Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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275
FXUS63 KFSD 241146
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
646 AM CDT Mon Jul 24 2017

.SHORT TERM...(Today and Tonight)
Issued at 437 AM CDT Mon Jul 24 2017

Comfortably cool night with surface high pressure nosing from
Minnesota into northeastern Nebraska.  Only concern out of the
starting block is the potential for some patchy fog. Had a few hints
on KSPW report, but webcams clear thus far.  Day of fairly decent
mixing could have resulted in too shallow a moist layer, but given
the small temperature-dewpoint differences and nearly calm winds,
could still see development of some mainly low-lying shallow type
fog through early morning, but have trimmed some areas west of the
ridge axis back toward I-29 where starting to see a little more
easterly gradient.

Interesting feature of the day is well-defined wave/MCV pushing into
northwest SD carrying with it a persistent area of convection.
Starting to trend downward in overall updraft coverage based on
decrease in lightning and warming cloud tops. Support in
southeasterly upslope flow, but airmass is progressively less
favorable for maintenance heading eastward, and through the day will
see mid level heights in a slow and steady climb mode, which should
begin to shunt the system a bit more northerly track as it shears
into the ridge axis. Cirrus shield will push out ahead of complex
early this morning, but dynamics suggest a slow demise to convection
will likely make it progressively more ragged. Trend in hi-res
models has been to pick up the pace a bit with diminishing
precipitation, now bringing into areas near KHON by around 18z. This
times better with progression of the upper level impulse than does
lower-resolution solutions which hold precipitation back quite a bit
through the morning, and take until late day to reach that same
area. Will carry only a small chance starting early afternoon near
KHON, as will have progression of some steeper mid-level lapse rates
into the region from the west along with wave. Still that small
chance toward evening that heating out will break capping and lead
to a couple of isolated storms toward south central SD. Motion on
these storms will be fairly sluggish toward the east and southeast,
so any threat would likely be fairly limited in area. If anything
approaching severe storms would occur, would only be a wind threat
with decent DCAPE the most notable convective feature.

In terms of temperatures, only area that is likely to get quite warm
today would be toward south central SD where readings should reach
well into the 90s. Some questions remain further north and east
toward the mid James valley of impact of early day clouds and
potential for some precipitation early to mid afternoon.  Clouds
sure to have some impact on temperature trends, and have slowed
warming some across the northwest for the morning hours. Southerly
winds will be on the increase through the day, with some gusts
approaching 30 mph this afternoon west of the James River. Fire
weather concerns are a bit higher, but with minimum humidity
generally 20-30 percent, will feature up to a high fire danger.

Southerly winds will keep things quite mild tonight, becoming gusty
along the ridge late.  There could be a couple of isolated storms
across southwest Minnesota later tonight as low level jet focuses
further northeast. Lows upper 60s to mid 70s likely.

.LONG TERM...(Tuesday through Sunday)
Issued at 437 AM CDT Mon Jul 24 2017

Tuesday evening into Wednesday afternoon still poised as the
potentially busiest over the next several days. As warm advection in
the mid levels moves through on Tuesday morning an isolated
thunderstorm will be possible in southwest Minnesota. After that the
remainder of the morning and afternoon should be dry for most
locations. The exception will be from about De Smet to Huron to
Chamberlain where strong surface heating could allow a few
thunderstorms to develop after 4 pm. If any storms do develop in the
area CAPE values around 2000 J/kg and marginal deep layer shear
values around 20 to 25 knots. This might hint at a few initial
supercells through about 0z to 1z, then all models suggest that the
LLJ will increase as the mid level winds remain fairly weak. This
might hint that any convection that is ongoing along and north of
Highway 14 will begin to settle south towards the stronger and
increasing LLJ convergence. Freezing levels look to be about 13000
to 14000 feet so as the convection settles south the hail threat
will likely diminish while the heavy rain threat increases. CAPE
profiles in the evening and overnight hours not overly impressive
either, but strong dry air below about 850mb would likely support a
wind threat.

So, from about 5 pm through 8 pm central SD towards De Smet along
highway 14 will see a threat for hail up to golf ball size as well
as wind gusts to 70 mph. From about 8pm to midnight the threat
should transition to heavy rain and wind with a skinny CAPE profile,
high freezing level and an increasing LLJ helping drive convective
propagation southward. Not looking like a perfect set up for
training but some hints that storm motion during this time could be
a bit southwest.

Wednesday should see a continued chance for showers and
thunderstorms with a bit of a decreased threat for severe weather.
Wednesday will depend upon how far south the more unstable air is
suppressed by the overnight MCS. Will see the better chance for
severe thunderstorms in northwest IA, with that threat quickly
diminishing during the evening hours.

Thursday through Sunday looks seasonal and mostly dry. A weak wave
will bring a small threat for showers and thunderstorms Friday night
to the Missouri River corridor.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 646 AM CDT Mon Jul 24 2017

Some patches of fog will continue through 14z mainly through the
lower Missouri River valley and into northwest Iowa, some with
MVFR visibility, but mainly of a shallow nature. VFR conditions
will be featured through the remainder of the period. A healthy
low-level jet will develop tonight, creating low-level wind shear
conditions starting around 04z at KHON, 07z at KFSD and 10z at
KSUX. Probability of high-based convection is non-zero around
KHON by early afternoon, and again this evening, but far below a
chance that need mention in TAFS, and likely would remain as VFR
as well.

&&

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

$$

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



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