Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
625 PM CDT Friday Jul 31 2015
Short term...(this evening through saturday)
issued at 257 PM CDT Friday Jul 31 2015
A weak surface boundary has settled across the far southern zones late
this afternoon. Generally this has allowed dew points to fall over
much of the County Warning Area...with temperatures hovering in the 80s.
Late this afternoon/evening...weak surface convergence along this
southward settling boundary will develop a weak cumulus field along and
south of the MO river near ksux...however much of this area remains
capped. Better forcing and moisture and forcing is found over
central and eastern Nebraska where a few storms are expected this
Otherwise...models all showing upslope flow developing over western
Dakotas and the Nebraska Panhandle as a weak wave enters the region.
Thunderstorms will likely form late this evening and track eastward
towards the MO River Valley by daybreak with the low level jet. Will begin to
increase probability of precipitation after midnight in the western and southwestern zones.
Saturday...should feature a considerable middle-level altocumulus castellanus field or
perhaps even decaying showers to start the day gradually spreading
east as low-level warm advection moves overhead. Still considerable
disagreement in GFS/NAM and other high-res solutions regarding
quality of boundary layer moisture in the late afternoon...both impacting
relative strength of cap aloft and instability. Averaging the
solutions...still allows for the development of 1500-2000 j/kg across a
broad area...but even moreso focused along a weak frontal boundary
over southern South Dakota and northern NE. With this wave approaching during
time of weakest convective inhibition...we should be able to develop
scattered storms by middle-late afternoon.
As far as hazards...shear and cooling middle-level temperatures would appear
adequate for the production of large hail...and depending on the
depth of afternoon mixing...steep 0-1 km lapse rates suggests a damaging
Long term...(saturday night through friday)
issued at 257 PM CDT Friday Jul 31 2015
Late Saturday storms should abate quickly to the southeast after the
early evening as upper and thermal support moves steadily
east/southeast. The severe storm threat will continue early in the
evening southeast. Sunday will be a bright and warm day with drier
air and unsupportive wind fields preventing storm development as a
cool front slips through during the day. Monday should be fairly
bright as well as moisture begins to build south of the area. After
Sunday upper 80s to middle 90s highs...Monday will be 5 to 10 degrees
The extended period...Monday night through Friday...is marked by a
wide split between the operational GFS and ec. Both models suggest
isolated to scattered storms during midweek as moisture begins to
pool just south and slowly spread into the area. The GFS would
suggest greater storm coverage due to better upper support...in the
form of short waves breaking across the upper ridge and moving over
the area. Both models suggest daytime temperatures running a little
below normal in spite of the ec rebuilding the upper ridge...as the
surface high depicted over the western Great Lakes helps feed a
relatively cool easterly breeze.
The main difference between the models shows up on the last day of
the forecast...next Friday...when the ec suggest decent warming
under the strengthening upper ridge...along with scattered storms...
while the GFS merges a westerly wave with a digging Canadian trough
to produce a dry and cooler scenario. Slightly favor the GFS
solution but will keep a small mention of storms along with the
below normal daytime temperatures for Friday.
By the way the ec eventually brings the dry and cooler scenario in...2 or 3
days later. This is apparently due to its temporary digging and
stalling next week of the Pacific upper low near the British Columbia coast before
bringing it east.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 625 PM CDT Friday Jul 31 2015
VFR conditions will dominate through Saturday afternoon...with the
primary question being convective chances late in this taf period.
Potential for isolated thunderstorms looks to increase after
01/21z...slightly more favored through the Missouri River valley.
However confidence in convection affecting taf locations too low to
include in tafs at this time. Light and variable winds overnight
into early Saturday will gradually increase to 10-15kt from the
south-southwest through the late morning/afternoon Saturday.