Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 130519
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1119 PM CST Thu Jan 12 2017

.SHORT TERM...(tonight through Sunday)
Issued at 401 PM CST Thu Jan 12 2017

Continued cold through Friday night and a slight warmup on Saturday,
along with a small chance for snow tonight in our north, are the
primary short-term forecast concerns before potential winter storm
moves into the region later in the weekend.

Upper low swinging into central Ontario this afternoon was
maintaining cold airmass over the central CONUS as arctic air
plunged south in its wake. Flow aloft was mainly zonal over Nebraska
and Iowa, with weak wave associated with rear entrance region of
upper level jet was producing at least modest lift in the High
Plains this afternoon. That feature will track through South Dakota
and northern Nebraska overnight, spreading light snow across those
areas. Lift will weaken with time, so not expecting a lot in the way
of snow, generally under an inch in far northeast Nebraska.

Otherwise mainly cloudy conditions are forecast tonight through
Saturday night as overrunning southwest mid level flow develops
ahead of upper low digging south and southeast into northwest Mexico
by Saturday afternoon. Mid and high level cloud cover should be
widespread through the period, but we may see a few more breaks
during the day Saturday before Gulf moisture begins working toward
our region. Thus we may gain a few degrees from the low to mid 20s
Friday to closer to 30 on Saturday afternoon.

.LONG TERM...(Sunday night through Thursday)
Issued at 401 PM CST Thu Jan 12 2017

Potential winter storm with ice, sleet and snow is the primary
forecast concern into early next week.

Upper low swinging through the southwest states and northern Mexico
Saturday night is expected to progress into the Texas Panhandle
region Sunday afternoon, into Kansas on Monday, then swing through
eastern Nebraska and western Iowa Monday night and Tuesday. There
has been relatively decent agreement between models in regards to
placement and timing with this system over the last couple of days,
with a trend toward slightly slower progression over the last day or
so. So confidence is relatively high that significant precipitation
will affect our area Sunday and Monday. However there is still much
uncertainty on exactly what the low level temperature profile will
look like during the bulk of the precipitation event.

Certainly cold air is in place now and will be so for the beginning
of precipitation Sunday. Magnitude and duration of sub-freezing
surface layer temps is still in question. All model output suggests
temps will be below freezing Sunday, but there are various ideas on
how much warming will occur Sunday night and Monday. GFS shows temps
above 32F along and south of I-80 around 12Z Monday as latent heat
processes in falling precipitation help to warm shallow cold layer.
Surface flow is initially easterly, a neutral advection regime,
before turning northeast or north on Monday with cold advection
setting in Monday afternoon. Latest 12Z runs of Canadian and ECMWF
are similar in the evolution of temperature trends, with ECMWF
probably the warmest of the solutions during the day Monday.
Forecast temperature grids will follow a GFS/ECMWF blend, weighted
more closely to colder solutions each period, and would a worst-case-
scenario of the major model output as far as ice potential goes.

Our potential precipitation begins Saturday night in southeast
Nebraska when northern fringe of ice storm affecting KS/MO will
skirt that area. Only have a few hundredths of an inch of ice
potential there for now, but a northward shift would increase
amounts. More likely precipitation will begin Sunday as broad warm
advection regime aloft encourages higher theta-e air transport over
cold surface dome. Precip should spread north of I-80 by early
afternoon, and likely to the SD border by late in the day.
Temperatures remain below freezing, so ice accumulation is likely
most areas, but highest QPF will remain south if I-80 in the
afternoon. However as main system approaches Sunday night, higher
QPF is forecast for all of the area, again with temperatures likely
remaining below freezing. Some warmer surface temperatures should
begin working into southeast Nebraska and southwest Iowa during the
day Monday, but that is no guarantee by any means. In fact Monday
could be quite interesting as GFS implies some convective bands near
center of upper low as it swings through, perhaps causing sleet
showers or bursts of heavy snow as atmosphere is turned over.

Though the most likely form of precip is freezing rain much of this
event, soundings will teeter between freezing rain, sleet and snow
much of the time Sunday through Monday night, with a slant toward
snow the farther northwest you go. Trying to nail down exact precip
types this far out is futile. Thus forecast grids will show a
variety of winter weather types for now. Suffice it to say, with the
combination of potential ice accumulations from a quarter to a half
inch I-80 south, possible snowfall of several inches, or a
combination of ice, sleet and snow across all the area, we have
elected to issue a Winter Storm Watch beginning Saturday night in
parts of southeast Nebraska, and beginning Sunday for the rest of
eastern Nebraska and southwest Iowa. Will run that through Monday
night for now when most of the precipitation should be moving to the
east.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1105 PM CST Thu Jan 12 2017

VFR conditions expected thru the fcst pd.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Storm Watch from Sunday morning through late Monday night
     for NEZ011-012-015>018-030>034-042>045-050>053-065>068.

     Winter Storm Watch from Saturday evening through late Monday
     night for NEZ078-088>093.

IA...Winter Storm Watch from Sunday morning through late Monday night
     for IAZ043-055-056-069-079-080-090-091.

&&

$$

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



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