Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 190448
AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1148 PM CDT Sun Sep 18 2016
.SHORT TERM.../This evening through Monday/
Issued at 301 PM CDT Sun Sep 18 2016
Fairly quiet for tonight, with the possibility of an isolated
shower/storm across the far northeast late. Large area of surface
low pressure situated across the central US with the sfc trough axis
extending southward through the Dakotas into central NE/western KS.
Southerly flow ahead of the trough axis across IA with WAA and
increasing sfc dewpoints. Zonal flow pattern in place aloft, with a
weak shortwave trough moving through the main flow across the region
tonight. This will shove as associated weak cool front, basically
acting as a wind shift line through the CWA late tonight. Weak theta-
e advection as well could help initiate some isolated
showers/thunderstorms across the far north/northeast late tonight as
the LLJ becomes oriented into NRN IA/MN. Stronger lift further north
closer to the sfc low and nose of the LLJ, therefore decreased pops
some more in areal extent with more of the activity expected closer
to the stronger dynamics. CAMS suggest this potential as well
keeping all of the activity across MN and barely a dribble into IA.
With the increasing low level moisture, models trying to indicate
some light fog/stratus developing near/behind the front as it slows
and stalls across IA into early Monday.
As for temperatures bumped overnight lows a nudge warmer given the
push of moisture with dewpoints expected to push into the 60s across
much of the CWA overnight.
.LONG TERM.../Monday night through Sunday/
Issued at 301 PM CDT Sun Sep 18 2016
Summary...Warming that began Sunday will continue into Monday and
result in above normal temperatures, into the mid 80s at times, for
much of the week. Chances for storms, potentially a few severe, and
heavy rain returns late Tuesday night and over the following
few days as waves move through.
Monday into Tuesday...Main concern through this period is warming
temperatures as large warm nose develops in the 925/850mb areas.
With temps into the low 20 C, surface temps should have little issue
approaching mid, even upper 80s across southern IA and into central
IA with plenty of mixing expected. Similar expected Tuesday and
through much of the week actually, though a couple degrees cooler.
Possibility remains, though weaker than previous forecast cycles, of
a few storms across far eastern/southeast portions of CWA with weak
boundary in the area as high pressure moves in. Soundings indicate
storms rooted around 850mb or higher, limiting severe chances.
Late Tuesday Night into the Weekend...Generally zonal CONUS upper
pattern comes to end and yields to SW flow as upper trough digs into
SW US. Will begin a period of active weather late Tuesday night into
the weekend. Currently, heavy rainfall appears the main threat with
PWats continuing to be progged in the 1.75-2.0 inch range, which is
on the very high end for this time of year. Localized flooding may
be a concern, primarily across northern IA. Additionally, severe
storm or two may come into play with a fairly favorable environment,
however limiting factor may be how widespread convection becomes and
how quickly. SW trough ejects into Upper Plains by Saturday,
sweeping across the region. GFS continues to be faster than Euro,
but generally expected to move cold front through overnight Saturday
and through Sunday. Pooling moisture ahead of front will boost PWats
once again into unusual territory for the time of year, around 2.0
inches, but progressive nature would limit any flooding concerns.
.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1148 PM CDT Sun Sep 18 2016
Conditions will be mainly VFR for duration of forecast period.
Still a period of low level wind shear overnight but this will
decrease toward daybreak as surface front approaches northern
Iowa. With winds weakening toward daybreak in the north, some
patchy fog may also develop, but don`t anticipate anything worse
than MVFR visibilities for a brief period around sunrise. Isolated
showers and storms are expected along the boundary, especially by
later Monday afternoon in central Iowa and this will have to be
monitored for possible impact on Taf sites. Winds will be variable
on Monday as the boundary passes south through the state.