Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 211516
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1016 AM CDT Fri Jul 21 2017

.UPDATE...
Issued at 1012 AM CDT Fri Jul 21 2017

Went ahead and shaved off the eastern edge of counties from the
heat advisory. This includes Kingsbury county South Dakota,
southeastward through Minnehaha county and into Clay county Iowa.
These areas will struggle with cloud debris through the morning
hours limiting heating potential. In fact at 15Z, the temperatures
are still 72 to 74 degrees in these locations, so we have already
lost 3 or 4 hours of heating. However it will still be warm and
humid with heat index values in the 90s, as high dew point values
will be in no short supply, but it should stay less than 100
degrees. Elsewhere left the advisory as is as they will clear out
sooner.

&&

.SHORT TERM...(Today and Tonight)
Issued at 455 AM CDT Fri Jul 21 2017

Convection continues to be an important but fairly difficult
parameter to forecast both spatially and temporally in the shortest
term, with much of the hi-res guidance shaky at best on the details,
and coming up somewhat short on the general trends. Secondary
concern with heat and heat headlines today, as yet another day of
high moisture content will drive the heat index into dangerous
territory today.

Somewhat correctly advertised was the development of warm advective
wing of elevated convection through the Missouri River corridor with
expansion northeast through the early morning. Development ahead of
the main area in immediate vicinity of the Missouri River has been
quite a bit quicker, and have tried to account for some lower
coverage PoPs in advance of main band. One element missing in most
solutions is the tail of convection back into central SD, with very
strong cell on the southern end of the line. Seems to be favoring
925-850hPa thermodynamic gradient at present and is associated with
a leading weak wave pressing across SD in advance of main wave back
across MT.  Tendency for this cell has been an effective east-
southeast movement toward the lower Brule vicinity, which times into
the area between 6 and 7 am at current pace.  Effective wind shear
is marginal for organization, and may be a bit better than modeled
given the current structure of storm. Large hail with a thicker
elevated CAPE profile along with brief heavy downpours would be the
main threat with this storm. A smaller threat for isolated severe
early this morning with the remainder of the warm advection wing,
mainly in the hail realm.  While most solutions take the warm
advection band and continue to lift northeast early this morning, a
rogue hi-res solution along with support of the NAM take a band of
convection west to east over the area through the entire morning,
seemingly with axis of weak PV anomaly At this point, have expanded
PoPs into the west early this morning, and only have a progressively
lower chance for convection through the morning as area works
eastward. Any longer existence of convection/convective debris will
have an impact on eventual temperature potential today.  With
passage of wave and trend toward breaking into sunshine, dewpoints
will favor lower 70s most areas this afternoon, with perhaps a bit
better mixing into the 60s toward south central SD as winds begin to
shift toward west/northwest late day.  Expanded the heat advisory
into the KSPW area, and while do see a couple hours of the heat
index to around 100 farther north, there are more questions as to
how much cloud/precip impact could occur toward southwest MN.

By late day, the heating out through the Missouri River corridor
will bring some readings near 100, and heat index from 100 to 110
with a potential mixed-layer instability reaching 2000-2500 J/kg.
The best threat for severe storms would likely occur in two main
areas by late afternoon and early evening. The first is likely the
most significant, on the southern edge of the MCS expected to impact
much of MN during the evening hours with stronger deep-layer shear,
with main threat as damaging winds.  While the greatest
southeastward moving complex should remain northeast of the area,
would have to watch degree of the 0-1km shear and potential for
enhanced circulations along any QLCS near KMML to KMWM line during
the evening. A secondary area of convection could develop along the
advancing frontal/pre-frontal boundaries into far northeastern
Nebraska, far southeast SD and into the KSUX area of northwest Iowa
by very late afternoon and early evening as heating likely erodes
capping near the boundary. The greater instability present here
produces a more hail friendly profile, a bit more DCAPE, but along
with this a lesser degree of deep-layer shear.  Could even see a
similar development back toward the James valley if wind shift
lollygags a bit more, with main cell motion dragging more to the
southeast as wave flattens flow aloft. Severe threat dwindles by mid
to late evening, with most convection over and done shortly after
midnight other than some lingering activity near mid-level
wave across northwest Iowa.  While wind flow will turn northwest,
any more significant drop in dewpoint is likely to hold off until
Saturday for most.

.LONG TERM...(Saturday through Thursday)
Issued at 455 AM CDT Fri Jul 21 2017

Saturday will be the lead in to cooler weather Sunday and Monday,
but right now still looking pretty hot. A somewhat mixy push of
drier air will work into the area which will bring a significant
drop in dew points and humidity, but with solid mixing highs will
still climb into the 90s in many locations. While the gradient is
not strong, the deep mixing will bring the potential for gusts of 20
to 25 mph in the afternoon, especially west of Interstate 29. This
is also the area that should be hottest and driest. In fact with the
ongoing dry conditions in central SD there may be a fire weather
concern with the winds and relative humidity values less than 25
percent.

By Saturday night into Sunday the upper level jet will dip south and
allow the cooler air to settle south. Will continue highs Sunday and
Monday of about 80 to 90 with the less humid conditions. Lows will
also be more pleasant from about 55 to 65.

Tuesday into Wednesday a marginally agreed upon upper level jet max
is expected to move along the Canadian border with upper level
ridging building behind. This should bring seasonally warm
temperatures to the area with a few chances for showers and
thunderstorms. Confidence very low on timing and location however at
this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1027 PM CDT Thu Jul 20 2017

Starting to see a few mid-level clouds form as the midnight hour
approaches. This could be the sign of increasing risks of
scattered thunderstorms after 4am for the terminals.

If this activity does develop, could see a few hours with showers
and thunderstorms in the vicinity, then improvement after mid
morning.

VFR conditions will persist for most of the daytime hours. Low
confidence on forecast details for any renewed convection in the
afternoon and evening, so will leave out of current forecast.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory until 8 PM CDT this evening for SDZ038-050-052>054-
     057>061-063>071.

MN...None.
IA...Heat Advisory until 8 PM CDT this evening for IAZ012-020>022-031-
     032.

NE...Heat Advisory until 8 PM CDT this evening for NEZ013-014.

&&

$$

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


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