Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 291133
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
633 AM CDT Sat Apr 29 2017

.SHORT TERM...(Today through Monday)
Issued at 332 AM CDT Sat Apr 29 2017

Rain and snow potential through Monday morning remains the primary
forecast concern in the short term period.

Widespread rain spread across most of eastern Nebraska and southwest
Iowa on Friday, with snow the primary precipitation type in much of
northeast Nebraska Friday morning. The snow occurred under shortwave
trough rotating north where 850 temperatures were in the 2-3C below
range. Likely some dynamic cooling under shortwave helped the
conversion to snow. Webcams and pictures from social media indicated
some accumulations of up to 2 inches likely occurred on grassy areas
by noon Friday, when temperatures were generally between 35 and 38
degrees. But by 3 PM Friday, all traces of snow were gone as
relatively warm ground temperatures at or above 40 and air
temperatures in the upper 30s melted the snow rapidly.

A somewhat similar situation is evolving for this weekend, likely
covering a larger part of our CWA, when temperature profiles will
lean toward a rain/snow mix, with pockets of dynamic lift helping
cool column enough for snow to reach the ground. The cooler
temperature profiles will favor our northwest CWA, but dynamic
cooling regime will likely be more pronounced over our southeast CWA
across southeast Nebraska and southwest Iowa.

The driving force responsible for all of this is an upper low
currently near the Four Corners region which will swing into the
Plains by this evening. Models are much more consistent this morning
in tracking core of low from the Texas Panhandle this evening, into
northeast Kansas by Sunday evening, then across northwest Missouri
and into eastern Iowa by Monday morning. 850 temperatures were below
0C across the northwest half of our CWA according to Friday evening
upper air soundings, but warm advection ahead of approaching low
should warm that layer a degree or two during the day today, with
only northeast Nebraska remaining below 0C. Thus given broad
warm/moist advection promoting increasing chances of precipitation,
expect liquid to be predominant type, except for a potential mix at
times in northeast Nebraska.

As upper low swings closer to our area tonight into Sunday, warming
of low to mid levels of the atmosphere should again keep predominant
precipitation type liquid save for parts of northeast Nebraska where
colder air is a bit more stubborn. Slight cooling of the surface
layer there will promote a mix with or change to snow from time to
time.

Sunday night into Monday is a bit more problematic as cooling aloft
under cold upper low overspreads more of eastern Nebraska and
southwest Iowa. There is consensus in model output in driving 850
temps below 0C for all of our area by 12Z Monday on back side of
upper low moving into eastern Iowa. There is also consensus in
decreasing critical partial thickness progs into the snow side of
precipitation type spectrum, with broad area of 536-540 dam 1000-
500mb thicknesses shown by GFS and ECMWF. Forecast soundings are
stubbornly indicating a sufficient near-surface warm layer to melt
snow in the southeast half of our area Sunday night, but given above
temperature and thickness forecasts, see no reason cooling of the
surface layer to allow snow will not occur until later Monday
morning when core of upper low moves east.

Accumulations of snow is another question. First of all, even where
snow occurs in most of eastern Nebraska and southwest Iowa, would
only expect a brief accumulation where rates can overcome melting
processes with air and surface temperatures remaining above
freezing. However in northeast Nebraska where surface temperatures
are cooling, and air temperatures will be closer to freezing, 2 to 4
inches of snow is not out of the question Sunday night into Monday
morning, especially in areas west of a line from Columbus to Norfolk
and Wayne. Am not expecting sub-freezing temperatures there, so
impacts are not forecast to be harsh with some limited visibilities
and slush on roadways the major hazards.

Precipitation will be waning by Monday afternoon as upper low and
attendant trailing trowal region lifts northeast. A rebound in
temperatures into the upper 40s and middle 50s is possible Monday
afternoon, rising from 30s and low 40s which are expected to persist
through the weekend.

.LONG TERM...(Monday night through Friday)
Issued at 332 AM CDT Sat Apr 29 2017

A much more quiet weather period is forecast for the rest of the
week, with gradually warming temperatures. A quick shot of zonal mid
level flow will follow Monday`s upper low before strong shortwave
drops into the Plains Tuesday night. This system could spark
scattered showers from Tuesday afternoon into Wednesday, but bulk of
expected significant precipitation should remain well south of our
area where moisture fields support widespread convection. Mid level
heights begin to rise by Thursday and Friday as upper ridging is
advertised to develop in the Rockies and possibly the High Plains.
Height increases along with more sunshine will help our temperatures
rebound into the 60s or lower 70s by week`s end.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 630 AM CDT Sat Apr 29 2017

Conditions area expected to deteriorate through the period as
upper low over the southern Rockies moves into the southern
plains. MVFR cigs expected to spread into southern parts of the
area 15Z to 18Z at KLNK and KOMA becoming IFR after 00Z. KOFK will
be on the northern edge of the system with MVFR cigs not expected
there until after 00Z.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&

$$

SHORT TERM...Dergan
LONG TERM...Dergan
AVIATION...Fobert


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