Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212341

Area Forecast Discussion
National Weather Service Des Moines IA
641 PM CDT Fri Apr 21 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 255 PM CDT Fri Apr 21 2017

Confidence: High

High pressure will be building south overnight with generally light
winds and clear skies.  This will once again result in very cool
temperatures over the region tonight...especially nearer the high
circulation across northern Iowa.  There will be a chance of some
patchy frost...mainly in low lying areas.  Lows tonight will drop to
the mid 30s over the north and into the upper 30s to lower 40s
elsewhere with light northerly flow overnight. The ridge axis will
expand across Iowa Saturday with light winds once again expected for
the northern areas while the south and east may see some mixing
during the afternoon...bringing the winds up to about 10 to 15 mph
during the peak mixing hours of 10 am and 4 pm in the afternoon.
Otherwise look for mostly sunny skies with a few cumulus clouds
around 4kft to 5kft.

.LONG TERM.../Saturday night through Friday/
Issued at 255 PM CDT Fri Apr 21 2017

Confidence:  Medium to High through Monday Night

The early portion of the extended remains high confidence.  The
large high will drift southeast with winds gradually becoming south
southwest with time by late Sunday afternoon.  Meanwhile...warm air
advection will have begun already by Sunday morning resulting in a
warming of the H850 temperatures from 5-6C to near 8-9C by 00z
Monday. Sunday looks like another great day with slightly warmer
afternoon highs reaching the lower 70s over most of the region. late
Sunday night a lee side low will form across the Western Plains.
This should help to increase southwest flow aloft...along with the
beginning of return of moisture from the Gulf.  By 12z Monday H850
winds will increase to 35 to 50kts across the area...with afternoon
H850 winds 30 to 45 kts.  Surface flow will likely be rather robust
as well with sustained winds of 15 to 20 mph and gusts from 30 to 35
mph across most of the region.  At this point the winds look sub
advisory...but will monitor in later forecasts.  The Western Plains
low will track northeast into Minnesota by late Monday resulting in
the passage of a cool front with little impact.  Models are showing
isolated to scattered showers/thunder by late afternoon.  Just ahead
of the boundary...H850 temperatures rise into the 9 to 11C range.
This will promote afternoon highs in the lower 70s north to mid 70s
over the central/southern areas.

Confidence:  Medium from Tuesday through Friday

The weak cool front will translate to just south of Iowa before
becoming nearly stationary.  Several upper level waves will impact
the region through the remainder of the week with periods of clouds
and showers and thunderstorms.  The first H500 wave will arrive by
Tuesday morning and provide support for showers and perhaps an
isolated thunderstorm as moisture is lifted north over the frontal
zone just south of Iowa. With clouds in place...highs will drop back
into the 50s/60s over the region.  There should be a brief break
from the rain Wednesday morning but by late Wednesday another weak
impulse will cross the region.  A more significant system will be
affecting the forecast area Thursday into Friday. Gulf moisture
...even today...remains locked in place over the southeast third of
the CONUS and will eventually be pulled back north with the much
stronger H500 wave moving across the US late next week.  Unlike some
of its predecessors...this system will be strengthening as it moves
out of the Southwest into the Central Plains from Wednesday night
into Friday.  A significantly strong upper level jet streak will
accompany the system. Early indications are for several rounds of
rain and thunderstorms as the low lifts northeast from Thursday into
Friday morning.  Though the surface low is expected to track into
Iowa by Friday is also occluded and there is some
uncertainty as to the track of the system. The Euro is now
trending farther south with time. Wind profiles and instability
suggest that some stronger storms may accompany the low as it
tracks toward Iowa...possibly bringing some isolated severe storms
into southeast/east Iowa Thursday night. Otherwise any significant
severe threat should remain south and east of the forecast area
based on current information. Rainfall will likely be moderate to
potentially heavy from time to time Thursday into Friday. Early
indications of amounts exceeding 2 inches are possible. Forecast
precipitable water nearing 1.5 inches will assist in possible
rainfall amounts of 2 or more inches during the period.

There remain some differences on the timing of the large scale pattern
at the end of next week and weekend. The GFS shows about 48 hours
of separation between two upper level waves while the Euro has a
24 hour break between the two storms/strong jet streaks tracking
over the CONUS. Thus...some uncertainty will remain in the forecast
from Thursday into Sunday until this can be resolved with consensus
by multiple model solutions.


.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 641 PM CDT Fri Apr 21 2017

High confidence in VFR conditions through the period with nothing
beyond high cloudiness. Winds will change little, staying
northeasterly for the most part.





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