Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Des Moines Iowa
622 PM CDT sun Apr 20 2014

Short term /tonight/...
issued at 336 PM CDT sun Apr 20 2014

A weak surface trough is currently draped across far northwest Iowa
into southern Minnesota...with a ridge upstream and a stronger cold front
approaching from the western Dakotas. As the latter front nears Iowa
overnight it will squeeze the preceding ridge and trough into a
nebulous weak flow region that will sink across our area overnight.
Scattered showers and storms should develop along the
north/northwest Iowa trough this evening...then increase in coverage
overnight as a modest low level jet kicks in concurrent with broad but weak
lift preceding a 500 mb shortwave moving over the region. Despite
the increase in coverage rain should still be scattered at any given
time...and have continued likely probability of precipitation but held off on categorical
for this reason. Instability and shear are limited and severe
weather is not anticipated. Did increase low temperatures a bit due
to anticipated cloud cover and weak southerly flow ahead of the
sagging trough.

Long term /Monday through Sunday/...
issued at 336 PM CDT sun Apr 20 2014

A weak surface low and boundary will shift east across Iowa on
Monday as an upper trough pushes east through Monday evening. There
is weak to modest instability and weak shear...enough to see
thunderstorms across the forecast area through the day but severe
potential is unimpressive. Soundings show that we lose our
moisture fairly quickly in the afternoon behind the system. Precipitation
should end across the northwest half of the state by 18z and linger
across the southeast through 00z but thunder should be more
isolated as the afternoon wears on. Heavier rainfall will occur
with thunderstorms but overall we are looking at between a tenth
and a quarter of an inch. Temperatures will be tricky Monday and I may be
too warm given the cloud cover and precipitation. We will have some
southerly flow in the morning then ahead of the system the winds
turn westerly before going north in the afternoon. Temperatures should
hit their maxes across the north by noon then remain steady or
fall. Across the south there will be a couple more hours for temperatures
to climb before frontal passage.

The system shifts east Monday evening with clearing progressing
across the state. Temperatures will be cool Monday night with lows
dropping back into the middle to upper 30s north to the lower to middle
40s south. Tuesday should be a nice day...sunny with a light north
wind and highs in the 60s. The upper ridge that will be responsible
for the decent day Tuesday will shift east through the day
Wednesday. A deepening low will dig a trough across The Rockies
with a surface low developing across the Lee of The Rockies. By
Wednesday the surface low will shift into eastern Nebraska/western Iowa.
Showers and thunderstorms should increase across the
area...especially late in the day Wednesday and overnight through
Thursday morning. Models diverge here as the Euro digs more of a
negatively tilted trough while the GFS is more progressive with
the low and pushes everything east in the afternoon.

After that we stay in a northwest flow and temperatures remain cool into
the weekend. Differences in the long term models continue for
next weekend with the GFS digging this huge trough west of The
Rockies while the Euro has a trough that is not quite as strong or
sharp and it swings up against a departing ridge in eastern IA/IL.
The Euro breaks out precipitation over a broad region...including Iowa
while the GFS is more narrow with it/S quantitative precipitation forecast forecast...owing to the
sharpness of the trough. As for the grids...the allblends keep
probability of precipitation in and confidence at this point is not strong enough to pull
or narrow probability of precipitation. Looks like a cool and wet weekend next weekend at
this point. If the GFS is right it will cool and drier.

&&

Aviation...21/00z
issued at 620 PM CDT sun Apr 20 2014

Boundary across northern Iowa will continue to push slowly southeast
throughout the period. Rain showers and thunderstorms and rain will remain possible along
boundary through period. May see MVFR or lower visibilities and
ceilings at times with thunderstorms and rain. System will push southeast by end of
period...with winds shifting from the south to the northwest
behind boundary...becoming gusty.

&&

Dmx watches/warnings/advisories...
none.
&&

$$

Short term...Lee
long term...fab
aviation...awb