Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
632 am CDT sun Mar 9 2014
Short term /today/...
issued at 410 am CDT sun Mar 9 2014
Northwest flow pattern aloft...with the upper ridge over the western
third to half of the US nudging eastward slightly today. Surface high
pressure to slide southeast exiting the area today...with return
flow setting up and strong warm air advection aloft. Surface winds already out of the
south to southwest across much of the County Warning Area with temperatures holding steady
or beginning to rise. 850 mb temperature climb to around +10c across the County Warning Area
today...with a warming surface flow out of the south to southwest.
Therefore temperatures should warm nicely today...but will be limited
somewhat by increasing middle/high clouds throughout the day with a
very weak shortwave dropping through Minnesota/northern Iowa. Little low level
moisture so no precipitation expected. Trended highest temperatures across
the usual warm spots and also in areas where less snow cover/no snow
present...with highs topping out in the middle 40s north/northeast to
the middle/upper 50s South/West central.
Long term /tonight through Saturday/...
issued at 410 am CDT sun Mar 9 2014
Primary concerns remain temperatures early and precipitation around Tuesday with a
fairly inactive extended forecast. Warm up will be well underway by
tonight with Monday morning mins likely the warmest since
Nov...widespread 30s. Made little change to Monday highs based on
soundings and projected mixing...taking the more aggressive end as
models typically over-emphasize snow effects in these instances.
Historically well performing European model (ecmwf) MOS has also changed
little including fresh 09/00z run...which while too late to
incorporate into the forecast...now suggests highs into the upper
60s from dsm south and west. These warmer temperatures will certainly
increase the potential for ice jams through the week...working
south to north as streams and rivers free...and will continue to
be highlighted in products and severe weather potential statement.
Attention will then turn to the system currently off the western
U.S./Canadian coasts...which will eventually cross The Rockies into
the MO valley spreading precipitation into NE/South Dakota by late Monday...and into northern
Iowa Tuesday morning. Much of our sensible weather will be driven by
kinematic and frontogenetic forcing as saturated column drifts north
to south through the state. Strong 850 mb/700 mb Theta-E advection is
noted to our south into MO...but is moisture starved and may not
produce much precipitation. One feature of note is the strong and deep
frontogenetical forcing which will be paired with fairly low
static stabilities increasing precipitation rates somewhat. All this
leads to increased confidence in precipitation supporting categorical
probability of precipitation.
The big question will be resultant precipitation type however. Model
soundings certainly show warm layer temperatures eventually crashing to
support snow. However with near surface temperatures hovering around
critical middle 30s and warm ground temperatures from Monday will make snow
accums quite problematic due to the rain/snow mix and degree of
melting...which may be enhanced by event occurring during the day.
Best scenario for now is an inch or two of snow over northern half of
the forecast area...and mainly on organic surfaces. The potential
is there for greater amounts with decent upward motion noted in
h6/500 mb or 305k dendritic growth zone enhanced by noted
frontogenetical forcing in same locations. Will certainly need to
monitor thermal trends which could result in lesser or greater
snow amounts. Higher snow rates may overcome melting resulting in
higher accums as well...so there are many variables here to
Winds will flip to the north behind this system dropping temperatures back
to seasonal norms into middle week. This will also include a dry
period with relatively inactive northwest flow. Decent short wave will
pass to our north through the northern plains and Great Lakes Thursday into
Friday...but with insufficient forcing and moisture for anything here.
With Iowa staying on warm side of the system...this will likely
introduce another round of above normal temperatures...albeit not as warm
as Monday. A stronger short wave eventually drops into the MO valley
by the end of the weekend...but would likely be just beyond the
valid period into Sat night or sun.
issued at 632 am CDT sun Mar 9 2014
VFR conditions expected throughout the taf forecast period. Some
middle/high clouds increasing through the day. Southerly winds to
increase toward late morning and then diminish around sunset.