Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 200343
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1043 PM CDT Tue Jun 19 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 359 PM CDT Tue Jun 19 2018

The short-term concern is the increasing risk of rain through
Wednesday.

There was general model agreement today with the evolution of the
large scale features through 00Z Thursday. In the mesoscale the
12Z NAM was the farthest north with the location of the 850 mb
front and subsequent precipitation through Thursday. The NAM
typically has a north bias with these type of systems. The GFS
and ECMWF which had more run to run consistency and remained south
of I-90 so followed these models.

Overall, a weak lead short-wave was forecast to move into western
South Dakota this afternoon and showers and thunderstorms have
developed over far western SD. As this waves moves east expect
additional storms to develop toward south central SD after
sunset. Expect these showers to focus near the 900 mb front.
Showers with embedded thunderstorms will spread slowly north and
east through the night with the greatest chance closer to the
Missouri River. The storms are likely to spread to I-90 after
midnight and may extend as far north as MML by sunrise. With
clouds and precipitation, lows tonight will again be in the 60s.

On Wednesday, the main upper wave will move from the Rockies into
Nebraska and South Dakota. The 850 mb front is expected to slowly
migrate south from Hwy 14 in the morning to near I-90 by the
evening. As the wave moves out, expect widespread rain with a
isolated thunderstorms to develop or move in from central South
Dakota during the morning. Rain will develop eastward through the
morning and is expected to be across most of northwestern Iowa and
southeastern South Dakota by early afternoon. The rain is
expected to persist through the afternoon as the 850 mb boundary
continues to slowly move south of I-90 by 00Z Thursday. With moist
adiabatic lapse rates from the surface to above 500 mb, this will
be a very efficient rainfall event. Add in precipitable water of
1.5 to 2 inches - rainfall amounts by Wednesday evening may be
approaching an inch along and south of I-90. The one question is
how tight of a gradient will exist on the north side of the band.
With this rain occurring within a frontal band, there should be
some gradient and we may be underdoing the magnitude of the
gradient. So expect a fairly rapid decrease in precipitation as
one moves northeast across southwestern Minnesota.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 359 PM CDT Tue Jun 19 2018

The main concern in the long-term is the potential for heavy rain
and possible impact on rivers later in the week.

The upper low will slowly continue to move southward Wednesday
night and Thursday. A TroWAL is expected to form north of the upper
low across northwestern Iowa and into the Missouri Valley of
South Dakota and Nebraska. PWs within the TroWAL are expected to
remain around 1.75" through Thursday morning. There is weak
frontogenesis along the northern side of the trowal with the front
expected to pivot near the lower Big Sioux Valley on the South
Dakota and Iowa. This will be the focus of where the heaviest rain
will fall due to the long duration of the moderate to heavy
rainfall in this area as the front is expected to remain over this
area for 12 to 18 hours with moist neutral lapse rates. Rainfall
amounts will decrease north of I-90. As the low continues to move
south on Thursday, ending north of I-90 by early Thursday
afternoon and in northwestern Iowa by Thursday evening.

With moist neutral lapse rates and frontogenesis within the
TroWAL, rainfall rates will be fairly heavy considering
convection will be minimal. Heavy rainfall is likely in some
location south of I-90 with the heaviest rain expected to be
centered over northwestern Iowa where 2-4 inches of rain is
possible. Rainfall will decrease to the north and west with less
than an inch of rain north of SD Hwy 34/MN Hwy 30. As with winter
frontal bands, the exact location of the heaviest rainfall is
still uncertain. The current forecast has it between Hwy 18 and
Hwy 20 but it is possible the heaviest rainfall could be farther
north and closer to I-90. Also, several deterministic models have
total rainfall exceeding 4" so local amounts greater than 4" are
possible.

With the heavy rainfall expected, there will be an impact on area
rivers - with some minor to moderate river flooding possible on
the Floyd, Big Sioux and Little Sioux Rivers. The degree of
flooding, timing and locations are strongly dependent upon where
the rain falls over the next 48 h. Persons with interests near
small creeks and rivers will want to stay aware of the latest
forecasts should rivers rise later in the week.

With the clouds and rain, temperatures will have a small diurnal
range through Friday. Highs will be in the upper 60s to mid 70s
with lows generally in the 60s with a few upper 50s west of the
James River.

The unsettled weather will continue through the weekend and into
next week. A wave is expected to move through during the weekend
bringing a chance of showers and thunderstorms. Both the GFS and
ECMWF show another cutoff low moving across the area early next
week which would again bring in more rain with embedded
thunderstorms. At this point, it is too early to say if heavy rain
is possible with either of these systems.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1034 PM CDT Tue Jun 19 2018

Showers and isolated thunderstorms will continue to lift across
the area tonight and continue through Wednesday. Heavy rain with
deteriorating visibilities will be possible south of Interstate
90 on Wednesday and Wednesday evening. MVFR/IFR ceilings will
dominate through the period.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...Schumacher
AVIATION...JM



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