Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
550 am CST Friday Dec 26 2014
Short term /today/...
issued at 343 am CST Friday Dec 26 2014
There is a lot to contend with today. A long wave trough on the Lee
side of The Rockies will lift northeast across the upper Midwest
late today through early Saturday. At the surface...a low over Kansas
will lift into Iowa along a frontal boundary in response to the upper
wave shifting east. Currently the boundary across Iowa runs from
about Mason City to Atlantic. South of the boundary there is middle
and high level clouds and southerly surface winds at 10 to 15 miles per hour
while north of the boundary low stratus has socked in northwest and
far western Iowa and winds are north at 15 to 25 miles per hour. This boundary
is going to slowly move across the state today thus timing clouds is
crucial to temperatures today. In general the system appears a few hours
slower than it did previously so I think the southeast portion of
the forecast area will still have fairly warm temperatures today. Not like
yesterday but very nice for late December. Northwestern portions of
the County Warning Area will see a return to stratus and colder temperatures...at least
relatively speaking. Having said that the stratus will need to be
monitored. If it comes in quicker then temperatures over the areas where
stratus cover was not previously forecast will need to be lowered.
Next we look at moisture. Soundings show a very dry middle level and
shallow low level moisture. Weak forcing does move across the
northwest this morning and with no ice introduction present in the
soundings until at least 18z there is at least a possibility of some
patchy freezing drizzle this morning where temperatures are below freezing.
Confidence is low on this occurring though due to the lack of
sufficient low level moisture. As middle and high level clouds stream
into the area from the trough to the west pushing in...the column
between -10 and -20 will moisten up and when that happens then
precipitation will change over to freezing rain or rain depending on how
much the surface layer can warm. Between middle and late afternoon the
front will push southeast and we will then finally see a changeover
to snow or a period of a rain/snow mix then snow. The change over
will occur this evening across the southeast.
As far as precipitation chances go for south central and southeast Iowa
GOES...I do not see sufficient moisture for precipitation coming in much
before 00z though after 21z moisture increase will be fairly rapid.
There is also not much forcing to speak of before this evening so I
did back off on probability of precipitation south and east.
Long term /tonight through Thursday/...
issued at 343 am CST Friday Dec 26 2014
Main upper system will eject tonight from The Rockies and pass into
the upper Mississippi River valley by Saturday morning. Strong middle
level kinematics will progress into western Iowa during the evening
as deepening surface wave lift into southeast Iowa. The column will
saturate quickly with the strong lift with snow developing across
the northwest and expanding in areal coverage during the evening. In
addition...as surface wave lifts northeast and moisture from the
Gulf becomes more entrained into the system...precipitation is
expected to develop near the track of the surface low and the low
level baroclinic zone. Much of this precipitation will be liquid
with surface readings holding above freezing especially southeast of
Grinnell to Lamoni line. However...west of this line...even where
temperatures are slightly above freezing...the strong forcing and
dynamic cooling will likely help transition precipitation to all
snow. This rain snow line will push southeast through the night as
near surface cold advection persist. Meanwhile...the snow in the
north will persist for much of the night and with steep middle level
lapse rates...a few bursts of heavier snow will be possible
especially in the far northwest. Snow amounts near Estherville will
likely be near 4 inches or so by daybreak with lighter amounts to
the southeast. Have added a Winter Weather Advisory to the far
northwest for the higher accumulations in those areas.
Snow will linger in the far north on Saturday morning from nearby
deformation zone. However...subsidence will quickly overspread the
state with precipitation ending by midday in the far northeast.
Otherwise...drier and colder air will overspread the state on
Saturday. Next system will arrive on Monday with an polar front
dropping through the region. The system will be lacking in moisture
with only a slight threat for precipitation at this time.
However...much colder air will push into the state behind this
system later Monday and last for much of the work week with an
impressive 1058mb high dropping into the US. This will provide a big
reality check after a few weeks of relatively mild temperatures.
issued at 550 PM CST Friday Dec 26 2014
LIFR ceilings moving in from the northwest will spread across the forecast area
through the morning. Light precipitation will develop across the north by
18z which will transition to a rain/snow mix or possibly some
freezing drizzle or brief freezing rain then changing to snow by
22z. Precipitation across southern taf locations will be several hours
later...with a transition to snow around 02z.
Winter Weather Advisory from 6 PM this evening to 6 am CST
Saturday for Emmet-Kossuth-Palo Alto-Winnebago.