, will see highs
relax into the 80s to around 90, and humidity levels decrease.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 102 PM CDT Mon Jul 18 2016

Showers and thunderstorms will become more likely after about 7z,
especially south of Interstate 90. The best chance will be near
KSUX. Outside of the thunderstorms VFR conditions are expected.
There will be a small chance for MVFR ceilings between about 10z
and 14z, but the chance is too small at this time.

&&

.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Watch from Tuesday afternoon through Friday
     evening for SDZ038-050-052-053-057>060-063>065-068-069.

     Excessive Heat Watch from Wednesday morning through Friday
     evening for SDZ039-040-054>056-061-062-066-067-070-071.

MN...Excessive Heat Watch from Wednesday morning through Friday
     evening for MNZ071-072-080-081-089-090-097-098.

IA...Excessive Heat Watch from Wednesday morning through Friday
     evening for IAZ001>003-012>014-020>022-031-032.

NE...Excessive Heat Watch from Wednesday morning through Friday
     evening for NEZ013-014.

&&

$$

SHORT TERM...08
LONG TERM...Chapman
AVIATION...08


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