Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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179
FXUS63 KOAX 160543
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1143 PM CST Fri Dec 15 2017

.SHORT TERM...(Tonight through Monday)
Issued at 230 PM CST Fri Dec 15 2017

The main forecast concerns in this period are the chance for some
light precipitation late Saturday night into Sunday morning and
temperatures through the period.

There were at least three jet streaks noted at 300 mb at 12Z this
morning, one over the TN valley area (180 knots) another from
southern Manitoba into MN (125 knots) and another punching east
from the Pacific NW coast (about 110 knots). Strong height falls
at 500 mb (up to 200 meters for the 12 hours ending at 12Z this
morning) were moving into western Canada. The height falls were
associated with a trough which was flattening the strong ridge
which has been in place much of the past few weeks along about
120-130 degrees west longitude. There was a closed low at 500 mb
over Mexico, and general model consensus forecasts that feature
or the remnants to track northeast. It is expected to become and
open wave track toward southeast NE and southwest IA for late
Saturday night into Sunday morning.

No precipitation is expected from tonight through Saturday
evening. We are looking at potential for a light rain and light
snow mix (small potential for freezing rain and sleet) moving up
into southeast NE late Saturday night. Lift and deeper moisture
increase, mostly for the southeast 1/3 of the forecast area.
Although the GFS keeps surface temperatures above freezing, some
other model guidance does not. Model forecast soundings indicate
mixed precipitation types and will stick with that thinking for
now. Will need to watch that as we get closer to the event. There
are also hints at possible flurries into parts of northeast NE
Saturday morning but did not include that in the forecast for now.

A weak cold front will push south across portions of the forecast
area on Saturday, then stall from parts of KS into southeast NE
and southwest IA. The GFS MOS guidance seems to have a decent
handle on highs Saturday, while a blend of model output was used
for lows tonight and lows Saturday night. As for highs Sunday, we
expect generally poor mixing, so kept a blend of GFS and NAM MOS
for highs Sunday (upper 30s to around 40 north and in the 40s
south).

A low pressure system tracking across parts of the northern plains
and southern Canada should bring a warmer westerly flow to the
local area Monday, with highs mainly in the upper 40s/lower 50s.

.LONG TERM...(Monday night through Friday)
Issued at 230 PM CST Fri Dec 15 2017

The main concerns in this period are the trend toward colder
temperatures and potential for some light snow or a mix of light
snow and light rain for the area from Thursday into Thursday
night. We will keep the forecast mainly dry from Monday night into
Wednesday night.

Thursday morning, the 12Z runs of both the GFS and ECMWF show a
closed low at 500 mb over UT, but the flow is split, with another
branch of the jet stream with a trough from Manitoba into eastern
MT. Through Friday, the models dig the closed low southward toward
the Gulf of California, while the northern stream trough slides
southeast. The result is a highly amplified meridional pattern
that is very likely to bring colder air into the region from the
north by Friday.

We will include POPs in the chance category for now with this
potential event. The 12Z GFS did not really generate much QPF,
while the ECMWF and Canadian models did generate some light to
moderate amounts. The confidence level on amounts at this point is
not very high, since there has been quite a bit of model to model
and run to run variation. That will likely continue until we get
into next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1130 PM CST Fri Dec 15 2017

VFR conditions with variable amounts high clouds fl150-250. Light
southeast to southwest winds 4 to 10kts to veer around to the
northwest with a wind shift. North flow will eventually usher in
deteriorating conditions at KOFK toward the end of the TAF period
or after 06Z.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&

$$

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



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