Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 230348
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD...Correction
1047 PM CDT MON AUG 22 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 409 PM CDT Mon Aug 22 2016

Tonight through Tuesday morning will remain quiet, with primary
focus on low level winds and the associated increase in moisture
that will accompany the low level jet starting late tonight. Low
level jet of 40-50kt will again develop by late evening, holding
temperatures in the mid 60s to around 70. Afternoon gusts will ease
a bit through the evening in most of the forecast area, though the
areas along/northeast of the Buffalo Ridge in southwest Minnesota
will again see some higher winds throughout the night.

Primary concern for the short term will be on increasing convective
threat Tuesday afternoon as strong wave lifts northeast through
Nebraska into northwest Iowa. Expect this wave to overcome weak
capping inversion across the Missouri River Valley during the mid to
late afternoon. Instability increasing to around 2000 J/kg and
modest low level wind flow should be sufficient to support severe
storms. However, weak mid-upper level winds resulting deep layer
shear only around 20kt, so expect primary severe threat with the
initial development to focus on damaging wind and large hail.

With continued deep southerly flow ahead of the wave, will see
dramatic increase in moisture through the day, with good model
agreement showing increase in precipitable water values from just
over one inch at daybreak Tuesday, to 1.75+ inches by late Tuesday
afternoon in parts of northwest Iowa, which is 50-75% above normal
for late August. With the weak upper level flow as mentioned above,
storm motion should be on the slower side, so will also have to
watch for flash flooding potential.

.LONG TERM...(Tuesday night through Monday)
Issued at 409 PM CDT Mon Aug 22 2016

The intermediate and longer term range of the forecast is becoming
fairly active with three disturbances to be concerned about. The
first disturbance and most vigorous one occurs late Tuesday and
Tuesday night. A leading short wave moves northeastward centered
across northeast Neb and northwest Iowa per mid and upper QG forcing
vectors late Tuesday and Tuesday night. Pushing this wave is a
strong upper low moving eastward throughout the northwestern high
plains and southern Canada. Strong low level jet still is progged to
exist of 25 to 35 knots feeding into the leading short wave along
with a veering wind profile. But in terms of limiting severe
weather, there are a couple of factors going on. First, the mid
level winds are still fairly weak, with only about 10 to 15 knots
noted at 500mb. Second, profiles still show a subtle but stubborn
inversion between 850mb and 700mb, and another one between 600mb and
500mb. This is limiting CAPE, for instance with 0-1km ML CAPE
averaging close to 1000 J/KG. Therefore believe the strongest
potential for severe weather will be confined mainly to our zones in
extreme northeast Nebraska and northwest Iowa where these zones are
coincident between the low level jet and strongest portion of the
short wave. Because in areas further away from the short wave, it
will become increasingly difficult for storms to tap into the
boundary layer where low level effective wind shear is very strong.
Elsewhere and interestingly, there could be a mainly non-severe
convective line developing just east of the James River valley in
the early evening hours along the wind shift just off the surface.
So there could be two organized areas of convection going on, one
along the short wave mentioned above, and the other in our western
zones along the wind shift. Otherwise, all the activity will rapidly
wane after evening as we lose daytime heating and the short wave,
with lows projected to be in the upper 50s to mid 60s.

One last thought about Tuesday night is the potential for areas of
heavy rain in northwest Iowa Tuesday evening. With the amount of
thetae convergence and moisture plume available in the lower levels,
there could certainly be some heavy rain in northwest Iowa. The good
news is that much of northwest Iowa has not received a ton of rain
lately and in fact some areas could take quite a bit of rainfall
without flooding, minus the usual urban flooding in poor drainage
areas.

Once the cold front moves through Tuesday night, temperatures will
be decidedly cooler Wednesday through Saturday. Later in the
forecast period, there is consensus in showing the next upper
disturbance moving through this area in the Friday night through
Saturday night time frames, possibly in two pieces. These are
synoptically more broad and cooler systems, so not too concerned
about severe weather with these systems.  This will potentially be
followed by yet another wave moving through at the end of the
forecast period where the deterministic ECMWF and GFS are showing
waves in the Sunday night and Monday time frames, but certainly a
lot of discrepancy in the strength of the wave between the two
solutions.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1037 PM CDT Mon Aug 22 2016

southerly winds will remain in the 10 to 15 kt range overnight. A
low level jet will result in low level wind shear at TAF sites
through around 12z. thunderstorms chances will increase after 21z on
Tuesday and linger into Tuesday evening. These storms will mainly
affect the KFSD and KSUX TAF sites.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...JH
LONG TERM...MJ
AVIATION...JM


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