Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 152043
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
343 PM CDT Fri Sep 15 2017

.SHORT TERM...(This evening through Saturday)
Issued at 343 PM CDT Fri Sep 15 2017

No easy forecast tonight with scattered thunderstorms possible in a
fairly weakly forced environment. Dry air aloft has moved into the
area with an enhanced EML in place. However just above this EML is a
little bit of moisture, enough that spotty showers and even spottier
thunderstorms are developing. Hard to tell for sure but it appears
there could be a little kink in the flow heading into southern
Nebraska at 19z, which does somewhat match up with the right
entrance region of a jet max swinging around the eastern side of the
upper level low over northeast WY. It appears that this will bring a
little additional mid level moisture, likely from about 800mb to
600mb, which will aid in increasing instability, utilizing the
fairly strong EML. Along with this there is a low to mid level
boundary gradually working east, which should begin to move a little
more quickly as this wave moves north. All in all, coverage should
not be extensive but do expect scattered showers and thunderstorms
to develop after about 23z, with the better chances between the
James River and Interstate 29 from about 7 pm to 10 pm CDT. In all
likelihood these will be elevated storms with about 1000-1500 J/kg
and 30 knots of deep shear to work with. So isolated severe storms
will be possible. While the low level jet is strong it does wane a
bit through the night so not expecting the convection to congeal
into an organized MCS at this time. But we should see scattered
development through the night into southwest MN and northwest IA.

Clouds will increase through the night with stratus working back
from central SD. This will help keep temperatures mild through
tonight with morning lows in the mid 60s across northwest IA to 50
to 55 in central SD. The diurnal range will not be very large on
Saturday with the lows clouds in place. There should be some erosion
from southwest to northeast through the day but much of the area
will remain in the thicker clouds through peak heating. One area to
watch will be from about Jackson MN south to around Ida Grove where
scattered strong to severe storms will be possible. These storms
also appear like they will be elevated but may have closer to 2000
J/kg CAPE to work with and while shear will still be closer to 30
knots there is a bit of directional change so might support a little
stronger updrafts.


.LONG TERM...(Saturday night through Friday)
Issued at 343 PM CDT Fri Sep 15 2017

Active weather pattern will continue into the mid and long range as
southwest to westerly flow aloft direct a series of waves into the
northern Plains.

For Saturday night, a frontal boundary will likely set up in
northwest Iowa into south central MN by late afternoon, with a
significant heating differential ahead of the front versus areas to
the west of I-29. With this unstable environment, scattered shower
and thunderstorm activity is expected to form near the boundary in
the evening and quickly track east through midnight. The question
will be how far east the boundary sets up in the afternoon. Good
instability topping 1500 j/kg and effective shear values around 30
to 35 kt should support a brief severe weather threat along and
ahead of the front before it exits the region. Best potential will
be in our far eastern counties in IA and MN.

Cooler high pressure lifts through the region on Sunday, allowing
the morning to be mostly sunny and dry. By evening however, a
shortwave moves into the western Nebraska and lifts into Missouri
Valley region late Saturday night. With the stronger forcing
remaining to our south, much of the region will remain dry through
the evening, however clouds will be on the increase Sunday
afternoon, with scattered showers and storms possible in far
southeast SD into northwest Iowa late Sunday night into Monday
afternoon.

Large upper level low centered over Alberta will wobble slightly
eastward mid week, however the models remain uncertain in the timing
of waves rotating northeastward into the northern Plains mid to late
week. Confidence is pretty low on the timing, however the overall
picture does look like there will be several rounds of showers and
storms impacting the area. Temperatures will likewise be tricky,
with the potential for stratus and precipitation. Temperatures
should stay seasonally mild, but don`t have enough confidence to
improve on the blended forecast.


&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1213 PM CDT Fri Sep 15 2017

Capping aloft has moved in and brought an end to most of the
activity. Some weak instability just above the cap will allow
isolated showers to develop. By early evening scattered
thunderstorms should develop between the James River and
Interstate 29, then progress east through the night. A few strong
storms are possible. Otherwise, IFR ceilings will be likely in
central SD through the afternoon, then these ceilings will surge
east during the evening and overnight hours.


&&

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

$$

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



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