Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
325 am CDT Sat may 23 2015
Short term...(today and tonight)
issued at 321 am CDT Sat may 23 2015
Another relatively pleasant day is expected across the area today.
Radar lighting up across the area...but echos fighting a lot of dry
air in the sub cloud layer. Expect this to be the story for the
better part of the day...with low level moisture trapped further to
the south. Moisture will continue to work north in the
southeasterly flow leading to an increasingly unstable atmosphere.
With that said...still very little instability so have dropped
mention of thunder until middle afternoon. Even then...only expecting
a rogue lighting strike or two. Tonight have some weak elevated
instability as short wave pivots in from the west. With increased
forcing and moisture...have raised probability of precipitation into the categorical range.
With relatively dry atmosphere...expect relatively strong mixing
today and have raised forecast highs to a NAM/Gem/BC cons blend.
This results in a fairly substantial adjustment to highs along and
east of the James where dry conditions are expected to prevail.
Didn/T change lows much for tonight.
Long term...(sunday through friday)
issued at 321 am CDT Sat may 23 2015
The various models including the NAM...GFS...Gem and European model (ecmwf) are in
pretty good and detailed agreement in ejecting Saturday nights short
wave northeastward across the forecast area Sunday morning. High
probability of precipitation are warranted in the morning...minus our south central South Dakota
zones...lifting to the north...with a dry slot aloft between 850mb
and 500mb then taking over much of the forecast area by afternoon.
There is not a lot of instability Sunday morning as the rain moves
through. However in the afternoon...0-1km ml convective available potential energy nudge close
to 1000 j/kg coupled with 925mb-850mb lifted index values of -3 to
-4c generally from near Yankton to Chamberlain. This area is in the
crux of the dry slot...so deep moisture is certainly limited. But if
there was any chance for some marginal severe weather in the late
afternoon or early evening it would be in those locations. Helicity
values are limited due to limited wind shear. Highs on Sunday should
be in the middle 60s to around 70 with what should be abundant cloud
cover through the day...with potential stratus under the dry slot.
On Sunday night and Monday...the large upper low across the Western
Plains lifts slowly northeastward into ND and northwest Minnesota. Yet
another discernible short wave lifts across our area Sunday
night... once again with marginal instability. Then the base of
the upper low looks to move through our forecast area on Memorial
Day afternoon. Finally this last wave will be associated with a
surface wind shift late Monday afternoon and evening which should
provide an additional focus for thunderstorms. The GFS at 18z
Monday does show around 1500 j/lg ml cape and 925-850mb lifted
index values of -4 to -5c. But just ahead of the wind
shift...these kind of values then shunt eastward through the
afternoon and by 00z Tuesday the instability is in our far eastern
zones. So perhaps another marginal severe threat for our eastern
zones late in the day as temperatures warm into the middle 70s. Wind
shear is moderate...at least in the 0-3km layer...not so strong in
the 0-1km layer. And 0-6km bulk shear vectors are averaging close
to 30 knots...so will mention this possibility in the forth coming
severe weather potential statement.
Further out...aside from an isolated Post upper trough light
shower on Tuesday...not much is going on Tuesday and Tuesday
night. Tuesday will feature a breezy northwest wind...but very
nice temperatures in the middle 70s. The GFS and European model (ecmwf) do have their
differences Wednesday through Friday...but in general show a wavy
pattern with scattered chances for showers and storms really about
everyday. Not too untypical for this time of year...as middle may to
middle June is our wettest 30 days climatologically.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1127 PM CDT Friday may 22 2015
For the most part...should remain largely VFR for much of the
period. Initial band of rain showers pushing slowly northward
and slowly dissipating through early morning...with ridging
aloft leaving a lower than mentionable level of rain threat for
much of the day. By the final 6 to 8 hours of the taf period...
Prospect for showers will increase...as will the chance of
developing some MVFR ceilings.