Scientific Forecaster Discussion
fxus63 kdmx 172341
Area forecast discussion
National Weather Service Des Moines Iowa
541 PM CST sun Nov 17 2019
Discussion.../tonight through Sunday/
issued at 325 PM CST sun Nov 17 2019
..light precip possible Monday, with no hazardous conditions
..widespread precipitation likely on Wednesday PM-Thu am...
..Generally near-normal temperatures expected through this week...
Light precipitation possible Monday...
Key points: light precipitation may move from west to east
throughout Monday morning and afternoon. Intensity should be very
light, with generally no accumulation expected. With daytime
temperatures above-freezing, this should be more in the form of
rain or drizzle. No hazardous impacts are expected.
Scientific discussion: 20z GOES-r water vapor imagery is picking
up on a weak impulse ejecting off The Rockies along the US/Canada
border over Montana. Models are converging on this feature and
generally have it propagating towards Iowa, crossing into the
western portion of the state near 12z Mon, and pushing east of the
state by around 00z Tue. There is some kinematic support and support
through 750mb to 700mb frontogenetical forcing. But the moisture
profile from sub-850mb highlights a fair amount of dry air. Surface
dewpoint depressions may be running around 10 degrees. Ultimately,
getting much precip beyond drizzle/light rain will be difficult.
Widespread precipitation expected Wed PM-Thu am...
key points: confidence is becoming high in precipitation moving
across Iowa during this timeframe. On Wednesday, temperatures for
this event should be well into the 40s and 50s, so this will be a
rainfall event. There is a chance of isolated thunder during
later Wednesday evening. As of now, rainfall amounts may be near
the half- inch range, so this will be more towards the moderate
end of the spectrum for this time of year. By Thursday morning,
temperatures will drop to near freezing, so there could be a light
rain/snow mix with any lingering precipitation. As of this time,
it appears intensity should be light by Thursday morning, meaning
any noteworthy snow accumulation appears unlikely. By late
Thursday morning, this system should be east of our area.
Scientific discussion: models have been converging on a system
that features decent westward tilt with height ejecting off The
Rockies and pushing through Iowa during this timeframe.
Interestingly, the GFS solution is the one the long-range models
have been trending towards. The GFS solution has been trending
slower/further northwest and stronger. Kinematics are very impressive with
this system. Will be watching how 06z Thu 850mb low level jet placement
evolves over next few days. As of now, heaviest precip looks to be
in northern/NE Missouri... but if this shifts slower and northwest,
some of this heavier precip could make it into southern Iowa. So,
have boosted precip chances... with expectation that future shifts
may continue boosting trend.
With strong warm air advection/positive vorticity advection ahead of this system, a signal for elevated
instability exists. Thus, have interjected thunder into the
forecast. The severe weather threat is effectively nil at this time.
Generally near-normal temperatures expected through this week...
key points: normal highs for this time of year are around the
mid 40s. Highs for Wednesday look to be well-above normal in the
50s... and even 60s in southern Iowa. There certainly is a good
chance the Des Moines Metro may reach 60 on Wednesday. Behind this
system, temperatures should fall back to the 40s as there is not
any "extreme" cold air projected to make it to Iowa.
Aviation.../for the 00z tafs through 00z Monday evening/
issued at 535 PM CST sun Nov 17 2019
Confidence in non VFR tafs continuing through much of the period
remains high. Predominantly MVFR conditions will prevail with
ceilings expected to primarily reside in the 1500 to 3000 ft
range. A few areas may experience visiblity reductions as well
with light winds overnight, but with cloud cover confidence in
widespread fog/reduced visibilities is low.