Scientific Forecaster Discussion
fxus63 kfsd 200335
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1035 PM CDT Fri Jul 19 2019
Short term...(this evening through saturday)
issued at 451 PM CDT Fri Jul 19 2019
One more day to be concerned with convection and heat. It's a
steambath out there today with 20z heat index values from 100 to 110
degrees across the area. The heat should reach its worst in the next
hour or two, and then we can look ahead to some gradual improvement
overnight, and largely out of the area by tomorrow. Heat headlines
continue through the evening.
At mid-afternoon, the outflow/warm front has surged well north of
the area in Minnesota and will be focus for very significant storms
this evening, while a prefrontal wind shift/cold front has slipped
southeastward to near a Katy-kmhe-kvtn line. A little cumulus
recently showing up along a good stretch of the boundary, but
tonight the mid level temperatures are at a stifling 16-17c at 700
hpa. The will essentially cap off the boundary (short range hi-res
showing average of 150-200 j/kg capping) and keep things largely
quiet this evening. More confident in this given the southwest winds
leading the boundary which is weakening the overall convergence
along the front. There is an outside chance for a couple things to
change this - outflow from central Minnesota complex of storms
working southwest, or heating out where winds are starting to back a
bit more across central NE and spreading northeast along the
boundary. Neither is strong enough to suggest a widespread low pop.
In the event a rogue storm does develop, atmosphere is extremely
volatile thermodynamically with MLCAPE of 4000-5000 j/kg, but less
so from a shear perspective with only 25-30 kts of deep shear.
Generally would be a concern with heavy rainfall, wind gust or brief
tornado, but again, this is a vastly unlikely occurrence.
The time period of concern for convection will evolve later in the
night as the mid-level front begins to move southeast into the area,
along with the advance of the significant lower level drying. The
elevated instability is still appreciable at around 2000 j/kg, and
the elevated shear will increase to 40-50kts, so storms should be
capable of becoming organized into small segments/embedded
supercells. 0-3km shear vectors suggest there is a small potential
for developing mesocyclones along any more east-west oriented
segments, which could serve to focus a damaging wind threat.
Otherwise, large hail would also be possible with stronger storms,
at least up to Golf Ball size. Expansion of slight risk over much of
the area across southwest Minnesota and southeast South Dakota appears warranted.
Exact timing of this increase in threat is still somewhat up in the
air, with latest generations of hrrr/NAM now indicating an even
slower progression to precip. Not sure full Faith can be put in this
with the frontal forcing seemingly a bit quicker and more in line
with that in the European model (ecmwf). At this point, higher pops from later
tonight into the midday on Saturday should capture the bulk of the
The boundary continues to drop southward on Saturday, and should be
clear of all except perhaps the Highway 20 corridor by 18z.
Additional push from convection should move any tendency to linger
out of the way, and will therefore keep heat highlights at Bay,
although southern parts of northwest Iowa should see a heat index at
least reach the lower to mid 90s. Will continue to find elevated
instability as storms/remnants progress east/southeast through the
morning, and could yet find a hail/wind threat with storms during
the morning into early afternoon.
Long term...(saturday night through friday)
issued at 451 PM CDT Fri Jul 19 2019
Lingering showers/storms into Saturday evening should be mainly
south and east of Sioux Falls, exiting gradually with collapse
southward of the baroclinicity in broad jet entrance. Should also
start to see a better decrease in overnight temps, down into the
upper 50s to mid 60s.
One change in the forecast comes on Sunday, when models are
converging on the passage of a fairly compact wave in northwest
flow. Good pv advection can likely make up for drier air in lower
levels, and with diurnal support should likely get a scattering of
showers/thunder pushing into southeast South Dakota by midday, and gradually
fading by early evening as attempts to push east of southwest Minnesota and
After Saturday, will move into a period of lesser humidity as the
western right/eastern trough becomes established early in the week.
A cooler and dry area of high pressure settles in over The Heart of
the region by Tuesday morning. A slow return to warming commences
later week as the ridge nudges eastward, but does not appear to have
any significantly organized waves pushing through to create a
meaningful chance for rainfall. Temps starting below normal in the
mid 70s to around 80 Monday, warming to mid 80s to around 90 by
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1033 PM CDT Fri Jul 19 2019
Models continue to trend slower with convection on Saturday as
cooler air started to settle across the region. Favored area
through 18z Saturday appears to be north of I-90, with more
scattered activity further south. Therefore, do not have
thunderstorm mention in kfsd and ksux. Behind the convection,
potential for some MVFR stratus.