Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
545 am CST Sat Feb 28 2015
Short term /today/...
issued at 313 am CST Sat Feb 28 2015
Arctic high pressure is departing off to the east though ridging
remains extended into northeast Iowa. Current temperatures over the
northeast forecast area are in the 10 to 20 below range and combined
with southeast winds of 5 to 10 miles per hour...have created wind chills 20 to
30 below. Will leave the wind chill headline as is even though it is
marginal on the western fringe.
A middle level short wave is currently moving across Kansas this
morning and will continue to progress east/northeast today. This
will bring modest qg forcing over the southern half of the state.
This combined with moisture advection lifting north on the backside
of the departing high with bring potential for light snow. Moisture
saturation across the south through a relatively deep dendritic
layer with vertical displacement through the layer aided by the
passing forcing is expected to generate an area of light snow by
early afternoon. Any snowfall amounts will be light with one half
inch or less expected. Extremely dry air over the north will limit
the northern extent of the precipitation. Temperatures will again be
well below normal today with clouds moving in and a lack of good
warm advection. Highs will be in the teens to middle 20s which again is
around 20 degrees below normal. This is a fitting end to February
which will be around 10 to 12 degrees below normal for much of
central and northern Iowa.
Long term /tonight through Friday/...
issued at 313 am CST Sat Feb 28 2015
Two main systems of concern for the extended. First will be
ongoing tonight as wave pushes east. Models continue to pull best
forcing and moisture south with this system...and have begun to
trim back on northern extent of precipitation. However...very
latest models runs are beginning to pull system slightly further
north once again. In addition...soundings indicating good
saturation through column through Sunday morning...with much of
this saturation occurring within the dendritic growth zone.
Therefore...even though forcing is limited have included
precipitation chances through Sunday. Expect precipitation to be
light...favoring a high pop/low quantitative precipitation forecast solution. System will pull
east through the day Sunday and may have hung onto precipitation
too long into the afternoon Sunday.
High pressure will quickly push through for Monday ahead of next
system. Next system will arrive Monday night into Tuesday and will
be main forecast concern for the extended. Two low pressure
systems will affect northern plains as one pushes east across
Canada and another lifts northeast from rockies. Current model
solutions appear to favor phasing of the two systems...which could
result in significant precipitation for Iowa as it is squarely
between the two systems at this time. However...timing of systems will be
crucial and therefore do not have high confidence yet in
significant precipitation. Speed of southern low will determine
how much moisture is available...as systems may not phase until
further east of central Iowa. In addition...track of southern low
may lift into central Iowa...allowing dry slot to track across
area. However...at this temporal length have kept precipitation
chances throughout County Warning Area for Tuesday through Wednesday night. Last
concern becomes precipitation type with this system. Temperatures
will be warm enough ahead of low to allow for a mix of
precipitation across much of the central and southern portions of
the County Warning Area beginning early Tuesday morning...through the day Tuesday
before transitioning to snow as frontal boundary associated with
northern low pushes south. Precipitation will likely switch to
rain across the south during the day Tuesday as temperatures warm.
Therefore mixed precipitation will likely be variable lifting
north through the day and pushing back south in the evening and
overnight as temperatures cool once again. Winds will increase
behind boundary Tuesday...which may allow for blowing of falling
snow or fresh fallen snow. System will need to be watched
closely...given current uncertainties and potential for impacts
across the area.
Behind this system...another area of high pressures pushes in with
colder temperatures once again for middle week. However...more zonal
flow will return for the end of the period...brining slightly
issued at 545 am CST Sat Feb 28 2015
Clouds will increase and ceilings will lower this morning and into the
afternoon. MVFR ceilings are expected at kdsm/kotm and will extend
farther north later today. In addition...light snow will develop
over southern Iowa with a period of restricted visibilities. Ceilings will
continue through the evening before lifting late.
Wind Chill Advisory until 9 am CST this morning for Black Hawk-