Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 162040
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
240 PM CST Thu Nov 16 2017

.SHORT TERM.../Tonight through Friday/
Issued at 215 PM CST Thu Nov 16 2017

Departing Great Lakes low pulled a backdoor cold front through the
region earlier this morning...resulting in low clouds...areas of fog
and a few reports of flurries over the north and central areas.
The trailing front is partially connected to a developing warm
front over South Dakota back northwest into the next upstream
system currently over Manitoba this morning. Hires models indicate
that some erosion in cloud cover will occur over the central areas
yet this afternoon into the early evening. However the clearing
over central and southwest areas will quickly be replaced by increasing
warm air advection/low clouds currently located over the lower
Mississippi River Valley. This will be the beginnings of increased
moisture return through Saturday all ahead of the next upstream
system. Tonights lows will most likely not fall too far from todays
readings due to clouds and eventual warm air advection mitigating
radiative processes. Light drizzle will begin to increase after
12z with influx of low stratus. Highs tomorrow will struggle...with
temperatures mainly rising only due to advective processes. Have
trended to the lower portion of guidance for the day with afternoon
readings in the mid 40s far northeast to the mid to upper 50s in
the southwest. Clouds...light drizzle and dreary conditions likely
to hold on through the remainder of the day hours.

.LONG TERM.../Friday night through Thursday/
Issued at 215 PM CST Thu Nov 16 2017

./Friday Night through Saturday Night/...Confidence Medium

Forecast for Friday night generally on track with models in better
consensus on timing and slightly higher amounts of qpf into Saturday
morning.  GFS now has also finally held onto the Saturday morning
precipitation a bit longer...though it still pushes the forcing and
light mix of precipitation out faster.  Both the NAM/Euro are
trending toward some enhanced lift/weak def zone along the trailing
boundary as the system departs between 12-18z. Unfortunately the
models continue to differ on the thermal characteristics of the
airmass. This lowers the confidence a bit on ptype - especially
Saturday morning. The GFS is trending colder by 12z south to about
I80...with the Euro trending cold enough for rain/snow as far
south as the border but closer to 15z. The NAM is more confident
on colder air arriving early...enhanced lift and the potential for
a band of accumulating snowfall with the departing system. Resolving
the best forecast will most likely take another 12 to 24 hours...but
in the mean time have trended toward a rain snow mix spreading southeast
through the morning hours Saturday. Earlier across the north there
may be enough cooling to provide for some light accumulations
from Carrol toward Cerro Gordo County. At this time...even the
worst case scenario the NAM/Euro paints would more likely result
in accumulations central and south on grassy areas only. Will
monitor trends going forward. No real changes for the remainder of
Saturday with strong northwest winds expected along with strong
gusts but generally sub advisory criteria during the day/afternoon
hours. The cold air advection will continue into Saturday night
with overnight lows dropping to the teens north to the lower/mid
20s south.

./Sunday through Thursday/...Confidence Medium

Initially Sunday northwest flow aloft/sfc will continue but as a
ridge of high pressure builds east...west/southwest flow quickly
returns into the mid/late afternoon hours. This will help temperatures
recover over the area with highs in the upper 30s northeast to
the upper 40s southwest. Overall the GFS today has come around to
the Euro model regarding the large scale H500 pattern. Yesterday`s
12z run appeared to be an outlier and today it mirrors the Euro
with several Alberta Clippers affecting the region next week...but
no large scale breakdown of the H500 pattern is anticipated
through at least Friday. By Monday afternoon warm air ahead of the
Canadian clipper will bring H850 back to 6 to 10C over Central
Iowa prior to frontal passage. Highs will reach the 50s but then
rapidly cool by Tuesday back into the 30s northwest to the mid 40s
southeast. The best forcing with the Monday/Tuesday system will
be far enough north and along with little moisture we will not see
any precipitation. A second system over northwest Canada will
break off and eventually push southeast toward the Central Plains
by the end of the period. This will also result in a transition
back to warmer weather Thursday into Friday highs Thursday
reaching the 40s to maybe 50 southwest. The Euro is hinting at
some light precipitation with the retreating cold air along a warm
front...but confidence is nil at this time. Likely some mid level
clouds will at least accompany the warmer air. Though just beyond
the period today...quiet and mild conditions are expected
Thursday night/early Friday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1108 AM CST Thu Nov 16 2017

Stratus deck over much of northern and eastern Iowa retreats
northward the afternoon. This will eventually lead to improving
conditions at KFOD, KMCW, and KALO. VFR is likely across the area
this evening and into the overnight hours before our next round of
low cigs and visbys begins to move in from the south by Friday
morning. High confidence in MVFR or lower cigs expanding north
after 10z. Drizzle may begin to develop after 14z, however
confidence is lower with respect to the spatial extent and it`s
impact on visibility. Left most sites at 4sm for now, but this may
shift once there is a better handle on forecast trends.


&&

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

$$

SHORT TERM...REV
LONG TERM...REV
AVIATION...Martin



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