Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 220302
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
902 PM CST Wed Feb 21 2018

.UPDATE...
Issued at 902 PM CST Wed Feb 21 2018

A short-wave trough presently over the southern Plains is
promoting a strengthening downstream belt of warm advection across
the central Plains into mid-MO Valley which is being manifest as
increasing echoes in mosaic radar data. 00z OAX/TOP soundings
revealed a considerable amount of dry air in the low levels which
will have to be overcome before precipitation begins to reach the
ground. Latest convection-allowing model guidance and 00z NAM data
indicate light snow developing within the next 2-3 hours mainly
south of I-80 and gradually spreading northeast through daybreak
Thursday. Snow accumulations with this initial wave of
precipitation (though about 8 AM Thu) will range from less than
half an inch generally north of I-80 to around 2" over southeast
NE.

The initial warm-advection-related precipitation should largely
shift to the east of the area by mid morning Thu with forecast
soundings indicating a developing warm nose aloft and loss of ice
in the dendritic-growth zone. The net result is a transition of
any light, lingering precipitation to freezing drizzle or light
freezing rain. The exception is over northeast NE where
predominant precipitation type may remain snow.

By late tomorrow afternoon into evening, large-scale forcing for
ascent re-intensifies in response to the approach of the next
disturbance aloft. At that time, we would expect to see an
increase in precipitation rates with predominantly freezing rain
along and south of I-80 with a transition to sleet and then snow
northward into northeast NE and west-central IA.

The current forecast appears in reasonably good shape and we have
made only minor adjustments.

&&

.SHORT TERM...(Tonight through Saturday)
Issued at 447 PM CST Wed Feb 21 2018

Timing precip and location of dominate type late tonight through
Thursday evening is initial concern followed by Snow event on
Saturday.

Regional mosaic already showing batch of precip beginning to lift
northward into eastern KS. HRRR/RAP13 indicate activity should
reach the southern CWA shortly before midnight. Per RAP13 MaxT
aloft prog...there will definitely be a fine line separating
freezing/frozen precip. As of now, appears likely snow will be the
predominate type between midnight tonight and early Thursday
morning as it advances northward through the CWA. However, as
layer MaxT go above freezing from south to north, and feeder-
seeder process begins to weaken, expect snow to give way to light
freezing drizzle from south to north, expanding across the rest
of the CWA the remainder of the day into the evening hours. Snow
accumulations should be relatively even over the CWA ranging from
about 1 to 2 inches. Brunt of ice accumulations will be found over
southwest IA south of I-80 with anywhere from one to two tenths
possible. Over the rest of the CWA, five hundredths to a tenth
looks possible. Given this, will post a Winter Weather Advisory to
be in effect from late tonight through Thursday evening.

Turning to Saturday, a potent system lifting into the central Plains
will be the focus for the next snow accumulation event over the
region. Synoptic scale forcing will increase substantially as
500mb hgt falls of 15dam precedes a quite potent vort max as it
pushes across NE/KS. Associated precip activity will develop
within the warm sector of the surface reflection which is progged
to be in western KS 12z Saturday. The GFS and CMC are in rather
good agreement depicting the axis of heaviest lining up from
southwest NE to southeast SD, just to the left of the 500mb HFC
track. And as of now, there appears to be a distinct possibility
for significant snow accumulations over the northern CWA where
dendritic omega will be maximized...per the GFS...advertising over
40 microbars of lift.

.LONG TERM...(Saturday night through Wednesday)
Issued at 447 PM CST Wed Feb 21 2018

GFS/CMC in agreement extended periods will be dry from Sunday
through at least Tuesday before the next chance for precip. Tuesday
night and Wednesday, model show a cold front dropping into the
central Plains with the possibility a snow/rain mix event.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 518 PM CST Wed Feb 21 2018

A series of weak mid-level disturbances will eject from the
southern Rockies into the mid-MO Valley during the forecast
period, supporting an increase in clouds and the eventual
development of mixed-phase precipitation later tonight through
Thursday. Initial precipitation in the form of light snow will
spread north across the TAF sites in the 06z-08z time frame and
linger into at least mid morning. There are indications in model
guidance that a phase change to freezing drizzle will be possible
by mid to late morning at KOMA and KLNK. Precipitation rates could
increase by late Thursday afternoon into evening as forcing for
ascent strengthens over the region. Expect a steady decrease in
ceilings and visibilities with the onset of precipitation with
prevailing MVFR conditions developing later tonight and persisting
through Thursday.


&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Weather Advisory from 3 AM Thursday to midnight CST
     Thursday night for NEZ034-044-045-050>053-065>068-090>093.

     Winter Weather Advisory from 3 AM to 6 PM CST Thursday for
     NEZ078-088-089.

     Winter Weather Advisory from 6 AM Thursday to midnight CST
     Thursday night for NEZ012-015-017-018-031>033-042-043.

     Winter Weather Advisory from 6 AM to 6 PM CST Thursday for
     NEZ011-016-030.

IA...Winter Weather Advisory from 3 AM Thursday to midnight CST
     Thursday night for IAZ043-055-056-069-079-080-090-091.

&&

$$

UPDATE...Mead
SHORT TERM...DEE
LONG TERM...DEE
AVIATION...Mead


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