Area forecast discussion
National Weather Service Sioux Falls South Dakota
1249 PM CDT Wednesday Jun 19 2013
Update...
issued at 1101 am CDT Wednesday Jun 19 2013
Shortwave can be seen on water vapor imagery from south central
South Dakota into central Nebraska moving off to the northeast. Maybe
even a weak mesoscale convective vortex type signature entering Gregory County. Question
becomes what happens with convection this afternoon. Skies are
clearer than forecast...thus did go ahead and raise highs a couple
more degrees across the whole County Warning Area...but probably still not high
enough ins some spots. Model dewpoints continue to run high...making
assessing afternoon cape a bit difficult. If we continue to mix out
our dewpoints into the middle 50s...cape would only be between 500 and
1000 j/kg. However if dewpoints in the low 60s are able to creep up
ahead of the wave cape would end up over 2000 j/kg. Currently have a
bit of a cap around 600 mb...however this is forecast to weaken
through the day with the approach of the wave. Dry air entrainment
above 700 mb will likely limit usable cape some as well. Either way
it appears that cin will become near zero this afternoon...and with
the wave timing right during peak heating...would expect there to be
scattered thunderstorm development this afternoon. Veering low level
wind fields introduce enough shear for some multicell potential.
Still think most storms will be below severe limits...with winds
gusting to 45 miles per hour and up to dime size hail. However if dewpoints stay
up and cape values begin to exceed 2000 j/kg...would anticipate at
least an isolated severe hail report. So will just have to monitor
the mesoanalysis through the afternoon and see if and when scattered
storms get going.
&&
Short term...(today and tonight)
issued at 337 am CDT Wednesday Jun 19 2013
Short wave drifting over area today from the west will provide
some lift and convergence for showers and storms as instability
steadily increases. Low level winds also increase slowly...through
have doubts that much surface based convection will initiate late
in the day as wave will almost be on its way out. Activity should
therefore be mostly elevated. With middle level wind fields marginal
at best...not outlooking severe storms but having the low 5
percent threat for our area as Storm Prediction Center does seems reasonable. With low
level warming...temperatures should be warmer...but will keep
highs below guidance values due to cloud Dover associated with
wave. This will also be a negative factor in the heating for any
surface based storms...and also any severe threat through tonight.
With wave moving out but low level winds continuing to
increase...a storm threat should continue through tonight...or at
least return late tonight. Temperatures tonight will of course be
warmer as low level moisture increases...and we start thinking
about modest summertime humid conditions.
Long term...(thursday through tuesday)
issued at 337 am CDT Wednesday Jun 19 2013
Thursday thunderstorm threat is likely to start with more non
severe elevated convection from tonight. Timing and coverage
uncertain as it usually is with these type of storms. However
later in the day conditions appear to become more favorable for
some surface based storms. With better heating and increasingly
favorable wind fields for storms...some modest severe threat seems
to be there for late Thursday over the area. By Thursday night the
best low level winds and instability become concentrated over the
southeast part of our area...with the resulting lift extending
into southwest Minnesota and perhaps up to bkx. Organized severe seems
possible east Thursday night after the late day storms affect the
area to the west. Again timing and coverage uncertain and it is
hard to squeeze out stronger than high chance or barely likely
probability of precipitation...but there seems a decent conditional severe threat for
where storms affect the area. Temperatures and humidity continue
to become a bit more summerlike Thursday.
Western USA trough approaching will bring continued humid
conditions and scattered storms at times Friday through the
weekend...with the better chances of storms and also the severe
threat mainly during the nighttime hours. Passage of this trough
is currently expected to bring mostly dry weather for early next
week though for now have followed guidance continuing some chance
into Monday. Some decrease in humidity seems likely but
temperatures will be decently warm.
&&
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1242 PM CDT Wednesday Jun 19 2013
Mainly VFR conditions expected through the taf period. Although will
see two chances of potential thunderstorm activity. One is later this
afternoon into this evening...mainly along and east of the James
River...when isolated or scattered activity is expected. The other is
after midnight into early Thursday...when a complex may move in from
the northwest. Reductions to MVFR or IFR will be likely in any storm.
However confidence on timing and areal coverage is too low at this
point to mention in the taf. Other concern is potential stratocu
development in the morning. GFS and NAM soundings both hint at
this...and with strong southeast flow...can not rule this out. But
again will keep tafs on the optimistic side until confidence
increases. Low level jet increases enough tonight that low level wind shear may
become a concern. Surface winds will probably stay up enough to
limit this threat somewhat...although future shifts will have to take
another look.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Update...chenard
short term...
long term...
aviation...chenard
