Scientific Forecaster Discussion
fxus63 kdmx 131137
Area forecast discussion
National Weather Service Des Moines Iowa
537 am CST Wed Dec 13 2017
Short term.../today through tonight/
issued at 324 am CST Wed Dec 13 2017
Main concern for short term will be the very windy conditions as
shortwave moves through the state today. Currently, thermal ridge
advecting across the state with decent southwest winds. Temperatures
have steadily risen during the early morning hours as the warm
advection persists. Surface front is rapidly approaching the state
and will pass quickly through the forecast area this morning. A few
sprinkles will be possible with this boundary, however any
precipitation with the front will be very limited as moisture
remains lacking with the system. There is an impressive trop fold
associated with this system as it drops southeast and this combined
with strong mixing on the backside of the front will lead to a rapid
increase in wind velocities across the forecast area this morning.
Most soundings across the forecast area show a decent mixed layer
with any where from 40 to close to 50kts at the top of the layer.
Therefore gusts of 45 to 50 mph seem reasonable across much of the
area although areas farther southeast will tend to be more marginal.
Even if winds don't gust high enough, sustained winds will approach
30 mph in many areas, therefore the Wind Advisory has been expanded
across the entire forecast area through 6pm. There remains the
threat of light rain/snow showers in far northern Iowa into the
afternoon where some instability in the mixed layer may produce a
few light showers.
The system will depart this evening with winds gradually decreasing
into the overnight. The cyclonic flow on the backside of the system
will help lower clouds linger for much of the night and this should
keep temperatures from falling too far. Surface ridging will
approach the northwest portion of Iowa toward the end of the period
with winds continuing to relax overnight.
Long term.../Thursday through Tuesday/
issued at 324 am CST Wed Dec 13 2017
Bottom line up front...
with the strong winds from today decreased to the 10 to 15 mph
range by the start of the long-term fcst period, no significantly
hazardous weather is set in the long-term forecast period at this
time. Temperatures look to be decently above normal much of the
next week, and any precipitation chances short-lived.
Thursday and Friday...
not much sunshine expected Thursday and Friday as an expansive
stratus deck should establish itself from the Ozarks northward
into central Canada. Models are all in agreement with the feature
of a an impulse racing down from northern British Columbia through
the upper Midwest, nearing Iowa by Thursday afternoon. As
mentioned in the afd from yesterday morning, models showing
disagreement with track of this shortwave, and to some extent, its
magnitude. The operational GFS continues to be the furthest east
and strongest solution, remaining somewhat an outlier. Thus have
leaned towards non-GFS consensus, merely grazing our western
counties with flurries Thursday afternoon into early Thursday
For the remainder of Friday, a reinforcing surge of stratus will
follow the above-mentioned shortwave... keeping Friday afternoon
generally cloudy. Descent/low-level subsidence processes in play, so
though cloud cover present, wintry precip seems very unlikely.
Temperatures trending slightly cooler, so have nudged down.
Saturday and Sunday...
active, but highly disorganized setup... a longwave trough
digging over the northeastern Continental U.S. To help place Iowa in west/northwest
flow aloft. Additionally, there is slated to be a closed upper-low
beginning to push northeast from Mexico into West Texas, a
shortwave pushing southeastward through Saskatchewan, and another
shortwave pushing through the Rocky Mountain range near
Wyoming/Colorado. As of now, the West Texas system appears to have
the potential to yield the biggest impact on Iowa, so will focus
on it. Current consensus is to push this wave just east of Iowa
during the day on Sunday, but the past few model runs of both the
operational European model (ecmwf) and Gem have been slowly pushing the wave
further west, into Iowa. Long-range models have the weak, not very
well-organized, Canadian shortwave still in Minnesota Sunday and
do not show any positive interaction between the two waves... will
have to monitor this to see if it changes. If these waves can
phase/sync up, then Sunday could be more active in Iowa.
Ultimately, have a buffer of light wintry precipitation across
the southeastern third of Iowa. As temperatures near 40 degrees,
any flurries/light snow will melt upon contact with the surface.
Next Monday and beyond...
yesterday morning's afd mentioned a cut-off low potentially
setting up somewhere near the southwestern Continental U.S.. the 00z Wed run
has pushed that low further west into the Pacific... allowing the
cold, northerly component side to drop down into Iowa. Still a
highly fluctuating and unresolved system, so a lot of wiggle room
between now and next week. Have given a slight downward nudge to
temperatures...however, with 850mb temps potentially still nearish
to 0 degrees, temperatures should still be a std dev or so above
Aviation.../for the 12z tafs through 12z Thursday morning/
issued at 537 am CST Wed Dec 13 2017
Strong northwest surface winds will be a big issue today with
surface wind gusts of 40kts expected at most sites with localized
higher values. In addition, widespread MVFR ceilings will
overspread much of the area this morning into the early afternoon
and continue into the evening before slowly lifting to VFR
ceilings. A few rain or snow showers will be possible in northern
Iowa today, otherwise obstructions to visibilities are not
Wind Advisory from 10 am this morning to 6 PM CST this evening