Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180926

Area Forecast Discussion
National Weather Service Des Moines IA
326 AM CST Sat Feb 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 322 AM CST Sat Feb 18 2017

Another mild and quiet day is in store across the area. A surface
high pressure area will slide eastward along the Iowa/Minnesota
border this afternoon and evening. As a result light surface winds
will be from the northwest this morning, then variable late in the
day, then turn to southeast overnight. There is a lack of
appreciable warm or cold air advection and the warm airmass
experienced over the last couple days will remain in place, but
without the aid of mixing/advection that pushed temperatures well
into the 70s across much of the area yesterday. Highs will range
more in the mid 50s northeast to mid 60s south today, which is still
far above normal for the season.

.LONG TERM.../Sunday through Friday/
Issued at 322 AM CST Sat Feb 18 2017

Sunday through Wednesday...Confidence Medium to High

Active weather to return this period by Monday.  Sunday will be
another anomalously warm day with near to record highs across
much of the forecast area by late day. Warm southwest flow with
good mixing will once again promote highs in the lower 60s north
to the upper 60s to lower 70s south. Currently the GFS is warmer
than the Euro but consensus between the two remains high for a
very warm day. By late Sunday night into Monday H850 moisture
plume rapidly increases across Iowa with PWATS increasing from
0.50 Sunday evening to 1.25 inches by 12z Monday through most of
the morning and early least over the east half of
the state. Sounding climatology suggests that this is 375 to 400%
above normal with record values of PWAT for this date in
February. The concern Monday will be for record moisture
availability and the speed of the boundary as it passes through
the area. The GFS continues to be more progressive with the deeper
moisture being utilized east of the area while the NAM and Euro
are slower with the trough and higher with qpf over the east where
moisture convergence across the boundary is maximized between
18-00z. Have raised qpf value over the east from 12-18z...but have
not significantly raised values at this time from 18-00z Tuesday
due to some uncertainty with timing of the boundary. Will need to
evaluate again with next few packages. Given the available
moisture/potential for some thunder...seems likely that Monday
afternoon qpf will need to be higher. Will be evaluating further
in the next couple of packages. This may impact river forecasts
over the area due to runoff and will need to be monitored for
potential minor flooding if the heavier rainfall occurs. Forecast
highs Monday will be highly dependent on duration of rainfall and
cloud cover. Though clouds are expected much of the
day...intermittent rain will allow for more warming as forecast.
A period of more persistent showers into the afternoon may result
in lower highs. By Tuesday and Wednesday a thermal ridge will once
again bring a period of dry weather and very warm temperatures
with near record highs once again possible. Overall quiet and mild
weather will continue into Wednesday evening with no
precipitation until Thursday.

Wednesday Night through Friday...Confidence Low to Medium

Though record warmth and above normal temperatures are persisting
into next week...some major changes are on the way.  Tonights water
vapor imagery shows an intense series of storms in the North Pacific
with one near 150W/44N and another upstream along the Aleutian Chain
near 170W/52N. Both of these cyclones will eventually work southeast
toward the US west coast...eventually crossing the Rockies next
week and impacting our weather by Thursday/Friday. About the same
time the significant waves reach the Central US...colder air is
forecast to reach the Northern Plains and get pulled into the
system. Both the GFS/Euro are similar in projecting this scenario
...and if the trends continue...the models are suggesting a return
to winter weather by weeks end with snow/wind over the northwest
and thunderstorms in the southeast. Uncertainty is high enough at
this point to mention and just maintain awareness for the potential.
Leading up to Friday ...Thursday is likely to be very warm again
with highs in the 40s/50s while Friday would be colder with 30s/40s
and rain/snow mix across the region then changing to all snow.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1151 PM CST Fri Feb 17 2017

Confidence is medium to high that VFR conditions will persist
through the period. Low level relative humidity is increasing
leading to some radiation fog potential however drier/cooler air
continues to gradually ooze into Iowa at 06z behind a weak SW-NE
cold front through the state. Thus the forecast remains VFR until
trends suggest otherwise.




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