Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221143

Area Forecast Discussion
National Weather Service Des Moines IA
643 AM CDT Mon Aug 22 2016

...Updated Aviation Discussion for 12z TAFS...

.SHORT TERM.../Today/
Issued at 336 AM CDT Mon Aug 22 2016

Forecaster Confidence = High

A relatively flat zonal flow pattern is expected today as the upper levels
undergo a pattern change.  At the surface...high pressure continues
to shift eastward while a developing sfc low begins to take shape
over the northern plains.

Seeing scattered mid-level clouds develop across western Iowa early this
morning in conjunction with an area of theta-e advection around 700mb.
Although model soundings indicate a layer of saturation...low
level are very dry so not expecting anything to reach the ground. Otherwise
expecting mostly sunny skies today with a modest warming trend.  H8 temps
increase about +5C today which should result in highs in the lower to middle
80s.  The only other issue to contend with is a breezy south wind. A fairly
stout pressure gradient develops across the state /about 10 mb from east
to west/ which will result in sustained winds from 10 to 20 mph with a few
gusts topping 30 mph.  The strongest winds are expected over northern and
northwestern Iowa.

.LONG TERM.../Tonight through Sunday/
Issued at 336 AM CDT Mon Aug 22 2016

There will be two precip windows of concern through the period, one
into midweek and another toward the weekend. Initially tranquil
weather can be expected tonight with little moisture or forcing.
However persistent return flow through the night will continue to
increase moisture setting the stage into Tuesday which will bring
another day of humid conditions with dewpoints back to around 70 in
many locations. The current WA/BC upper low and weaker AZ/NM short
waves will both contribute to our weather by later in the day. The
northern upper low should reach Saskatchewan by midday with the
associated frontal system into the eastern Dakotas and Nebraska by
afternoon. The AZ/NM wave should reach the Missouri Valley by this
time with larger scale forcing and another surge of moisture/theta-e
advection possibly generating weaker elevated convection in the warm
sector. Farther west it will be a different story with MLCapes 3000
plus j/kg just ahead of the front across eastern Nebraska. The shear
will not be extreme but may be sufficient for peak heating supercells
to our west, which is supported by a few high res models elevated
updraft helicity. This will be mainly fueled by insolation however
and the convection will likely weaken moving east, especially if
elevated convection develops ahead earlier. Our main concern could
very well be heavy rain rather than severe as outlined in the
separate Hydrology section.

There may be only a small break into Wednesday with synoptic
scale forcing ahead of the northern Plains long wave trough
sustaining things through the day with additional thermodynamic
and kinematic contributions. The front and low level convergence
will still be over the southeast third of the forecast area during
peak heating, but it may be difficult to generate any significant
surface based convection due to lingering precip and convective
debris. The significant convergence should be to our south into
Wednesday Night but weaker elevated convection may still linger
south just ahead of the H85/H7 trough.

The precip should end later Thursday with cooler, drier high
pressure settling into Iowa to end the work week. Although
confidence is still fairly high in precip sometime over the weekend,
the confidence in the timing and details has diminished somewhat
from yesterday at this time. The GFS and ECMWF seem to be having
difficulties handling short waves in the low amplitude flow and
remnant weak long wave trough left behind our mid week northern
Plains wave. Although the deterministic runs now seem closer at 00z,
the GFS and ECMWF ensembles have wide member spreads and similar run
to run changes from their previous deterministic runs so have a
prolonged period of chance pops to account for this uncertainty.


.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 627 AM CDT Mon Aug 22 2016

VFR conditions are expected this period. Seeing an area of mid-
clouds /bases aoa 8k feet agl/ over northern Iowa this morning
and expect these will slide off to the northeast by mid-morning.
Otherwise the only other concern will be increasing southerly
winds today. Expect sustained winds from 10 to 20 kts at
KDSM/KALO/KOTM and 15 to 25 kts at KFOD/KMCW...with a few
stronger gusts. Should see a reduction in gustiness towards sunset
due to loss of insolation.


Issued at 336 AM CDT Mon Aug 22 2016

The Nam, GFS and ECMWF all suggest elevated moisture/PWATs and
warm cloud depths could lead to heavy, efficient rainfall Tuesday
Night with mean wind parallel to the front. Soundings show the
slightly elevated effective inflow region around the 305K
isentropic surface with around 40kts of flow providing for strong
moisture transport and convergence and also trying to anchor the
convection somewhat with weak Corfidi vectors. This heavy rain
scenario is also depicted by the model QPF with the Nam, GFS,
ECMWF and CMC all showing at least 1-2 inch/6 hours through the
night over sections of central Iowa. Although the confidence in
location is not great, changing run to run, the increased
confidence of this occurring somewhere in the forecast area has
led to heavy rain wording being added to the forecast Tuesday
Night. Agree with the recent WPC Day 2 Excessive Rainfall risk to
slight. This period will be monitored for potential Flash Flood
Watch in coming forecast cycles.




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