Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
625 am CDT sun Jul 24 2016
Short term...(today and tonight)
issued at 359 am CDT sun Jul 24 2016
Anxiously watching frontal zone clear the far southeastern corner of
the County Warning Area around Storm Lake in the wee hours of the morn, for this
means that the relentless humidity will be coming to an end.
Dewpoint readings already into the lower 60s west of the James
River, and trend downward likely to continue today with
northwesterly flow continuing to feed in drier air, albeit a fairly
slow change across northwest Iowa. Today will end up much more
pleasant on a heat and humidity front, however, there is a fly in
the ointment to address for the weather today. Positively tilted
upper trough will continue to progress eastward through the northern
plains, with secondary wave from Montana to Wyoming shearing through
during the midday and afternoon pulling jet entrance region across
the area. Evolving fairly strong mid-level frontogenetic zone will
enhance cloudiness quickly from west to east during the morning, and
could be enough lift to overcome dry air by a few showers this morning
toward k9v9/khon. Already see impact of forcing in two areas early
this morning, increasing showers in southeast Mt, and more
recently a few showers developing southeast of the Black Hills.
Band will continue to stream eastward along frontogenetic axis
through the afternoon, and with a few hundred j/kg of elevated
instability, showery nature should be enough to overcome low level
dry air. However, radar will probably look much more ominous from
a coverage perspective than what will actually fall, primarily due
to continued feed of dry air at lower levels. Have also included a
chance for isolated thunder per degree of high- based instability.
Main uncertainty with this will be the north/south location of the
main precipitation area, with a great diversity between cams
mainly near and north of I-90, and others closer to the Missouri
River. Given recent trends, and gradual southward focus of the
larger scale forcing, have trended solutions to begin near/north
of I-90 and gradually spread east/southeast through the afternoon.
Forcing will drop off fairly rapidly from west to east by very late
afternoon, with perhaps just a few showers remaining by early
evening toward northwest Iowa. High pressure will settle into The
Heart of the area overnight, allowing for the coolest night in
nearly a week.
Long term...(monday through saturday)
issued at 359 am CDT sun Jul 24 2016
Monday: as high pressure slides southeast on Monday, we'll continue
to see a very pleasant day, albeit a touch on the warm side.
Temperatures west of the James River should climb into the 90s, with
areas eastward in the middle to upper 80s. With dew points in the
60s, heat indices shouldn't be an issue.
Tuesday-Tuesday night: moisture transport will increase throughout
the day on Tuesday, becoming focused over northwestern and north
central South Dakota. Early morning activity may be possible on the
nose of the jet over central SD, but higher rain chances will hold
off until later in the day and the overnight hours. There will be
increased potential for convection to form as a stronger shortwave
enters the plains. Depending on the orientation of a warm frontal
boundary lifting northward, convection should form into one or more
clusters, eventually evolving into a forward propagating mesoscale convective system Tuesday
night. Where this mesoscale convective system tracks is a bit in question, but location of
the Theta-E axis would suggest areas along the Missouri River valley
will see the highest potential. I would not rule out a severe
weather risk at this point, but the highest risk seems to be with
heavy rainfall given precipitable water values upwards of 1.75" thanks to Stout
850:700 mb moisture transport.
Wednesday: will again be monitoring track of overnight MCS, but low
level wind field is expected to be very chaotic on Wednesday. Would
anticipate some stabilization in the wake of the overnight activity,
but soundings suggest rather sharp destabilization by Wednesday
afternoon, especially across MN/IA. The Fly in the ointment could
be finding a specific trigger for convection. Temperatures with
increasing cloud cover should rise into the lower 80s.
Thursday-saturday: again a very unsettled weather pattern setting
up, as broad troughing persists over the Great Lakes early in the
period, with ridging trying to return by the weekend. It's hard to
dig too deep into the fine details at this point, given substancial
uncertainties in the mesoscale features. However, anticipating
several chances for thunderstorms through the end of the month. As
far as temperatures are concerned, a brief break from the heat
through the end of the week, but signs of returning above normal
temperatures are apparent by next weekend.
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 625 am CDT sun Jul 24 2016
Scattered mid-level showers and a few thunderstorms will spread
from west to east across the area today. While there is some
uncertainty in the north/south location of the main band, it will
likely initiate in areas north of I-90 toward khon, then gradually
refocus progressively southward through the afternoon. Conditions
will remain VFR within the band, and for much of the area through
12z Mon. Spotty MVFR visibilities and ceilings will be possible
on the western flank of the Buffalo Ridge from east of kbkx toward
kotg through 14z.