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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
147 PM CDT sun may 3 2015

issued at 147 PM CDT sun may 3 2015

Have seen scattered sprinkles/light showers develop over the
western County Warning Area along elevated boundary ahead of the middle level wave.
Updated grids to progress this activity eastward across the area
this afternoon...with convective potential gradually increasing
across southeast half of the forecast area as the elevated front
works into more unstable air mass. Surface based storms may be
tougher to come surface convergence somewhat ill-defined
with broad area of weak/variable flow ahead of the boundary...
located south of kykn-kfsd-ktkc line at 18z. Isolated severe still
appear possible across the southeast...with recent hrrr model runs
indicating a greater potential for wind threat associated with
expanding cold pool ahead of elevated storms as they track across
extreme southeast South Dakota/northwest Iowa.

Otherwise forecast appears fairly well on track. Post-frontal winds
became stronger than anticipated with initial deeper mixing across
northwest parts of the County Warning Area in the late morning...but seem to have
tamed down closer to expected ranges now.


Short term...(today and tonight)
issued at 358 am CDT sun may 3 2015

Concerns this today and tonight deal with convection chances this
afternoon along with Post frontal fire danger. Early this
morning...low level jet continues to veer and push convection eastward
and away from the County Warning Area into central Iowa. There remains an outside
chance for a few showers to popup over the eastern zones...but
looking less likely.

The main story today will be focused on a frontal boundary dropping
southeast through the area this evening. Early this morning...the
front was draped across central South Dakota...allowing low to middle
50 dew points to pool ahead of it. This front will slowly press
southeast stretching from Worthington to Yankton by 4pm. Models
continue to show modest destabilization through the day...with
upwards of 1500-2000 j/kg MLCAPE values developing along and behind
the boundary by late afternoon. There will likely be two main
forcing mechanisms this afternoon...increasing frontal convergence
late this afternoon across the northeastern half of the front in
Minnesota...and a shortwave rotating across Nebraska. While difficult to
pickup in WV imagery...this shortwave was beginning to enter the
Western Plains early this morning...evident by veering npn/vwp. The
difficulty thus assessing the strength of this
shortwave...which models have been struggling with.

Soundings indicate a relatively capped environment...which should
hold off surface based convection until middle-late afternoon...when
convergence along the boundary over central Minnesota should be sufficient.
Latest guidance suggests this boundary may surge a bit faster over
northern and central Minnesota with rather high pressure rises
across the western Dakotas. This in return could limit the
convective potential over our SW Minnesota counties. However...further
southwest along the line...into far southeast South Dakota...northwest Iowa...and NE...initial
isolated convection may be more Post frontal and elevated in
nature...developing along a secondary 925:850 convergence zone.
More isolated to perhaps scattered surface based convection could
develop once forcing aloft catches up to the surface front toward 00z.
At this point...forcing associated with the shortwave may be the
more dominate focus for convection as opposed to the front...with
the best chances for thunder along a Yankton to Spencer line. That
said...would not be surprised if convection is just not that

Again...instability is moderate...but shear profile seems marginal
for the event. Best shear is actually in the Post frontal
environment. That said...would anticipate fairly isolated to
at most scattered convection...with isolated severe storms.
Relatively steep lapse rates...and pocket of middle-level dry air would
support damaging wind threat. Cape profile fairly elongated...but
skinny in nature. With the relatively low shear
environment...looking at a marginal hail threat. Tornadic threat
appears low...given poor 0-1km shear...and rather high LCLs.

The shortwave will move quickly eastward in the evening...with
convection pushing the elevated moisture layer south of the County Warning Area.
Depending on the clearing...looking at lows in the 40s.

Long term...(monday through saturday)
issued at 358 am CDT sun may 3 2015

While stronger ridging will attempt to become reestablished across
the northern plains on Monday...will have to deal with lift
associated with exiting upper jet entrance region pulling across the
area Monday into Monday evening. The morning hours will see main
boundary at lower levels continuing the push to areas between I-80
and I-70...with fairly strong push of lower level dry air through
much of the County Warning Area. Middle level moisture should stall across the area...
but no more than a minimal threat of a middle based shower along the
extreme southern border through the morning. Instability displaced
well southward to preclude any thunder mention...but this should
start to change during the afternoon as stronger upper forcing will
combine with increase in isentropic lift to bulge precipitation back
northward through the Missouri River corridor along with weak
elevated instability for an isolated thunder mention.

Going through Monday night and Tuesday...larger scale broad
isentropic lift will spread gradually northward to the north of
strong surface warm front lifting gradually north of I-80. Most
solutions do suggest that may see a midday minima in qg support for
precipitation with ridging strengthening aloft to east...and ahead of next
lobe ejecting from southern rockies closed low pushing toward
western County Warning Area. However...with persistent isentropic lift...expect at
least a scattering of showers to be present. Instability is just to
the weak side of thunder will be far from predominant
in coverage...and have generally kept a category or two below any
precipitation mention.

On Tuesday night...could actually see a decrease in coverage of
precipitation cycling northward through the eastern County Warning Area...with main
isentropic band lifting north of the County Warning Area...and lull ahead of
moisture conveyor lobe which will wrap rapidly north and eastward
toward the Missouri Valley by late night. The main dynamic forcing
will swing northeastward later Wednesday and Wednesday night as
upper low lifts from Colorado through the Dakotas...and should focus
period of most significant rainfall across the area. Instability
just a bit stronger...but deep layer shear is quite
Prospect of severe storms minimal with wave out of phase with
diurnal cycle as well. Thursday will see the low level boundary
pushing eastward across the County Warning Area as upper low lifts north of the
region. Depending on timing of of potential
warming ahead...may see severe parameters fall out a bit stronger
near frontal zone. Much of the Tuesday to Thursday period will have
quite a bit of cloudiness and moisture...with lower diurnal range.
Still should see temperatures at or above normal with upper 60s to middle 70s
common for highs...and 50s to around 60 for lows.

The end of the longer range for Thursday through Saturday continues
to advertise some significant differences in details...while a
larger scale agreement of western troughing to eastern plains
ridging remains in play. European model (ecmwf)/Canadian suggest a bit more southward
and quicker push to baroclinic zone by Thursday night and Friday
resulting in a drier period perhaps lasting into Saturday...and
would not be surprised to have things shake out in that manner.
Another fairly strong lobe will be moving in just beyond the end of
the forecast period...and may have somewhat slightly better shear
and instability at hand.


Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 147 PM CDT sun may 3 2015

Convective chances through the afternoon/early evening...along
with gusty Post-frontal surface winds...are primary concerns for
this taf period. Convective threat somewhat ill-defined as cool
front works through moderately unstable air mass in northwest Iowa
this afternoon. Scattered thunderstorms still appear possible in
areas south/east of kfsd and will maintain dry forecast for kfsd
as this point...though potential for isolated elevated rain showers/thunderstorms and rain
for an hour or two in the late afternoon. Greater chance for the
ksux area in the early evening as elevated front works through the
area. Late afternoon/evening thunderstorm activity could produce
variable gusty winds...possibly in excess of 40kt...though lower
confidence in timing/location of wind threat precludes mention in
tafs at this time.

Post-frontal winds gusting 20-30kt more widespread and expected to
persist into the evening before diminishing after dark.


Fire weather...
issued at 358 am CDT sun may 3 2015

High to very high fire danger expected today...primarily along
and west of a Marshall to Yankton line. The driving force behind
todays high fire danger will be strong Post frontal winds up to 20
knots at times. The air immediately behind the incoming frontal
boundary will not be all that relative humidity values will only fall to
around 25 to 30 percent. Thus...not anticipating red flag warning
conditions but any burning should be done with caution.


Fsd watches/warnings/advisories...
South Dakota...none.


short term...dux
long term...Chapman
fire weather...dux

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