Area forecast discussion
National Weather Service Sioux Falls South Dakota
659 PM CDT Sat may 25 2013
Short term...(this evening through sunday)
issued at 343 PM CDT Sat may 25 2013
Main forecast challenge is convective initiation and evolution this
evening and tonight. Thermal cap has held thus far along the warm
front this early afternoon. Surface warm front is stretched across
southwest South Dakota and southeast across north central Nebraska
to southeast Nebraska. This warm front will be the focus for
convective initiation across southwestern South Dakota this
afternoon as approaching vorticity maximum /currently across eastern Colorado per
water vapor imagery/ provides needed synoptic forcing. The other
sources of lift for convection is expected this evening as diurnally
induced southerly low level jet impinges warm frontal boundary across eastern
Nebraska/western Iowa. Edge of instability axis per NAM MUCAPE will
continue to sit from northwest to southeast across the area through
tonight. Convection will become elevated as it pushes into the area
both from the south with the sloping warm air advection...and with
the short wave convection from the west. Western convection may
develop into an mesoscale convective system and will likely follow instability axis/middle
level heights in an east-southeast direction over the County Warning Area overnight. As any
convection approaches areas into SW Minnesota...instability will decrease
quickly and storms should weaken. There is still a possibility of
surface based/strong storms across south central sodak and toward
the Sioux City area this early evening along and just north of the
warm front. Deep layer shear is approaching 55 knots across south
central sodak suggesting the potential for organized storms
/supercells/ with large hail and damaging winds with any convection
that may initial this evening there. Elevated instability of
1500-2000 j/kg across the forecast area except for SW Minnesota should
support potential for small to marginally severe hail and some
stronger winds gusts to 50 miles per hour with mesoscale convective system. As decoupling
occurs...model forecast soundings showing a strong low level
inversion and low level stratus over the region...especially east of
the MO river...but also including sux.
Convection should be exiting region Sunday morning with good portion
of the forecast area under low stratus. Destabilization expected to
occur across south central South Dakota and MO river again...as
mixing occurs and more sunshine prevails. Afternoon initiation looks
lacking as there are no strong signals in flow aloft of vorticity maximum or
short wave...and upper ridge looks to dominate. Better chance comes
in the evening and overnight again...so have kept just chance probability of precipitation
in for the afternoon. Farther east...stratus is expected to persist
for much of the day again with persistent east-southeast low level moist flow.
Long term...(sunday night through saturday)
issued at 343 PM CDT Sat may 25 2013
The active weather pattern continues throughout the middle and long
range with a continuation of precipitation mention in each period.
Still seeing quite a bit of uncertainty in the timing and evolution
of the numerous shortwaves tracking along the dirty southwest flow
as well as the placement of any boundaries.
Precipitation chances increase once again Sunday evening through the
overnight hours as daytime destabilization and a strengthening low
level jet nosing into the area lead to sustained convection
overnight. The best potential for more organized convection appears
to be near the Missouri Valley near the quasi-stationary boundary and
in the area with the best shear. A few storms may become severe with
large hail threat in the evening likely shifting to more of a wind
threat overnight. Convection should move out of the area Monday
morning...and much like Sunday...modest mixing in the afternoon
should allow some sunshine to peak through the clouds in time to
heat temperatures into 70s...and near 80 in the Missouri Valley.
With southerly flow continuing to tap into plenty of moisture and a
pair of shortwaves expected to track into the central and northern
plains providing upper level support...expect thunderstorms to again
become likely late afternoon and overnight. With instability
potentially as high as 2000+ j/kg and decent shear profiles...
especially in the late afternoon through early morning
hours...severe weather will be a possibility for much of the
forecast area. But still a few question Marks on the timing and
location of initiation and whether enough clouds can scour out
during the daytime.
Moving into the extended...the main upper low slowly wobbles across
the inter-mountain west through late next week. While models are
showing some major differences in the speed and strength of this
progression...all roads lead to a continued wet pattern for the
central and northern plains. While persistent rainfall is not
expected...periodic showers and thunderstorms seem likely through at
least Saturday with varying chances with each passing shortwave in a
similar fashion to what we have been seeing recently. Not a lot of
confidence in pop values in each period so went with allblend
numbers...but the screaming message is for a persist wet pattern
with near seasonal temperatures.
&&
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 659 PM CDT Sat may 25 2013
MVFR-IFR ceilings will have greatest aviation impact through much of
this taf period...as stratus over southwest Minnesota/northwest Iowa expands
westward again this evening on east-southeast low level flow. Low
ceilings expected to persist much of the morning...scattering slowly
from south to north through the late morning/early afternoon...though
this timing may be impacted by location of residual boundaries and
associated stable layers from thunderstorms expected to move across
the region overnight. At this time...04z-10z window looks like most
favored time for prolonged thunderstorms to affect taf locations...
though timing of best thunder threat still tentative as watching for
existing activity across the western High Plains to spread into the
area...as well as potential for additional development to our south
in Nebraska. Will watch closely and amend timing as needed through
the evening.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...
long term...
aviation...jh
