Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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004
FXUS63 KFSD 190354
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1054 PM CDT MON JUL 18 2016

.UPDATE...
Issued at 900 PM CDT Mon Jul 18 2016

For better or worse, decided to eliminate our pops in the Missouri
River valley in the near term. Satellite imagery does not look
very convective looking. In addition, both the operational and
experimental HRRR appear to be having a decent handle on the
western plains convection at this time. What both these models do,
is weaken the convection as it approaches our far western zones in
the overnight hours. This is plausible as the effective wind shear
is quite weak, not tapping the strongly veering profile that we
have in the lowest 1.5 to 2.0 kilometers. However as the short
wave moves eastward late tonight and encounters ample moisture
pooled aloft, the HRRR models then develop some fairly widespread
convection in the scattered to likely categories similar to what
we already had in there. This would be for late tonight, and
primarily for areas along and east of the James River Valley,
lingering into Tuesday morning east of I 29. If the ARW and NMM
happen to pan out closer, then the convective chances east of the
James River valley will be even slower, mainly for a while Tuesday
morning. But for now, hedging closer to the two HRRR outputs.
Severe weather is not a very big threat at all late tonight or
Tuesday morning again due to weak wind shear in the effective
layer.

&&

.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 06Z TAFS through 06Z Tuesday night)
Issued at 1046 PM CDT Mon Jul 18 2016

06Z TAFs will reflect convection as the main concern, primarily
in the 10Z to 14Z time frame early Tuesday. Leaned heavily on the
operational and experimental HRRR outputs as those models have a
fairly decent handle on the current western plains convection. Not
buying the aggressive convective NAM solution during the day on
Tuesday at this time. NAM dew point temperatures are projected to
be close to 80 degrees in some locations across the forecast area
on Tuesday, and that has allowed it to go very convective. That
said, morning convection is certainly a threat at least for the
Sioux Falls and Sioux City TAF sites, and lingering eastward
deeper into Tuesday morning.

&&

.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.

&&

$$

UPDATE...MJ
SHORT TERM...08
LONG TERM...Chapman
AVIATION...MJ



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