Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 261628
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1128 AM CDT Wed Jul 26 2017

.SHORT TERM...(Today and Tonight)
Issued at 359 AM CDT Wed Jul 26 2017

Current upper air analysis shows a large upper ridge of high
pressure extending across the southern United States, generally
along and south of the 40th parallel. A 500mb short wave is
currently moving eastward across our southern zones on the northern
periphery of this upper ridge. There is also a broad short wave at
700mb moving up from the southwest generally through Nebraska, and a
jet streak to our northeast rounding the base of an upper trough in
the western Great Lakes region. All of this is combining to create
an active morning of showers and embedded TSRA across much of our
southern zones, generally along and south of I 90. Mid level QG
forcing in the 700-500mb layer is slow to exit areas south of I 90
through the day, which basically follows the 700mb trough. In
addition, the jet streak to the northeast really does not move
either, and keeps much of our forecast area in the right rear quad
which enhances large scale ascent with the mid level trough.
Therefore despite the low level winds turning to the north all the
way through 2km south of I 90, there could be stubborn redevelopment
of rainfall through the afternoon along 850-700mb frontogenesis.
However any TSRA will not be severe, including the Sioux City area.
Highs today will be greatly moderated by cloud cover. In fact
stratus may develop at least across the southern half. Highs still
look to generally range in the upper 70s to lower 80s, perhaps near
85 around Chamberlain SD.

Tonight looks quiet as a light northerly wind continues with
decreasing clouds. Thought about fog in low lying areas. One can
never totally rule out shallow low lying fog on a night like tonight
with chilly air in clearing skies, especially after some rainfall
today. However am banking on enough mixing with the northerly flow
tonight to keep the fog from becoming too widespread. Lows tonight
will be a couple of degrees either side of 60 degrees.

.LONG TERM...(Thursday through Tuesday)
Issued at 359 AM CDT Wed Jul 26 2017

Story of the mid range period of the forecast will be generally
quiet conditions, with temperatures largely a shade below normal.
Should be a little flat cumulus development each diurnal cycle
mainly near/east of I-29, and perhaps even a little patchy shallow
fog Thursday night.  A fairly decent wave will dig through in the
northwest flow aloft on Friday, but airmass is so dismally dry below
the mid- to upper-level moist layer and lapse rates are far from
unstable, so for now will settle on just a bit thicker cloud cover
and hold off on mention of spotty light precipitation.  Will have to
begin to watch convection forming in upslope return flow over the
western high plains later in the day Friday to wander into areas
west of the James River as a weak wave pushing out into the Dakotas.
Overall, day to day the off surface temps will vary only slightly
through Saturday, with an abundance of upper 70s to mid 80s expected
for highs, and use of BC grids to slightly increase the temp fall at
night near surface ridge lingering over Minnesota and Iowa, likely
to find some mid to upper 50s.

Further out in the long range, will continue to see the mean
longwave ridge retrograde into the intermountain west, and begin to
amplify again. This will keep the region in a northwest flow aloft,
which is fairly poor in predictability of individual wave timing.
Fortunately, the overall pattern seems to have a high degree of
agreement. Thus, for the most part, the low-level synoptic pattern
will be slow to change, as day by day the surface ridge will relax
eastward and allow a gradual increase in the low-level return flow
as well as a slow warming to off-surface temperatures. Readings will
nudge back a bit toward normal through the period.  With low
confidence on where and when any of the minor convective threats
will occur, have not quibbled on the periodic slight chance PoPs.
Certainly does not appear to be any solid threat for rainfall
through Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1127 AM CDT Wed Jul 26 2017

MVFR ceilings will linger around Sioux City through the afternoon
hours, with widely scattered showers trying to redevelop between
FSD and SUX.

By this evening, clouds should begin to clear from north to south,
leaving VFR conditions through Thursday.

&&

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

$$

SHORT TERM...MJ
LONG TERM...Chapman
AVIATION...Dux


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