Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
302 PM CDT sun Apr 19 2015

Short term /tonight/...
issued at 302 PM CDT sun Apr 19 2015

South of Iowa...short wave continues to rotate east while upper
level trough progresses east with Pacific and secondary Canadian
cold front now plowing into southeast South Dakota late this
afternoon. Frontogenetical forcing has increased along the Pacific
boundary/trough over central and northern Iowa. Southeast still in
the warm sector and needing to monitor for scattered convection this
afternoon...a few may reach severe limits as surface based cape now near
1000 j/kg with bulk shear around 20-30kts as depicted by Storm Prediction Center RUC
model analysis at 17z. As Pacific front trough edge east this
evening...forcing shifts east and area over southeast Iowa fills in
with showers and storms as well this afternoon/evening. Over
northern Iowa the secondary boundary will increase forcing over the
east/northeast through 02z with lingering showers through 06z east.
Winds increasing behind the system with 20 to 25 miles per hour winds gusting
to near 35 miles per hour at times this evening. Rainfall totals will again
reach about one third of an inch through 00z and another third of an
inch over the northeast/east through 06z. Upstream clouds continue
with cold air advection...likely to see low stratus overnight across
the area with some breaks toward 12z northwest as drier air moves
into the region.

Long term /Monday through Sunday/...
issued at 302 PM CDT sun Apr 19 2015

Lingering showers and thunderstorms tonight...discussed in the
short term section above...should clear out to the east by Monday
morning as a cold front sweeps across Iowa. This will usher in
considerably cooler weather for the bulk of the coming week. A
large middle/upper atmospheric gyre moving from central Canada into
the upper Midwest...the system responsible for the passage of the
aforesaid surface front...will slide over Minnesota and the Great
Lakes region on Monday and Tuesday. As it does so...a series of
subtle shortwave impulses rounding the cyclone will assist in the
generation of a large cloud shield and some sprinkles/light
showers at times. These will primarily affect northern and eastern
Iowa during the afternoon and evening hours on Monday and possibly
Tuesday. Have maintained 20 to 30 probability of precipitation in our northeastern
counties on Monday afternoon...but have not yet introduced them on
Tuesday afternoon as confidence is lower on whether the
cloud/precipitation shield will extend as far southwest as our forecast
area by that time depending on the speed with which the large low
moves away over the Great Lakes. In any event...light showers that
do occur will have little to no sensible impact. Of greater
interest will be the strong winds generated as the gyre moves
slowly by to our northeast. These winds will peak with daytime
mixing from late morning through late afternoon each day...and
will likely be highest on Monday when the parent system is at its
closest approach. Forecast soundings continue to indicate a mixed
layer up to about 650 mb by middle afternoon Monday with winds at the
top of this layer in the 35 to 40 knots range. Have continued trend
of bumping up forecast winds...and suspect that a Wind Advisory
may become necessary for Monday later tonight. Advisory level
winds may also be possible on Tuesday...but again as with the
probability of precipitation...variance in solutions of the progression of the cyclone
leads to a lower confidence in the threshold being met at this
time. All in all Monday and Tuesday will be cold...blustery...and
with some light spits of rain north and east.

As the large middle/upper level low moves eastward away from the
Great Lakes later in the week it will allow a cold surface high
pressure area to spill down from Canada across the Midwest and
Iowa...dominating our weather from late Wednesday into Friday.
This will cause a considerable diminishing of winds but also
reinforce the cold airmass in place and likely lead to better
radiative cooling conditions on Wednesday night and Thursday
night...when frost may be possible especially in the north and low
lying areas such as the nish valley. We will be watching this
closely for the possibility of headline issuances although it is
very early in the growing season so any impact should be minimal.

Looking farther ahead toward next weekend...there has been some
consistency in long range model solutions scooping a broad
southwest U.S. 500 mb trough up across or near Iowa around
Saturday or Sunday...however the 12z GFS and European model (ecmwf) runs today
split off from this previous general consensus and in opposite
directions...resulting in the current depiction of clear and dry
weather by the GFS as opposed to the large low pressure system
moving overhead depicted by the European model (ecmwf). With such large spread and
variability there is little to hang our hats on a week from
now...except that no noticeable warmup is expected by that time.


issued at 1233 PM CDT sun Apr 19 2015

Complicated forecast for the next 6 the shortwave
drifts across the area showers and isolated thunder with IFR/LIFR ceilings will
fill in along the upper level/surface boundary moving southeast across
western/northern Iowa this afternoon. Better chances for isolated
thunder southeast with VFR/MVFR ceilings southeast through 02z. Then
with increasing northwest winds...gusts to 25 to 30 kts possible
this evening and again after 14z Monday as mixing increases with more
unstable boundary layer. Monday will see generally MVFR/VFR ceilings
with some ceilings around 3000 at times. /Rev


Dmx watches/warnings/advisories...none.



Short term...rev
long term...Lee

National Weather Service Glossary of Abbreviations