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Area forecast discussion 
National Weather Service Des Moines Iowa
1143 am CST Friday Dec 19 2014

Short term /today/...
issued at 329 am CST Friday Dec 19 2014

Low level moisture surge is lifting northeast across central Iowa
this morning. Weak forcing within this saturated layer has produced
a mix of light freezing drizzle or flurries with saturation tops
currently between -7c and -9c which is on the edge between good ice
introduction and super cooled droplets. The saturation depths
decrease from the top of the layer through the morning and thus
should decrease the amount of ice introduction which would lead to
more freezing drizzle processes. Not expecting a widespread impact
though localized icy patches could occur on roadways and walkways.
Sky conditions will remain cloudy today and will hold temperatures
in the 20s to lower 30s once again.

Long term /tonight through Thursday/...
issued at 329 am CST Friday Dec 19 2014

We have much to discuss.

This weekend will be characterized by general stratus and
haze/fog within a broad warm air/moisture advection regime in
light southerly surface flow. This will also lead to a slow
warming trend...with high temperatures in the middle 30s to lower 40s
by Sunday. During the nighttime and early morning hours...both
from Friday night into Saturday and from Saturday night into
Sunday...there will be increasing chances for fog and drizzle as
the near surface cloud layer gradually saturates and deepens. On
Friday night the layer depths are still marginal for drizzle and
there is no discernible source of lift to generate this light
precipitation. However...even early this morning /Friday/ and the
previous night we have seen patchy freezing drizzle around the
area and the saturation...while shallow...seems even better on
Friday night. Given the persistence of the pattern...have opted to
include drizzle across the area between about midnight and
sunrise. Temperatures will be below freezing during this period so
any drizzle that does form may cause travel
impacts...however...confidence in occurrence is easily too low to
warrant any headline consideration at this time. On Saturday night
a strong surge of Theta-E advection will move across the
area...mainly after midnight...with an attendant increase in the
depth of the warm cloud layer. This leads to a considerably higher
confidence in drizzle occurrence late Saturday night into Sunday. then temperatures will be near or above freezing in
many areas so it is difficult to differentiate where the drizzle
will freeze and where it is not...with the best chances for
freezing drizzle across the northern half or so of the forecast
area. Complicating this further...forecast soundings and
conceptual models suggest that the advection surge that will
trigger drizzle generation may push temperatures from below
freezing to above freezing and it is difficult to tell how quickly
this will be reflected in surface/Road temperatures. In any
event...those traveling this weekend should keep an eye on the
forecast for updates on the potential for freezing drizzle during
the nighttime/morning hours this weekend.

By Sunday night our much anticipated larger storm system will be
approaching in the form of a 500 mb shortwave and associated
surface low pressure center moving quickly southeast across the
Dakotas. As this system reaches Iowa we will see precipitation
break out across the state from west to east...mainly after
midnight Sunday night and into Monday. Temperatures will be above
freezing leading to rain across most of the area...with the
exception being in our far north where temperatures near freezing
may lead to a wintry mix of precipitation but with little to no
snow or ice accumulation through Monday. The stacked low pressure
center should be located over northern Iowa/southern Minnesota by
Monday night...thereafter occluding and moving slowly to the east.
This will lead to a rain to snow transition from west to east on
Monday night...followed by a day or two of light snow within the
large cloud and precipitation shield on the backside of the system. While
chances for snowflakes are high across much of the area from
Monday night through Tuesday...given that the low will be slowly
filling and moving away the quantitative precipitation forecast is fairly low and any snowfall
amounts will probably be light. This may still produce some travel
impacts but these are very difficult to diagnose at this range. We
will continue to closely monitor this system in the coming days.

The light snow trailing the early week system should clear out of
the area roughly around Tuesday night or possibly Wednesday. It
does appear that another storm system will approach our area from
the west late next week...just beyond the current seven day
forecast...with some chance for precipitation possibly in the form
of additional snowfall.


issued at 1143 am CST Friday Dec 19 2014

Low ceilings continue to be an issue with several locations reporting
IFR ceilings and MVFR visibilities across the area. Low ceilings may lift some
this afternoon to MVFR...but are expected to drop again overnight
tonight. Kotm remains VFR...but may lower throughout the period.
Winds will remain light out of the southeast. Other issue is the
potential for very light fzdz or flurries tonight into early
Saturday. Still some uncertainty...but given the conditions
produced some light precipitation this morning and look even better
tonight will likely have to add a mention in in subsequent taf


Dmx watches/warnings/advisories...none.



Short term...donavon
long term...Lee

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