Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200919

Area Forecast Discussion
National Weather Service Des Moines IA
319 AM CST Mon Feb 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 312 AM CST Mon Feb 20 2017

Regional radar imagery depicts a cluster of showers and
thunderstorms moving from northeast Kansas over the
Nebraska/Missouri border region at this hour. Radar extrapolation
and high-resolution models bring this slug of rain across our
forecast area from southwest to northeast between about 11Z and 16Z
this morning. The broad area of vertical forcing associated with the
forward flank of an approaching mid-level trough, combined with the
low-level convergence and warm air/moisture advection ahead of the
corresponding surface trough approaching from Nebraska/South Dakota
today, will support redevelopment of showers and thunderstorms
behind the initial cluster, persisting through the afternoon and
into the evening especially across about the east/northeast half of
our area. Precipitable water values are off the chart for this time
of year and current upstream observations indicate relatively
efficient rainfall rates, so widespread QPF of half to three
quarters of an inch is expected in a swath across our area today.
However, there should be relatively little impact from this,
hydrologically. Have added as much detail as possible to POP trends
today into tonight, with the rain ending by midnight as the surface
trough sweeps through. Also of note is that despite the clouds and
showers percolating today, the morning start temperatures are so
warm and low-level warm air advection during the day sufficient
enough, that we are still forecasting record or near record high
temperatures in the 60s across most of the area. To achieve record
highs on a rainy day in February is unusual to say the least.

.LONG TERM.../Tuesday through Sunday/
Issued at 312 AM CST Mon Feb 20 2017

...Significant Changes on the Way...

Tuesday through Thursday...Confidence High: Models continue in very
good agreement through Thursday with spring-like weather to continue
over the region. Record to near record highs are once again expected
for tomorrow and Wendesday as another in a series of troughs amplifies
west southwest flow aloft and warm air returns quickly into Tuesday.
H850 temperatures rebound tomorrow to 10 to 12C across the area and
by Wednesday temperatures peak by midday at 14C from northeast to
southwest. By 00z a weak cool front will drop southeast as low pressure
tracks east northeast into northern Michigan. Highs will once again
be in the 60s/70s both Tuesday and Wednesday. The passage of the
boundary Wednesday afternoon will begin the change to cooler air
late Wednesday night into Thursday as well as a return of more northerly
surface flow.

The long awaited significant two-part system which had been in
the North Pacific and near the Aleutian Islands last Friday night
will be moving into the southwest/western US by 00z Thursday. The
southern stream system is expected to move into the Rockies by 12z
Thursday and begin to amplify an already present weak lee side
low pressure system over the Texas Panhandle. As height falls
continue ahead of the upper level system and the sfc wave strengthens area of showers and thunderstorms will form over Missouri
into Illinois Thursday along a developing warm front. Strong QG
forcing associated with the H500 low over Colorado/Wyoming will
begin to develop widespread snow spreading east northeast toward
northwest Iowa Thursday evening.

Thursday Night through Sunday...Confidence Medium

The trends in the models continue to suggest the potential of a
major winter storm for the region by Thursday night into Friday.
The exact storm track and timing details of precipitation changeover
will take a few more days to finalize...but confidence is increasing
on the occurrence of the strength of the storm. Significant strengthening
of the upper level low over the Rockies is expected between 12z
Thursday/ 12z Friday as cold air is entrained into the storm from
the northern Plains. The resulting baroclinic zone/ isentropic
lift with strong 30 to 40 kt H850 inflow that forms over southwest
to northeast Iowa will support a band of ample precipitation with
an area of rain/snow changing over to potentially heavy snow by
Friday morning northwest of the H850 low/near the H500 low as it
crosses northwest to northeast Iowa Friday. Both the GFS/Euro show
the most rapid amplification on Friday as the storm crosses Iowa
with it slowly weakening east of the area Friday night. With that
in mind...the storm may actually slow enough on Friday to prolong
snowfall northwest of the H850 as this process occurs and a deformation
zone/trowal elongates back to the west. Over central and southeast
Iowa...a period of rain and potential thunder will occur Thursday
night into Friday with rain changing over to snow during the day
as cold air advection is forced southeast behind the system. The
central/southeast may briefly experience a dry slot...though eventual
track placement will determine that in the coming days as well.
Current projections from the GFS/Euro suggest qpf of 0.8 to 1.25
inches of water equivalent with this event. The northwest half of
the forecast area has the higher threat for significant snow and
wind given todays model data. Both the GFS/Euro are forecasting
+45 to 50 kts H850 winds with an increase in strength over northeast
Iowa on Friday. With wind and heavy snow potential and blowing
snow ...more details regarding this event will be the focus of
subsequent discussions and products as this week progresses. After
the early week warm temperatures...we will be back in the 20s/30s
for highs Saturday then in the 30s/40s for Sunday. The models
suggest the active weather will continue into next week.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1144 PM CST Sun Feb 19 2017

Delayed the onset of IFR ceilings and MVFR visibility a couple
hours as hires models as well as current trajectory of storms in
OK look to impact the TAF sites closer to 12z Monday. Much of the
IFR to MVFR conditions will exit by around 21z.




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