Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190534
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1134 PM CST Thu Jan 18 2018

.SHORT TERM.../Tonight through Friday/
Issued at 336 PM CST Thu Jan 18 2018

Confidence: High

Weak trough moved through the region early this morning with little
if any impact to warm air advection. In fact the H500 pattern
continues to flatten in the next 24 hours leading to a more west
southwest flow from today`s northwest flow aloft. some high clouds
have moved into the region in the northwest flow and will be with
the area tonight into Friday morning. As had been advertised for
the past several days the first influx of warmer air has been from
downslope adiabatic warming from the Canadian Rockies moving
southeast into the Plains. The next push will arrive tomorrow with
a more highly modified airmass. H850 temperatures are expect.

/Friday Night through Monday Night/...Confidence Medium

Initially...confidence ahead of the main system is better than the
actual system development at this point.  GFS continues to trend
farther northwest which has been the case for the past several model
runs. With the Euro continuing to show a slightly more southerly
track for now...confidence on some of the details remains low.

Ahead of the system models continue in decent agreement regarding
increasing low level moisture with clouds/lift commencing in the
lower layers between 00 and 06z Sunday. Soundings still offer no ice
introduction aloft...so an area of drizzle/perhaps freezing drizzle
will develop and spread north with time overnight into early Sunday
morning before changing over to all liquid precipitation as
temperatures warm.  Any freezing drizzle which occurs should lead to
only light ice accumulations with a couple hundredths of
precipitation expected. The track of the low remains in question
though the 12z Euro is trending a bit farther northwest than the 00z
run though not as far northwest nor as fast as the GFS. In either
case the better mechanical forcing and frontal lift occurs back in
Central to Northeast Nebraska northeast into northwest Iowa and
Minnesota through 12z Monday.  Warmer air wrapping into the low
should prevent any real changeover to snow until after 06z...
possibly later if the evolution of the timing of the Euro is more
correct. This adds another layer of complexity to the timing of the
higher impact conditions. The trends have been split between the two
models...Euro slower and GFS maintaining a faster translational
speed.  For now will continue to lean toward the slower Euro model
for impacts...with Monday being the more likely timeframe for any
accumulations of snow and potential for blowing snow. Still a few
days to iron out the details of amounts. The other forecast
challenge will be the extent and location of the dry slot which is
expected to track over central Iowa at some point. The timing of
this feature varies from Sunday evening (GFS) to Monday morning
(Euro). Once the system pulls a bit farther east/northeast into
eastern Iowa...light to moderate snow and blowing snow will occur on
the backside.  Winds may be strong enough to reduce visibility to
under a half mile with the heavier snow areas...so will be
monitoring that potential as well. Onto more mundane aspects...highs
will remain mild through Sunday in the 30s/40s then fall Monday into
the 20s northwest to an early high near 40 in the southeast.

./Tuesday through Thursday/...Confidence Medium to High

Little impactful weather is expected from Tuesday through Thursday
with only a weak disturbance possibly affecting the area Wednesday.
Highs should be mainly in the 20s/30s and 40s south by Thursday.ed
to warm from near 6C to 10C (east to west) today and end up near
10C to 15C (north to south) on Friday. This combined with mixing
should help to warm the boundary layer into the 40s. Soundings
show some limitation to warming tomorrow with deep inversion
advertised by both the GFS/NAM through Friday afternoon. Despite
that low level warm air advection from Kansas and areas south may
compensate for the lack of deeper mixing during the daytime hours.
Overnight tonight expect the usual decoupling and lighter wind
fields across the region. This may lead to some patchy fog
overnight into the morning hours Friday primary over the north and
into some of the river valley areas of northeast Iowa. Currently
the signal for fog is not particularly strong but increasing
melting today along with an approaching trough by 18z may be
enough to cause some moisture convergence and increased risk of
some fog north. Will leave to evening shift to monitor and make
adjustments as necessary. Winds will quickly increase during the
morning hours helping to dissipate any fog that may develop. Highs
Friday should recover all areas by afternoon to the around 40
north to the mid to upper 40s south.

.LONG TERM.../Friday night through Thursday/
Issued at 336 PM CST Thu Jan 18 2018

./Friday Night through Monday Night/...Confidence Medium

Initially...confidence ahead of the main system is better than the
actual system evolution at this point. GFS continues to trend
farther northwest which has been the case for the past several
model runs. With the Euro continuing to show a slightly more
southerly track for now...confidence on some of the more important
details remains lower at this time.

Ahead of the system models continue in decent agreement regarding
increasing low level moisture with clouds/lift commencing in the
lower layers between 00 and 06z Sunday. Soundings still offer no ice
introduction aloft...so an area of drizzle/perhaps freezing drizzle
will develop and spread north with time overnight into early Sunday
morning before changing over to all liquid precipitation as temperatures
warm. Any freezing drizzle which occurs should lead to only light
ice accumulations with a couple hundredths of precipitation expected.
The track of the low remains in question though the 12z Euro is
trending a bit farther northwest than the 00z run though not as
far northwest nor as fast as the GFS. In either case the better
mechanical forcing and frontal lift occurs back in Central to
Northeast Nebraska northeast into northwest Iowa and Minnesota
through 12z Monday. Warmer air wrapping into the low should prevent
any real changeover to snow until after 06z... possibly later if
the evolution of the timing of the Euro is more correct. This adds
another layer of complexity to the timing of the higher impact
conditions. The trends have been split between the two models...Euro
slower and GFS maintaining a faster translational speed. For now
will continue to lean toward the slower Euro model for impacts...with
Monday being the more likely timeframe for any accumulations of
snow and potential for blowing snow. Still a few days to iron out
the details of amounts. The other forecast challenge will be the
extent and location of the dry slot which is expected to track
over central Iowa at some point. The timing of this feature varies
from Sunday evening (GFS) to Monday morning (Euro). Once the
system pulls a bit farther east/northeast into eastern Iowa...
light to moderate snowfall rates and blowing snow will occur on
the backside. Winds may be strong enough to reduce visibility to
under a half mile with the heavier snow areas...so will be
monitoring that potential as well. Onto more mundane aspects...
highs will remain mild through Sunday in the 30s/40s then fall
Monday into the 20s northwest to an early high near 40 in the
southeast.

./Tuesday through Thursday/...Confidence Medium to High

Little impactful weather is expected from Tuesday through Thursday
with only a weak disturbance possibly affecting the area Wednesday.
Highs should be mainly in the 20s/30s and 40s south by Thursday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1134 PM CST Thu Jan 18 2018

VFR conditions are expected through the TAF period with modest
southwest breezes and thick cirrus clouds. The threat of patchy
fog overnight has diminished and it now appears unlikely. However,
low-level wind shear has been included at OTM/DSM in the next
several hours and may become more widespread Friday afternoon and
evening.

&&

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

&&

$$

SHORT TERM...REV
LONG TERM...REV
AVIATION...Lee



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