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Area forecast discussion 
National Weather Service Des Moines Iowa
629 am CDT sun Apr 19 2015

Short term /today/...
issued at 405 am CDT sun Apr 19 2015

Precipitation chances will continue to be main forecast concern for
the period. Progression of large upper level low across Colorado/Kansas border
will be the main driver for precipitation chances today. Low begins
to become more open wave as day progresses and system pushes east
and low in southern Canada deepens...bringing more trough-like
system through Iowa. Boundary associated with northern low is
currently located from eastern South Dakota through central NE. This will
bring the best chances for precipitation to the area generally in
the afternoon as boundary pushes south. Feel models are too
aggressive with speed of system...and have therefore trended a bit
slower with progression. However...for the morning will continue to
see areas of showers and isolated thunderstorms as system lifts
north. However...do not have high confidence in morning
probability of precipitation...though high res models all include probability of precipitation and have therefore
left precipitation chances in forecast...though generally cut back on
extent. Instability remains week with system...with better
instability further south into Missouri. However...enough
instability with forcing to warrant isolated thunder mention again
in the afternoon. Once again...cannot rule out an isolated severe
thunderstorm in the afternoon with large hail and damaging winds the
main threats.

Near and behind boundary winds will shift to the northwest and will
increase...becoming gusty with top of mixed layer near 30kts. Colder
temperatures will push in for end of the day...and have kept highs
generally near guidance.

Long term /tonight through Saturday/...
issued at 405 am CDT sun Apr 19 2015

Overall system departure and precipitation ending times a bit
delayed through the evening with the upper level system and
associated surface reflection are a bit further east that previously
expected. The primary area of cold advection will be from near
Mason City to Des Moines to Bedford early this evening and will be
through the area by midnight. The precipitation will be ending
with the frontal passage however gusty northwest winds will be developing in
the wake of the boundary. Some convective instability will remain
through at least 03z and have left isolated thunder mention in
through that time.

The well advertised northern stream upper low will become closed
off overnight and will remain the dominant weather feature through
much of the week. This system will track very slowly east across
the Great Lakes region during the week leaving Iowa in cool
cyclonic flow. Expect gusty northwest winds on Monday with some
showers developing over northeast Iowa by the afternoon while
farther south a more flattened cumulus field is likely. Similar
conditions on Tuesday with a slight chance of a few showers
northeast once again. One more significant impulse is expected to
wrap around the upper low around Wednesday and will bring another
round of cold advection to the state. Slightly warmer conditions
toward the end of the week as the upper low shifts further west
while another upper level system over the southwest Continental U.S. Will
begin to shift northeast. This will create a default ridge between
the two systems and will allow for weak warm advection into the
state. The southwest system will bring precipitation chances to
southern Iowa Friday into Saturday.

The primary weather impact to monitor this week with be the
potential for a frost and/or freeze. The warm April has
vegetation and fruit trees ahead of schedule and will be
susceptible to cold temperatures. Dewpoints will drop well into
the 20s Monday and Tuesday nights. Winds of 10 to 20 miles per hour Monday
night should prevent a frost though a few location may reach the
lower 30s. Tuesday and Wednesday nights will have a better
potential for freezing temperatures over northern Iowa and farther
south through the Nishnabotna valley with lighter winds of 5 to 10
miles per hour expected. Very dry dewpoints may limit frost potential but
also if winds become much lighter than expected...would result in
readings bottoming out.

&&

Aviation...19/12z
issued at 625 am CDT sun Apr 19 2015

IFR or LIFR ceilings and visibilities may affect sites briefly for
beginning of period before lifting to primarily MVFR through
18z and lifting to VFR beyond. Rain showers and thunderstorms and rain will affect sites
through 00z...with periods of MVFR or lower ceilings and
visibilities possible with precipitation. Ts will be possible at
sites...especially eastern sites kdsm/Kalo/kotm after 18z...but
have not included at this time due to uncertainties in timing and
location. Winds will shift the northwest and become strong and
gusty by end of period.

&&

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

$$

Short term...awb
long term...donavon
aviation...awb

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