Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131741

Area Forecast Discussion
National Weather Service Des Moines IA
1241 PM CDT Fri Oct 13 2017

Issued at 952 AM CDT Fri Oct 13 2017

Have updated the forecast today for a number of issues. First was
to add thunder mention in for today with a few storms already
present instability should just increase through the day.
Additionally wanted to get pop trends better through the day with
the area of showers working its way into the northwest as well.
With the clouds/showers around the northwest and temps currently
in the mid 40s, neutral to CAA aloft see it hard for the far
northwest to reach into the mid 50s. Therefore lowered highs in
the far northwest to west behind the frontal boundary, remainder
of highs looked ok so far. Also tried to trend in a break in pops
some for late tonight into early Saturday as activity for Saturday
looking to be trending slow moving into the state, with majority
of showers/storms spreading across the forecast area by late
morning into the afternoon hours rather than a by early morning as
the previous forecast had in place.


.SHORT TERM.../Today through Tonight/
Issued at 333 AM CDT Fri Oct 13 2017

GOES-16 Nighttime Microphysics once again showing an active night
of clouds across much of central Iowa, especially over eastern
Iowa where stratus has bases between 4000 and 5000 feet. A few
stations are also reporting fog, though so far isn`t as widespread
nor as dense as what it became last night. Otherwise, a cold
front has entered the northwest part of the state and will be the
focus for rain chances today and tonight. Have noted some higher
level clouds trying to form in GOES-16 low level water vapor as
well as Nighttime Microphysics around the Carroll area, but so far
development has been slow and a struggle. Cloud cover should
become widespread across all of central Iowa this morning as the
front pushes more into central Iowa. Forecast soundings would
suggest rainfall would begin shortly after daybreak over west
central Iowa and gradually expand northeastward in a somewhat
narrow band this morning. By this afternoon, warm air advection
will increase allow for rainfall to become more widespread. Rain
amounts will be on the light side averaging less than a quarter of
an inch. Overall, not much instability to work with until this
afternoon and even then best chances for thunder would be over
southern Iowa later this afternoon. With the boundary draped
across the state today, temperatures will range from the upper 50s
over far northern Iowa to the low 70s near the Missouri border.

Showers and thunderstorms will continue over roughly the southern
2/3rds of central Iowa this evening as the front stalls. A few
strong to perhaps severe elevated storms may be possible tonight as
the low level jet increases. The best chance for these storms would
be over far southern Iowa where forecast soundings show MUCAPE
values of 500 to 1000 J/kg in the GFS and 1000 to 1500 J/kg in the
NAM with both models showing 50 to 60 knots of shear. DCAPE values
would be supportive of strong wind gusts and some hail may be
possible as well with not large amounts of CAPE in the hail growth
region and modest lapse rates.

.LONG TERM.../Saturday through Thursday/
Issued at 333 AM CDT Fri Oct 13 2017

.Long Term /Saturday through Friday/

Saturday through Sunday: Confidence Medium to High

Few changes to previous thinking regarding Saturday.  With the
strong trough aloft and attendant shortwave approaching in the
afternoon hours...a slightly strengthening area of low pressure will
be moving northeast into central Iowa from north central Kansas. The
evolution of the entire system is now rather similar between the
GFS/Euro/NAM with some intensification of the sfc/upper level
features as the trough moves through Iowa...with the faster/more non-
linear accelerations taking place as the system exits our area and
proceeds east.  Once again the southeast/east sections of our county
warning area should be in the warm sector by the afternoon. An early
morning round of mainly elevated convection with additional nearly
continuous development into the late morning/afternoon hours should
occur. This will most likely have a limiting affect on the width of
the corridor of severe weather coverage and the new Day2 outlook has
trimmed the corridor somewhat and also has shifted it farther
southeast. Convection should be maximized in the afternoon hours as
the cold front edges east while the slowly intensifying sfc low
pushes east northeast into eastern Iowa and the more robust wind
fields aloft move across the region.  In our south/southeast the GFS
once again forecasts H850/H700/H500/H300 winds of 62/63/58/70 kts
respectively while the Euro forecasts H850/H700/H500/H300 winds of
51/55/60/75 kts by 00z Sunday. In each model the more robust
H500/H300 winds of 80/100kts lags the better sfc lift/convergence by
about 3 hours. Despite a worst case scenario for any higher end
convection...there is enough shear/cape available for an area of
strong convection across the south/southeast northeast toward the
Quad Cities in the afternoon/early evening hours. The main threats
still appear to be wind/hail.  0-3km bulk shear vectors suggest
there may be a smaller risk of an isolated tornado east/southeast of
our area between 20-00z if a linear convective system develops.  As
expected from last night...H850 subjective analysis shows a narrow
ribbon of +10C dewpoints already moving into eastern Nebraska south
to the Gulf States with 15C dewpoints beginning to creep north into
northern Texas. GFS precipitable water forecasts for 18-00z Sunday
continue to show a large area of 1.5 to 2 inch values over much of
the east half of Iowa with warm cloud depths of 11kft or higher.
Efficient rainfall with brief heavy rain will be likely especially
along the cold front as convergence becomes maximized as the
front moves across our southeast during the late afternoon hours.
After highs ranging from the cooler 50s in the northwest to the
mid 70s in the southeast...lows Saturday night will fall into the
mid to upper 30s northwest to the mid 40s in the southeast. Winds
Saturday evening and night will increase from the northwest to 15
to 22 mph with gusts increasing to near 35 to 40 mph. Bufr soundings
show a well mixed layer through the night with GFS H700 subsidence
of 4 to 5 microbars/sec. Should clearing take place we may realize
the forecast higher near 45 kt gusts at the top of the mixed layer
during the overnight hours. This would be near advisory criteria.
It will take another package or so to determine if the trends
become more relevant to the forecast.

Sunday Night Through Thursday: Confidence High

Once again a stable outlook for next week.  With high pressure
settling across the region for Sunday night into Monday morning...
chilly overnight lows are anticipated. The models are suggesting
some return west/southwest flow between 06-12z so some minor
adjustments may be needed to overnight lows early Monday.  For now
will leave lows in the mid 30s north to the upper 30s south. If
winds remain decoupled through sunrise without any increase in speed
overnight...some frost in low lying areas may be possible. Other
than that the remainder of the period will see some warming and a
nice stretch of quiet and generally sunny weather until late in the


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1241 PM CDT Fri Oct 13 2017

Boundary in place across the forecast area and will continue to
waiver around into Saturday impacting wind directions at KALO,
KDSM and KOTM. CIGS across the north to remain VFR and fall into
MVFR to IFR tonight as moisture increases ahead of the wave
impacting the area Saturday. Showers expected north this
afternoon, then tapering off with some drizzle possible late
tonight as CIGS lower and rain moving back in late Saturday
morning. Then across the south/east chances for showers and storms
through much of the night/drizzle possible then more widespread
showers/storms for Saturday.





SHORT TERM...Ansorge
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