Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 220537

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1137 PM CST Tue Feb 21 2017

.SHORT TERM...(tonight through Friday)
Issued at 253 PM CST Tue Feb 21 2017

Today likely is the peak of warm February conditions for most
locations. Subjective 12Z upper-air analysis indicates a broad
upper-level low centered over the Arklatex area. Another upper-
level trough was digging in upstream off the Pacific coast. A
135kt upper-level jet extended into northern CA/northern NV, with
smaller 130kt southwesterly jet streak across WI. Warm dome at
850mb continued, with 10C+ temperatures as far north as central SD
to southeast MT and as far west as the NV/UT border. Surface low
at 19Z was centered in north central SD, with a weaker surface low
centered in northern AR, and a ridge axis extending between from
southwest TX/western OK through eastern KS to southern and eastern

Main forecast concerns are temperatures tomorrow and winter weather
system on Thursday-Friday. A weak and fast-moving shortwave will
slide across the northern Plains tonight into Wednesday, dragging
the first shot of cooler air into the central US. Surface to mid-
level winds will turn northwesterly from morning through early
afternoon across the CWA as the cold front slips into the area.
Temperatures will still be quite warm, but in northeast NE where the
airmass changes earlier, temperatures will likely remain in the
lower 60s.  The lower temperatures should hold RH values into the
mid 20s or higher, keeping us just out of red flag territory, and
will not issue a fire weather watch for tomorrow.

By Thursday morning, a deeper western US trough will dig, with
precipitation expanding from WY/CO into western NE/western SD.  This
area should expand eastward through the day as the western trough
digs and closes into a low.  Surface temperatures should remain
above freezing across the CWA through the day, with model solutions
having various speeds of dropping 850mb temperatures below 0C into
the northwestern CWA. Given the warm surface layer, even if 850mb
temps do fall below 0C, rain or a rain/snow mix would be likely
through the day, with no accumulation.  Temperatures in northeast NE
should fall quickly after sunset, with slow progression of
subfreezing temperatures from 850mb to the surface through the
night. Even so, middle-of-the-road GFS and ECMWF solutions (between
slower GEM and way-too-fast NAM) keep above-0C temperatures in
western IA to southeast NE through around 12Z Friday. Above-freezing
surface temperatures remain as far north as the I-80 corridor by 12Z
in both solutions, with falling temperatures on Friday finally
taking freezing temperatures across the rest of the CWA by around
afternoon (ECMWF) to evening (GFS).  All in all, the trend is toward
a warmer solution in the south, while still cold and wet in the
north.  Not likely to be able to pin down the location of the
heaviest snow band yet, but ECMWF/GFS solutions are fairly
consistent in taking it across northern to northeastern Nebraska and
then into SD/MN/northwest IA. ECMWF would have a broader swath of
snowfall amounts of at least a few inches, while GFS has a
tighter gradient on the south side of the band due to the warmer
temperatures. Have reduced snowfall forecast in the southeastern
CWA for now and nudged higher in the northwest. Solutions still
look rather breezy on the back side of surface low pressure as it
deepens and ejects across KS/southeast NE/IA, with winds switching
from northeast to northwest as the inverted trough progresses
eastward. Have opted to issue a winter storm watch for the
northern five counties of northeast NE, where potential is
highest for accumulating snowfall of 6 or more inches.

.LONG TERM...(Friday night through Tuesday)
Issued at 253 PM CST Tue Feb 21 2017

The late-week storm system will usher in a return to near-normal
temperatures, along with a more active weather pattern. Upper-level
flow will become zonal this weekend and turn more southwesterly
through the week as another upper-level trough takes shape in the
western US.  Its depth and speed are still different among model
solutions, but the pattern should favor gradual moderation and a
progression of weak shortwaves that may generate brief, low-end
precipitation. Saturday should remain cold and blustery behind the
system, with temperatures lingering near or below freezing in
northeast NE/northwest IA to the upper 30s elsewhere. Temperatures
in areas with little or no snowpack should return to highs in the
mid-40s into the work week, with slower but gradual warming in
snowier areas. Have kept a slight chance of snow in the southern
CWA on Saturday night/Sunday, as well as a small chance of
precipitation on Monday.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1136 PM CST Tue Feb 21 2017

VFR conditions are expected through the TAF cycle. Surface winds
will become northwest Wednesday morning as a cold front pushes
through the region. Some gusty winds to 25kts are possible in the
afternoon but should subside by evening.


NE...Winter Storm Watch from Thursday evening through Friday
     afternoon for NEZ011-012-016>018.



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