Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180528

Area Forecast Discussion
National Weather Service Des Moines IA
1128 PM CST Sun Feb 17 2019

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Monday/
Issued at 304 PM CST Sun Feb 17 2019

Negatively tilted mid-level trough and PV anomaly continues to move
off to the north and east. While short term guidance paints the
better frontogenesis and ageostrophic processes east of the CWA,
light to moderate snow showers will continue in the northeast along
an area with isentropic ascent and non-negligible amount of
convergence occurring. This will provide sufficient lift through a
layer with a decent amount of available moisture left. This has the
ability to produce between 1 and 3 inches of additional snowfall
north of U.S. Highway 30, with the greater amounts centered closer
to the U.S. Highway 20 corridor. Later this evening, drier air from
H85 and upward enters Iowa which will begin to limit ice
introduction and inhibit snow growth potential. Eventually any snow
showers will weaken, giving way to a few scattered flurries while
the near surface layer remains saturated, before completely drying
out. Temperatures this evening have been bumped down a few degrees
to account for the switch to northerly flow from the surface through
H7, as well as the snow pack. Winds will not be an issue this
evening, and thus wind chill values may drop just a tad below zero
but will not present any hazards. Monday, a few flurries may be
spotted in the far northeast but at this point the bulk of the
activity will be well off to the east. Most of the area will remain
within cyclonic flow on the back side of the exiting low pressure,
which will keep skies cloudy for most of Monday. There will not be
enough AVA over the upper Midwest to provide enough subsidence to
clear skies out. With cloud cover, new snow, and weak northerly
flow, temperatures have been kept slightly cooler than guidance for
Monday afternoon. Mid to upper-level flow switches from
northwesterly to more zonal, which will keep conditions cool but
quite across Iowa for most of Monday. However, that will be short
lived as another trough digging across the Pacific Coast will bring
southwesterly flow aloft by Monday evening in this overall active
weather pattern.

.LONG TERM.../Monday night through Sunday/
Issued at 304 PM CST Sun Feb 17 2019

Broad western trof will continue to make the weather interesting
across the state in the longer term.  While Monday night into
Tuesday will bring a brief respite from the snow, the next in a
series of shortwaves is expected to eject from the southwest United
States on Tuesday. A jet segment associated with the wave will be
rounding the base of the trof and moving northeast into Missouri and
Illinois on Tuesday night.  This will support upper divergence
across Iowa overnight into Wednesday morning.  Strong accent is also
expected in the mid levels with strong Q-vector convergence over the
state along with an strong initial surge of isentropic lift.
Therefore, this lift will lead to saturation quickly late Tuesday
afternoon/evening from southwest to northeast across the state. Snow
will expand rapidly across the state during the evening and persist
overnight as the forcing increases ahead of the shortwave.  There is
a good 12 hour period where max forcing is close to the dendritic
layer and snow production should be decent during the overnight.  By
daybreak on Wednesday, the bulk of the forcing will be from northern
Iowa into Minnesota and Wisconsin with precipitation ending from
southwest to northeast during the morning.  On the backside of the
precipitation, mid level saturation lowers substantially and there
may be a brief period of freezing drizzle prior to the cessation of
all precipitation.  Winds will increase on Wednesday but should not
become overly strong as cold advection remains in check along with
the pressure gradient.  However, given the amount of snow already on
the ground, it won`t take much to get some blowing snow and would
expect at least some limited blowing in rural areas.  Amounts across
the forecast area range generally from 5-7 inches and looks like a
high end advisory to low end warning storm at this time.

The aforementioned lack of cold advection will mean that daytime
highs on Thursday into Friday will remain at least as high as
Wednesday if not higher as low level thermal profiles warm into the
end of the work week.  GFS and Euro have substantial differing
solutions into the weekend which continue to change but suggest some
sort of precipitation, mainly light at this time from later Friday
into Saturday.  As was the case yesterday, there is low confidence
in any solution at this point.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1128 PM CST Sun Feb 17 2019

Light snow persisting late this evening and have it lingering
awhile longer at most sites with high end MVFR ceilings. Ceilings
will lift into VFR, though confidence on timing is low. Could
foresee the clouds lingering a few hours longer given climatology.
Winds from the north will be 5 to 10 knots.




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