Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
632 am CDT Monday Mar 10 2014
Short term /today/...
issued at 405 am CDT Monday Mar 10 2014
A weak surface trough axis pushes through the County Warning Area through middle
morning...with winds shifting around to the northwest to north
today. Aloft have near neutral advection to weak cold air advection with 850 mb temperatures
still around the low teens celsius. High clouds currently in place
across the County Warning Area should push south of the state throughout the early
morning/middle morning hours...with high clouds then increasing again
late today. Given air mass in place and some mixing expected should
still see very mild temperatures across the County Warning Area today. Expect temperatures in
relative snow-free areas to be much warmer...climbing into the upper
50s to middle/upper 60s. Areas where the deeper snowpack
remains...warming will be less but still relatively mild in the middle
to upper 40s. Some concern given the high clouds today...and winds
shifting around to the northwest to north coming off the deeper
snowpack...however feel this will be minimized some by the mild
start with early morning temperatures a good 20 to 25 degrees warmer this
morning than those yesterday so 10 to 20 degree rise in temperatures does
not seem unlikely.
Long term /tonight through Sunday/...
issued at 405 am CDT Monday Mar 10 2014
With Monday temperatures now past on to the short term...only real sensible
weather concern for several days will be the Tuesday/Tuesday night precipitation
event. System in question is just coming onshore into the Pacific northwest
and should spread precipitation into Iowa by Tuesday morning. Model consensus is
a bit slower than this time yesterday with precipitation maturation not
occurring until late morning or midday. Much like previous runs
much of our lift will be middle level and kinematic with Theta-E/warm
advection farther south in somewhat limited moisture. This shows up
well in isentropic analysis as well with isobars parallel to flow
where sufficiently low pressure deficits are present. Raw model
Omega is still noted in the middle levels however...including in the
dendritic growth zone /dgz/. This also appears to be driven by
deep and at times moderate frontogenetical forcing.
Big question for this event will not be if...but rather precipitation
type...and will be quite problematic with low confidence. Rain to
snow changeover and degree of melting all make accumulate forecasts
difficult. Question has been relegated to rain vs snow with
soundings not supporting sleet or freezing rain. Cold layer is not
present to refreeze anything and surface temperatures do not fall below
freezing until warm layer has been well eroded. The big fly in the
ointment will be near surface temperatures. Soundings and dgz lift
support moderate snowfall at times with only thing holding back
snow relatively warm but quite shallow near surface temperatures in the
30s to near 40f. This could result in a cold rain or quickly
accumulating wet snow...even with temperatures in the 30s. Have generally
not changed snow amounts much with widespread one to two inches
possible over the northern half of Iowa...and locally three inches or so.
However have spread inch or so accums farther south into central
and southern Iowa into the evening as cold advection kicks in.
Amounts mentioned in the forecast will mainly be on organic
surfaces...but still expect wet...slushy Road accums. Road temperatures
were into the 60s sun and remain in the 30s or 40s early this
morning...with similar trends expected today into tonight. However
past events have shown that intense snowfall rates and melting
will quickly cool Road surfaces where accums will begin.
Precipitation should be mainly snow into the evening south and east before
ending overnight. Passage of this trough will introduce a period of
dry northwest flow. Surface high will settle into the region Wednesday with
temperatures back below normal. There will be some moderation by
the end of the week...but the short wave crossing to our north Friday
should bring gradual cooling into the weekend with overall trend
toward lower heights. Next window for precipitation looks to be toward the
end of the period...Sat night or sun...with better chances appearing
to be down the immediate MO valley. This event could be rain or
snow depending on timing and its relation to diurnal temperature cycle.
Certainly nothing to suggest a return to temperatures of the past few
days in the foreseeable future.
issued at 632 am CDT Monday Mar 10 2014
VFR conditions throughout the taf forecast period. Southwest to
westerly winds shifting around to the northwest to north through
the period. Some thinning in the high clouds/clearing likely
toward midday...with increasing high clouds again late this
afternoon into the evening.