National Weather Service Raw Text Product

715 
FXUS63 KOAX 281130
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
530 AM CST Mon Dec 28 2020

...Updated Aviation Forecast Discussion...

.DISCUSSION...
Issued at 323 AM CST Mon Dec 28 2020

.Impact Summary:

A winter storm will affect the region beginning tonight and
continuing into Tuesday night. Accumulating snow and ice will 
result in minor to moderate travel impacts.

.Today:

Surface high pressure will quickly build through the area ahead 
of a deepening lee trough over the High Plains. There is some 
signal in model guidance that clouds could briefly diminish before
thickening again later this afternoon. High temperatures will 
range from mid/upper 20s north to lower 30s south.

.Tonight through Tuesday night:

The 28/00z global models are now in reasonable agreement in the 
base mass fields associated with winter storm that will impact the
mid MO Valley beginning tonight. The general consensus of these 
data now indicate the amplification of a northern-stream trough 
over the northern Plains on Tuesday with that system progressing 
into the upper MS Valley by Tuesday night. Ahead of the mid-level 
trough, a strengthening low-level jet (LLJ) will advect an 
anomalously moist air mass poleward with the related warm 
advection/isentropic upglide being the primary forcing mechanism 
for this snowfall event. 

Latest CAM solutions depict the initial warm-advection shield of 
snow evolving over the central High Plains today before spreading 
northeast into our area late tonight into Tuesday morning. The 
greatest moisture/thermal fluxes will likely occur from mid 
morning on Tuesday through late afternoon as the LLJ migrates 
through the region. Snow rates will likely peak at that time, 
which is depicted by an increase in 28/00z HREF 1"/hr 
probabilities over eastern NE and southwest IA. 

The zone of enhanced warm advection along the LLJ will likely 
foster a transition from snow to sleet to freezing rain over 
portions of southeast NE and southwest IA on Tuesday afternoon. 
Elsewhere, the snow could mix with or change over to light 
freezing rain by mid to late afternoon with the arrival of a mid-
level dry slot from the west/southwest. The wintry precipitation 
should end from northwest to southeast across the area on Tuesday 
night as the mid-level system and associated surface front 
continue east. 

Based on the latest available information, we will indicate 
storm-total snowfall amounts of 2" to 4" over southeast NE and 
3" to 6" elsewhere across the area. Some of the high-resolution 
model solutions suggest the potential for bimodal swaths of 
higher snowfall totals; one from near Omaha and Plattsmouth into 
southwest IA, and another over our northwest counties. Flat ice 
accumulations should remain less than a tenth of an inch over much
of the area. The exception will be over portions of far southeast
NE where some localized flat ice accumulation could approach a 
quarter inch. Gusty southeast winds on Tuesday will likely result 
in areas of blowing/drifting snow. 

Given the above expectations, we have opted to issue a Winter 
Weather Advisory beginning at midnight for the western half of our
area, and 6 AM Tuesday for the eastern half of our counties. 
Portions of the advisory could be upgraded to a Winter Storm 
Warning later today or tonight if confidence in higher snowfall 
totals increases.

.Wednesday and Thursday:

Blustery northwest winds on Wednesday could result in some areas
of blowing snow. Otherwise, highs on both days will be mainly in
the 20s.

.Friday: 

A vigorous shortwave trough (or closed mid-level low) is forecast
to move through the mid MS Valley into Great Lakes. The glancing 
influence of that system could result in some light snow potential
for portions of southeast NE and southwest IA. 

.Saturday and Sunday:

Dry conditions are forecast with moderating temperatures.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 522 AM CST Mon Dec 28 2020

MVFR ceilings will likely continue to linger through the morning
hours before scattering out this afternoon. Light northwest winds
will become variable around 19Z today, and eventually turn to the
southeast tonight ahead of an approaching winter storm. Snow will
move in from the west, bringing deteriorating conditions to KOFK
and KLNK around 09Z Tuesday morning, but shouldn't reach KOMA 
until closer to 12Z.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Weather Advisory from 6 AM Tuesday to 6 AM CST Wednesday 
     for NEZ015-034-045-052-053-067-068-090>093.

     Winter Weather Advisory from midnight tonight to 6 AM CST 
     Wednesday for NEZ011-012-016>018-030>033-042>044-050-051-
     065-066-078-088-089.

IA...Winter Weather Advisory from 6 AM Tuesday to 6 AM CST Wednesday 
     for IAZ043-055-056-069-079-080-090-091.

&&

$$

DISCUSSION...Mead
AVIATION...KG