Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 172349
AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
649 PM CDT Sat Sep 17 2016
.SHORT TERM.../This evening through Sunday/
Issued at 328 PM CDT Sat Sep 17 2016
As upper trough axis finishes pivoting through the region, trailing
upper level forcing will continue to keep low stratus, around 1500-
2500ft, across fair portion of northern Iowa into this evening.
With clouds unable to break up as quickly as anticipated,
temperatures across the north remained on cool side of original
forecast highs and could see lows into the lower 50s even with
southerly flow returning as clouds eventually clear out. Have
reflected that thinking in grids accordingly dropping temps a
degree or two below guidance. Patchy fog may be possible again
with clear skies and light winds overnight, with highest
likelihood over far eastern CWA as surface ridge axis moves
across. Though dry air will be limiting factor with fair amount of
doubt enough nocturnal cooling will occur.
.LONG TERM.../Sunday night through Saturday/
Issued at 328 PM CDT Sat Sep 17 2016
Summary...Models continue to be in fair agreement with placement and
timing of major features through the long term. Temperatures will
warm into the mid 80s early in the week with 925mb temps to low 20s
C. Number of waves will move through flow with limited precip
potential until mid week as a more robust wave ejects out of the SW.
Followed by trough digging across northern plains to end week
sweeping cold front through Saturday.
Sunday through Tuesday...Relatively quiet and warming period.
Surface high pressure ridge axis will pass this evening, returning
southerly winds and WAA. With 925mb temps around 20 C, expect highs
around 80 across much of CWA. Warming continues into Monday with
925mb temps into lower 20s C, resulting in mid 80s across at least
southern half of the area. A wave will move through zonal flow
Monday, reflected with a relatively weak surface low moving across
the state. As a result, warm front should lift through during the
day, providing an opportunity for storms. NAM has perked up in
recent runs compared to GFS/Euro/Canadian, but with model continuity
on feature placement, have introduced low end POPs. Will be a threat
of severe weather should storms initiate with good instability and
decent helicity and shear depicted. Quiet Tuesday ahead of larger
Wednesday through Saturday...Zonal upper flow will transition more
southwesterly through this time frame, with a rather robust wave
remaining progged to move through Wednesday into Thursday. With
southerly flow still in place, there will be ample moisture as
depicted by PWats approaching the 1.8-2.0 inch range, which is on
the extreme end for this time of the year. Heavy rainfall appears
main threat with broad forcing and lack of surface feature to shunt
storms off. May be an opportunity for a storm to organize early
ahead of multi-cellular development with decent shear anticipated.
However, given how far out, details may change significantly. After
wave moves through, upper trough to dig across northern plains.
Models diverge at that point with Euro slower, bringing cold front
through late Sat into Sun, while GFS depicts stronger low and more
progressive with front through the state by Sat evening.
.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 648 PM CDT Sat Sep 17 2016
Generally good aviation weather for the duration of the forecast
with widespread VFR conditions. Visibilities will be unrestricted
for much of the period although some patchy ground fog will be
possibe around daybreak on Sunday. Any ceilings should remain from
high clouds. Winds will be light overnight but increase out of the
southwest on Sunday.