Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 152353

Area Forecast Discussion
National Weather Service Des Moines IA
553 PM CST Wed Nov 15 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 303 PM CST Wed Nov 15 2017

Afternoon surface analysis shows an area of low pressure over the
upper peninsula of Michigan with a cold front around Highway 30
moving southeastward through the forecast area. This is the
secondary push of cold air advection, which has resulted in a non-
diurnal temperature trend behind the front. Stratus clouds with
bases between 1500 and 2000 feet AGL have slowly moved southeastward
into north central and northeast Iowa per surface observations and
GOES-16 imagery. As the area of low pressure over the Great Lakes
pulls farther away from the area tonight, these clouds will be
pulled away as well. The gusty winds from the northwest that have
prevailed today will decrease this evening as an area of high
pressure moves into the state. With the high becoming centered over
the state near daybreak, winds will be light with a fairly clear
sky. This should allow temperatures to drop to around 20 degrees
across far northern Iowa with low to middle 20s elsewhere.

As the area of high pressure moves away on Thursday, higher level
clouds will begin to arrive ahead of a strong shortwave trough
arriving over the western US coast. Surface winds from the southeast
will be common as air rotates clockwise around the departing high.
This will mean that temperatures will be influenced by this high and
will be cooler than Wednesday with highs ranging from the upper 30s
over northern Iowa to the 40s elsewhere across central Iowa. Aloft,
winds from the southwest will develop and begin to draw low level
moisture toward the region.

.LONG TERM.../Thursday night through Wednesday/
Issued at 303 PM CST Wed Nov 15 2017

./Thursday Night through Saturday Night/...Confidence Medium to High

Models settling in on a more consistent solution with the upcoming
event for late Thursday into Saturday morning though there are a few
minor differences between the Euro/GFS with the departure of the
system. Both the GFS/Euro continue to model an open wave structure
with little amplification until the system gets into the eastern
Great Lakes where ample cold air advection finally catches up and
enhances deepening over eastern Wisconsin to Michigan. Initially a
weak wave will propagate northeast into southern Iowa by 18z. Along
with the weak lift...low level moisture will be increasing across
the south between 12 and 18z though midlevels will remain dry.
Soundings and cross section confirm that all of the moisture will
remain below 800mb with dry air and an inversion above that level.
This should lead to increasing chances for drizzle initially with
rain showers filling in during the afternoon as both frontal forcing
and lift from a weak sfc low increase shower chances over the east
during the afternoon hours. Lows Thursday night will remain mild
with southerly flow and highs on Friday will be definitely be mild
in the south...but at the northern areas may not see the mild air
ahead of a frontal zone which will be setting up across the forecast
area Friday afternoon. Guidance on Friday varies quite a bit but
given the lowering stratus and increasing saturation throughout the
day will lean toward cooler highs than MOS output for the day.  It
is possible that low level warm air advection may account for the
bulk of the heating Friday...but any hint at drizzle or light rain
would also tend to cap afternoon temperatures.  By Friday night into
Saturday morning the focus then shifts to the system as it begins to
exit the state to the east.  The GFS is now speeding up the
translation of the front with little overall support aloft for an
area of enhanced precipitation later Friday night into Saturday
morning while both the NAM/Euro indicate some lift along the exiting
boundary with as tenth or two of precipitation. This will probably
need to be evaluated again tomorrow...but for now will stay with the
trend of some precipitation Friday night as the front departs...with
colder air mixing into the system for a rain/snow mix changing to
light snow/flurries prior to ending early Saturday. As the system
departs significant subsidence and cold air advection will lead to
rapid mixing during the morning hours through the afternoon.
Adiabatic lapse rates support wind gusts of 30 to 35 mph on top of
gradient sfc winds of 15 to 25 mph during the day.  In the evening
as an inversion develops...winds will slowly relax through the
midnight hour.  The remainder of the night will allow for stronger
radiational cooling.  Unlike yesterdays 12z`s run shows
little in the way of upstream cloud production so most of the night
now looks clear.  This will allow for lows in the teens to lower 20s
after daytime highs in the 30s north to the mid 40s in the south.

./Sunday through Wednesday/...Confidence Low to Medium

Confidence for the remainder of the outlook period has lessened
today. The models are now diverging by Tuesday...though so far the
Euro is holding onto a warm solution while the GFS today has now
taken a 180 degree turn.  At 500 mb both models remain in lock step
until about 12z Sunday when the Euro begins to shed a piece of
energy from a 498 dm vortex located just north of the Northwest
Territories of Canada near the Bank Shelf in the Beaufort Sea. As
the shed area of cold air and associated energy propagate southeast
the parent low continues to retrograde west southwest into Alaska.
The shed wave tracks east southeast across the Northern Plains to
the Great Lakes and induces a Clipper system with a glancing blow
of cold air across mainly northeast Iowa. This is similar to
previous days runs of the Euro so for now...some day to day
consistency remains present. The GFS however...sheds a larger
piece of the cold air southeast along with a more significant
portion of energy into Alberta. The parent H500 low initially
retrogrades west but then pivots east southeast following the
lead wave. This results in a phasing evolution with the eastern
Canada H500 large scale trough...along with an expansion of
colder air southeast and a rapidly deepening H500 vortex that
spreads across most of the Central Conus by day 10. The differences
between the GFS/Euro are about as extreme as is possible regarding
next week`s forecast. Given that reality...and the likelihood
that the trigger point for the change is Sunday morning "in the
model world" our forecast will continue to reflect a warmer...more
persistent Euro and previous GFS centric solutions for the present.
This trend will keep temperatures in the 30s and 40s from Sunday
onward with little precipitation through the period. It is interesting
to note however...that last Friday 11/10...the 12z Euro had been
trending toward a cold pattern at the very end of the 240 hr
forecast. This would have put the pattern change near next Monday
the 20th at the time of last Friday`s forecast. The next few days
will be quite interesting since it appears that a very subtle
change far away from our area is causing a divergence in medium
range model solutions at present.


.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 552 PM CST Wed Nov 15 2017

The primary concern will be the fate of NE Iowa stratus this
evening. Models are not handling or recognizing this well but
those that are closer still suggest it will dissipate and/or move
off to the east later this evening. Thus have only depicted +2KFT
MVFR conditions for several more hours before becoming VFR area
wide for the rest of the valid period. NW winds should diminish
and then become light SE by Thu afternoon.





SHORT TERM...Ansorge
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