Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200847

Area Forecast Discussion
National Weather Service Des Moines IA
347 AM CDT Fri Oct 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 342 AM CDT Fri Oct 20 2017

Large area of high pressure is now centered over the southeast CONUS
and ridged northward to the eastern Great Lakes.  In addition, a
trough of low pressure is developing on the lee side of the Rockies.
This is allowing going moisture flow from the Gulf of Mexico to
stream north. Currently the best moisture transport is from Texas
into southern Nebraska. This moisture transport will continue to
intensify today and gradually shift east into Iowa later today and
into tonight. In the meantime, very dry air is in place across
Iowa with PWAT values in the one third to one half inch range and
large dew point depressions up to 20 kft. Above this layer, some
cirrus is beginning to move northeast into Iowa and this will
continue through the day. Profiles will begin to saturate down to
near 10 kft by late afternoon as the mid-level moisture advection
arrives. Overall, another pleasant day though breezy/gusty with a
few gusts near 30 mph this afternoon. Temperatures will be near to
slightly warmer than Thursday.

The low level jet will intensify this evening to over 60 kts and
will bring another wave of theta-e advection and increasing
instability and steepening mid-level lapse rates to Iowa from west
to east. Review of sounding profiles across the area still continue
to show a drier layer from 850-700 mb and would suggest any
convective development will not initiate in this layer. The best
point for initiation would be above 10 kft which would reduce any
instability to work with and does bring into question the potential
for showers and storms late tonight. A few of the CAMs including
ESRL HRRR are much more bullish on convection late tonight but seems
overdone given the scenario and discontinuity with the best forcing
and best moisture being separated within the vertical profile.

.LONG TERM.../Saturday through Thursday/
Issued at 342 AM CDT Fri Oct 20 2017

The well advertised upper trough and cold front make their way
across Iowa Saturday and Saturday night.  Initially Saturday morning
some weak theta-e advection behind the initial surge Friday
night lifts through the state coincident with some weak forcing.
However moisture is lacking and we have a stout cap in place. This
would still limit instability to work with so if we see any precip,
it will likely be in the form of light showers with an isolated
elevated storm or two. The cap remains in place until the front
pushes through the state. Model consensus shows the front through
about the western third by early afternoon, central Iowa between
mid to late afternoon and early evening and the eastern third from
mid evening into the overnight hours of Sunday morning with about
a 4 hour window in each area for convection. Based on forecast
soundings, the peak time for convection around the metro area
would be from 22Z until about 02Z. Instability, if we can realize
all of it, is pretty impressive for late October with around 2000
J/Kg and there is also some very impressive shear along the
boundary with deep layer shear between 40 and 60kts. SPC day 2
outlook now has a broad area of slight risk over nearly all of
Iowa, tied to the frontal boundary. Storms on Saturday would have
a damaging wind potential. By later Saturday evening and Saturday
night we lose a lot of our frontogenetical forcing and instability
really wanes pretty fast so storms over the east Saturday night
may produce moderate rainfall but the severe threat will diminish.

On Sunday the upper trough pushes east ending any residual precip
chances and bringing in some cooler air.  For the rest of the long
term, not much change to the forecast as we still get caught up in a
couple of stronger waves moving across the Northern Plains into the
Great Lakes and deepening over the region.  This will bring even
cooler air into the region...especially Tuesday.

The current model trend is to bring a ridge into the Upper
Midwest a little quicker on Wednesday in response to yet another
strong wave digging into the Pacific Northwest and Northern
Rockies. This will temper the cooler air for mid to late week so
highs will be a touch warmer than previously forecast, with the
warmest temps over the west.

Then, Friday into next weekend, that deepening trough makes it`s
way into the Upper Midwest and this will mean very cool temps for
this period. Timing any disturbances in the northwest flow that
far out is difficult so I did not add PoPs at this point but I`m
sure this will change in the coming days.


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1206 AM CDT Fri Oct 20 2017

Widespread VFR conditions are expected for the entire period
although ceilings will thicken by the end of forecast timeframe.
Visibilities will be unrestricted with strong south to southwest
winds, especially during the day.




SHORT TERM...Donavon
AVIATION...Cogil is the U.S. government's official web portal to all federal, state and local government web resources and services.