Area Forecast Discussion
Issued by NWS North Platte, NE

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593
FXUS63 KLBF 051818
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
1218 PM CST Mon Dec 5 2016

.SYNOPSIS...
Issued at 331 AM CST Mon Dec 5 2016

H5 pattern tonight has a tandem of closed lows over central and
western Canada with east to west flow noted from Oregon east into
the upper midwest. A closed low was also located over northern
Mexico. High pressure aloft remained anchored off the coast of
southern California and over the the western Caribbean. Ht. falls
tonight were most pronounced over the northwestern CONUS and were
100 to 140 meters from western Montana west to the Oregon coast.
An arctic cold front in association with this feature has begun to
push into Montana overnight and as of 3 AM CST, had made it as far
south as southeastern Montana and north central Wyoming. The
coldest air was located from central into northern Montana with
readings in the teens along the US/Canadian border. Further south
across western and north central Nebraska, a combination of mid
clouds and westerly winds have led to mild temperatures overnight.
Temperatures as of 3 AM CST were in the lower to middle 30s.

&&

.SHORT TERM...(Today and tonight)
Issued at 331 AM CST Mon Dec 5 2016

Over the next 24 hours, a significant drop in temperatures and a
small threat for light snow in the northwestern forecast area are
the main forecast concerns.

For today: The arctic cold front will approach the northwestern
forecast area by late morning, passing through the forecast area
by early evening. Highs today will not follow the typical diurnal
pattern in the northwest as they should occur near midday.
Elsewhere, highs should peak in the mid to late afternoon hours.
With the frontal passage, northwesterly winds will increase and
this could lead to some localized enhancement for precipitation in
the Pine Ridge and northwestern Sandhills. The latest NAM12 and
GFS solutions from 00z earlier this evening, seem to capture this
in their QPF forecast. There is some lift in the moist layer
across the northwest, albeit weak, this afternoon immediately
behind the front, so the threat for light pcpn seems reasonable.
The inherited forecast did have a slight chance mention in the
northwest and was continued with this mornings package. As for
precipitation type, decided to keep it all snow. Dew points across
northwestern Nebraska are in the lower to middle 20s. Any pcpn
which does make it to the ground should cool the column quickly to
the freezing or subfreezing mark. Further south, the front will be
crashing into a drier airmass with dew points in the teens, so a
dry forecast will continue. Highs today will vary widely across
western and north central Nebraska with mid 30s in the Pine Ridge
to the lower 50s in the southern forecast area.

Cold arctic air will push into the forecast area overnight
tonight. Lows will range from the single digits over northwestern
Nebraska to the lower teens in the southeast and eastern forecast
area. Lows were trended downward toward the latest MET and MAV
guidance as the forecast blender was some 5 to 10 degrees above
guidance. Wind chills over northern Nebraska will approach 5 to 10
degrees below zero tonight as winds are expected to remain in the
10 to 15 MPH range.

.LONG TERM...(Tuesday through Sunday)
Issued at 331 AM CST Mon Dec 5 2016

Primary forecast concerns through the long term is the threat for
accumulating snow late Tuesday through Wednesday, in addition to
unseasonably cold temperatures for the middle and second half of the
work week.

A broad longwave H500 trough will be in place over the western two-
thirds of the CONUS to begin the long term, with a closed H500 low
over Northern Minnesota Tuesday morning and a quick moving shortwave
diving southeast into the Great Basin. This quick moving H500
shortwave with an associated 85-100knot jet max, will move from the
Great Basin Tuesday morning and across the Rockies towards the
Plains by late Tuesday evening/Wednesday morning.

Isentropic analysis of the 285K surface on Tuesday suggest strong
isentropic downglide for most of the morning and afternoon hours,
which will all but eliminate precipitation chances for the daylight
hours across Western and North Central Nebraska. As the evening
progresses on Tuesday, a switch from isentropic downglide to upglide
will commence over portions of the Central High Plains and into
portions of Southwestern Nebraska. While mixing ratio values within
this region of isentropic upglide are meager at best (1 to 3 g/kg),
the moisture return at the 285K surface in-conjunction with strong
700mb frontogenetical forcing, should produce a period of light
snowfall across portions of Southwestern Nebraska and Northern
Kansas late Tuesday evening and into Wednesday. The extent as to how
far north this frontogeneticaly forced band extends, will be heavily
dependent on the track of the H700 low. At this time, current 00z
model consensus has the aforementioned H700 low moving from North
Central Colorado, southeast over West Central Kansas, and eventually
into Southeastern Kansas as an open wave. This current track would
place the strongest frontogenesis over extreme Southwestern Nebraska
and Northern Kansas, and help focus the heaviest precipitation over
these areas.

While snowfall:liquid ratios will be high during the event
(~16to20:1), am concerned with how much moisture is able to return
to the region over such a short time duration (~12 hours from 00z
Wednesday until 12z Wednesday at 285K). For this reason, believe
that snowfall accumulations will be limited to 1-3 inches generally
south of the Interstate 80 corridor. This is due in part to the
limited amount of upper level moisture available and southerly track
of the H700 low over Central Kansas (a west to east track of the
H700 low across Northern Kansas would be more ideal for snowfall
over a good portion of the Southern Sandhills and Southwestern
Nebraska). In addition to these concerns, most of the lift (omega)
across Southwestern Nebraska will be above the dendritic growth zone
(DGZ), which in itself will be fairly shallow and will be
detrimental to dendrite growth.

The closed H700 low will then become an open wave and quickly move
into Southeastern Kansas by Wednesday afternoon, which will bring an
overall decrease to the areal coverage and intensity of the
snowfall. Given that snowfall accumulations are only expected to be
in the 1-3 inch range and with light winds of around 5-15knots
expected, am not currently anticipating the need for any headlines
with this package. However, it should be noted that any accumulating
snow would be occurring during the morning commute on Wednesday,
which could impact travel at times on local roadways.

A second surge of Arctic air will then move across Western and North
Central Nebraska during the morning and early afternoon hours on
Wednesday, with 850mb temperatures falling to -14C to nearly -20C.
This will limit high temperatures to the teens, which is 20 to
nearly 30 degrees below normal for the first week of December.
Depending on the areal extent and depth of snow, low temperatures
Wednesday night could easily fall below zero across a majority of
Western and Southwestern Nebraska. Furthermore, given light
northwest drainage flow, past events with this setup would suggest
that model guidance is usually too warm with low temperatures. At
this time, CR builder is giving Western and North Central Nebraska
lows of -6F to 3F, which could be 5 to 10 degrees to warm given
recent observed warm bias. Despite the light northwest winds, wind
chill values may approach 5 to 20 degrees below zero Wednesday night
and Thursday morning.

Temperatures will remain well below normal for the day on Thursday,
with a "slight" warmup into the 20s to low 30s on Friday as surface
high pressure moves into the Ozarks region and return flow setups
over the Plains. Overnight low temperatures will be cold once again
Thursday night into Friday morning, with the single digits above
zero to near zero degrees. Wind chills may once again approach 5 to
20 degrees below zero over this time frame.

Light precipitation will be possible early Saturday morning as a
weak upper level disturbance crosses the region. However, at this
time, amounts look to be very light given the lack of upper level
support. PWATs over this time frame look to be around the
climatological norm of 0.30in, however as stated above, lack of
upper level support would only support light precipitation.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1215 PM CST Mon Dec 5 2016

A strong Arctic cold front has already pushed across the far
northwest Sandhills shifting the winds out of the northwest. This
cold front will continue to track across western and north central
Nebraska throughout the day. Behind the front winds will become
strong out of the northwest around 15 to 25 knots and cloud
ceilings will lower with incoming mid cloud deck. In general, VFR
conditions are expected to be observed over the next 24 hours.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SYNOPSIS...Buttler
SHORT TERM...Buttler
LONG TERM...Moldan
AVIATION...Gomez



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