Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
627 am CDT Wednesday may 6 2015
Short term...(today and tonight)
issued at 407 am CDT Wednesday may 6 2015
Isentropic lift continues to increase this morning...as large upper
trough ejects eastward into the plains. The diffuse nature of the
lift has led to little organization in the precipitation overnight.
At the surface...still watching fog/stratus bank lifting northward
into southwestern Minnesota. With the warm front continuing to lift
northward...will likely cancel a few counties in Iowa from the
advisory prior to 4 am.
For today...precipitation will remain very scattered through
daybreak. Higher precipitation chances will begin to build after
daybreak and especially by middle/late morning as lobe of energy
currently over eastern Kansas lifts towards the area. Hrrr/RUC
support this wave spreading showers and embedded thunder over the
area by late morning.
After the initial wave passes at mid-day...we should begin to see
additional destabilization through the afternoon. Given the
persistent strong lift overhead...would anticipated scattered to
numerous convection across building from south to north through the
late afternoon. Really no focusing mechanism...so hard to pinpoint
where rain will actually fall at any given point.
Soundings do suggest fairly thin cape...upwards of 500 to 800
j/kg...slightly higher along and south of the MO river. Could not
rule out a marginal hail threat with any stronger elevated
thunderstorm storm later today.
By tonight... models consensus supports the idea that robust
convection will develop along and north of a dryline across western
Nebraska/Kansas. Large scale forcing for assent will drive
additional convection further north along developing inverted
trough/cold front. As the upper wave pivots northeast...this
convection should rotate northeast into the County Warning Area during the late
evening and overnight hours. Some potential that convection further
south on the dryline could drive highest rain amounts south and east
of the County Warning Area. Again...not much in the way of instability advecting
northward as this activity approaches...so severe threat low.
Long term...(thursday through tuesday)
issued at 407 am CDT Wednesday may 6 2015
Organized precipitation band will continue to pull east and
northeast early in the morning as tail of upper wave drags across
the eastern County Warning Area. Low level jet will be in process of veering toward
the southwest in support of the exit of precipitation early in the day.
Behind wave...heights once again will subtly rise and expect that
much of the area will be devoid of precipitation...or have some
isolated activity at best through much of the day. Plenty of low
level moisture ahead of the boundary could start to thin some and
allow area of broken cloudiness to develop near the surface boundary
during the midday and afternoon. While this surface boundary will be
working across southwest Minnesota and northwest Iowa during the
afternoon...the upper levels are fairly non supportive to lift
forcing that far east...with secondary wave lifting back through
central toward northeast South Dakota by evening. Front is only
minimally convergent...so hard to picture any more than a scant pop
for some surface based instability assistance as boundary moves
across northwest Iowa middle to late afternoon into very early evening.
Little pulse of 40 to 45 knot middle level winds will stretch the
hodograph out a bit...but quite unidirectional flow limiting shear.
Not out of question if can get a bit more surface heating...could
get into a marginal severe weather environment in terms of hail or
even a brief updraft phase spin ups with shallow cape...but this
would be a very conditional set up.
A few solutions indicate another weak impulse moving northeast on
Friday...perhaps brushing far southeastern areas with a small
rainfall threat. Present in enough solutions to keep from removing
from forecast. Overall...will be looking for the Friday/Saturday
period to be largely dry and cooler...with varying amounts of
cloudiness...but again under increasing influence of the north to
northeast low level flow. Friday night could have just enough
clearing to be problematic to temperatures...but also will maintain at
least some north to northeast wind.
Extended range once again looking to be active as deep western U.S.
Trough will send out another significant piece of energy into the
central and northern plains in the form of a closed low. Solution
runs the gamut of track and to a lesser degree timing...but appears
to be potentially a fairly significant precipitation producer for
the end of the weekend and start of work week. Will likely see
fairly broad precipitation shield work north and eastward on
Saturday night...most likely impacting areas through the Missouri
Valley by later night. Dry slot should be rotating northward Sunday
and may decrease precipitation across the south and east for a while
during the afternoon...but bulk of precipitation will wrap back to
at least the North/Western County Warning Area as 700 hpa gradient backs into
western South Dakota. Tremendous lower to middle baroclinicity will be
in the mix...so likely that some areas could get a decent amount of
rainfall on Sunday and Sunday night. Strong elevated instability so
would also be looking at thunder for areas south/east of the main
deformation band. If evolution is closer to stronger and slightly
quicker European model (ecmwf)...would even bring a severe weather threat to at least
the lower Missouri Valley/northwest Iowa toward Sunday evening.
Temperatures challenging with cooler drier air providing evaporational
component north...while warming and saturating within strong low
level baroclinicity to the south...which may or may not creep toward
the far southern County Warning Area. Potential for large errors...so nudged away
from the mean on both ends of the County Warning Area a bit...cooled the north and
warmed the south.
Common feature of solutions will be the intense pressure gradient
between the Central High plains low and strong surface ridge nosing
in from Canada. Gradient is supporting much stronger winds than
forecast would suggest...and if can get some evaporative cooling to
support wind on Sunday morning...would not be surprised to find some
40 to 50 miles per hour wind gusts on leading showers...especially with
easterly winds coming off the Buffalo Ridge.
Much cooler on Monday with perhaps 10 to 15 degrees below normal
temperatures...with a bit of recovery Tuesday as clouds and precipitation
pull out of the area.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 621 am CDT Wednesday may 6 2015
Tricky aviation forecast this morning. We will continue to see
showers lift northward into the area later this
morning...becoming prevailing late morning into the early
afternoon. More scattered convective activity may be possible
this afternoon...with what appears to be widespread convection
chances later this evening and overnight.
Ceilings will initially remain VFR/MVFR...but may turn more
prevailing MVFR through the day. A high degree of uncertainty
regarding ceilings overnight and into Thursday morning tied to
convection. For now...will plan on MVFR ceilings and
visibility...but further reductions in heavy thunderstorm activity
Minnesota...dense fog advisory until 8 am CDT this morning for mnz080-081-
Iowa...dense fog advisory until 8 am CDT this morning for iaz002-003.