Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 240327

Area Forecast Discussion
National Weather Service Sioux Falls SD
1027 PM CDT Sun Jul 23 2017

.SHORT TERM...(This evening through Monday)
Issued at 308 PM CDT Sun Jul 23 2017

High pressure at the surface will move off to the east tonight,
shifting the forecast area`s winds around to an east to southeast
direction. Used bias corrected guidance values for tonight`s lows
which captured the warmer readings in the 60s in our eastern zones
due to the return flow. Conversely, cooler readings in the 50s are
warranted along and east of the I 29 corridor with lighter winds. It
still looks like patchy or areas of fog will develop across the
eastern half late tonight and very early Monday due to a light flow
of air and small surface dew point depressions. If the ground were
wet over more more locations, the fog potential would likely be
worse. But as it stands now, it looks to mainly affect low lying
areas and locations favored by cool air drainage and it should be
fairly shallow.

Monday will be a warmer day, and fairly breezy along and west of the
James River valley as a southeast flow of air increases. Our zones
in south central SD easily mix to about 97 degrees. Manually raised
guidance temperatures east of I 29 into the lower 80s as 850mb
temperatures do not justify mid to upper 70s at this time. And it
appears after perusing over the soundings that not a lot of cumulus
will develop in our east Monday afternoon to hinder a rise in
temperatures. Put in a slight chance of TSRA late Monday in our far
western zones due to a short wave passing across this area which is
currently in northern WY. Quite a nice vort max is showing up on the
water vapor imagery. In addition, the flow at 700mb shows a second
wave moving slowly northeastward which is currently in the southern
Rockies which will help to aid in ascent. It is dry below 700mb
however, so if any activity develops it will only be isolated.

.LONG TERM...(Monday night through Sunday)
Issued at 308 PM CDT Sun Jul 23 2017

Potential for isolated elevated showers/storms beginning in our far
southwest late Monday afternoon will continue through the Missouri
Valley Monday night. Soundings remain quite dry below 700mb, but
with the mid-upper level support and elevated instability, cannot
rule out some very light precipitation and/or clap of thunder.

Main focus for precipitation will be late Tuesday through Wednesday.
100+ kt upper jet has trended a little farther north, tracking near
the Canadian border in the latest models, but will still be looking
at associated mid-upper wave dragging a cool front to near our far
northwest corner around Huron by late in the day. Forecast soundings
(aside from overly mixed GFS) remain capped ahead of the boundary,
but this front should become a focus for storm development by late
Tuesday/Tuesday night as it pushes across the forecast area. Better
forcing/shear focused north of the forecast area, but pockets of
moderate shear combined with CAPE values exceeding 2000 J/kg should
support isolated strong-severe storms across our northern forecast
area. Ahead of the front, strong thermal ridge in south-southwest
low level flow should result in the last day of hot temperatures we
will see in the near future. Tempered the warming/mixing potential
across far northern areas/Highway 14 corridor, where we have seen a
fairly widespread 2-4 inches of rain over the past week. However,
even there low level temperatures suggest highs should top 90 again
for the first time since this rainy stretch began. In contrast, our
persistent dry locations in the southwest CWA should more easily
climb into the upper 90s to around 100.

Cold front pushes slowly southward late Tuesday night/Wednesday,
with models showing a more potent shortwave lifting out of the
Central Rockies and interacting with the boundary through the day
Wednesday and into Wednesday evening. Depending on timing of the
boundary, and even more, the extent of any rain on Wednesday, could
again see an isolated strong-severe storm in our far southeast late
Wednesday. With most of the area north of the front, and potential
for quite a bit of cloud cover/rain across the area, temperatures on
Wednesday will be quite a bit cooler with highs in the upper 70s to
mid 80s.

Late next week, surface high begins to dominate across the Upper
Mississippi Valley, with any significant low level moisture cutoff
by a boundary pushing through the central/southern Plains. This
should lead to a fairly dry period Thursday-Sunday, with daytime
temperatures near to a few degrees below normal.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1026 PM CDT Sun Jul 23 2017

With winds becoming nearly calm later tonight, cannot rule out
shallow fog development in areas from the Interstate 29 corridor
and eastward after 10Z. This may affect KFSD and KSUX through 14Z
on Monday morning. Otherwise, outside of this concern, VFR
conditions are expected through the remainder of the TAF period.




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