Scientific Forecaster Discussion
fxus63 kdmx 201109
Area forecast discussion
National Weather Service Des Moines Iowa
609 am CDT Fri Sep 20 2019
..updated for 12z aviation discussion...
Discussion.../today through Thursday/
issued at 435 am CDT Fri Sep 20 2019
-generally dry conditions today, with continued above-normal
-Widespread showers and storms expected Saturday and again
Sunday... potentially bringing heavy rainfall to southern/southeast Iowa.
-Above-normal temperatures and precipitation expected for next week.
Ongoing morning convection from Tama through Emmet counties will
gradually drift northeastward and diminish... with impacts likely
being capped off by mid-morning. Scattered clouds and well-above
normal temperatures are expected once again today. With dewpoints
near 70 degrees, heat index values from central to
southern/southeastern Iowa will likely top out in the low 90s.
Well-advertised showers and thunderstorms remain forecast for
Saturday and Sunday. For Saturday afternoon and evening, a threat
for severe weather exists. Overall, the ingredients look slightly
weaker and slightly further west for this event than the forecast
models were showing yesterday. Damaging winds and large hail are the
primary threats with any storms that may become severe.
For Saturday night into Sunday morning, the threat shifts to heavy
rainfall... particularly across southern to southeastern Iowa.
The challenge here is that while confidence is growing higher that
heavy/significant rainfall will quite possibly occur during this
timeframe, the specific location of said precip is somewhat low.
Will have to monitor closely.
Looking ahead to next week, Iowa seems to be setup for westerly to
southwesterly flow. This suggests above normal temperatures will
likely continue, with several rounds of precip possible. As such,
have begun to boost daytime temperatures versus model guidance. Will
likely continue this trend with the next overnight update.
*/ Technical discussion...
At 09z Fri, a subtle shortwave/impulse... picked up on the 325k pv
sfc was propagating northward through Nebraska. Tied into this
impulse was a subtle 950mb frontogenetical forcing associated
with a boundary oriented from northwest to southeast across north-central Iowa.
With several hundred joules of cape to work with, instability
sufficient to support convection. By 15z, this boundary loses its
definition, so am expecting convection to rapidly wane around
this timeframe. Aside from lightning, no significant hazards
expected with this precip.
For this afternoon, plenty of instability for peak-heating storms...
however, vertical wind profile is terrible. In short, basically any
measure of shear is around 10 kts at best... which obviously will
not support updraft/downdraft interaction to yield well-sustained
storms. So, am not making too much of a mention of this.
Northeastern Iowa and far SW Iowa would be favored locations for
A huge trough/upper low pushes eastward across the upper Midwest...
arriving in the northern plains by 00z sun, and exiting towards the
Great Lakes region by 12z Mon. With very broad, deep high pressure
near the Carolina coastline through this weekend, the Gulf of Mexico
is wide open for moisture transport. Further, remnants from the
tropical system impacting Texas are expected to be assimilated into
this trough around Saturday.
Severe weather potential...
strong to severe weather potential exists Saturday as a boundary
oriented north-S is slated to push across Iowa from west to east
during peak heating. With a huge low level jet bringing 40 to 50 knots winds at
850mb with this system, shear will be much better than it has
been the past several days. Overall parameter space slightly less
impressive in today's model output than yesterday's output. Also,
a slight trend to slow the boundary... keeping it further west
during peak heating. Low-level lapse rates with the boundary very
high, and dcape values exceed 1000 j/kg. Our far SW cwa, near
Creston, may be the most favored area to watch at this time.
Damaging winds and large hail main hazards. Tornado threat cannot
be ruled out either, but wind and hail threat much greater.
Heavy rainfall overnight...
long-range and now medium-range models have been keying in on
reinforcing rounds of notable moisture Saturday night through
Sunday morning. Pwats with this are forecast to reach and slightly
exceed 2 inches. Warm cloud depths over 14 kft, 0 to 6 km relative humidity
values of 90 to 100 percent, and 0-6 km mixing ratios over 8 g/kg
all support potentially heavy, efficient rain producers. The
challenge with this setup is the location... the convection
expected Saturday could very well shift the mesoscale setup to
where precip misses much of the dmx County Warning Area to the south and
southeast. In terms of flood-related headlines, certainly cannot
rule out the need for headlines with this event. However,
southern Iowa has had 25 percent or less of their normal
precipitation over the past 7 days, and 1 to 2 inches below normal
precipitation for the past 14 days. Ultimately, this suggests the
ground is quite dry at this time. Portions of southeastern Iowa
are currently in a d1 drought status. Combining this setup with
the lack of confidence of where the heavy rain will setup (it
could very well be nudged south and east of iowa), am holding off
on any flood-related headlines at this time.
Aviation.../for the 12z tafs through 12z Saturday morning/
issued at 608 am CDT Fri Sep 20 2019
Medium-high confidence ongoing storms impacting kmcw and Kalo will
dissipate/move out of the area by 15z. Slight chance of storms at
kmcw this afternoon, but confidence not high enough to include from
Confidence increasing in MVFR conditions arriving towards 12z Sat
in advance of precip. Saturday could feature showers and thunder
through most of the day.