Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 110146

Area Forecast Discussion
National Weather Service Des Moines IA
746 PM CST Sun Dec 10 2017

...Updated for Near Term Weather Trends...

Issued at 746 PM CST Sun Dec 10 2017

GOES-13 infrared satellite imagery shows upstream Alberta Clipper
approaching the North Dakota border from the southern Canadian
prairies with cloud shield and precipitation ahead of the clipper.
Current observational and model guidance trends would suggest this
area of precipitation would clip the far northeast so have
trimmed PoPs back in areal extent over north central Iowa this
evening in collaboration with neighboring offices. Also have
delayed the timing of the precipitation a few hours reaching
northern Iowa toward mid-morning. Remainder of the forecast
appears on track at this time.


.SHORT TERM.../Tonight through Monday/
Issued at 342 PM CST Sun Dec 10 2017

Confidence: Medium to High

System crossing the region today has already brought northwest
surface flow back to most of the area...with some gusts as well.
Area of lower stratus over northeast Iowa will drift to the east by
evening. In the mean Alberta Clipper is rapidly moving
southeast toward the Northern Plains and Central Plains/Great Lakes
once again.  Larger scale pattern continues to tell the story:
Northwest flow predominates but also the ridge of high pressure over
the west continues to cut the Gulf of Mexico off from any moisture
transport north.  Even more importantly...for the Clipper systems
originating from Canada...this ridge which extends north along the
west coast to eastern Canada/British Columbia is limiting moisture
transport southeast into the US.  The longer residence time along
the Rockies due to the southeast trajectory appears to be squeezing
out more of the moisture with each system.  Unfortunately the models
have not quite caught on to the situation and continue to
overforecast precipitation beyond about 3 days.  The amount of
moisture available is not likely to change for our region until the
ridge breaks down and systems are allowed to track more east with
time. The good news is that there is a hint at 228-240 hours though
the GFS and Euro are not in great agreement regarding the amplitude
of the ridge by that time.  Upshot is that the system tomorrow will
once again be moisture limited...though we may be able to see a
light amount of precipitation. As clouds increase later tonight
another push of milder air aloft will move into Iowa with H850
temperatures peaking ahead of the system during the overnight hours.
Both the GFS/NAM show H850 temperatures from 0C to about 4-5C from
north to southwest by 12z tomorrow morning.  This will keep
overnight lows rather mild...with the warm air advection then
quickly coming to an end by 18z. With the passage of the front lift
and stronger mixing will follow. Our best chances for any
precipitation will accompany the upper level front around H850
during the afternoon hours. Bufr soundings show sufficient shear...
saturation and boundary layer shear to produce banded HCR
precipitation following the front. Coverage looks to be somewhat
limited but the the potential for snow flurries/showers there may be
brief periods of light snow combined with gusty winds.  The lack of
any appreciable available moisture will limit any impacts.  Overall
the system is not packing much moisture...even cross sections
showing extent of moisture limited over time at mid-levels with best
mid level moisture between 15-21z north and 15-21z south though lift
remains rather meager through column. A layer of dry air also works
quickly into the system from the north by afternoon and essentially
cuts off chances for precipitation...other than maybe a few flurries
or a brief period of light drizzle.  Any precipitation will then
diminish quickly into the mid to late afternoon hours with
increasing subsidence. Subsidence values behind the boundary are
about 1-2 microbars/sec with average H850 winds around 35 to 40kts.
Both gradient winds and winds aloft remain brisk through the evening
with subsidence supporting gusty winds Monday evening as well. With
colder air replacing the warmer air tomorrow highs will not be quite
as warm as today but still above average for this time of the
year...topping out in the 30s to mid 40s from north/south.

.LONG TERM.../Monday night through Sunday/
Issued at 342 PM CST Sun Dec 10 2017

As the upper wave departs Monday night and Tuesday the gradient
still remains pretty tight over the state so while no precip is
expected, winds will remain quite brisk.  With colder air in place
it will feel pretty cold all the way into Wednesday.  The exception
being the far southwest where warmer air will begin to work in and
highs will start approaching the upper 40s or even 50.

Except for the Euro...which is weaker...all the medium range models
are suggesting that Wednesday will be dry as the better forcing gets
shunted to our northeast.  The Euro does still bring the shortwave
right across central Iowa which would indicate some precip potential
but soundings do not support precip over the area...too much dry air
in place.  Northern Iowa...if some forcing could work down far
enough would maybe have enough moisture to squeeze out some precip
but confidence has slipped enough to not include precip at this

There is little change to previous forecast runs to the overall
pattern past Thursday with the west coast ridge flattening some as
a Canadian low treks eastward. This will produce a zonal flow
over the region and allow for modification of temps, but mainly
over the southwest third to half of the state. The northeast
cannot seem to break out of the 30s to lower 40s due to the
proximity of the Great Lakes trough impacting that area, while
the southwest will see temps well into the 40s and possibly lower
50slate in the week or weekend. Systems that do drop through the
mean flow seem to stay in the Northern Plains and northern
portions of the Upper Midwest grazing possibly northeast Iowa and
locations north and east.


.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 552 PM CST Sun Dec 10 2017

Main forecast concerns this period will be cloud cover,
precipitation, and winds as a fast moving Alberta Clipper moves
out of Canada through the region tonight into Monday. MVFR
ceilings have stayed north and east of the terminals late today so
expect VFR conditions to prevail until late morning at all
terminals. As the aforementioned clipper moves through, four impacts
are forecast.

1) High confidence that winds will become gusty from the
northwest through the end of the TAF period.

2) Moderate to high confidence that ceilings will lower into MVFR
restrictions from north to south from late morning through the
afternoon. Once MVFR restrictions set in, they are expected to
last through the end of this valid TAF period with the exception
of KDSM. There is also low confidence and therefore it is not
mentioned that IFR conditions could develop at KMCW.

3) Low to moderate confidence that rain and/or snow will impact
the terminals Monday with only VCSH mention at KMCW at this time.
This will probably be incorporated into future TAFs farther

4) There is low confidence of a brief window of low level wind
shear (LLWS) at KFOD and perhaps KDSM Monday morning. However,
this looks marginal and fleeting so left out mention at this time.




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