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Displaying AFOS PIL: AFDDMX Received: 2017-10-13 11:38 UTC

FXUS63 KDMX 131138

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

...Updated for 12 Aviation Discussion...

.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 12Z TAFS through 12Z Saturday morning/
Issued at 638 AM CDT Fri Oct 13 2017

Main concerns this period will be ceiling and visibility 
restrictions with showers and storms at times throughout the TAF
period. To start, low clouds have persisted overnight and some
patches of fog have formed with restrictions at KMCW and KFOD.
Fog will dissipate this morning while the clouds will lift a bit
into MVFR or VFR. Showers will develop as a cold front sinks 
through the state with visibilities remaining above 6SM. However,
ceilings should fall into MVFR and perhaps IFR category as the 
rain passes over KALO, KDSM, and eventually KOTM this afternoon. 
Late this evening, did insert a brief window of dry weather per
short range guidance, but ceilings are expected to stay in the 
IFR range over the southern terminals. For the northern
terminals, ceilings will gradually fall from VFR to MVFR with 
KMCW following this trend shortly after this TAF period.




SHORT TERM...Ansorge