Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 210359
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
959 PM CST Sat Jan 20 2018

.UPDATE...
Issued at 952 PM CST Sat Jan 20 2018

Winter storm still on track with the brunt of the storm`s impacts
from central into northeast Nebraska. The storm is beginning to
bring clouds into the region with moisture advection...however a
surface boundary near LNK/OMA is making temperatures and dewpoints
a difficult forecast with mid 20s dewpoints at TQE at 03z and
dewpoints around 40 at Plattsmouth. Some fog beginning to develop.
Generally raised low temperatures in southeast Nebraska and
southwest Iowa. Better chance for measurable precipitation Sunday
morning. Precipitation overnight should mostly be drizzle with
the clouds and the warmer dewpoints/temperatures.

&&

.SHORT TERM...(Tonight through Tuesday)
Issued at 250 PM CST Sat Jan 20 2018

The main forecast concern is still the potential for heavy snow with
the winter storm system that will affect parts of the northern and
central Plains Sunday through Monday. Still expect somewhat of a mix
of precipitation on the southeast side of the heavy snow band, which
is messy. After coordination, decided to convert northeast NE to
a Winter Storm Warning at 3 pm Sunday, where probabilities are
highest. That may be just a little early, but accumulating snow
could start in parts of Boone, Antelope and western Knox counties
a little after that. There is still an area of uncertainty on the
southeast flank of the heavy snow area, so will keep a Winter Storm
Watch going there. It may need conversion to another product
later.

Primary features of interest from the 12Z upper air maps this
morning included the following. Jetstream at 300 mb (with winds over
100 knots) curved from southern CA into northwest UT, southeast MT,
then to Lake Superior. Strongest values noted were around 130 knots
over ND and northern MN. Max 12 hour height fall center (HFC) at
500 mb was about 150 meters over southern CA. A modest thermal
gradient was in place at the 700 mb level from the southern Rockies
out into the Plains. A wedge of warm air at 850 mb was across the
southern and central Plains, with a big gradient noted from western
SD to western KS (Rapid City was 0 C and Dodge City was +20 C).

Surface analysis early this afternoon showed a low pressure center
of around 1000 mb over southeast CO and a warm front/nearly
stationary front curved from the low east across KS into IA. Low
clouds were over parts of southeast NE and southwest IA but there
has not been much drizzle reported and fog had mixed out. The
depth of moisture is somewhat in question, and that will be
important for development of any drizzle tonight into Sunday
morning.

Surface low should track from southwest KS at noon Sunday to far
southeast NE at midnight Sunday night, then toward eastern IA by
noon Monday. HFC should still be back over CO and NM at midnight
tonight, track to near the OK panhandle by noon Sunday, then
probably to eastern KS or southeast NE by midnight Sunday night.
HFC values are mainly 200-250 meters which are a good sign of the
strength of the system. There is also a strong TROWAL shown in the
700 mb Theta E fields.

There is still some model spread, so it is not a perfectly clear
picture. Previous shift delayed the ending time of the watch and
that was good, since blowing snow will probably still have some
impact even after the falling snow ends. Will continue to mention
near blizzard conditions in the WSW product.

Mixed pcpn and ice present aloft on the southeast side are concerns
if surface temperatures are below freezing tonight into Sunday.
It is possible that an advisory may be need for that, but for now
thoughts were that if there is much that it would be light. Main
period with accumulating snow is after 6 pm Sunday, except
possibly in Boone, Antelope and western Knox counties a few hours
before that.

Models show some mid level frontogenesis, which suggests some
mesoscale banding of the snow Sunday night into Monday morning.
700 mb vertical velocities at times are in excess of 20 microbars
per second. In regards to moisture values that we typically look
at for snow, mixing ratios/specific humidity values are generally
2.5 to 4 g/kg in the 700-750 mb layer. Lift is over 12 hours
duration, so amounts could reach over a foot in northeast NE.
Thunder appears possible in parts of southeast NE/southwest IA
Sunday evening, but did not include that in the gridded forecast
at this time. Kept POPs pretty high into Monday afternoon, but
decreasing in our western areas. Snow should decrease from west to
east Monday afternoon into Monday night, but blowing snow may
continue into the evening.

.LONG TERM...(Tuesday night through Saturday)
Issued at 250 PM CST Sat Jan 20 2018

This period looks mainly dry from Tuesday into Friday morning,
with highs moderating from the 30s to the 40s at most locations.
Will include some small chances for rain and snow from Friday
afternoon into Saturday. More on that in later forecasts.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 547 PM CST Sat Jan 20 2018

The stratus is making northward progress.  MVFR ceilings this
tonight at KLNK and KOMA. Deteriorating conditions at KOFK later
tonight...especially after 10Z. The clouds will hold temperatures
up and for now think any of the light precipitation would be
drizzle. North of the stratus...temperatures should drop off, but
this looks more to be north of KOMA and KLNK and low confidence
there is enough lift as far north as KOFK and have some mvfr fog
then mvfr vsbys. Progs lower cigs to IFR and will need to monitor
trends, however so far have been way too low on cig forecasts.


&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Storm Watch from Sunday evening through Monday afternoon
     for NEZ015-033-034-043-044-050-065.

     Winter Storm Warning from 3 PM Sunday to 6 PM CST Monday for
     NEZ011-012-016>018-030>032-042.

IA...Winter Storm Watch from Sunday evening through Monday afternoon
     for IAZ043.

&&

$$

UPDATE...Zapotocny
SHORT TERM...Miller
LONG TERM...Miller
AVIATION...Zapotocny


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