Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1227 PM CST Sat Mar 8 2014
issued at 1227 PM CST Sat Mar 8 2014
Forecast is generally on track as clouds depart to the southeast.
However...an area of cloudiness across Minnesota and Dakotas will
edge across northern Iowa this afternoon and have increased clouds
there to reflect the movement of these clouds. Otherwise...
temperatures on track as sunshine has helped readings recover
despite ongoing weak cold advection.
Update issued at 656 am CST Sat Mar 8 2014
Low clouds already eroding across west central Iowa...with other
area of cloud cover eroding across South Dakota as well. Therefore expect
clearing faster than previously expected today...thus have updated
the sky grids to account for the latest trends on satellite. Will
have to monitor temperatures...as could possibly see them warm a little
more than expected as well due to less cloud cover.
Short term /today/...
issued at 330 am CST Sat Mar 8 2014
Area of surface high pressure to build into the County Warning Area today...with
winds diminishing and the chilly airmass in place to moderate
through the day. Stratus shield in place with models suggesting a
slow erosion of the clouds from northwest to southeast throughout
the afternoon hours. Currently a break in the clouds across
northwest Iowa into southeast South Dakota/SW Minnesota...although satellite trends show
this area filling in as cloud shield expands. If have some breaks in
the clouds toward daylight...could see clouds erode faster as drier
moves into the County Warning Area and stronger March sun Burns some of the clouds
off. This makes the temperature forecast tricky with cloud cover
keeping temperatures a bit warmer than expected overnight...which should
allow temperatures to warm a bit compared to the previous forecast. However
with clouds expected to linger much of the day and north to
northeast surface flow...warming will still be limited. Therefore opted
to keep maximum temperatures in the middle 20s north to middle 30s south. As for
precipitation...current band of very light snow/flurries will
continue to push east southeast through the next few hours. Band has
been diminishing in intensity over the last few hours...and expected
the trend to continue thus pulled mention of probability of precipitation and stuck with
flurry wording through 11z...with dry conditions across the entire
County Warning Area for today.
Long term /tonight through Friday/...
issued at 330 am CST Sat Mar 8 2014
Although there is a window for precipitation around Tuesday...the primary
concerns this period will be temperature trends and potentially ice jam
effects. Surface high pressure will settle into Iowa tonight with
return flow beginning overnight leading into significant warming
sun and Monday. Looking back at model verification during decent warm
up in Feb showed forecast cool bias and suggested that European model (ecmwf) MOS
performed the best. European model (ecmwf) suite points toward even greater warm up
than going forecast so have increased sun and Monday highs several
degrees...which makes sense with dry SW flow sun and proximity to
thermal ridge Monday. More aggressively mixed and seemingly more
realistic GFS soundings also support warmer. Expect low 60s over
southwestern half of Iowa Monday and this may start period of heightening ice
jam potential with additional details on that below.
Remnants of current eastern Pacific/Gulf of Alaska trough reach the central
Continental U.S. By Tuesday. This will drop temperatures back closer to normal and also
increase precipitation chances. GFS/European model (ecmwf) still have some timing
differences with GFS a bit faster...but are generally similar with
trends. Both models suggest much of the forcing for Iowa will be based
in the middle levels so would tend to favor lighter European model (ecmwf) quantitative precipitation forecast. Much of
the 1-3km forcing either brushes southern Iowa /ECMWF/ or stays to
the south /GFS/ and is relatively moisture starved either way.
Soundings suggest either rain or snow for precipitation type with warm
layer and surface temperatures essentially dropping at the same time.
Dry and mild northwest flow looks to be in place to end the work week with
temperatures back into the 50s for Thursday and Friday. Although a short wave does
traverse southern Canada into the Great Lakes Thursday into Friday...there is
little evidence of sufficiently phased moisture and forcing for Iowa
so forecast remains dry. Models do potentially point toward light
precipitation just beyond the valid period however with a transient period
of light rain or light snow possible Friday night.
issued at 1227 PM CST Sat Mar 8 2014
Generally VFR conditions expected through Sunday as clouds depart
the southeast this afternoon. Some middle level clouds will pass
across the north this afternoon and evening with weak middle level
warm advection...however any ceilings will be VFR. Surface winds
will become light and variable overnight before increasing quickly
from the southwest on Sunday.
issued at 301 PM CST Friday Mar 7 2014
Ice jams and resulting flooding will become a concern by early next
week as temperatures warm up. Area of most concern initially
includes streams south of Highway 30...but will shift northward
with time to encompass much of the County Warning Area by the end of the week.
Timing would initially be as early as next Monday night or Tuesday
for the southern County Warning Area...and the middle to later parts of next week
Runoff from snowmelt will make its way into our streams as
temperatures increase over the next few days. The Deep Ground frost
depth will minimize infiltration of any water from the
snowmelt...and would thus help maximize runoff into the streams.
If ice jams do occur early next week they would be driven primarily
by snowmelt alone. Studies of such past ice jam events in and near
the National Weather Service Des Moines County warning area have yielded some common precursor features.
For ice jams driven totally or primarily by snowmelt only...the ice
jams were preceded by 3 to 5 consecutive days with the daily maximum
temperature in the 40s or higher. In addition...the ice jams
occurred with overnight lows in the preceding 3 to 5 nights which
were both above and below freezing. Some overnight lows were as low
as the middle to upper teens.
The studies also showed that when the daily average temperature
reached 42f or higher...ice jams almost always occurred within a day
or two. They sometimes occurred during the night immediately
following the daytime high which resulted in the daily average
temperature of 42f or higher.
Locations that have historically experienced ice jams and resulting
flooding would be of most concern. Those locations include but are
not limited to the Des Moines River in The Fort Dodge area...the
Raccoon river /including in the Des Moines area/ as well as portions
of the south Skunk River. Given the unusually thick nature of the
ice...however...ice jams and resulting flooding may occur in other
locations as well. As has happened with past ice jams...river levels
may rise rapidly thus leading to flash flooding.
Flash flood watches and warnings may be issued if concern becomes
high enough for ice jam flooding.