Area forecast discussion
National Weather Service Sioux Falls South Dakota
326 PM CDT Thursday may 23 2013
Short term...(this evening through friday)
issued at 305 PM CDT Thursday may 23 2013
High pressure across the region will begin to push off to the east
overnight...and the main effect from this will be to bring winds
around to a southeasterly direction in the returning southerly low
level flow on the backside of the high. Will see the development of
a low level jet over western South Dakota during the night time
hours...and this will begin to nudge into our far west by Friday
morning as a middle level shortwave begins to track out of The Rockies.
With increasing midlevel Theta-E advection via the aforementioned
jet and weak elevated instability...may see showers and
thunderstorms develop in our far west early Friday morning...then
push to the east during the day on Friday as the shortwave and a
surface warm lift up over the area. Would not expect any severe
storms with only relatively weak instability in the middle levels and
effective shear in this area only around 15 to 20 kts. Friday will
be a breezy day with winds of around 30 kts at the top of the mixed
layer and a tightening gradient in our west as the surface low
pressure tracks out of the western High Plains. In terms of
temperatures...thermal profiles will begin to warm from the west
tonight...so will see lows in the upper 40s through south central
South Dakota/James River Valley...to cooler lower 40s in our east.
In spite of the warming thermal profiles...fridays temperatures will
be similar to today with the clouds and showers...topping out in the
upper 60s to around 70.
Long term...(friday night through thursday)
issued at 305 PM CDT Thursday may 23 2013
Friday night is a bit of a mess to decipher through the models. An
elevated band of rain and thunderstorms will likely exit the eastern
County Warning Area during the evening associated with something close to the
700-750mb layer. From about 3z through 6z the low level jet will
crank up at about 850mb and may allow for more thunderstorms to
develop around the Missouri River and track eastward and likely
track into northwest Iowa. The initial elevated band will likely not
have quite enough instability or shear to produce any severe weather
but the second area of development may see cap values around 1500
j/kg so not out of the question to produce some hail...especially
with about 30 to 40 knots of shear. The freezing level is at about
11000 feet so pretty average for this time of year. An interesting
note...the NAM BUFKIT soundings are not matching up with the NAM
plan view output in AWIPS...resulting in pretty big differences in
shear values.
The set up Saturday into Saturday night may pose a threat for severe
weather and possibly some heavy rainfall. While upper level flow on
the latest runs of the models has increased a bit...which is what
brings more of a threat for severe weather with 0 to 6km bulk shear
about 40 to 45 knots and a fairly strong cape gradient along the
Missouri River valley with 2500 j/kg convective available potential energy to the south of the
boundary. What also makes this an interesting set up is that the
timing of the wave allows for an increasing low level jet through
the evening so if storms can develop late afternoon along and
north of the boundary there should be a steady or increasing feed of
deeper moisture and better instability. Right now surface based
thunderstorms appear unlikely with a fairly significant cap so
believe any thunderstorms that can develop north of the Missouri
River will be elevated so hail and heavy rain will be the main
threats.
By Sunday the threat for thunderstorms will continue as boundary
looks to linger around. While instability may still be fairly high
around 2000 j/kg in the southern County Warning Area shear looks to drop off
considerably. Kind of a wait and see for this day as with convection
many times the boundary locations will be dependent upon the
convection or lack thereof the previous night. Otherwise the
expectation is that cloud cover will keep temperatures in check a
bit as will the threat for showers and thunderstorms through the
day. Lows middle 50s to lower 60s with highs from 65 to 70 in southwest
Minnesota to near 80 along the Missouri River.
Monday through Thursday a very unsettled period with the West Coast
trough moving into The Rockies and then potentially onto the plains
late in the period. This allows the Gulf to remain fairly open and
any waves that eject onto the plains will have a good chance to
produce thunderstorms. This makes it pretty difficult to pick any
periods of truly dry weather. So unfortunately will have the chance
for thunderstorms in pretty much every period. As the trough moves
onto the plains shear will also increase so the potential for severe
weather will likely be a little higher with any storms that can
develop.
&&
Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1216 PM CDT Thursday may 23 2013
VFR conditions will predominate through the taf period. Increasing
southeasterly winds aloft will result in low level wind shear at
khon later tonight. Showers and thunderstorms will develop through
the James River Valley on Friday morning...and southeasterly winds
will pick up across the entire area...gusting to 25 kts at times.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...jm
long term...08
aviation...jm
