Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181155

Area Forecast Discussion
National Weather Service Des Moines IA
655 AM CDT Mon Sep 18 2017

Issued at 644 AM CDT Mon Sep 18 2017

Updated aviation discussion for 12z TAFs below.


.SHORT TERM.../Today through Tonight/
Issued at 353 AM CDT Mon Sep 18 2017

Today...Forecast Confidence: Medium

00Z upper air analysis and water vapor imagery indicated an upper
level trough pivoting across the central and northern Plains...with
a second stronger trough coming onshore along the Pacific NW. At the
low to mid levels, regional RAOBS suggested a plume of drier air
over Iowa /H8 Td around -8C/ with significantly more moisture poised
just to south over MO/KS /H8 Td near +12C/.  MSAS showed the large,
dry sfc high over the Midwest gradually shifting ewd while weak
pressure falls were occurring over the High Plains.

At 0830Z...radar mosaic showed an area of showers and embedded tstms
over NEB/KS gradually moving towards the east/northeast.  Due to the
aforementioned dry air in low/mid levels believe some of the precip
will likely go into top down saturation as the echoes move east into
IA. As such, have delayed the onset of precipitation by a few hours
from the previous forecast in most locations. However, combination
of continued WAA/moisture transport and large scale ascent from
upper level PV anomaly should allow the precip to gradually make
headway into the forecast area after about 09z-10z this morning.
Still expecting the heavier precip to impact most areas from mid-
morning into the afternoon hours. Overall instability today is
rather meager due to poor, moist adiabatic mid-level lapse rates.
However most of the operational and CAM solutions continue to show
100-500 J/kg of MUCAPE - thus decided not to try and get too cute
and kept the mention of thunderstorms in the forecast.  Latest
consensus of model QPF indicates most locations should receive a
tenth to half inch of rainfall with a few heavy amounts possible.
With abundant cloud cover and a good chance for precipitation, high
temperature forecast is very low confidence. Have stuck with highs
in the mid 60s north and lower 70s south...but this could be
generous if rain cooled pcpn is more widespread.

Tonight...Forecast Confidence: High

The 18/00Z operational models and recent HRRR/ESRL HRRR/NAMNEST show
the band of precip being fairly progressive this evening with precip
ending from west to east across the forecast area between 00z to 06z
as large scale subsidence overspreads the region.  A few models are
showing stratus or fog developing in the wake of pcpn but due to low
confidence will leave out of the forecast for now.  Lows tonight are
expected to range from the mid 50s north to lower 60s south.

.LONG TERM.../Tuesday through Sunday/
Issued at 353 AM CDT Mon Sep 18 2017

./Tuesday through Friday Night/

Confidence: Medium to High

Active weather week ahead but beginning to see some consensus in
model data moving forward...thus confidence increasing in forecast.
Weak system from today will keep a weak warm front over the region
Tuesday...though upper level support will be only in the form of
warm air advection.  This will likely result in some clouds during
the day at 5-6kft...especially north of the boundary.  Have adjusted
highs a touch in the south reflecting better warm air return by late
afternoon over the south/southwest.  Otherwise expecting a dry day
with sun/cloud mix during the day hours.  Strong upper level system
and sfc reflection plowing into the Dakotas/Minnesota by Wednesday
will result in area of strong storms northwest there...but also will
pull the warm front north of the region by Tuesday night.  Models
continue to focus some thunder development along a trailing cool
front into Iowa by 12z Wednesday.  The boundary is likely to further
weaken during the day but some residual convection will move through
the region Wednesday afternoon/evening.  Temperatures both days will
be warm...especially south where mid to upper 80s will be possible.
Once again the exiting boundary will stall over the south/northern
Missouri and then move north Thursday. There is a signal for some
convection as the boundary lifts north again by 18-00z Friday into
northern Iowa. Sufficient warm air advection and mixing will quickly
bring back rather unseasonably warm afternoon readings into the
upper 80s once again.  Overnight lows from Tuesday night through at
least Friday morning will be more like summer...mid 60s to near 70.
The warm front eventually washes out with just strong south
southwest flow taking over Friday into Friday night as a rather
large H500 upper level trough sets up shop over the western CONUS.

./Saturday Through Sunday/

Confidence:  Medium

Meanwhile...the pattern over the eastern CONUS/Atlantic is in a log
jam with a blocking high over the Great Lakes/Quebec back into the
Atlantic and both Jose and Maria getting caught up in the blocking
large sub-tropical high that keeps the two systems pin wheeling
off the east coast from the mid Atlantic to near Long Island
through next weekend. This blocking high and Atlantic tropical
activity will bring the progression of the western trough to a
standstill by Friday. Thus... have been able to remove/trim back
much of the PoP from Friday into most of Saturday. There is some
hint that the weakening cool front and progressively sheared upper
level trough will finally reach our forecast area Sunday/Sunday
night with showers/iso to scattered thunderstorms. With more
clouds and increasing chances for precipitation along with falling
heights/thickness and cooler temperatures aloft...highs will cool
into the 70s/lower 80s by Sunday as the system slides our way.
Though our wx will be interesting to forecast...the bigger/more
interesting story will be Jose/Maria and the odd interaction of
the tropical systems with the blocking high in the east.


.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 644 AM CDT Mon Sep 18 2017

Numerous showers and scattered thunderstorms will push across
central Iowa today. VFR conditions at FOD/MCW early this morning
will likely deteriorate to MVFR CIGs/VSBYs as precip moves in
with periodic IFR conditions possible. At DSM/ALO/OTM the precip
will be delayed until later this morning or early afternoon. Like
further north...VFR conditions are likely to transition to MVFR
at times in heavier showers/tstms. The precip should gradually
push east of the terminals by late afternoon or evening. Later
tonight fog development is possible, but held off mention in TAFs
at this time due to lower confidence.




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