Scientific Forecaster Discussion
fxus63 kdmx 192334
Area forecast discussion
National Weather Service Des Moines Iowa
634 PM CDT Sat Aug 19 2017
Short term.../tonight through Sunday/
issued at 343 PM CDT Sat Aug 19 2017
The two primary concerns through the period will be upstream
elevated convection into the early morning hours and then the
potential for additional surface based development northwest into peak
heating Sunday. There will be two large scale forcing areas of note
upstream, a weaker wave currently crossing The Rockies into the
Western Plains, and other more significant lift ahead of the western
Canada trough with response into Montana and ND.
A weak surface boundary noted in the 19z objective surface analysis
and several GOES-16 products extends from southeast Kansas into SW Nebraska
and has triggered isolated convection. This is coincident with a
broad area of warm/Theta-E advection ahead of the aforementioned
weak rockies wave. While the airmass immediately upstream is fairly
moisture starved at the moment, deeper moisture and instability
should advance north and east into the immediate MO valley by early
Sunday. This may fuel a weak elevated warm advection mesoscale convective system into Iowa
into the morning hours, possibly lingering into midday, on the
remnants of any NE/Kansas surface based convection this evening.
A nice forcing couplet associated with the Canadian long wave trough
will push west to east and eventually drag its current Dakotas
surface reflection into northwest Iowa by afternoon. MLCAPE projections are
2500-3500 k/kg 21-00z but convergence will be weak with some
inhibition to overcome so the better potential may not develop under
beyond this period. If something does develop, a few strong to
severe storms would be possible considering the degree of
instability with 0-6km shear on the increase.
Long term.../Sunday night through Saturday/
issued at 343 PM CDT Sat Aug 19 2017
Bottom line up front...
strong to severe potential, along with the possibility of at
least moderate rainfall exists for Monday. At the very least,
there continues to be a decent likelihood of widespread cloud
cover to impact the solar eclipse. By mid-week, high pressure
takes over and cool, dry weather returns to Iowa.
Monday through Tuesday...
large-scale setup features an upper low spinning just west of the
Hudson Bay and a broad, well-defined area of anti-cyclonic flow
that has a core which makes it to 300 mb and encompasses most of
the deep south. This high is expected to slowly propagate eastward
the next few days... making it towards northern Mississippi by
12z Mon. Meanwhile, high pressure will be building over The Four
Corners region. Putting this all together, Iowa will be under
westerly flow aloft and southerly to SW flow at the sfc. Moisture
will be easy to access for our anti-cyclone, and a 30 to 40kt low level jet
will more or less reinforce transport of this moisture directly to
Iowa from Sunday morning through Tuesday morning.
By 12z Mon, models starting to come in line with a sfc low over
Nebraska and an attendant boundary with strong low and mid-level
frontogenetical forcing oriented west-east across northern to north-
central Iowa. This, along with models showing several rounds of
Theta-E advection pulsing through Iowa puts thermodynamical
forcing into play. Monday PM MUCAPE values respond by reaching
4000 j/kg. Low to mid-level lapse rates are decently
steep...especially across central to western Iowa...placing large
hail as primary threat. With low local heights and helicity off the
charts from the above-described strong vertical veering a tornado
or two would certainly be plausible as a secondary threat, along
with damaging winds. Though it will be cloudy and possibly rainy
much of the morning, still think enough forcing present to warrant
at least a slight risk for severe weather potential.
Tuesday and beyond...
long-range models and ensemble members have been in strong
agreement with a 1020mb or so area of high pressure coming
through the Dakotas by Tuesday afternoon and pushing across
Minnesota by the latter half of the week. Trend has been to come
in a little stronger with the high...thus a downward trend in
temperatures through the remainder of next work-week. Overall, a
very persistent forecast Wednesday through Friday. Given the
magnitude of this high pressure, would not be surprised to see its
propagation slowed even further, meaning cool and dry conditions
would persist into next weekend.
Aviation.../for the 00z tafs through 00z Sunday evening/
issued at 634 PM CDT Sat Aug 19 2017
VFR conditions will prevail through the taf period. A few showers
are approaching fod from the west at this time, but should
dissipate before reaching the terminal and even if they do not,
would be of little to no impact. Additional showers and possibly
thunderstorms are forecast toward sunrise Sunday and at intervals
during the day, but expected probability/coverage is too low to
include in the tafs at this time.