National Weather Service Raw Text Product

493 
FXUS63 KOAX 130920
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
320 AM CST Thu Jan 13 2022

.DISCUSSION...
Issued at 320 AM CST Thu Jan 13 2022

Overview: 
Although there have been some minor changes, we still expect a
snow event, potentially a significant one for parts of the area.
Chances for 6 inches or more of snow are highest in western IA and
will keep the previously issued Winter Storm Watch in place. There
was a small eastward shift in the axis of the heaviest amounts.
That axis is from southeast ND to central IA. We should get a
better sampling of the energy with the shortwave troughs that will
create the precipitation as they merge/phase over the central US
on Friday. And then hopefully a better picture of just how much
precipitation the system will produce. 

Recent past and current pattern: 
A 500 mb ridge was building over the western US last evening, but
model data initialized a strong system off the Pacific Northwest
coast with 12 hour height falls of over 150 meters. There were a
couple of jet segments in place, with a 150 knot max extending
southeast from Alberta into northeast SD and another around 100
knots off the coast of OR and northern CA punching northeast. The
850 mb chart last evening did have some warm air over the high
Plains, but colder air was across MN and WI northeastward. Decent
baroclinicity was noted at 700 mb and 500 mb from MT southeast to
NE and IA. High temperatures yesterday in our area reached the
50s, with several spots in the upper 50s. That was a bit warmer
than expected. 

Today and tonight: 
Northwest surface winds in the local area will turn more to the
north today, and then to the northeast early this evening.
Previous shift bumped highs up a bit and will stay close to those
numbers or even raise just a bit more, closer to the higher end of
guidance. Winds turn to the southeast overnight as low pressure
strengthens over eastern CO and an inverted trough extends from
the low up into SD. Thickening cloudiness and the general weak
return flow later tonight should help lows from getting too cold.
We will go with mid 20s to around 30.

Friday....Friday night and Saturday morning: 
This is still expected to be the most active period of the next 7
or so days. Soundings still show mixed precipitation possible
during the morning and even early afternoon for parts of the Most
recent GFS soundings for the Omaha area appear to saturate by
around mid day with a rain/snow mix looking most likely. With
surface temperatures initially above freezing, snow may not start
to accumulate right away during the middle of the day. Models
suggest that there will be lower tropospheric warm air advection,
but looking at thickness profiles, those values actually decrease,
suggesting the warm air advection going directly into lift and
saturation of the column. Sometimes once the precipitation type
changes to snow in these events, it never goes over to all rain. A
feature that may not have been discussed too much yet with this
system is a trowal structure (trough of warm air aloft) apparent
in the 700 mb Theta-E progs. This is over eastern NE during the
day Friday, then shifts southward out of our area by Saturday
morning. It seems to correlate fairly well with where we would
expect a deformation zone (north and east of the axis). 850-700 mb
specific humidity values average 2 to 3 g/kg during the event. The
duration of the snow is probably in the 12 to 18 hour range. That
would suggest snow amounts potentially 4 to 9 inches for parts of
the area. Also looked at the old school 200 mb thermal advection,
which might yield around a max of 6-7 inches of snow. Max 700 mb
vertical velocities are not all that strong, seeming to get as
high as maybe 8 microbars per second in the GFS output over Omaha
(NAM output looked weaker). So after looking over all of the above
(and other stuff) did not make many changes to the forecast we had
going. There may be a pretty sharp cutoff to low snow amounts on
the western edge of our area. Hopefully we can refine that over
the next 24-36 hours. 

We may still need some adjustments to the higher end snow amounts
as things get closer to Friday afternoon. As mentioned earlier, 
sampling of the mid level energy and moisture coming in from the 
eastern Pacific will get better with the next few model runs.
And the whole event will soon get fully into the shorter range
high resolution model output like the RAP13. Some things that 
could inch amounts upward include if there is stronger upper level
divergence or if the mid level system slows down and we get a 
longer duration of snowfall.

Saturday afternoon into Wednesday: 
This period looks mainly dry. A cold airmass will move in behind
the departing precipitation producer with daytime highs mostly
around 20 to the mid 20s Saturday. Sunday through Tuesday should
see a slow moderation back to highs in the 40s for eastern NE by
Tuesday and 35 to 40 in Iowa (where snow may keep it cooler a bit
longer). Cold air starts to move in for Tuesday night with a 
reinforcing shot Wednesday into Thursday. That could produce some 
light snow on Wednesday. 

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1130 PM CST Wed Jan 12 2022

Broken deck at FL090 will slowly clear overnight. VFR conditions 
persist overnight and through the day Thursday. As the sun rises, 
wind shear will develop with winds of nearly 40 knots at FL020. 

Expect a period of gusty northwestern surface winds on Thursday 
afternoon.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...Winter Storm Watch from Friday morning through late Friday night 
     for IAZ043-055-056-069-080-091.

&&

$$

DISCUSSION...Miller
AVIATION...Nicolaisen