Area forecast discussion
National Weather Service Sioux Falls South Dakota
639 am CDT Sat may 18 2013
Short term...(today and tonight)
issued at 338 am CDT Sat may 18 2013
Unstable conditions again on tap today for the region. Breezy
southerly late morning and afternoon will continue tapping into
plenty of moisture and should hold dewpoints around the 60 degree
mark throughout the forecast area. Although we should see some
residual clouds from overnight convection linger this morning...we
should see enough afternoon sunshine for temperatures to easily
warm into the 80s. Shear looks pretty modest into the
afternoon...however lapse rates and cape values increase
significantly through the day. With the trough just beginning to
move out of The Rockies today and surface convergence likely remain
outside of the forecast area...convection seems unlikely for our
forecast area with strong capping in place. Have removed
precipitation chances for today with the exception of a slight
chance in our extreme western counties late afternoon. /
Heading into the evening hours...likely that the warmer temperatures
aloft will provide just enough capping for most of the warm sector
to relegate the convective threat for the early evening to
generally the James Valley and west...closer to location of the
low level boundary and any residual outflows from earlier day
activity. Rap and to lesser degree GFS indicate that may be as
much as 100-150 j/kg lid over the fairly moist surface mixed
layer. Above...a very unstable elevated mixed layer...which will
set up an atmosphere with 1250-1750 j/kg instability in Reserve.
Likely that coverage of storms may be somewhat lesser until late
evening and early in the overnight...when should finally start to
feel impact of wave currently swinging through the Desert
Southwest. Any more scattered development James Valley and west in
the evening...even somewhat elevated...would certainly have enough
effective shear of 25 to 35 knots in order to organize and present
a threat for isolated severe storms with hail up to two inches...
damaging wind gusts...and a tornadic threat near discernible
boundaries...especially as shear increases with strengthening low
level jet early this evening. The increase in low level 0-3km
shear strongly suggesting that will get some upscale organization
and development of bowing segments in the overnight hours...
perhaps a fairly widespread damaging wind event with gradually
lesser hail potential through 08-09z as development in Nebraska
streams northeastward in tandem with upper forcing pulse. Much
more coherent band of thunderstorms will swing eastward overnight...
with severe weather threat diminishing after 08-09z. Some areas
of heavy rainfall over the past couple of days will heighten
concerns somewhat of flash flooding...with some areas of 1 to 1.5
inch per hour /1.5 to 2 inches per 3 hour/ thresholds along and
east of i29.
Long term...(sunday through friday)
issued at 338 am CDT Sat may 18 2013
Expect to see an exit early in the day of the main convective band...
with leading wave along boundary lifting northward...and synoptic
north/south boundary drifting through areas between the James
Valley and i29 through the day. Deep trough will start to swing
negative tilt energy around the base and into the region in the
18z-21z time frame. Not nearly as capped for this time around...
and should initiate storms along any boundaries quite a bit
earlier in the day...perhaps as early as 18z-20z across eastern
Nebraska where it somewhat more unstable and will find the lift
approaching sooner. Model based deep bulk shear of 30 to 35 knots
in place...and 0-3km shear of 20 to 25 knots is very indicative of
organization and supercell potential. There are a lot of questions
regarding the potential for thunderstorm development tonight to
redistribute the threat area. However...at this time...would
appear that locations from just west of I 29 eastward would have
the greatest threat for severe storms...including the risk of large
hail and isolated tornadoes...especially the lower Missouri Valley
eastward into northwest Iowa...transitioning toward a gradually lesser
hail and greater wind threat farther eastward through northwest
Iowa...with the threat lasting through early evening.
On Sunday night...negative tilt energy swinging northward will
start to close off the middle level trough over Nebraska and South
Dakota...and it will be a long and drawn out process to finally
rid ourselves of this feature through midweek. This spells out
an unsettled period with waves of scattered to fairly widespread
showers and a few thunderstorms. Fairly moist neutral by Tuesday...
and gradually limited the isolated thunder mention toward the east.
Will have to monitor the threat for spin up given the proximity of
the closed low on Monday...and even in the east on Tuesday...but
much will depend on being able to break out of the clouds and more
widespread precipitation...and generate a decent surface based
lapse rate during each diurnal cycle.
Gradually...will see a decreasing and more diurnal threat of
showers by Wednesday... still with the area under influence of
cyclonic flow as upper low wobbles away through Iowa. End of the
week did finally open up the diurnal ranges a bit more with impact
of drier easterly flow from Keewatin high.
&&
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 627 am CDT Sat may 18 2013
Showers and thunderstorms in south central South Dakota expected to diminish by
around 15z. Could see a few gusts to around 35kt with thunderstorms
in the khon area this morning. Also watching a band of MVFR stratus
across western and central Nebraska which is spreading northeast this
morning. Expect this band to largely dissipate through middle/late
morning as it approaches it Missouri Valley...with little to no impact
in our forecast area. Otherwise...expect showers and storms to push
in south central South Dakota after 00z and become more widespread and they
track east into the i29 corridor around 06z. Strong to severe storms
will be possible this evening and overnight with large hail...gusts
to 70 miles per hour possible. Best threat for large hail will occur from 00z to
06z and may transition to more a damaging wind threat late evening
and overnight. MVFR/IFR ceilings possible in stronger storms.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...Chapman/
long term...Chapman
aviation...
