Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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350
FXUS63 KFSD 250917
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
417 AM CDT Sun Jun 25 2017

.SHORT TERM...(Today and Tonight)
Issued at 416 AM CDT Sun Jun 25 2017

The main focus for today`s forecast is potential for showers and
isolated thunderstorms this afternoon and overnight.

In general, models are in agreement in evolution today and tonight.
Weak upper level wave currently on the Saskatchewan and North Dakota
border is forecast to move southeast through the day. As it does a
weak front will develop around 850 mb. Moisture remains very
limited but with 700 mb temps below zero several CAMs as well as NAM
show no convective inhibition with MUCAPE around 200 J/kg. The
combination of forcing from the upper wave and front should be
enough to get a few showers to develop in northeast South Dakota
later this morning and then spread southeast through the afternoon.
Moisture is very limited and this will limit coverage of any
showers and storms that develop. After discussing with
surrounding offices, we also decided to add thunder in this
afternoon and early evening with the idea that there will be
strong enough ascent with the limited CAPE to produce a
couple of thunderstorms within the area of showers. The loss of
diurnal heating should end the threat for thunder. Also there
is still a lot of dry air below cloud base which will allow some of
the rain to evaporate before reaching the ground. Therefore have
kept PoPs less than 40 percent due to uncertainty on coverage. With
the best chance along and south of Interstate 90 after sunset when
the 850 mb boundary and upper wave appear to have the best
interaction. The precipitation should move south of Sioux City and
Storm Lake by 12Z Monday.

Otherwise, it will be less windy than yesterday as a ridge of high
pressure builds into central South Dakota. There is enough gradient
east of I-29, especially in southwestern Minnesota, that gusts could
again approach 30 mph this afternoon. Highs today will be similar to
yesterday mid 60s along the Buffalo Ridge to the upper 70s in south
central SD.  Overnight, lows will again fall into the 40s with a few
lower 50s near Highway 20 where more clouds will inhibit radiational
cooling.

.LONG TERM...(Monday through Saturday)
Issued at 416 AM CDT Sun Jun 25 2017

Any lingering thicker mid clouds and isolated showers will certainly
be on the way out of the lower Missouri River valley by early
Monday, as boundary continues to push southward and surface ridge
settles across the heart of the area through the afternoon. Temps
will again be similar to even a shade cooler with mainly upper 60s
to mid 70s. With the surface ridge residing across the area through
the evening, and lingering longest in northwest Iowa into the early
overnight, would expect temperatures with cool and dry airmass to
again undercut most guidance, and have lowered several degrees into
the mid to upper 40s for lower locations across northwest Iowa and
nearby locations.

Toward south central SD, return flow will strengthen considerably
post 06z Monday night, and will likely start the day on the breezy
side at more exposed sites in lower Brule area. Southerly winds will
increase rapidly and expand their reach eastward through the day,
with 25 to 35 knots in the mixed layer most areas west of I-29 at
some time during the day. Fairly dry lower levels with trajectory
out of exiting ridge lingering through NW IA and SW MN, but should
begin to see increasing low level moisture below the strongly
warming temperatures aloft into southeast SD. Concern in that with
700-500 hPa lapse rates of 7.5-8.5 C/km, there could be some
tendency for mid-level accas type showers/isolated T develop during the
afternoon across southeast SD, but really do not see a great deal of
larger scale support to lift along the advancing instability
gradient.

At this time, period of greatest concern in the forecast will arise
on Tuesday night, when there will be a much improved prospect for
convection. Unless some mid-level convection develops late afternoon
into early evening as elevated mixed layer shoves eastward to I-29
by early evening. Warm layer aloft will mean focus for convection
will likely be closer to the low-level convergence boundary across
western SD and in lee of Black Hills late afternoon into very early
evening as upstream trough approaching from the west will flatten
the flow aloft and knock back warmer temps aloft from west to east,
also increasing directional shear overnight. 45 to 55 knot south to
southwest low-level jet and westerly flow punching across in the mid
to upper levels does generate a fairly potent deep-layer shear,
although most guidance suggests will have some weakness to winds
around the middle of the 0-6km profile. Nonetheless, there is
sufficient shear to support storm organization.  Shear is highly
boundary parallel, so expect western discrete storms to evolve
fairly quickly toward a more linear and widespread structure, which
will also be supported by good dynamical support to lift with PV
anomaly push, meaning most areas will be likely to see at least some
precipitation as the line moves east. Elevated instability centering
on generally 1000-2000 J/kg, and provided storms can abandon more
surface-based roots heading later into the night, should see a
fairly decent MCS press eastward across the area from the mid
evening toward south central SD, into southwest MN and northwest IA
after midnight. Potential threats would generally be larger hail and
damaging wind gusts.

Very progressive mid-level feature will keep convection moving
along, and may exit even faster than forecast on Wednesday morning,
abandoning the synoptic frontal boundary pushing toward the James
River valley early morning.  While have maintained some lower chance
PoPs on Wednesday through northwest Iowa during the afternoon, there
is uncertainty if the modification of atmosphere by earlier storms
can be overcome in time to produce afternoon storms along the
boundary in northwest Iowa.  Mixing behind the front should push
readings well into the 80s, especially through lower Brule areas
which could approach 90.

Beyond Wednesday, forecast becomes a bit more muddled with various
impulses timing through the west/northwest flow into next weekend.
One such series of waves moves across on Thursday into Friday,
dragging dynamic lift forcing across the area mainly afternoon and
overnight. While most solutions knock the main synoptic boundary
well south by Thursday night along with the lions share of
instability and moisture, the ECMWF does attempt to dig a sharper
northern stream trough, pick up a piece of southern energy and wrap
back northward toward northwest Iowa on Thursday night with a much
higher chance for convection. For now, have settled on modest chance
PoPs to see how intervening convection affects the pattern. After
sharp trough passes Friday, should be generally dry through
Saturday, although hints that return flow could initiate by later
Saturday toward south central SD allowing for a low chance of storms
by late day along advancing mid-level instability once again. Temps
should settle out fairly close to normal again by the weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1030 PM CDT Sat Jun 24 2017

VFR conditions are expected through the TAF period.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...Chapman
AVIATION...



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