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Area forecast discussion 
National Weather Service Des Moines Iowa
940 PM CST sun Nov 29 2015

issued at 919 PM CST sun Nov 29 2015

Challenging forecast remains with plenty of issues to contend
with. Radar mosaic at 930pm continues to show a widespread area
of precipitation along and east of the I-35 corridor. Up until
now...all of the precipitation has been either been rain or freezing rain with
surface temperatures hoovering around 32f. Recent calls to law
enforcement have indicated mainly wet roadways thus far. This
makes sense as RWIS Road temperatures continue to indicate readings
generally in the middle/upper 30s. At the office...weve noted the
same trend with wet pavement. However...elevated objects such as
tree branches and wires are do appear to be accreting light
ice...generally a couple hundredths. Latest guidance continues to
indicate low level temperatures hoovering in the lower
made some minor tweaks to the forecast. Bottom line...expect a
continuation of light ra/fzra...but with little impact on roadways
for at least the next couple hours. Could also see the precipitation
transition to snow across the north as deeper saturation occurs in
the ice nucleation zone...and the warm layer aloft begins to cool.

Latest short range models continue to agree that the bulk of the
heavier precipitation will move into the region towards daybreak
Monday. Still looking at a combination of snow to the west and
north...transitioning to freezing rain and rain to the east and
south. Snow totals look on track...however a bit concerned that
less icing will occur as temperatures flatline in the low/middle
30s...with little in the way of reinforcing cold air. At this
point...have opted to let the advisory/warning ride.



Short term /tonight/...
issued at 400 PM CST sun Nov 29 2015

Plenty of concerns overnight as large western trough begins to emerge
into the central United States. Moisture transport has begun to
increase from the south during the day with broad area of precipitation
across MO/Kansas lifting back north into southern Iowa this afternoon.
Precipitation has been mainly rain although the northwest edge has seen a few
reports of mixed precipitation. This precipitation is supported by isentropic
lift/warm advection ahead of the system and will continue/expand
across the southeast this evening. However...the bulk of precipitation is
expected to move in after midnight as strong forcing and moisture
transport arrive ahead of the surface low and precipitation type will be the
biggest issue overnight as freezing line will bisect the forecast
area. Some concern that those locations that are currently seeing
rain may not revert to freezing rain this evening. It will be a
fight from drier air pushing in from the northeast near the surface
helping to keep readings at or below freezing vs the warm advection
and gradual increase of dewpoints from the south. Ultimately...the
southeast should remain rain all night where readings will be above
freezing. Farther west and north...there will be a better treat
of cooler air holding on overnight as precipitation expands into the area.
Locations in west central Iowa and then eastward along the Highway
30 to Highway 20 corridor have best threat of substantial icing
approaching warning criteria. Highest output is near the
Nishnabotna river valleys into the western Raccoon river basins
where up to one quarter to three tenths of an inch of icing occur.
These will be covered by a warning later tonight into Monday.
Farther north...the warm nose will not be as pronounced and with
deeper cool air...the threat for freezing precipitation will be minimized
with snow much more likely. However...precipitation farther north will be
quite light as the bulk of forcing and lift come on Monday when
the higher snowfall rates occur. Temperatures will fall some this
evening as precipitation falls and temperatures drop toward the
wetbulb...however once things are saturated...temperatures will be
steady to slowly rising toward daybreak.

Long term /Monday through Sunday/...
issued at 400 PM CST sun Nov 29 2015

Concerns center around the storm system to impact the County Warning Area Monday
into Tuesday...with dry conditions expected the remainder of the
forecast period. Upper low currently spinning across The Four
Corners region to be centered over the Central Plains with the
associated surface to track into northwest Iowa by late Monday afternoon.
Southerly flow ahead of the system in place Monday...with strong
isentropic lift and increasing moisture. Deeper saturation noted in
the soundings during the day...with a warm nose aloft pushing into
much of central and southern Iowa. Therefore expect any snowfall to
be relegated to the far northwestern County Warning Area...with rainfall across the
other two thirds of the County Warning Area. Initially surface temperatures to be
near or below freezing across much of central and southwestern
County Warning Area...which will result in a period of freezing rain before surface
temperatures warm enough for mainly rain. Some very weak instability could see some convective tendencies and brief moderate
to heavy rainfall. This could result in around a quarter inch or
more of ice accumulation in portions of the County Warning Area along the I-80
corridor from Dallas County westward. Therefore will issue a Winter
Storm Warning for that area. Further to the south and
amounts less from a light glazing to maybe one to two tenths so will
go with an advisory for that area. In the Des Moines metropolitan...some
light freezing rain is possible around the morning commute time...but should
see a quick transition to rain through the morning hours with warmer
air moving in. Of other concern with the ice amounts will be the
Road temperatures. Much of the Road temperatures across the central and
southern County Warning Area remain above could see more of the ice
accumulation on elevated surfaces such as trees and power lines and

Further north and west...precipitation to spread across the County Warning Area
Monday with mainly snow to the northwest. Across the
northeast...precipitation may be a mix of freezing rain or snow...with a
change over to a mix of freezing rain/rain depending on how warm the surface temperature
gets as the warmer air wraps into the state ahead of the surface low.
This will also coincide with the dry slot wrapping into the County Warning Area for
Monday afternoon into the could also see a period of
dry weather in between precipitation periods as well. Colder air then moves
in later Monday night with precipitation to transition back to freezing rain
and then snow by early Tuesday. Snow fall to continue as the def
zone/wrap-around precipitation area and the upper low move through the
state lifting into the Great Lakes region by Tuesday night into
Wednesday. Strongest lift in the dendritic zone and resulting better
snowfall rates to occur across the far northwest and further to the
north/west of the County Warning Area. Expect heavy snowfall across the northwestern
third of the County Warning Area...with a sharp gradient of snowfall to the
east/south where more liquid precipitation is expected. Will issue a
Winter Storm Warning for snow for much of the northwest with 6 to 10
inches of snow. Fringe counties of the warning...will see more of a
mix of snow and ice accumulation resulting in the warning. Otherwise
areas to the east/southeast of the warning may still see some
smaller amounts of ice...and a chance for some snow so went with an
advisory in those areas. Light snow to linger into Tuesday
afternoon...and taper off during the evening Tuesday. Expect stratus
to stick around into Wednesday...with upper ridging to build into
the central US for the latter portion of the week and into the
weekend. Thermal ridge builds aloft as expect moderating
temperatures for the rest of the period. Also not a real cold push
with the backside of this expect relatively mild temperatures
into Wednesday/Thursday already.


issued at 603 PM CST sun Nov 29 2015

Expect deteriorating aviation weather conditions this period.
Fairly widespread area of precipitation continues to move northeastward
across central Iowa. Ceilings remain primarily VFR at this time...although
expect a gradual transition to MVFR and then IFR/LIFR on Monday as
moisture continues to increase. Still a considerable amount of
uncertainty regarding exact precipitation type as temperatures continue
to Hoover close to the freezing mark. Expect a combo of rain/freezing rain
tonight. On Monday...kfod/kmcw should transition to
snow...possibly heavy at times...with visibilities below one mile at
times. Kdsm/Kalo/kotm should remain on the warmer side of the
system with primarily rain...possibly mixing with freezing rain
at times.


Dmx watches/warnings/advisories...
Winter Weather Advisory until 6 PM CST Monday for Adams-Madison-

Winter Storm Warning until 6 PM CST Monday for Adair-Audubon-

Winter Storm Warning until 6 PM CST Tuesday for Boone-Calhoun-
Carroll-Cerro Gordo-Crawford-Emmet-Franklin-Greene-Hamilton-
Hancock-Hardin-Humboldt-Kossuth-Palo Alto-Pocahontas-SAC-Webster-

Winter Weather Advisory until 6 PM CST Tuesday for Black Hawk-



short term...cogil
long term...beerends

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