Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1103 PM CDT Wednesday Jul 23 2014

issued at 950 PM CDT Wednesday Jul 23 2014

Forecast for tonight still looks in pretty good shape. Thetae
advection still prognosticated to increase tonight...with the combination of
this increasing moisture and steepening lapse rates aloft resulting in
increasing instability. Based on soundings...appears like convection
should be based around 700 mb. As by later tonight...parcels from
this layer should be uncapped...and realize between 1000 and 1500
j/kg of cape. These storms should be based above the best
shear...however rap soundings still suggest they may be able to tap
into 30 to 40 kts of effective shear. Thus still seems like small
hail would be a possibility with any storm...with a few of the
stronger ones possibly pushing quarters. Leaning towards the
hrrr...rap and European model (ecmwf) for timing...bringing storms into the west
around 08z to 09z...approaching Interstate 29 by 12z. Do think storms
will be in a weakining state as they approach Interstate 29 and points
east as the low level jet slackens with daybreak. The 0z NAM seems
off and an outlier so was discounted at this time. Not a sure bet we
see convection these elevated events are always will stay below likely probability of precipitation...but am definitely leaning
towards US at least seeing scattered activity.


Short term...(this evening through thursday)
issued at 317 PM CDT Wednesday Jul 23 2014

Main focus of the short range forecast will be late tonight and
Thursday morning. Dealing with a lot of thetae warm air advection
moving into...then across our forecast area...first hitting our far
western zones overnight tonight...then exiting our east Thursday
afternoon. The thetae advection is maximized in the 800-750mb layer
at the top of the inversion. The inversion is strong in the low
levels...therefore it is likely that the strong directional wind
shear in the lowest 1.5km will not be tapped...leaving elevated
showers or storms which will be tough to hit severe criteria late
tonight and early Thursday west of I 29. As far as probability of precipitation...still did
not have enough evidence to go likely probability of precipitation at any particular
location from 06z to 18z Thursday. The various hires models are
surprisingly dry...possibly due to the aforementioned reason of the
precipitation being elevated. Therefore only the global models are bullish
in developing convection right to the west of the middle level ridge
axis...which does not lead to a lot of confidence is going likely or
categorical probability of precipitation yet. Will just have to monitor how this morphs
out. As the precipitation tries to move eastward by midday Thursday...less
middle level moisture looks to be available in our eastern zones by
Thursday afternoon with weaker thetae advection.

Temperatures still look pleasant tonight. However 850mb temperatures
are very warm west of the James River Valley on Thursday behind the
thetae advection. Therefore as skies clear in the evening...still
looking at lower 90s approaching central South Dakota...with a dramatic drop
off in temperatures heading east of the there. In fact east of the
James...we are still looking at readings only a couple of degrees
either side of 80 as the cool high pressure system over the Great
Lakes still greatly influences the weather.

Long term...(thursday night through wednesday)
issued at 317 PM CDT Wednesday Jul 23 2014

A ridge flattening short wave may bring some redevelopment of
storms Thursday night...isolated west and scattered east...mainly
far east where most moisture and instability will linger.
However...this moisture and instability will be limited and most
of the activity should end late. Isolated storms still seem
possible far east Friday with some weak upper and thermal support
associated with the lingering moisture. Another such weak system
may bring a few showers or storms to the Missouri River area late
Friday night and Saturday while most of the area stays dry.
Temperatures Friday and Saturday should be modestly warm and close
to normal with 80s highs.

What I described yesterday as a secondary short wave is nor a
cooling Canadian upper low which will dig toward the western Great
Lakes Sunday and begin to carve out a large scale trough over the
central and eastern USA. A few weak showers/storms seem possible
north and east of fsd Sunday and will mention with slight chance
probability of precipitation. Sunday will of course be cooler with mainly 70s highs...80
or so southwest. A mostly dry and somewhat lasting cool pattern
then seems in store for the area...but went with the extended
guidance slight chance of storms for Wednesday as the ec does show
a short wave with the right timing...GFS being a little faster.
However nobody is going to get very much rain with the moisture
very limited.


Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1055 PM CDT Wednesday Jul 23 2014

VFR conditions are expecetd through the taf period. Scattered
thunderstorms are expected to move into south central South Dakota late tonight
and slowly track eastward across the area through the day Thursday.
Expect activity in the khon area from 09z to 14z and into the i29
corridor...including fsd and sux after 13z. Again...coverage will be
scattered however did leave tempo mention at all 3 taf sites. The
high based storms will likely keep conditions predominately in
VFR...but stronger gusts and very brief MVFR conditions will be
possible in the stronger storms. Breezy south southeast winds pick up
after 12z and perist through the end of the taf period.


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


short term...mj/
long term...

National Weather Service Glossary of Abbreviations