Scientific Forecaster Discussion
fxus63 kdmx 211112
Area forecast discussion
National Weather Service Des Moines Iowa
612 am CDT Sat Sep 21 2019
Discussion.../today through Friday/
issued at 431 am CDT Sat Sep 21 2019
..updated for 12z aviation discussion...
-scattered storms through most of today, followed by slight
risk of severe weather this evening for SW Iowa
-widespread showers and storms expected tonight through Sunday
morning... potentially bringing heavy rainfall to southern/southeast Iowa.
-Above-normal temperatures and precipitation expected for next week.
Scattered showers and storms ongoing across Iowa
should yield no hazards aside from lightning. For Saturday late
afternoon and through the evening, a slight risk of severe
weather exists... primarily in southwestern Iowa to west-central
Iowa. Overall, the ingredients continue to look slightly weaker
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. A brief spin-up tornado or two
cannot be completely ruled out either. The general track of the
storms along and north of I-80 should be west to east. In SW
Iowa, the storms may have a slight more SW to NE component of
motion to them.
For Saturday night into Sunday morning, the threat shifts to heavy
rainfall... particularly across southern to southeastern Iowa. A
widespread 1 to 2 inches of rainfall is forecast from around
Creston to Pella and south/southeast. Nearing Ottumwa and
southeastern Iowa, total forecast rainfall amounts may near 3
inches. This will likely come in two larger surges... the first
surge from around 7 PM through midnight tonight... the second
surge tomorrow morning near sunrise through mid-morning.
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. At
this time, Tuesday into Wednesday, and both Friday and Saturday are
the most favored time-frames for precipitation. Further, there is a
non-zero severe weather threat for Friday and Saturday.
*/ Technical discussion...
08z Sat water vapor imagery shows two upper lows more or less
going through a fujiwara effect over the intermountain west. By
00z Mon, the resultant trough of this interaction should be
entering into Minnesota. Meanwhile, incredibly broad anti-cyclonic
flow from 850- 300mb is over the southeast Continental U.S.. this large-scale setup
is producing a strong fetch of SW winds that GOES into the baja
of California. Currently in the baja of California, more or less
stalled out, is Hurricane Lorena. GOES-r water vapor channels show
moisture from this hurricane already being advected into West
Texas towards the Permian Basin.
with the sfc reflection low beginning to take form over North
Dakota via the aforementioned upper low interaction, the warm
advection Wing- highlighted by several northwest-southeast ribbons of Theta-E
advection coupled with distinct 850 moisture flux convergence are
slated to be advected through Iowa during the morning hours...
supporting continued scattered showers and thunderstorms across
most of the state.
as has been well-advertised a north-S oriented attendant boundary to
the parent low over North Dakota is slated to propagate across
Iowa from west to east during peak heating. Over the past several
days, the timing of this boundary has been delayed to being over
west-central Iowa during peak heating. The major inhibitor of
significant convection the past few days during peak heating has
been an abundant shortfall of shear/0-6km wind profile support.
With the strong SW flow arriving, a 30 to 40 kt low level jet ends the lack
of shear. Strong winds and large hail are in play as low-level
lapse rates near the boundary are rather impressive for this time
of year. SW Iowa, from Creston through around St. Joseph, MO, is a
particular area of intersect as pocket of strong forcing and
vertical ascent- supporting processes arrive in the 21z sun to 03z
sun timeframe. After Sundown, these storms will likely continue
to move into NE Iowa, but the severe wx threat should wane.
Tonight through Sunday morning...
heavy rainfall potential continues as advertised. Will keep
mesoscale details brief, other than to say that heavy and
efficient rainfall processes are very well supported. Pwats over 2
inches are highly abnormal in Iowa for this time of year. Models
generally converging on a solution that brings northern edge of
slug of heaviest precip into the southern to southeastern two or
so tiers of counties in the dmx County Warning Area. Thus, have made no changes to
location of Flash Flood Watch. Mitigating factor to flash
flooding is the fact that this part of Iowa has received little to
no rainfall the past 7 to 10 days. In the past 11 days, the
Lamoni ASOS has recorded 0.01 inches of precipitation... obviously
suggesting very dry ground. For flash flooding to stick, it would
most likely be caused by multiple rounds locally intense rainfall
over a short period of time... versus a slow, steady, drawn-out
rainfall event. The heaviest rainfall should have fallen by noon
Rest of next week...
as mentioned above, westerly to SW flow favored, with above-
normal rainfall and precipitation chances. Rainfall chances
Tuesday into Wednesday may need to be amplified as signal for rain
has been in the forecast for the past 5 days... implying solid
run-to-run consistency. Pwats for this timeframe appear to be in
the 1.25 to 1.5 inch Max range... implying much light precip
potential than this weekend.
Towards next weekend, long-range models are hinting that the Gulf of
Mexico may be reopened again... suggesting the potential for strong
moisture advection and potentially more heavy rainfall and
strong/severe storms. Will keep an eye on this.
Aviation.../for the 12z tafs through 12z Sunday morning/
issued at 610 am CDT Sat Sep 21 2019
Highly active fcst period. Have left precip mention out of tafs
this morning through the afternoon, except for kotm. Non-kotm
sites may be briefly impacted by showers and a rumble of thunder,
but confidence in timing and location too low to mention.
Confidence increases in showers and thunderstorms moving from
west to east across Iowa this evening through Sunday morning. Kotm
and kdsm especially to be impacted. Vsbys will need to be
monitored, as periods of heavy rainfall may drop flight categories
to IFR during periods of heavier rain.
Flash Flood Watch from this evening through Sunday evening for