Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 170856
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
356 AM CDT Thu May 17 2018

.SHORT TERM.../Today through Tonight/
Issued at 354 AM CDT Thu May 17 2018

Bottom Line up Front...
Today looks to be close to a carbon copy of yesterday more or
less, with sunshine and warm temperatures into the mid 80s
expected across Iowa.


08z GOES-R Water vapor imagery clearly picking up on compact
shortwave spinning near the Kansas City area. Meanwhile, there is
broad anticyclonic flow over the Great Lakes region, and a broad,
weak area of cyclonic flow over the southeastern CONUS. These
latter two features are effectively hindering the departure of
this shortwave. GOES-R "Upper-level" Water vapor imagery clearly
picking up on high-level clouds covering most of Iowa that are
tied into this shortwave.

For the rest of today, agree with models in keeping ci/cs over Iowa
most of today, especially southern Iowa. 850mb temperatures are
around 2C warmer than yesterday, so have generally went with
persistence plus a few degrees for max temps today. Though we mix to
above 800mb, fcst soundings show winds aloft are not much above 10
kts in the low levels, so expect another day of light winds. Quiet
weather expected into Friday.


.LONG TERM.../Friday through Wednesday/
Issued at 354 AM CDT Thu May 17 2018

Bottom Line up Front...
Shower and thunderstorm potential continues for this upcoming
weekend. Models have slowed progression of system, so Saturday is
trending to be drier for longer into the day, and Sunday may
feature increased precipitation chances into the afternoon hours.
Temperatures will likely be above normal for the next 7 days... at
least.


This Weekend...
08z Water vapor imagery picking up on well-defined, closed upper low
propagating southeastward along the California/Nevada border. 500mb
height analysis shows longwave trough dropping all the way through
the SW conus, reaching down towards Baja California. Models in
decent agreement with propagating this broad trough/closed low
northeastward towards the Midwest, placing the upper low near
northeastern Colorado by 12z Sat. The overall consensus trend has
certainly been to slow the progression of this system. Through 12z
Sat, confidence increasing in dry forecast as lack of significant
forcing mechanism should keep atmos capped. After 12z Sat, model
evolution discrepancies begin to crop up...

Confidence in forecast from around 12z Sat-12z Mon has diminished
since yesterday as model-to-model agreement is not great. the 00z
GFS has established itself as the slowest model... keeping the
closed low in eastern Nebraska at 12z Sun, and propagating it into
western Wisconsin by 12z Mon. The 00z ECMWF and the 00z UKMET are
much faster with the progression of this low, not only opening it up
sooner, but placing it into western Iowa by 12z Sun and well into
the Great Lakes region by 12z Mon. The 00z GFS and the 00z ECMWF
both initialize well. Other models, such as the 00z NAM, are more or
less in the middle. Ultimately, have stuck with a 00z model
consensus blend for the moment as struggling where to find
opportunity to add value to determine if the GFS or the ECMWF track
should be favored. GFS seems to be more of an outlier overall with
the strength of the low, so a slight nudge away from it may be
prudent.

With going forecast, best moisture does not arrive in Iowa until
around 00z Sun - 06z Sun timeframe. Moisture amounts certainly
decreased from what they were progged to be 24 hours ago. 850-300mb
wind analysis picking up on anticyclonic flow directly over
Louisiana at 12z Sat... effectively shutting off the Gulf of
Mexico for notable moisture transport to Iowa. 00z GFS, the
strongest model with the low now struggles to even get a 30kt LLJ
into Iowa... continuing with the theme of being a stark difference
from 24 hours ago. Ultimately, potential for heavy rainfall has
surely taken a hit. If the GFS solution wins out, will have to
watch for severe weather potential during peak heating Sunday.
Boundary associated with baroclinic zone may be over central to
southern Iowa somewhere, serving as a focusing mechanism for
convection. With so much uncertainty in track, confidence low
enough to where it is not worth getting into potential mesoscale
details for Sunday.


Monday and beyond...
Eventually this system pushes east of the DMX CWA. Long-range
models attempt to establish what appears to be a rex block over
the western US. Correctly placed is a thermal ridge that builds
across the Great Plains to the Canada/US border. Models break down
this rex block by Wednesday... but it would seem plausible to
hold it into Thursday/Friday. When this rex block breaks down, it
would seem plausible the thermal ridge would crash over Iowa...
likely around the end of next week/into next weekend. When that
happens, near-record highs in the 90s are coming to Iowa.
Precipitation- wise, though not likely, highly scattered peak-
heating convective showers may be possible during peak heating.


&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1144 PM CDT Wed May 16 2018

Widespread VFR conditions across all sites for this period with
few clouds and unrestricted visibilities. Winds increase slightly
for Thursday afternoon, from the east and southeast.


&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Hagenhoff



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