Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
407 am CDT Thursday Aug 21 2014

Short term...(today and tonight)
issued at 406 am CDT Thursday Aug 21 2014

Main focus for the short term will be the convection chances this
morning and again late this afternoon and tonight as well as heat
this afternoon.

For this morning...all models show the low level jet that has been
across the area much of the night will continue to veer and weaken
through the morning. This is expected to decrease convergence on the
nose of the low level jet as well as decrease the warm advection and
associated lift. Therefore expect that storms over south central
Nebraska at 08z will gradually turn more to the east. The northern
edge of this complex could impact Sioux City and into Ida County but
expecting it will primarily remain south of there. Other storms in
northwest Iowa and SW Minnesota may linger after sunrise but expect these will also
move to the east of the area by 15z. With MUCAPES well above 1000
j/kg...some isolated severe hail will still be possible through
sunrise but with weakening trend in storms after not
anticipate additional severe storms this morning and will not
mention any morning severe threat in the severe weather potential statement for today.

After that...expect skies to become partly to mostly sunny. There is
a strong inversion in place east of the wind shift that will near
the James Valley. This will act to suppress convection much of the
day and also trap moisture in the boundary layer. Therefore...expect
dew points to remain steady or slowly rise throughout the afternoon
in most locations. This will result in dew points into the lower to
middle 70s everywhere but in south central South Dakota where light
northerly flow will slowly bring drier air in and keep dew points in
the 60s. With a decent amount of sun expected today...highs should
reach the upper 80s to lower 90s in all areas with the warmest temperatures
in the Missouri Valley. As a result heat indices are forecast around
100 in the lower big Sioux and middle Missouri Valley. Considered a
heat advisory but with some uncertainty on how long clouds will
linger decided to hold off until cloud trends are more certain.
Should also note that if temperatures or dew points are a couple of
degrees warmer...most areas east of the James River would reach a
heat index of 100. It will remain fairly humid into the evening
before the boundary moves through after sunset and lowers dew points
back into the 60s.

There is a lot more uncertainty about thunderstorm chances for later
this afternoon and tonight. Certainly with all of the heat and
humidity...MLCAPES will be over 3000 j/kg and may exceed 4000 j/kg
in places. noted above...there will be a strong
inversion in place. Neither the GFS nor European model (ecmwf) produce much convecion
through tonight. The higher resolution models all produce convection
but the timing and location varies from model to model. This is
because the synoptic scale actually favors weak middle level subsidence
which acts to maintain the inversion. Also the low level jet is much
farther south tonight and really stays southeast of the area. That
leaves convergence along the weak wind shift line that will be
between the James and Big Sioux River by late afternoon. The NAM and
4 km NAM decrease the cin to 80 j/kg by late afternoon. That may be
just weak enough that isolated storms can develop late afternoon or
early evening in the vicinity of i29 or east of there. The best
chance will be in northwest Iowa where the greatest instability will exist
but even there the probability is only from 20 to 30 percent.
However...if storms can develop...the effective shear is greater
than 40 kts which would support supercells so large hail and
damaging winds would be possible. So will mention the threat of
severe storms in severe weather potential statement. Any storms that do develop are expected to
move east of the area after midnight.

Long term...(friday through wednesday)
issued at 406 am CDT Thursday Aug 21 2014

Overall Friday should see a mostly dry day with an increasing threat
for thunderstorms late in the afternoon. The better chances will
likely be confined to the Missouri River into central South Dakota. Height
rises through the morning should keep any activity at Bay. Some
minor height falls begin late in the afternoon but are most
substantial Friday night as a jet maximum moves northward through
central South Dakota into North Dakota. Decent instability and marginal shear
values should support some isolated severe thunderstorms but one of
the main threats could be heavy rainfall. Upper level winds are
fairly parallel to the low and middle level boundary and with strong
forcing and precipitable water values of about 1.75 inches not out of the question
to see some training and 3 to 5 inch rainfall amounts.

The better chances for showers and thunderstorms will likely shift
into the northern County Warning Area on Saturday as troughing and a middle level
thermal gradient remain close by. Highs on Saturday likely to be
pretty warm in the southern County Warning Area...near 90 while the northern County Warning Area
will be more in the clouds and precipitation and closer to 75 to 80.

In the outer periods(sunday through wednesday)...a strong wave in
southwest flow aloft will move northeast into southern
Canada...keeping the local area within the low to middle level thermal
gradient. This makes temperature forecasting a bit tricky through
this time frame...although the models do all seem to agree that by
Wednesday we will be well below normal. The GFS is a bit of an
outlier on how warm it keeps the low to middle levels compared to the
European model (ecmwf) and Gem. This is also seen on the statistical output as the
men numbers for all of these days are cooler than the mex numbers
even though the mex is already below climatology. So...will continue
the trend of below normal temperatures but maybe creep them back up
a bit in northwest Iowa where even the European model (ecmwf) gets slightly warm air
in on Monday. The threat for precipitation will be in place most of
these days as well as fairly strong troughing remains in place to
the west.


Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1119 PM CDT Wednesday Aug 20 2014

Convection a difficult forecast for shortest elevated
activity starting to pick up along the I-29 corridor then
southwest toward konl. Next few hours will be most likely time
for any convection to affect kfsd...and perhaps khon if stronger
northern complex can build far enough south. For now...have kept
out of ksux taf...but will be close to the north/west. Still
expecting an area of lower MVFR to upper IFR conditions in
fog/stratus to develop after 09z mainly near and north of
Interstate 90...where surface gradient weakens as well as
potential cooler outflow pool settles into the County Warning Area. Should be
persistent into middle to late morning before mixing/lifting to a
developing cumulus field. Depending on location of convectively
reinforced outflow...may have a mentionable convective threat
again by late afternoon for kfsd and ksux...but warming temperatures
aloft and uncertain location suggests holding off on additional
mention for now. Could start to see seeds of additional MVFR to
IFR conditions for just outside the taf period starting to develop
toward 06z Friday.


Fsd watches/warnings/advisories...
South Dakota...none.


Short term...schumacher
long term...08

National Weather Service Glossary of Abbreviations