Scientific Forecaster Discussion
fxus63 kfsd 190341
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1041 PM CDT Thu Jul 18 2019
Short term...(this evening through friday)
issued at 440 PM CDT Thu Jul 18 2019
Short term focus remains consistent on the convective threat as well
as the heat.
The first day without appreciable outflow concerns from the
overnight and early morning has yielded an uncomfortably hot and
humid day across the area. Afternoon heat index is mainly from 95
north to 105 south. A subtle surface boundary has pushed back
northward through the day, and is currently identifiable by a
southeast to south wind shift, a fine line on radar, as well as a
marked difference in cumulus coverage. Atmospheric thermodynamics
suggest a fairly strong cap is in place with 700 hpa temps from +12
to +14 across the area. Cumulus field has been increasing in depth a
bit over the last couple hours, and there is a signature of a subtle
Twist near the kmhe area along the western edge of the thicker
cumulus field. Heading into the evening, not expecting anything
near a significant coverage threat for storms given the warm layer
aloft and only the most weak of forcing. However, given a very
subtle wave moving through the evening, is possible for perhaps a
slightly elevated isolated storm to develop toward southwest Minnesota.
Deep shear parameters and the somewhat thicker cape profile suggests
a conditional severe weather threat. Strong dcape as well, so would
generally be a hail/wind threat, although the toasty temps aloft
should limit potential hail size as well.
Continued warm and humid tonight with south/southeast flow, with
most lows in the lower to mid 70s.
Friday will find a wave moving over the ridge, with stronger lift
forcing well being a boundary which will trudge slowly southward
across the area through the day and overnight. With a lack of
convective support to force a more aggressive southward trend, a
later day boundary location from kmml-kfsd-kykn should allow much of
the area to sizzle again on Friday. Should have quite a bit of sun
with lesser extent to any restrained cumulus, and overall pattern of
wave over Flat Ridge in this axis location is one of maximizing the
potential warming. Most of the area likely to reach the lower to
mid 90s again, with dewpoint readings in the lower to mid 70s.
Possible that could even get a couple degrees warmer than forecast,
but since this would also in all likelihood be accompanied by a
slightly lower dewpoint, there should be minimal impact on the
forecast heat index. Expected combo of heat/moisture in the air
suggests that there is a need to expand the excessive heat warning
northward roughly a tier of counties, with a buffer of heat advisory
during the afternoon north of this area. Could end up needing a
further northward expansion, but still a decent amount of
uncertainty in thermal fields north of I-90.
In terms of Friday convection, the cap should hold for most, if not
all of the daytime to early evening period. Kept an ever so slight
chance of the boundary sparking an rogue storm, which would be most
probable if could manage to heat out.
Long term...(friday night through thursday)
issued at 440 PM CDT Thu Jul 18 2019
Threat for convection should slowly increase through the night.
Potential for convection should come in two waves, smaller with the
first with the isolated threat along and just north of the boundary
during the evening and early overnight, and second as the mid-level
cooling approaches through Post-frontal areas later night. Evening
storms would contain a little better threat for severe weather given
a slight increase in shear and the more extreme instability,
especially if any storms would develop along the otherwise capped
frontal boundary. Spotty heavy rains would also be possible. Later
night storms would have less but sufficient effective shear, but
still a greater instability, keeping up with a somewhat lesser
Saturday, the boundary lingering near the southern border of the County Warning Area
will exit, taking worst of the heat impacts with it. Continued
southeastward progression of the mid-level cooling and stretched
upper lift forcing dragging across the area should maintain a threat
for storms, but gradually shifting southeast through the day and
into the evening. Temps in the upper 70s to mid 80s will feel
better, but will still need to shake some lingering moisture by
later into Sat night/Sunday before becoming truly comfortable.
Most showers and storms should be south of kmwm-kfsd-kykn line by
early evening, and pretty much clear of northwest Iowa by very early
Cooler and drier air will settle southward through Sunday into
Sunday night. Could still be a little more cumulus develop on Sunday
along/east of I-29, but sunshine should be a fairly great quantity
working into early week. The general pattern heading into next week
is amplification of the western ridge/eastern trough early in the
week, with wave by Thursday pushing flat ridging back into the area.
Good news is that organized precipitation threat is minimal after
Saturday night, although could have to watch for something drifting
off the High Plains toward south central South Dakota by mid-late week. Temps
cooler than normal to start the week, climbing back toward and a bit
above by late week.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1040 PM CDT Thu Jul 18 2019
VFR conditions expected through the taf period.
South Dakota...excessive heat warning until 8 PM CDT Friday for sdz068>071.
Heat advisory from 1 PM to 8 PM CDT Friday for sdz059>065.
Excessive heat warning from 1 PM to 8 PM CDT Friday for sdz066-
Minnesota...heat advisory from 1 PM to 8 PM CDT Friday for mnz081-089-090-
Iowa...excessive heat warning until 8 PM CDT Friday for iaz020>022-031-
Heat advisory from 1 PM to 8 PM CDT Friday for iaz002-003.
Excessive heat warning from 1 PM to 8 PM CDT Friday for iaz001-
NE...excessive heat warning until 8 PM CDT Friday for nez013-014.