Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251108

Area Forecast Discussion
National Weather Service Sioux Falls SD
608 AM CDT Tue Oct 25 2016

.SHORT TERM...(Today and Tonight)
Issued at 332 AM CDT Tue Oct 25 2016

A weak shortwave is sliding through northern South Dakota early this
morning. This in conjunction with midlevel theta e advection lifting
up across the area may kick off a few light showers through the
northern Highway 14 corridor by mid morning where moisture profiles
are a little deeper. Farther to the south, still seeing a fairly
significant dry layer in the mid levels, and even drizzle is now
looking unlikely over those areas. Otherwise, models are trending
toward a drier forecast into mid day, before rain chances begin to
increase in the later afternoon as a more significant upper level
shortwave presently over Wyoming lifts into the Northern Plains.

Moisture profiles begin to increase by late afternoon, and with
enhancing lift as the aforementioned shortwave nears, expecting
scattered showers to develop in areas mainly from the James River
Valley eastward. There will be enough weak elevated instability
across the area that isolated thunderstorms will be possible over
most areas; though bona fide thunderstorms will be possible through
the Sioux City to Storm Lake corridor by late afternoon with CAPE
values reaching into the range of 800-1000 J/KG - with the higher
values trending southward toward the edges of our southernmost CWA.
Most recent SPC Convective Outlook brings a Marginal Risk just to
Sioux City and southward, and this seems reasonable considering the
aforementioned CAPE values/steep lapse rates/and moderate shear,
however the better convective parameters remain to the south of our
CWA so think the risk over our area is pretty low. Wind profiles are
not especially impressive, so think small hail would be the main
threat with any stronger storms. Temperatures today will run close
to seasonal normals with highs in the 50s to lower 60s.

By tonight, the upper level wave lifts across our area, with strong
moisture transport developing east of Interstate 29 thanks to a 45
to 50 kt low level jet which will develop over that area in the
evening. Could have some decent rainfall amounts there, running from
around half an inch to an inch, though not looking at an extended
period of rainfall so the flood risk remains on the low side. The
veering low level jet and upper level wave pull eastward by 06Z
tonight, with showers and isolated thunderstorms tapering off during
the late night. It will be another fairly mild night with lingering
clouds, with lows mainly mid and upper 40s.

.LONG TERM...(Wednesday through Monday)
Issued at 332 AM CDT Tue Oct 25 2016

Wednesdays weather is highlighted by the departing short wave. There
are a few issues on Wednesday. First is the surface low itself. It
is forecast to move across Iowa, then into west central Illinois
toward evening. To the north, high pressure filters down behind it
into our area from southeast Canada. This scenario begins to tighten
up the surface pressure gradient across our eastern zones, with 15
to 25 knots of wind forecast not too far off the surface, especially
in northwest Iowa. Therefore increased the surface winds a few knots
across our east as the blended guidance looked too light. Second,
both the NAM and GFS show abundant moisture near 900mb through much
of the day east of the James River valley. The blended guidance
rarely picks up on stratus so increased the sky cover for these
areas. This will have an impact on temperatures, with the ECMWF and
bias corrected highs much cooler east of the James River valley on
Wednesday then the blended highs. Therefore trended toward these
cooler readings with expected stratus in place.

On Wednesday night, a return southerly flow is in place for most
areas, minus our zones in the southeast corner of northwest Iowa
which helps to keep our lows from cooling off too radically. One
thing of concern however is fog potential. Anticipating a wet ground
after tonight in our eastern zones, and it is these zones which will
be the last to see a southerly wind, with light winds in place
before then. Therefore mentioned some patchy or areas of fog, most
pronounced in northwest Iowa. But also mentioned some fog in some of
the river valleys such as the Big Sioux and James. One item which
could inhibit fog are mid clouds which are forecast late Wednesday
night and Thursday morning. However, both the GFS and NAM keep much
of the mid clouds a bit too the north of our forecast area, scraping
our northern zones where fog should be limited by the southerly flow
anyway. Any fog that develops will likely not stick around very long
early Thursday as the southerly flow increases. For highs on
Thursday, the aforementioned southerly flow will help to push highs
into the 60s, even lower 70s in south central SD. These readings are
backed up by the forecast 900mb temperatures from the NAM, GFS and

Organized measurable chances for precipitation will be hard to find
from Thursday onward. The northwest flow aloft gives way to more of
a zonal flow by Friday, with an upper trough digging toward the
Pacific coast. The chilly blast that the GFS was showing yesterday
coming down for Friday and Friday night is not there right now,
therefore the deterministic ECMWF and GFS are much closer with the
late week and Saturday temperature forecast when compared to
yesterday. After a much above normal day on Friday, a weak cold
front will move through Friday night giving highs much closer to
normal with readings in the lower 50s to lower 60s. Temperatures
remain at or above normal from Sunday through next Tuesday with no
chilly air forecast to move down yet from the north.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 603 AM CDT Tue Oct 25 2016

Rain chances will increase by this afternoon with ceilings
lowering into the MVFR range by late this afternoon or evening.
Heavier rain and isolated thunderstorms will develop east of
Interstate 29 this evening, with MVFR visibilities in heavier
showers. Ceilings will lower into the IFR range by late tonight.




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