Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
631 PM CDT sun may 3 2015
Short term...(this evening through monday)
issued at 323 PM CDT sun may 3 2015
Have seen scattered sprinkles/light showers develop over the
western County Warning Area along elevated boundary ahead of the middle level wave.
Updated grids to progress this activity eastward across the area
this afternoon...with convective potential gradually increasing
across southeast half of the forecast area as the elevated front
works into more unstable air mass. Surface based storms may be
tougher to come by...as surface convergence somewhat ill-defined
with broad area of weak/variable flow ahead of the boundary...
located south of kykn-kfsd-ktkc line at 18z. Isolated severe still
appear possible across the southeast...with recent hrrr model runs
indicating a potential for wind threat associated with cold pool
ahead of elevated storms as they track across extreme southeast
South Dakota/northwest Iowa. Not wholly convinced this will pan out...but does
not seem entirely unreasonable given higher-based storms with drier
air undercutting behind surface boundary...so will definitely need
to monitor through the late afternoon/early evening.
Overall convective chances should wane after 02z-03z as the wave
passes by...with overnight hours into early Monday morning expected
to be dry across the area. Increasing isentropic lift ahead of broad
wave should begin to spread precipitation back into southern parts of the
area Monday afternoon...with main question being how quickly it
expands to the northeast the dry low level east-northeast flow
firmly in place. Will hold generally southwest of khon-kfsd-kspw
line...supported by greater model consensus of NAM/ECMWF/Gem. Monday
should be a cooler day...with thicker clouds holding temperatures in the
60s through the Missouri Valley...with lower 70s more prevalent to
Long term...(monday night through sunday)
issued at 323 PM CDT sun may 3 2015
On Monday night and Tuesday a warm front begins to lift northward
through Nebraska...with increasing midlevel Theta-E advection across
our area during the period. With this will have increasing
precipitation chances...especially in our south and east.
Instability is not especially impressive so stayed mainly with
isolated thunder...although it does look to be a little more
unstable in our south on Tuesday afternoon. Highs on Tuesday will
mainly be upper 60s to lower 70s...though areas through the lower
MO River Valley may approach 80.
On Wednesday a more significant vorticity lobe begins to lift out of the
western Continental U.S. Upper level trough and into the central and northern
plains. This will mostly likely bring more significant precipitation
to the area for later on Wednesday into Wednesday night. While it
will be fairly unstable on Wednesday...shear is rather
unimpressive...but there will most likely be thunderstorms around.
Highs again will be running lower to middle 70s. Rain chances then
continue into Thursday when a surface frontal boundary swings across
the area as the upper level shortwave pulls off into the western
Great Lakes region.
May finally begin to dry out by Thursday night and Friday as high
pressure begins to build back into the region behind the exiting
system. Slightly cooler air filters into the area for
Friday...though still looking at highs middle 60s to lower 70s. Rain
does look to return by next weekend however as another upper level
low lifts out of the western Continental U.S..
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 631 PM CDT sun may 3 2015
Main focus is convection this evening. For khon...conditions
expected to be VFR through 00z Tuesday. For kfsd...also expecting VFR
conditions through period with winds dropping off to 10 to 15 kts
after 02z. Small concern that high res models are showing some
convection developing between 03z and 06z and NAM/GFS soundings do
show instability above 800 mb. At this point...probability is too
low to include in taf but will need to monitor for storms later
tonight. For ksux...in general VFR conditions expected. There is
a chance of storms this evening although no evidence on satellite
or radar that storms are imminent so have will not include in taf
at this time. Otherwise...similar issue to kfsd in that hi-res
models move these storms near ksux between 05z and 09z tonight.
Again...probability too low to include in taf. Next round of rain
will be approaching ksux around 00z. At this point...better
chances after 00z Tuesday so kept conditions VFR.