Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202029

Area Forecast Discussion
National Weather Service Des Moines IA
329 PM CDT Sat May 20 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 328 PM CDT Sat May 20 2017

Surface low has tracked into west central Iowa this afternoon with
the bulk of moisture advection shifting northward into Minnesota
along with the rainfall.  A surface trof axis is swinging through
southwest Iowa and into the forecast area with a narrow line of
light showers/sprinkles along the interface. While this overall
setup would typically be cause for concern, the lack of overall
instability will likely keep the event in check.  However, the past
hour has seen some increased insolation near this trof as cloud
coverage has decreased in vicinity of the wind shift.  SPC
mesoanalysis also indicates some increase in mixed layer CAPE and
will continue to watch the evolution of this into the late
afternoon.  If the cap erodes sufficiently, then a few cells may
develop with the potential for a brief tornado spin-up before
sunset in a local maximum of effective helicity near the trof axis.

The low will lift northward into Minnesota overnight with forcing
waning across the state.  Any shower activity tonight will be
limited across the far north with much of the area remaining dry.
Otherwise, broad status field associated with the upper low will
cross the state overnight into Sunday with partial clearing only
toward the end of the period as the cyclonic flow subsides.
Temperatures will remain below normal through Sunday with thermal
trof passing through the state tonight as any warming limited on
Sunday given the cloud cover.

.LONG TERM.../Sunday night through Saturday/
Issued at 328 PM CDT Sat May 20 2017

Sunday night should remain dry though we will still be under the
influence of the low.  A shortwave pivoting around the low will drop
down across Iowa beginning late Sunday night/Monday morning.  The
question is whether more subtle waves that the models are hinting at
will pass through the state in the overnight.  At this point I`m
thinking that even if this does happen, it will result mainly in
cloud cover versus precip.  During the day Monday the shortwave
pivots into the state and we will actually be dealing with this for
several days.  A ridge will be steepening over the Rockies which
will act to deepen the trough over us and slow it`s progression. The
effect of which will be to keep precip going over us Monday
afternoon through Tuesday.  On Monday a cold front will sweep across
the state and while shear is marginal, the instability is sufficient
for thunderstorms.  In addition, we do have some forcing and a
narrow ribbon of theta-e advection ahead of the boundary.  SPC
outlook for Monday has a marginal risk in place for hail which
appears reasonable.

On Tuesday we`ll see some wrap around precip across mainly northern
Iowa as the upper low pulls out and we`ll see relatively cool temps
Tuesday and Wednesday.

We will then be under the influence of the upper ridge until Friday
when a stronger shortwave breaks down the ridge and brings a chance
for precip to the area mainly in the afternoon.  This will allow a
stronger low over the Rockies to shift into the region however that
far out models have pretty large discrepancies but it will lead to
periods of rain over the Memorial Day weekend.  For now I went with
model blends since there is no favored solution yet.


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1239 PM CDT Sat May 20 2017

Large upper low will gradually cross the upper Midwest through the
forecast period. Widespread MVFR to IFR ceilings are expected for
much of the forecast although some improvement is forecast late in
the period, especially in southern Iowa. Surface winds will
gradually veer from the east to southwest overnight and westerly
by Sunday. Precipitation the is restricting visibilities in the
north will gradually end by this evening as the surface low pulls
out of the state.




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