Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 211218
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
618 AM CST Sun Jan 21 2018

.SHORT TERM...(Today through Tuesday)
Issued at 329 AM CST Sun Jan 21 2018

Major winter storm to impact much of Nebraska tonight and Monday,
and will impact northeast Nebraska with Blizzard conditions in heavy
snow and strong winds. Have hoisted a Blizzard Warning for areas
along and northwest of a line from Columbus to West Point and
Macy.

Models remain in fairly decent agreement again this morning
regarding track and timing of system, and with axis of heaviest
snowfall stretching from southwest to northeast Nebraska and into
southern Minnesota. Upper system was currently deepening in the Four
Corners region, and will emerge into western Kansas later this
afternoon before swinging through northeast Kansas then northern
Missouri on Monday. Attendant surface low is forecast to take a
similar path, placing much of Nebraska in favored precipitation
regime left of the track.

Increasing theta-e advection was noted this morning from the
Southern Plains into the Mid Missouri River Valley region as
diffluent mid level flow was overspreading the Plains. Areas of
low clouds and fog have resulted as surface dew points in the
middle and upper 30s have overspread much of the southern half of
our CWA. In fact, much of eastern Nebraska and all of southwest
Iowa were solidly above freezing this morning, and this should
hold most of the day. With deepening low level moisture and mid
level ascent, expect increasing chances for drizzle over much of
our area this morning and afternoon. However freezing drizzle
should not be an issue save for a small part of northeast Nebraska
where temperatures may hang close to freezing today.

Strong isentropic upglide leading to enhanced precipitation
potential will begin today in western Nebraska and spread into
central Nebraska this afternoon, just a bit slower than earlier
thinking. Forecast soundings suggest a late afternoon but more
likely early evening saturation of atmosphere in northeast
Nebraska when snow is expected to begin there as layer
temperatures cool. Impressive ageostrophic vertical circulation is
noted between 06Z and 12Z when cross-sections of theta-e lapse
rates and saturated EPV suggest convective snows are likely.
Perhaps a rumble of thunder? Various snow accumulation methods
including Garcia, Cobb, and straight QPF/Snow Ratio calculations
indicate double digit snowfall is likely, with a large part of
northeast Nebraska northwest of a Columbus to Macy line seeing 8
to 15 inches of snow before ending Monday afternoon.

Also, strong north winds are expected to begin this evening when
forecast soundings show unidirectional and increasing flow with
height, producing an efficient momentum transfer regime. Boundary
layer average winds of 30-35 mph with occasional gusts over 45 mph
are likely, especially during time of heaviest snowfall. Travel will
be impossible after midnight, and will likely continue to be,
well into Monday morning before snow begins to taper off a bit.
Blowing and drifting snow should continue through the afternoon as
well.

To the southeast things are much more tricky. Strong warm advection
through a deep layer of the atmosphere will maintain liquid
precipitation for good part of the evening, with a gradual change to
mixed precipitation then snow overnight. Will likely see a stripe of
sleet or freezing rain as colder air begins to undercut warmer air
aloft as strong north winds overspread the area. The combination of
the mixed precip potential initially, then a change to snow with
accumulations in the 2 to 5 inch range on Monday, and the strong
north winds, mean at least a Winter Weather Advisory is needed just
southeast of the Blizzard Warning. If the change to snow occurs
earlier, then an upgrade to winter storm or blizzard warning may be
needed there.

Then farther southeast, areas around Interstate 80 in Nebraska, and
much of southwest Iowa should remain mainly liquid through 12Z
Monday before the change begins. We could also have a rumble of
thunder or two overnight as models paint and area of elevated
instability across southeast Nebraska and southwest Iowa. A brief
period of freezing rain or sleet will change to snow through the
morning, and in the early afternoon in far southeast Nebraska and
southwest Iowa. Given the later change to snow, most areas near
and south and southeast of the Interstate will likely hold at an
inch or two of snow. This may be enough for an advisory on Monday
given the strong north winds. And if there is a little more
freezing rain than expected, or the change to snow occurs earlier
leading to increased snow amounts, headlines will definitely be
needed.

The snow will end from west to east through the afternoon and
evening Monday, and may linger past midnight in western Iowa. Colder
temps will follow for Tuesday, but not drastically colder as with
the few systems we`ve had this winter.

.LONG TERM...(Tuesday night through Saturday)
Issued at 329 AM CST Sun Jan 21 2018

A milder pattern is setting up for much of the work week. Mid level
ridging is expected by Wednesday and Thursday ahead of another
trough swinging through the Rockies. 850 temps rise above 0C and
southerly flow should dominate both those days. The next shot of
precip comes Friday into Saturday as Rockies trough moves through
the Plains. It looks warm enough for rain Friday, with perhaps a
little snow on the back side for Saturday. Still a long way out.
Let`s get through Monday first.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 600 AM CST Sun Jan 21 2018

A potent pushing across the central Plains will be the focus for
deteriorating conditions over ern NE. Latest obs already
indicating IFR/LIFR conditions were slowly lifting nwd over sern
NE. Expect onset of pcpn initially will be a RN/SN mix with a
transition to all SN at KOFK by early evening, and at KOMA/KLNK
shortly after midnight. KOFK will see the brunt of +SN
accumulation through tonight with 3 to 6 inches possible by 12z
Monday with an inch or so at KOMA/KLNK



&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Weather Advisory from midnight tonight to 6 PM CST Monday
     for NEZ034-043-044-050-051-065.

     Blizzard Warning from 6 PM this evening to 6 PM CST Monday for
     NEZ011-012-015>018-030>033-042.

IA...Winter Weather Advisory from midnight tonight to 6 PM CST Monday
     for IAZ043.

&&

$$

SHORT TERM...Dergan
LONG TERM...Dergan
AVIATION...DEE



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