Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130833
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
333 AM CDT Fri Oct 13 2017

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

Short Term -- Friday and Friday night -- 10/13/2017

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
week.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1152 PM CDT Thu Oct 12 2017

Slight tweaks to the 00z aviation package. Have bumped up category
at KALO. At this moment, it appears the best threat of IFR will be
just east of KALO, impacting KCCY. However, will monitor closely
for fog potential near sunrise. Added light precip to the TAFs
during the afternoon. Precip should be light enough to generally
keep vsbys above 6 SM, but will monitor this as well and adjust
later this morning if it appears heavier precip will affect TAF
sites. high confidence in winds under 10 kts.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Ansorge
LONG TERM...REV
AVIATION...Kotenberg



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