Area forecast discussion
National Weather Service Sioux Falls South Dakota
631 PM CDT Wednesday may 22 2013
Short term...(this evening through thursday)
issued at 305 PM CDT Wednesday may 22 2013
Upper low finally departing the region to the southeast and
currently located over the Iowa/MN/WI state lines per water vapor
imagery. However...a vorticity maximum on the west side of the low /currently
over the tri-state area/ will continue to generate showers this
afternoon/evening...slowly ending from west to east as the upper low
and vorticity maximum both sink southeast. No focused mesoscale forcing looks
present...as showers are synoptically driven...so only light showers
are expected through this afternoon/early evening.
Overnight...airmass drying works from middle/upper levels down and
advects south across the forecast area. Low level winds will veer to
east-northeast as trough moves southward. This winds direction
typical favors lingering moisture/stratus...however...with such
extraordinary dry airmass in mid-levels/as seen on water vapor over
ND and northwest MN/...expect that scouring out the stratus should not be
an issue. Light winds and recent rains will likely result in some
radiational fog late tonight where clearing occurs first...and have
expanded fog coverage for late tonight across the area. After a cool
night...temperatures will begin to rebound Thursday as upper ridging
amplifies and warm air advection sets in for the afternoon. Will
also have plenty of sunshine /maybe a few cumulus/ and a dry airmass in
place as high pressure axis builds across the upper Midwest. 850 mb
temperatures increase to 5 to 10 c from east to west across the forecast
area by 00z Friday. Despite this warm-up...temperatures still will average
nearly 5 degrees below normal for highs Thursday.
Long term...(thursday night through wednesday)
issued at 305 PM CDT Wednesday may 22 2013
Friday will see a huge changeover aloft as dew points around 700mb
at about -40 degrees c Thursday night will increase to about +5
degrees c by fridays afternoon. With 700mb to 500mb lapse rates also
running about 7 degrees or so...a bit tough to gauge as
precipitation breaks out in the model...there should be a decent
chance for an elevated band of showers and thunderstorms to develop.
Will continue with some Middle Range probability of precipitation but maybe increase just a
bit. Elevated cape from around 750mb to 700mb running about 500 to
700 j/kg which supports thunder chances.
Saturday looks like a bit of a breezy day as a warm front becomes
established over the southern County Warning Area into parts of northern Nebraska. A
decent amount of instability low level shear exists but winds aloft
not very supportive of organized convection. Not to mention capping
looks pretty strong and convection will likely need to develop
outside of the area and advect in. Will decrease probability of precipitation a it on the
north thinking capping will have everything shut down.
No easy answers for Sunday into Wednesday as a boundary looks to
linger around much of the time. Relatively weak upper level flow
should preclude any major severe weather outbreaks Sunday into
Monday but enough low level shear and instability to promote the
possibility for a few severe storms. As we move into Tuesday and
Wednesday the models begin to shift the faster southwest flow aloft
onto the northern and Central Plains which if we still have a
boundary around there may be a better chance for severe
thunderstorms with deeper enhanced shear. Otherwise...the European model (ecmwf)
continues to lock in the cooler air Sunday and Monday more so than
the GFS and Gem. Tuesday and Wednesday will see a better chance for
warmer conditions and with the low level ridge in the south
shifting east the Gulf opens up a lot more and deeper moisture will
likely exist. Will unfortunately have probability of precipitation in every day but leaning
towards better chances south Sunday and Monday then anywhere Tuesday
and Wednesday...maybe a little better in the eastern two thirds.
&&
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 630 PM CDT Wednesday may 22 2013
MVFR conditions with some localized IFR will gradually erode
southward through tonight. Behind the stratus...VFR conditions
should take hold but there is one caveat. A surface high pressure
ridge axis will extend from central South Dakota to central Minnesota by late
tonight and early Thursday. Therefore the winds will likely go
very light in our northwest zones including the khon taf. With the
moist ground...am worried about fog potential late tonight and
early Thursday for those areas...so kept the mention of some IFR
visibilities for khon. Otherwise VFR should prevail for most areas
on Thursday. Would not be surprised to see some scattered to
broken ceilings from evaporation off of wet ground in the late
morning and afternoon. But latest NAM model soundings show that
any broken ceilings should be in the lower VFR category.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...
long term...08
aviation...mjf
