Area forecast discussion
National Weather Service Sioux Falls South Dakota
646 am CDT Friday may 24 2013
Short term...(today and tonight)
issued at 412 am CDT Friday may 24 2013
Sharp ridging aloft currently across the County Warning Area will gradually lose
hold as middle level feature shifts eastward and impinges a bit more
southwesterly flow through the day. Keewatin high continues to
filter drier air at low levels into the area on easterly flow...
but moistening on strong low level jet through the Western Plains
leading to expansion of lower to middle clouds across western South
Dakota and western and central Nebraska gradually building eastward.
More middle level clouds showing up of late in southwest Nebraska...and
area correlates quite well with development of stronger isentropic
lift. Lift spreads rapidly east and northeast and will be across
much of southeast South Dakota between 12z and 15z. However...likely
the leading middle level showers will diminish lifting northeast while
running into drier air with perhaps a couple of sprinkles somewhat
ahead of the main area of precipitation...expected to start to
develop by later morning from Lower Brule areas into the lower James
Valley. Much of timing for precipitation defined by 305k net
isentropic lift...considering where drier air at low levels will
impede progress somewhat. Continued to introduce an increasingly
better coverage for showers as lift forcing spreads east towards and
east of i29 during the afternoon hours. Generally 200-500 j/kg
elevated instability from in and around 750 hpa level initiation...
so while thunder will be far from prevalent...does warrant a more
isolated mention.
Should also note that it will be quite a windy day along and west of
i29...especially from the James Valley west. At this time...likely
will be marginal for issuance of a Wind Advisory...but would not be
shocked to touch 30 miles per hour sustained or even gust to around 40 miles per hour at
times.
The surface winds will gradually decrease through the evening
hours and will drop more after midnight as winds aloft gradually
weaken with time. The band of warm air advection/Theta-E advection
precipitation is gradually expected to shift off to the east. Not
much in the way of instability...and toyed with the idea of
reducing thunder mention in the forecast.
Long term...(saturday through thursday)
issued at 412 am CDT Friday may 24 2013
Tough forecast through the extended with active pattern and little
certainty in how everything will play out. On Saturday...model
soundings appear to be very saturated especially in the low
levels. Models continue to warm the low levels despite this...but
am concerned it would take till at least middle day if not longer to
erode stratus in the lowest 1500 feet. As such...lowered forecast
highs some especially in the north and east portions of the
forecast area where stratus will be the most stubborn. This also plays
into the amount of instability. Surface based cape in southwest
corner of the forecast area climbs to near 1500 j/kg throughout
the day...but still appears to be effectively capped at least from
the surface. Unlike last night/S model runs...models have backed
off some on the low level jet Saturday night...bringing forcing
into a question. Agree with previous forecaster that tall thin
cape profiles and precipitable water values near 1.30 create great
antecedent conditions for flash flooding. With weak middle level winds...
precipitable water values well above the 95 percentile and the
potential for a nearly stationary boundary...field appears to be
set...however...one thing missing appears to be a trigger across
the area. Can/T rule out convection moving into the region...but
at this point am beginning to question probability of precipitation and have trended down
slightly.
Similar set up Sunday with low level saturation...however...appears
to be more shallow and with slightly stronger winds may be able to
mix out stratus with day time heating. With warm front hanging
across the area...have a focusing mechanism but atmosphere still
appears to be capped at least from the surface. Models really try
to erode the cap by late in the day as wave approaches from the
southwest...question becomes whether elevated inversion will erode
that quickly. Still appears to be a good set up for flash flooding
with precipitable water values near 1.3 inches...especially for any storms that
would be able to train in the same area. Shear profiles Don/T
support a widespread outbreak of severe weather....but may be able
to get an isolated storm or two to produce marginally severe hail
based on instability...especially if storms are able to develop in
the near surface layer.
Much of next week continues to remain active with dirty southwest
flow aloft and short wave train moving across the area over the
next several days ahead of a upper level low that is expected to
impact the region next weekend. Models are much weaker with upper
ridging in the middle week...and therefore...kept medium range probability of precipitation
through the extended with the chance of thunderstorms.
&&
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 645 am CDT Friday may 24 2013
Khon on edge of stronger gradient aloft this morning...and will
keep a couple hours of non convective low level wind shear to
start the morning before the wind profile starts to mix out.
Otherwise...mass of lower to middle level clouds developing in
warm air advection zone through much of Nebraska and South Dakota.
As a weak wave slides up through the Western Plains...this will
help to ignite showers and a few thunderstorms in focused warm air
advection zone pushing across the area today. All indications are
that lift forcing will become more focused as area shifts to near
and east of Interstate 29 by afternoon. With instability profiles
fairly narrow...prefer to keep overall thunder mention with band
minimal. Favorable set up for developing stratus field behind
precipitation overnight tonight...and likely to get into lower
MVFR to upper IFR ceilings along with some MVFR visibilities.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...Chapman
long term...
aviation...Chapman
