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000
FXUS63 KDMX 210504
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1204 AM CDT Mon Oct 21 2019
.UPDATE...
Issued at 1053 PM CDT Sun Oct 20 2019
Issued first shot at Wind Advisory across northern Iowa earlier
this evening. Looked at NAM/GFS/4km NAM/HRW ARW and NMM soundings
and all suggest at least advisory criteria gusts /45 mph/ Monday
Night. GFS suggests 50kt gusts are possible, so headline may need
to eventually be expanded in area and possibly magnitude
(Warning).
Regarding convective trends tonight, instability is lacking but
0-1km shear and SRH are extreme and favorable for tornadoes if
adequate CAPE is present. Southeast sections would be most
concerning based on MLCAPE forecasts and extrapolation so trends
will be watched closely overnight.
&&
.DISCUSSION.../Tonight through Sunday/
Issued at 338 PM CDT Sun Oct 20 2019
*/ Highlights...
-Another round of showers, with a few rumbles of thunder, likely
this evening into the overnight hours.
-Wind advisory level winds very possible for Monday PM into Tuesday.
-Below-normal temperatures likely for the rest of this week.
*/ Summary...
A large area of low pressure will push across Iowa and towards the
Great Lakes Region this evening through Monday. A few rounds of
scattered showers, with a few rumbles of thunder are likely around
sunset in western Iowa, spreading eastward across the state through
the midnight hour Monday morning. By sunrise Monday morning, most
banded rainfall accumulation with the initial surge should be east
of our area. While not making it down to central and southern Iowa,
"Wrap-around" precip may bring an additional one-tenth of an inch or
less of rain to northern Iowa. Total forecast rainfall amounts are
around the one-quarter of an inch range... A few locations,
especially in northeastern and northern Iowa, may receive closer to
the one-half an inch range, where as a few locations in southern
Iowa may receive closer to the one-tenth of an inch range.
Strong winds will likely follow this area of low pressure, impacting
Iowa. Peak winds could very well be in the 20 to 30 mph range, with
gusts of 35 to 45 mph. Important to note that winds will be largely
from the west, so travel along north-south routes, such as
Interstate-35, will likely have amplified impacts. It appears
increasingly likely the strong winds will last overnight Monday
night into Tuesday.
For the rest of the week, fairly persistent conditions are expected
before the next best chance of precipitation arrives towards the end
of the week/into next weekend. Daytime Temperatures will likely be
stuck in the 50s... which is around 5 to 10 degrees below normal for
this time of year.
*/ Technical Discussion...
Synoptic-scale setup...
20z Visible & water vapor imagery both clearly depict an area of
low pressure ejecting off the Rockies and pushing into western
South Dakota/Nebraska. This low is generally vertically stacked,
with SFC analysis showing the central sfc pressure of this low to
be around 994 mb. Short-range models keep the central sfc pressure
around 991 to 994 mb as it nears Lake Superior Monday afternoon.
In the wake of this system, a 1020mb or so sfc high pressure is
slated to eject southeastward off the Rockies. As is fairly common
with such wrapped- up systems like this one, overall progression
has trended slower.
This Evening/Tonight precipitation...
There is a strong band of theta-e advection leading a surge of
low- level moisture with the progression of the "warm" advection
wing phased with the cyclonic system mentioned above. Trailing
this leading band of forcing is another band of forcing. Overall
kinematic forcing is stronger with this second round, however the
moisture advection/support is weaker/more narrow. So... the first
round of precip will likely be more broader in scale and lighter
in magnitude/rate... and the second round will likely be more
intense, but more narrow in terms of coverage and duration.
Overall, PWATs are briefly around 1 inch. With decent low-level
saturation, expecting moderately efficient rain producers... so
total QPF nearing half an inch for much of the DMX CWA. The
highest amounts should be in the north.
Monday PM-Tuesday's High Wind Potential...
The above-described synoptic-scale setup will pave the road for
strong winds Monday afternoon as the sfc pressure gradient dips
below 3mb/100km. Further, a broad area of low-level subsidence
coupled with 3 mb/3hr pressure rises is slated to push eastward
across Iowa during peak heating. With the delayed propagation of
the cyclonic system, the arrival of these mesoscale support items
is pushed back until later Monday... and lasts through the
overnight hours, into Tuesday morning. Transport layer winds
within the mixed layer top out around 40 to 50 kts. Given the
mesoscale setup described in the previous sentences, it would seem
plausible these 40 to 50 kt winds should be able to make it to
the sfc. With the strong amplification the westerly component to
the winds will have on north-south travel such as I-35, a Wind
Advisory certainly not ruled out in the next forecast update.
Given the delayed nature of the arrival of the strong winds,
think best service would be to let the overnight forecast shift
assess and issue a Wind Adv. should it be necessary.
Rest of the Week...
The pressure gradient relaxes some for Tuesday, but is still
strong enough to support strong WNW winds of 20 to 25 mph, with
gusts of 30 to 40 mph through much of the early to mid-afternoon.
For Wednesday... models are converging on propagating a shortwave
across the Upper Midwest... in particular, northern Iowa. Low-
level analysis shows what looks like may be a multi-pronged, low-
impact event... the first round supported by a W-E oriented band
of frontogenetical forcing leading a band of low-level moisture
flux convergence. The second round looks to come overnight via a
cold front. With PWATs nearing 0.75 inches for the passage of this
event, am thinking around one tenth to one quarter of an inch may
be a good starting point for total QPF in the next forecast
update.
A strong signal for CAA behind this system to last into much of next
week. Long-range models have 850mb temps of 0 to -10C settling in
across the Upper Midwest, which is -1 to -2 std dev and suggests
below-normal temperatures for next week.
&&
.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1202 AM CDT Mon Oct 21 2019
VFR/MVFR showers and isolated storms will lower to IFR/MVFR
conditions overnight. Thunder is certainly possible and have added
mention where confidence is higher. A break in precip is expected
early Monday with another round of scattered light rain north
Monday. Winds will increase by late afternoon and especially into
the night north. Winds gusts of 35kts or more are expected. MVFR
ceilings are likely north Monday Night as well.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 7 PM this evening to 4 PM CDT Tuesday for
IAZ004>007-015>017.
&&
$$
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