Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1228 PM CDT Thursday Jul 24 2014

issued at 1121 am CDT Thursday Jul 24 2014

Have tweaked probability of precipitation this morning and raised them in east central South Dakota...
southeastward into parts of southwest Minnesota and northwest Iowa
accounting for the current cluster of elevated storms moving
through. A few cells get a bit angry aloft...but then cores
collapse so thus only one warning so far. The cells are rooted
near 750mb so the wind shear in most of the lowest 2km is not
being tapped. But they are holding together along positive thetae
advection and will certainly impact southwest Minnesota and far northwest
Iowa through early afternoon. More uncertain is what the cluster in
eastern ND and extreme northeast South Dakota will do. Had to keep some
decent probability of precipitation going in southwest Minnesota for the middle afternoon hours and
may have to raise them if that activity holds together. Highs
still look very warm in our far west...cooling to around the upper
70s to near 80 in our east.


Short term...(today and tonight)
issued at 420 am CDT Thursday Jul 24 2014

Fairly Sharp Ridge with axis just east of The Rockies early this
morning. On water vapor loop...can see several waves rotating
through the ridge...all riding on the edge of a fairly strong middle
level frontal boundary. Strongest wave back over Pacific northwest...but one
nice wave pushing from eastern Montana into western ND. Patchy clouds in the
9-10 kft range have developed from south central ND toward
southeastern South Dakota...and increasing low level jet winds on mrr
profiler. All recent runs of hrrr and WRF-arw have been fairly
insistent on development of convection over the next couple hours
from 09z-11z out from the James Valley westward...and most solution
have at least some development occurring this morning. Given a decent
middle level Theta-E advection focus...and middle level frontogenetic
axis...seems would be favored for a few storms developing this
morning. However...will be increasingly difficult for activity to
build eastward as forcing with weak middle level waves drops southeast

Most likely location for convection looks to be the middle to southern
James Valley then southeast toward ksux area during the morning
hours. By afternoon...continued building heights and warming temperatures
aloft as wave moves past will start to wear on convective threat
with coverage expected to dwindle as builds east.

With elevated instability perhaps as high as 1000 j/kg and 2-6km
shear of 30-40 knots...potential is there for some organized storm
development and updraft helicity on hrrr indicates some need for
concern for marginally severe hail with any stronger storms through
at least middle to late morning.

A pretty sharp gradient will again today be present between warm and
humid less mixed conditions...with a hot...dry...and more mixed
airmass... likely to remain just west of the County Warning Area. Have raised temperatures
many areas by a couple degrees...with mainly 80s...but some upper
70s in more persistent clouds/potential precipitation axis. Have lower 90s
crawling back into the west...and with recent dry conditions and
winds gusting 25 to 35 miles per hour...will start to bring a bit more concern
to fire behavior...but starts will still be very difficult with
dewpoints in the middle to upper 60s holding back to Lower Brule area.

Not buying in to development of complex of storms tonight across
southeast South Dakota...which would almost require that development
to storms occurs back to the west where terrain/convergence would
have a better chance to overcome the massive warm layer aloft /+15
to +16 c at 700 hpa/ by evening. Retained a low probability of precipitation during the
evening in the event of outside chance development can evolve toward
more elevated as spreads east. A much better chance that a more
organized complex will take shape just north and east of the County Warning Area...
within warm advection zone ahead of low pressure pushing into the
eastern Dakotas...with perhaps a scattered to low likely chance by
later evening and early overnight in eastern parts of SW Minnesota/northwest Iowa.
Fairly south southerly gradient and very warm airmass was convincing
enough to raise overnight lows a few degrees.

Long term...(friday through wednesday)
issued at 420 am CDT Thursday Jul 24 2014

Expect convective activity to pretty much be east of our County Warning Area after
12z Friday. Should thus be a partly to mostly sunny sky...with just
some cumulus possible. Will have quite a bit of instability...but a
strong inversion aloft will keep US capped. Model thermal fields
have a decent spread...but overall have shown a warming trend on
recent runs. A bit concerned that the strong inversion aloft and
decent low level moisture of 60s and 70s dewpoints...will prevent US
from fully mixing...especially across our eastern areas. So did go
ahead and warm highs a few degrees across the board into the middle 80s
and low 90s...but did not go too extreme yet. Will have to monitor full mixing would result in highs a few degrees warmer
than currently forecast. This seems most probable west of Interstate
29...and thinking we may need to warm that area up on future
forecasts if trends continue.

Convective activity may become more active Friday night into
Saturday morning. A strong middle and upper jet will push across the
northern plains...accompanied by pretty good pv advection as well.
This large scale forcing will interact with a low level boundary and
easterly upslope flow across central South Dakota. Thus thinking is
we may see an area of elevated convection form...with storm motions
taking this activity east southeast into our County Warning Area. Actually pretty
good agreement between the European model (ecmwf)...NAM and sref on this solution.
GFS is a dry outlier...and seems to represent a less likely
solution. Thus did go ahead and increase probability of precipitation...moving into south
central South Dakota after midnight...pushing towards the Interstate
29 corridor through the morning. Amount of instability available to
this elevated parcel remains uncertain...but could end up around or
just above 1000 j/kg. Given the strong jet...speed shear should be
pretty good. Thus should storms least some severe hail
threat would seem to exist...possibly a wind threat too with the dry
sub cloud layer possibly helping organize storms along an outflow.
These elevated nocturnal thunderstorms are tough to
forecast...thus confidence on evolution remains low...but
something to keep an eye on.

Expect any convection would exit the area by way
to partly to mostly sunny skies. Temperature forecast probably a bit
lower confidence than normal due to differences in how models handle
morning convection and debris clouds. For now stayed near consensus
of low to middle 80s...but would expect adjustments up or down a few
degrees may very well be needed as we get closer. A strong upper
wave will dive southeast out of Canada into the Great Lakes
region...pushing a cold front through our area by Sunday. This will
result in a cooler end to the weekend. Looking at middle 70s to around
80 on Sunday...with blustery northwesterly winds gusting around 30
miles per hour.

This cooler and drier air is expected to continue into the middle of
next week as the pattern becomes more stagnant...with a deep trough
over the eastern USA and ridge over the west. Appears we will stay
on the cooler side of things...with the ridging unable to build far
enough east. Thus temperatures should stay several degrees below
normal through middle week...with highs in the 70s and 80s and lows in
the 50s and 60s. Not much forcing...but moisture increases
enough...that combined with the cool temperatures aloft...we may be
able to generate enough instability for a few diurnally driven showers
and storms by Wednesday.


Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1220 PM CDT Thursday Jul 24 2014

Overall...VFR will prevail for most areas through the taf period
ending at 18z Friday. But thunderstorms moving southeastward
across areas east of the James River Valley will certainly need
watched into late this afteroon and a Stout low level
jet remains juxtaposed right in this area. This will give the
possibility of continued scattered redevelopment. For now...this
is very difficult to time so left out of the three taf sites the
mention of thunderstorms and rain. Kfsd may be the most impacted by this scenario so
do not be surprised if you see amendments involving convection.
But also need to keep a close eye on ksux for redevelopment but
much of their activity may stay to the north and east. Khon may
also see most of their activity off to the east. So most concerned
about kfsd through the evening. In any strong storms...brief hail
and MVFR conditions are possible. Late tonight and Friday is expected that the convection will stay north of
our area. Concerning winds...expect frequent gusts from the south
and southeast along and east of the James River Valley this
afternoon of around 25 to 30 knots. Furthermore...a strong low
level jet will exist for much of the area tonight...some of which will
translate down to the surface. But for now left the mention of non
convective low level wind shear out of the tafs because surface
speeds may be strong enough to preclude the mention of it.


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


short term...Chapman
long term...chenard

National Weather Service Glossary of Abbreviations