Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
633 am CDT Wednesday Oct 1 2014
Short term...(today and tonight)
issued at 314 am CDT Wednesday Oct 1 2014
Stratus continued to develop early this morning as warm and moist
air advects northward. With surface low moving almost due north into
North Dakota overnight...the eastward progression of a frontal
boundary has stalled with the front lingering just west of Huron and
Mitchell. Ahead of this boundary...dew points have crept back into
the upper 50s to lower 60s leading to a bit of fog through daybreak.
Temperatures have also leveled off and will remain in the low 60s
Aloft...water vapor imagery indicates shortwave energy rotating east-northeast
across western Nebraska this morning. Impressive lift ahead of this
shortwave will continue to produce convection over west central Iowa
and southeast Nebraska. As this wave moves northeast into the local
area...convection will begin to advect into north central Iowa by
daybreak...and the northern edge of the stratiform region of the
developing mesoscale convective system should skirt the southeastern zones. Further
west...beginning to see light echoes developing along and west of
the James River Valley. These echoes are likely tied to a weak
corridor of 850:700 mb warm advection that is expected to continue
into the daybreak hours as the wave passes through the area.
With the shortwave moving NE of the area this morning...the early
afternoon hours should remain dry. A stronger upper trough will
force the front eastward in the afternoon. A few showers or
thunderstorms will be possible along and ahead of this boundary into
the evening hours...but activity will generally remain low in
coverage. Rain will gradually spread towards the southern zones
late tonight...but may struggle to move into the area given
influence of dry air aloft.
Long term...(thursday through tuesday)
issued at 314 am CDT Wednesday Oct 1 2014
Thursday is looking increasingly interesting as a vigorous short
wave impacts our area. This short wave is currently situated west
of The Rockies in the Idaho and Utah area. The northern side of
the short wave passes across Nebraska on Thursday. Initially the
low levels are dry due to a north to northwest fetch of surface
air. Therefore early in the day...the best chances for rain are
across our far western and southern zones. Strong middle level
frontogenesis is firmly planted along an axis in our west...with
850mb frontogenesis across our south. As the wave shifts eastward
Thursday afternoon into eastern Nebraska and southeast South Dakota...moisture
increases dramatically near 850mb...with the 850mb and 700mb
frontogenesis becoming more juxtaposed across our southeast zones.
Therefore likely probability of precipitation are still warranted through the afternoon
generally along and east of a Vermillion South Dakota to Windom Minnesota line. One
final note about the precipitation...went ahead and took out the mention
of isolated thunder. Double checked both the NAM and GFS stability
parameters and at least on the 00z run...elevated mu convective available potential energy based
in the 925-850mb layer were very thin...on the order of 200 j/kg
or less in northwest Iowa. Furthermore...elevated lifted index
values were positive until getting towards Omaha. Concerning
highs...they are a bit of a challenge due to anticipated cloud
cover. Mixing does not look overly high either. Mainly along and
east of Yankton to Marshall...went with the cooler raw guidance
values and then decreased a few locations a degree or two from
there. The western zones still look a bit more mild with some
partial afternoon clearing possible.
On Thursday night...the well advertised cold upper low dives
southward into the northern plains from Canada with an attendant
surface cold front. Preferred the cold frontal timing of the NAM
and European model (ecmwf) which is a bit slower than the GFS. It appears the crux
of the cold air wedge is west of our forecast area Thursday night.
Coupled with increasing winds producing a well mixed lower
atmosphere along the surge of cold air advection...sided with the
warmer bias corrected values for lows.
Friday still looks like a foul day weatherwise. Stratus is likely
to follow the cold front across much of our forecast area. Not
sure if our MO River Valley counties will stratus in that far
south...but becoming increasingly confident that the rest of the
forecast area will. In time...a Wind Advisory will likely be
needed. By 18z...all models show 35 to 45 knots at 925mb...and 40
to 55 knots at 850mb. The lighter winds aloft are across our
western zones. However our western zones will likely mix higher
due to some possible partial sunshine...with mixing to 850mb in
our west quite likely. Under the stratus...mixing to 925-900mb
will probably hold east of the James River Valley with scattered
showers through the day. Will continue to mention the strong winds
in the forthcoming severe weather potential statement. High temperatures will be very chilly...
feeling worse with the wind. Went with straight raw model values
as mixing even as high as 850mb will not reach 50 degrees in many
of our northern and eastern zones.
For Friday night...there is still a degree of mixing despite the
winds decreasing dramatically. But there should be just enough
breeze to help keep the lows from tanking too hard. Not sure if a
frost advisory will be needed because we may not frost due to the
dry air and wind. With lows in the lower 30s though...it is
possible a freeze warning may be needed in the future...but at any
rate will mention this possibility in the severe weather potential statement also.
In the extended portion of the forecast...the GFS and European model (ecmwf) are in
good agreement Saturday and Sunday. The cold upper low which moves
across our area Thursday night and Friday deepens even more east
of here. Then the center of the low in southeast Canada actually
retrogrades into western and central Ontario by Sunday and Sunday
night. So moderating temperatures are in store...but would not be
at all surprised if we will need to shave a few degrees off of
sundays highs if the cooler European model (ecmwf) is closer to being correct. By
Tuesday the reverse happens...as the GFS keeps the cold upper
low/trough in place over eastern North America...with the European model (ecmwf)
becoming more zonal with the upper flow. Therefore the European model (ecmwf) is
much milder with the temperatures by Tuesday than the GFS will be.
Experience tells ME that the GFS may be better in this situation
as once these large...cold upper troughs become established over
eastern North America...they usually do not disappear as fast as
the European model (ecmwf) has advertised. Therefore kept highs closer to the GFS
on Tuesday. Left the extended dry for now as not seeing a lot in
the way of organized precipitation yet.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 632 am CDT Wednesday Oct 1 2014
Aviation concerns are abundant for this morning...especially for
the ksux taf site. Although hard to see on the infrared fog satellite
curve due to high clouds...observations suggest that ksux is on
the edge of of stratus...ranging anywhere from LIFR to lower end
VFR close by. For now...opted to go predominant IFR through the
sunrise hours but will have to monitor closely for conditions that
get worse. After sunrise...model soundings suggest that MVFR
ceilings should takeover for that site...before possibly burning
off in the afternoon hours. The surface winds and winds just off
the surface are light today...so mixing potential will be limited.
But but there does appear to be an overall trend of higher
ceilings upstream in southern Nebraska and southern Iowa...at
least when some of their fog Burns off in those locations.
Further north...kfsd runs the risk of some IFR ceilings through
sunrise. Then MVFR should takeover for the balance of the day...
possibly breaking up in the late afternoon. Khon appears to be on
the edge of stratus right now...and their winds will shift to the
west behind a weak cold front. So fairly confident they should go
VFR later this morning.