Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
641 PM CDT Friday Aug 22 2014
Short term...(this evening through saturday)
issued at 245 PM CDT Friday Aug 22 2014
Several forecast concerns in the short term this afternoon...from
heavy rain to severe weather potential on Saturday. A stationary
boundary has taken shape across portions of eastern Nebraska into
north central Iowa this afternoon. A largely uncapped environment
now persists along and south of this boundary...with scattered
convection developing along weak convergence zone. Generally feel
this diurnally based activity should remain just south of the County Warning Area
through evening...but cannot rule out a storm drifting north.
Elsewhere...beginning to see slight uptick in convection over
portions of norther Nebraska this afternoon...linked to a weak
shortwave embedded in the flow on nose of increasing 500 mb jet axis
beginning to nose into the plains. This activity is slightly more
troubling as it could drift into the southwestern County Warning Area by this
evening...with light showers and isolated thunderstorms persisting
into early evening. Will bump up probability of precipitation to account.
The main story into the overnight hours is still focused on a much
stronger shortwave trough beginning to pivot out of the four corner
region at this time. Models continue to trend slower and more meridional
with the track of this shortwave into the region after midnight.
Nevertheless...anticipating convection to Blossom across western
Nebraska/Kansas as the wave begins to eject into the Western Plains
later this evening. An increased low level jet will also begin to produce a
linear band of scattered thunderstorms further east along a retreating
baroclinic zone through eastern Nebraska and into western Iowa.
Severe weather chances continue to appear low overnight...but would
not discount it given the gradual northward advection of MUCAPE some
1000-1500 j/kg toward daybreak and increased effective shear. Large
hail would be the main risk...mainly along and south of the Missouri
River. Also question the areal coverage of storms over the eastern
County Warning Area towards daybreak as models suggest the low level jet will actually back
and focus the best lift west of the County Warning Area.
The highest risk for heavy rainfall looks to remain west of the
James River overnight. Strong low-level moisture transport will
bring precipitable water values approaching 2 inches to portions of central and
west central South Dakota overnight. I would anticipate any storm to
be an efficient rain producer...and wouldn't rule out a flash flood
risk especially along the Missouri River. Rain totals could approach
1 to 3 inches.
The forecast become more difficult for Saturday...and a conditional
severe weather risk could become evident by Saturday afternoon.
Mesoscale guidance continues to lift a developed surface low
northward into north central South Dakota by Sunday afternoon...pulling the
warm sector...and potential 90 degree readings...well north into the
Missouri River basin. Gradual back building or upscale growth of
lingering convection along a faint moisture discontinuity and
quasi-warm front would be possible by mid-aftn...with moderate
instability and stronger middle-level winds supporting severe weather.
Again...this is highly conditional and dependent on how convection
evolves overnight. But agree with the sentiments of the swody2 that
current slight risk could be expanded. Any convection that does
form Saturday afternoon will again pose a flash flood risk...mainly
over the northern zones and into portions of northeast South Dakota and west
Long term...(saturday night through friday)
issued at 245 PM CDT Friday Aug 22 2014
Somewhat active pattern will continue into the extended time
frame. Cold front should be in central South Dakota by Sunday
morning allowing for lingering thunderstorms and rain ahead of it. Front is slow to
make its way through as the 500 mb low and associated surface low
moves north/northeast dragging the front through by Sun afternoon
into the evening.
Next chance of more widespread and potentially heavy rain looks to
be Monday night into Tuesday night. Longwave trough will move from pacnw
into the intermountain region. In the forefront...a series of waves
pass through as precipitable water increases to near 2 inches by
Tuesday morning. Low level jet increases enough to provide additional
support for periods of heavy rain. Model quantitative precipitation forecast ranges from 1-2
inches with the GFS most aggressive with amounts. Depending on the
outcome of tonight/saturdays rainfall...these additional amounts
could be a problem so will continue to monitor.
European model (ecmwf)/GFS differ on timing of 500 mb trough and subsequent ending of
rain...especially over southwest Minnesota and northwest Iowa.
The slower European model (ecmwf) seems to make more sense as the strong high
anchored over the southeast US would tend to slow things down
upstream. With that in mind have lingered probability of precipitation into Wednesday evening
for the aforementioned area.
At this time from Wednesday night into Friday the area will remain dry.
Temperatures throughout the extended period will remain below
normal in the middle to upper 70s.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 641 PM CDT Friday Aug 22 2014
Primary aviation concern through this taf period will be potential
for convection...and associated MVFR-IFR ceilings/visibility...
mainly expected to affect South Dakota portions of the forecast area. More
widespread convection should lift through Nebraska and western
portions of South Dakota through the evening...with a band of thunderstorms and rain
expanding into eastern South Dakota by late tonight/early Saturday.
Brief restrictions in visibility and ceilings will be likely as this
activity moves through.
Period of MVFR-locally IFR stratus to follow any convection Saturday
morning...improving to VFR as warm front approaches from the south.
Additional convective development possible with the warm front
Saturday afternoon...however considerable uncertainty remains on the
timing/location of the front...and thus have not included mention of
thunderstorms and rain after 18z in this taf issuance.