Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200457

Area Forecast Discussion
National Weather Service Des Moines IA
1157 PM CDT Sun Mar 19 2017

.SHORT TERM.../Tonight through Monday/
Issued at 333 PM CDT Sun Mar 19 2017

Warm sector has expanded into the area this afternoon with
readings well into the 70s in the west with 60s across the
remainder of forecast area although 50s linger in the far
northeast. This is in conjunction with 850mb thermal ridge
expanding into the state from the Plains ahead of an approaching
cold front. Surface front is approaching northwest Iowa currently
with higher dewpoints just entering far southwest Iowa. As the
evening progresses, the boundary will slip into the state with
increasing mid level forcing as shortwave approaches. Eventually,
sufficient lift for parcels to reach the LFC will occur in eastern
Iowa near the Highway 20/30 corridor later this evening,
according to meso-models. This convection will be elevated but
have some modest instability and shear with which to work and may
produce marginally severe to severe hail. The convection will
likley linger near the far east/southeast for much of the night s
the surface low slowly moves east near the Iowa/Missouri border.

Drier and cooler air will push into the state on Monday on the
backside of the system with partly sunny skies. Although
temperatures will be cooler than today, readings will still be
several degrees above normal with the bigger surge of cold
advection arriving Monday night.

.LONG TERM.../Monday night through Sunday/
Issued at 333 PM CDT Sun Mar 19 2017

From Monday night through Wednesday will will lie beneath the
influence of a large high pressure area sliding by to our
northeast, keeping us fairly cool with highs in the 40s to lower
50s. On Tuesday a band of frontogenetical forcing will translate
from northwest to southeast across Iowa, generating some
precipitation, but the location and intensity are in question. The
EC has consistently depicted lighter precipitation, probably as a
consequence of fighting dry low-level air, over about the
southwest half of our forecast area. However, the NAM and GFS are
much more robust in their QPF and further north with both the
forcing and associated precipitation. In any event, it appears
likely from forecast soundings and surface temperatures that the
bulk of the precipitation will fall as rain or melting snow. There
is some opportunity for accumulating snowfall, mainly across our
northwest to north central counties, particularly of the stronger
forcing depicted by the NAM/GFS pans out and is able to
dynamically cool the warmer near-surface layer.

During the latter half of the week a messy, prolonged
precipitation event is still in store. On Wednesday night and
Thursday a deep 500 mb trough will move from the southern
California coast to the Four Corners region, with the southwest
flow ahead of the trough placing Iowa within a large zone of warm
air and moisture advection and modest but broad lift. This will
result in increasing low-level saturation, clouds, and light
precipitation. On Wednesday night and Thursday this will likely
come in the form of a light wintry mix, but then transition over
to all rain before noon Thursday as temperatures warm. By late
Thursday the southwestern U.S. trough will close off, then meander
east as a large, stacked cyclone eventually moving over Missouri
around Friday night or Saturday morning. As the low approaches
late this week, an effective warm front will stretch northeast
from it up over Minnesota, and from late Thursday through Friday
most of our forecast area will remain within a broad warm sector
in which long range models depict waves of instability and forcing
moving through. This should result in periodic showers and
probably a few thunderstorms during this time frame. Then as the
cyclone finally moves to our south, it will pull the front through
and we will transition to stratiform rain, possibly mixed with a
few snowflakes, before precipitation ends on Saturday. Behind this
system it appears that another longwave trough will move through
the high-amplitude steering pattern in place, probably resulting
in more precipitation early next week, around Monday or so.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1157 PM CDT Sun Mar 19 2017

Scattered thunderstorms developing over east central Iowa should
avoid most sites though the period. Local MVFR conditions may
occur late tonight early Monday morning in vicinity of KOTM.
Otherwise southeast winds will become breezy from the north on




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