Area forecast discussion
National Weather Service Sioux Falls South Dakota
639 am CDT Sat may 25 2013
Short term...(today and tonight)
issued at 430 am CDT Sat may 25 2013
Thunderstorm complex over Nebraska overnight continues early this
morning. Seeing showers and storms beginning to expand across
northwest Iowa as well as low level thetae advection increases. High
res model suite look to have a pretty good handle on this activity.
Expect it to continue this morning and possibly expand a bit further
north into portions of southwest Minnesota. Lowered probability of precipitation across
southeast South Dakota...as all indications are that these storms
will not develop northwest of their current location. Most of the
storms should be east of the County Warning Area by 16z. Main impacts will be
briefly heavy rain and possibly small hail with these cells...as
effective shear and cape are too low for any severe threat. Much of
the afternoon should then be dry across the County Warning Area with an area of
subsidence likely behind this mornings shortwave. Warm frontal
boundary will position itself south of the Missouri River this
afternoon. Hrrr and rap try to break the cap their this
afternoon...but both these models seem a bit overdone with dewpoints
and instability. Feel more likely scenario is activity waits for the
low level jet to kick in after 0z before it gets going.
Other issue today is cloud cover and high temperatures.
Interestingly the hrrr and rap are complete opposite of each
other...giving around a 20 degree difference in highs...with the rap
having more stratus and cooler temperatures. The Gem and European model (ecmwf) side
closer to the rap...with the NAM in between. Since the areal
coverage of low stratus is currently less than expected...did go
ahead and raise highs several degrees for today in our central and
west. Also the hrrr has a good handle on the current situation...so
one would think its short range forecast should be somewhat
reliable...and it has consistently been warm for today. Lowered
highs a few in the east where it is raining...and where stratus
should indeed hold in for much of the day. Again this is a pretty
low confidence temperature forecast for anywhere east of the James
River. But feel going forecast is a decent starting point...with the
day shift likely having to adjust up or down depending on cloud
cover trends.
Quiet short term conditions from the afternoon hours should turn
quickly to more stormy conditions overnight. Gave a fair amount
of credit to the consistency of the 00z high resolution namdng5
and WRF-arw solutions...which fit the overall conceptual model for
convective initiation tonight. Boundary from current night mesoscale convective system
will be festering and drifting northward...with likely 1000 to
1500 j/kg elevated cape lifted from 1 to 1.5 km above ground level. As 35 to 40
knot low level jet develops tonight through the Central Plains...
should become enough to initiate elevated thunderstorms near/north
of the boundary which will be drifting northward of Interstate 80
during the evening hours. Effective shear in areas north of the
boundary will be very concentrated in the 1-3km layer with strongest
winds in low levels and weakening middle level winds roughly 20-30
knots...with lesser deep layer shear contributions from Above. All
suggest at least a minimal threat for isolated larger hail or
damaging winds through the Missouri River corridor and lifting
somewhat northward through 066-08z. Other significant convective
initiation is likely to occur back through the western High Plains
where stronger shear exists with a bit stronger middle level winds
to around 25 knots...as well as the deep upslope transport of moisture
to result in stronger instability. Eventually...when supercell motions
which will take action more southward than east start to ease...and
low level jet veers eastward...will find a more linear mesoscale convective system likely
taking shape. Many hi res models suggest this mesoscale convective system will zipper down
the location of the warm advection Wing spreading northward...moving
toward east central South Dakota and southwest Minnesota late night. Severe threat
with this feature would seem to be somewhat lower...but likely to
get some gusty winds and areas of heavy rainfall.
Long term...(sunday through friday)
issued at 430 am CDT Sat may 25 2013
Mesoscale convective system expected to weaken and shift east on Sunday...and with middle
level forcing showing tendency to force subsidence around
midday...again expect that much of the daytime period will
probably end up precipitation free. However...clouds likely to
persist with stratus developing in wake of system. Most likely to
get breaks through the western and southern County Warning Area during the
afternoon...and have pushed expected temperatures to near 80 there...
while more middle to upper 60s linger in the far east. Result of this
will be generation of decent potential instability of 1500-2000
j/kg across the area...with weakest of the capping inversion near
the Missouri River corridor. Shear becomes a bit more distributed
through the effective layer...and fairly close to those values
from this evening from 30 to 35 knots. A few more severe storms
would be possible mainly through the southwest half of the area
during the later afternoon and evening hours...as initiation
likely to be a result of weak impulse sliding up back side of
ridge in the later afternoon and early evening.
The further one GOES into the forecast...the less reliable any
guidance becomes given the abundance of convection...and the
increasingly great uncertainty as to where critical boundaries may
lie. One thing for sure...given the closed low across the western
U.S. And the southwest flow aloft...difficult to pinpoint any time
from Monday on that one could say there was absolutely no chance
for precipitation...but on the same front it certainly will not be
an all time washout with many breaks defined on the mesoscale.
Memorial Day and Monday night will place the area on the southern
flank of a couple of waves moving over the ridge...so likely to
have enhancement of precipitation in the nocturnally favored
periods. While a few severe storms are again well within
reason...high precipitable water and fairly slow moving wind
profiles bring the concern of areas of heavy rainfall...at this
range hard to define. At the moment...the Prospect for more
significant convection and severe weather would appear to be late
day Wednesday into Thursday...as deep western closed low kicks
stronger jet energy into the plains. Moisture through lower levels
/thus instability/ will continue to benefit from persistent
southerly flow. Temperatures will likely gravitate to somewhat
above normal readings through the upcoming week.
&&
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 619 am CDT Sat may 25 2013
Low confidence taf forecast. Many guidance solutions showing IFR to
MVFR stratus this morning. These solutions show it developing over
Nebraska and Iowa and advecting into our area. Winds are
southeasterly from 1500 feet and below and southwesterly above that.
That southwesterly wind is advecting in dry air...thus only stratus
below 1500 feet would be able to advect into the area. Currently not
seeing much of that out there. Thus cut back on the low ceilings and kept
most of our area VFR. However will have to monitor things through the
morning. Not out of the question that low stratus still forms...or
that we get some MVFR stratocu after sunrise. Next problem is
convection. This mornings activity should remain east of the taf
sites and exit the County Warning Area by middle to late morning. Tonight...expect
activity to get going in the vicinity of a low level warm front after
03z. Where this exactly happens remains uncertain. At this point it
appear that areas near the Missouri River have the best shot. Also
expect a thunderstorm complex to form to our northwest...and dive
through the area after 7z into the morning hours. Main threats will
be reductions to IFR visible/ceiling in the heavier rain...but some hail and
gusty winds are also possible. Stratus may also try to form
tonight...but that will be dependent on how well we mix out the low
level moisture today. Did put MVFR ceilings in the tafs starting this
evening...but this is also unfortunately uncertain.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...Chapman/chenard
long term...Chapman
aviation...chenard
