Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1105 PM CDT Thursday Jul 24 2014
Short term...(this evening through friday)
issued at 251 PM CDT Thursday Jul 24 2014
Favorable pattern for storms will continue to shift slowly east
tonight...including wind fields and resulting Theta-E advection.
Current storms will likely continue moving over eastern part of the
area...from north of fsd and over southwest Minnesota and northwest Iowa the
rest of this afternoon. Storms should decrease or briefly end early this
evening before some redevelopment as low level jet kicks in. This
development could get back close to fsd though will keep probability of precipitation here
just at slight chance for now. Isolated storms still seem possible
further west as increasing instability and moisture fight a
cap...though with that low level jet shifting east am temped to drop.
Storms should definitely decrease over our eastern area late tonight
and any isolated storms will quickly end west. The southerly warming
pattern should hold low temperatures in the middle to upper 60s.
Thermal and wind patterns favorable for storms will continue to
shift east of the area early Friday as low pressure exits the area
with the flow becoming very weak north of the low. Will keep only a
small mention far east early otherwise it looks dry. Fairly warm
temperatures in the 80s with around 90 along the Missouri River
Long term...(friday night through thursday)
issued at 251 PM CDT Thursday Jul 24 2014
A weak short wave passing over the area Friday night into Saturday
may take advantage of a fairly tight thermal gradient south of
digging Canadian upper low to produce a few storms over the
southwest part of the area and along the Missouri River to the Sioux
City area. Will mention as slight chance to low chance probability of precipitation mainly
late Friday night and early Saturday...keeping dry in the north.
Temperatures will show some slight cooling Saturday but will still
be mild. Any storms far south should end by the end of the day as
more stable air sets in.
The Canadian upper low will pass southeast across the far western
Great Lakes late Saturday night and Sunday and bring cooling. Will
keep a few showers northeast Sunday given the cooling aloft being
stronger than at lower levels.
Monday through Thursday look generally dry as the Canadian low digs
a broad upper trough east of the area. Extended guidance keeps a
slight chance of showers or storms near the end of the period and
will keep in given the models are still showing a northwesterly
short wave passing on the west side of the trough. This feature
looks quite weak and hard to time and am tempted to keep the
forecast dry through the extended period. However we are almost
doing that anyway given the low probability of precipitation. Temperatures should be quite
mild...that is below normal...with at most weak warming given the
anchored nature of the large scale patter with the ridge to the
west and trough to the east. Beyond the forecast period...the GFS
and ec both suggest warming in slightly different ways...the GFS
with a building of the ridge into the plains...and the ec with a
flattening of the upper flow south of a huge developing Hudson Bay
Aviation...(for the 06z tafs through 06z Friday evening)
issued at 1105 PM CDT Thursday Jul 24 2014
Currently mainly VFR across the area. Will have to watch convection
currently across the Western Plains as it moves east through the
night. Still looks like most of this activity will split our
County Warning Area...and then reform to the east of the taf sites after midnight.
So will continue to leave mention of thunderstorms out of the
taf...but will continue to monitor...as the northern South Dakota
storms may build south and impact khon. As the low level jet
increases tonight...low level wind shear could become an issue.
Surface winds may stay up enough so left mention out of the
tafs...but something to watch.
Last thing to monitor is potential stratus and stratocu development
late tonight and tomorrow morning. Seems like we may have enough low
level moisture ahead of the cold front to allow for some development.
Confidence is low...but signal in the GFS...NAM and several sref
members is strong enough to keep some MVFR ceilings in the kfsd and khon
taf. If morning stratus does form it should be east of the James
River and be based between 500 and 1000 feet. This will gradually
lift after sunrise...but scattered to broken cumulus will likely linger
at least into middle afternoon....based around 2500 to 3500 feet.