Area forecast discussion
National Weather Service Sioux Falls South Dakota
317 PM CDT Wednesday Jun 19 2013
Short term...(this evening through thursday)
issued at 315 PM CDT Wednesday Jun 19 2013
Shortwave continues to work across the area this afternoon. Modified
soundings suggest cap should be broken by this point. However
current cumulus field is clearing being choked off. Guess is that dry air
aloft entraining into the updraft...combining with slightly warmer
middle level temperatures than modeled is the culprit. Also the lack of any low
level focus for storms to get going. Still think the threat warrants
isolated to scattered mention in the grids through this evening...as
still could get something as we continue to destabilize ahead of the
cooling middle level temperatures associated with the wave. Small hail
and wind gusts to 45 miles per hour continue to be the main threats with any
storm that is able to form. Low level jet really picks up tonight
resulting in strong thetae advection and elevated thunderstorm
threat. Model consensus is that most likely area for this convection
to form would be north of our County Warning Area closer to the nose of the jet and
cooler middle level temperatures. If this convection became organized
and was able to establish a surface connection...it would follow the
corfidi vectors south southeast into our County Warning Area. While if ot stays
mainly elevated it will follow more of the mean layer wind and go
northeast. Will have to watch what California form in northwest or north
central South Dakota as the low level jet kicks in and how it starts
moving. Best chance of seeing something appears to be southwest
Minnesota based on the above potential storm motions...and will keep
the higher chance probability of precipitation there.
Uncertainty continues into Thursday. Seems like we will have to
deal with some leftover cloud cover from whatever happens
tonight...and possibly some stratocu that forms with our strong
southeast flow. Combined this with a pretty decent cap...and
convection during the afternoon seems unlikely. 12z NAM was really
the only model aggressive with quantitative precipitation forecast on Thursday...and even the NAM hi
res version is dry. Thus went ahead and lowered probability of precipitation. Will keep the
slight chance to chance given uncertainty. Cap does weaken as you
get east...so that appears to be the area that would have the best
shot. Cape and shear profiles appear favorable for severe weather if
a storm is able to form...but again this seems like a lower
probability event at this time...at least through 0z. Highs are
tricky...and will depend on cloud cover. Our previous forecast was
above consensus...and while this makes ME a little uncomfortable
given cloud cover concerns...it does not take much sun for our temperatures
to jump rapidly. Thus ended up not changing the going forecast much
at all with regards to temperatures.
Long term...(thursday night through wednesday)
issued at 315 PM CDT Wednesday Jun 19 2013
Thursday night should see a few thunderstorms develop in across the
area with the better chance in southwest Minnesota. Capping will
likely play a role in keeping thunderstorms at Bay so only low probability of precipitation
over the western and southern parts of the County Warning Area. Parts of southwest
Minnesota may be fighting off a worked over atmosphere but if some
decent heating can occur cape...around 2000 j/kg and surface to 3 km
bulk shear values of about 30 to 35 knots would support a few severe
thunderstorms. Otherwise breezy and a bit humid with lows from the
upper 60s to lower 70s.
By Friday the better chances for thunderstorms will be in the far
eastern County Warning Area near the instability axis and the veered low level jet
while another chance will exist near and behind the front in the far
western County Warning Area. By late afternoon and early evening the atmosphere
becomes favorable for severe thunderstorms...especially in the
northern County Warning Area where the cap is much lower. Cape values likely to
approach 3000 j/kg with shear values not overly impressive.
Regardless expect a few severe storms north of Interstate 90 late
Friday afternoon and Friday night as +14 at 700mb likely to be a
little too warm in the south to allow any updrafts to get going.
Highs Friday from near 95 in Sioux City to the middle 80s along Highway
14. Lows Friday night again very warm and mainly upper 60s to lower
70s.
Will continue the thoughts that most of the activity will be north
and west of the area on Saturday with a burly cap in place and the
area beginning to get into more of a deep southerly flow regime with
little convergence. The better chances will be along a front in
central South Dakota. Will go with hot and breezy conditions...highs
from the middle 80s T middle 90s...then lows again Saturday night upper
60s to lower 70s.
For the period Sunday through Wednesday...an upper level wave will
pass north of the area Sunday night into Monday which will continue
the threat for a few thunderstorms. After this the models begin to
diverge a bit but still support a relatively fast west to southwest
flow aloft across North Dakota into southern Canada. This should
start to decrease the threat for thunderstorms a bit as the area
could get deeper into the warm and dry air aloft. However the GFS
and European model (ecmwf) both bring a wave through Monday night...more vigorous on
the GFS...so no great certainty that it will be dry.
&&
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.
&&
$$
Short term...chenard
long term...08
aviation...chenard
