Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Hastings NE
403 PM CST Thursday Dec 25 2014
Short term...(this evening through friday)
issued at 357 PM CST Thursday Dec 25 2014
As a cold front transitions through the region today...expect cloud
cover to increase and the potential for accumulating snowfall for
much of the County Warning Area. Consequently...a Winter Weather Advisory will be in
effect for our western and northwestern counties. The advisory will
be for snowfall amounts near one to three inches through tomorrow
At 21z...a cold front was located just to our north beginning
to pass through Ord...with snowfall ongoing across the Nebraska
Panhandle. Expect this cold front to continue pushing south
throughout the day and evening. By Friday morning...the cold front
will have made its way completely through the County Warning Area...and snowfall
should begin across western parts of the outlook area.
With the absence of a strong surface low/middle level low...snowfall
should be pretty consistent during the event being driven primarily
by an upper level wave. Meaning...chances for snow banding to occur
is limited under these circumstances. However...confidence is high
regarding overall snow potential and leaned towards wpc for
quantitative precipitation forecast...which resulted in total amounts ranging around 0.10 and 0.25
inches. Several sets of guidance indicated quantitative precipitation forecast around these values.
The sref probability ensemble for 0.25 inches or greater was near
60%-80% through Friday night across our west...which further lead to
high confidence in regards to quantitative precipitation forecast. Under a cold atmospheric
profile...I basically used a 15 to 1 snowfall ratio for
collaboration purposes...but seriously contemplated a higher ratio
near 20 to 1. There are decent Omega values in place within a moist
dgz layer. Three inches of snowfall is expected along and north
of a line from roughly Beaver City...to Kearney...to Greeley. I
would not be surprised if more counties were added eastward into
the advisory over the next couple of shifts. Friday night we lose
all forcing and moisture...so snowfall should end southwest to
northeast by this time.
Behind the cold front...highs tomorrow will be drastically different
than what we experienced today. Highs will be in the middle to upper
20s for most of the outlook area...with perhaps some Kansas counties
reaching the lower 30s tomorrow afternoon. These temperatures will
not fluctuate much from lows tonight. Overcast skies will keep
temperatures pretty steady from tonight through tomorrow.
Long term...(friday night through thursday)
issued at 357 PM CST Thursday Dec 25 2014
Aloft: the primary shortwave trough will lift NE out of the western USA
longwave trough and cross Kansas/Nebraska Friday night. This trough will be sheared
and weakening with no closed circulation aloft. The trough heads for
the Great Lakes Sat with broad/flat flow left behind over the region
into sun. Meanwhile...the next shortwave trough will dive into the western
USA sun-Monday and close off over the desert SW. Mean westerly flow
over the Central Plains will become northwest by middle-week with confluent flow
Winter cold: the pattern is looking colder...favoring below normal
temperatures the next 10 days....and 2-3 days of much below as a ridge
builds over Alaska and unloads significant Arctic cold into the USA.
Multi-day ec/GFS/Gem ensemble means continue in unison with a strong
signal...a +tilt longwave trough from Hudson Bay SW into the western USA.
The initial push into the western USA guarantees temperatures will plummet in
the 12/29-1/1 time frame. Thereafter...the track of the Alberta
clippers will determines if the cold eases and we remain on its western
fringe in downslope flow. There are signs the Alaska Ridge will be
undercut by the westerlies.
Surface: Canadian high pressure will continue to slide southeast over the plains
Friday night and will be overhead Sat. In the meantime...an Arctic cold
front will press S...arriving at the northern USA border at dawn sun.
This front will sweep through here late Sun night with Arctic high pressure
sliding down the Front Range behind. This high will be overhead Tuesday
and slip S Wednesday with downslope west flow developing. Another Arctic
front will drop into the northern USA Thursday. This next surge of frigid air
will probably make it in here late next week.
The daily details...
Friday eve: light snow will be in progress across the entire forecast area
at sunset and will gradually end from SW-NE.
Believe we're looking at a 7-10 hour period of snow at any one
location. It ends first over north-central Kansas...by midnight. Then over S-
central Nebraska 12am-4am.
The greatest lift will occur the front half of the event and will be
done by 02z and that means the greatest accums will occur before
then. So while snow will linger into the night...there will be
little additional accumulate after 8 PM.
Snowfall: the ingredients just are not there for this to amount to
much. No warm air advection or closed middle-level low and deformation.
This is primarily driven by the approaching trough and a subtle jetlet
outside the main core of the upper jet. Believe an advisory will
handle it. Went a little below wpc quantitative precipitation forecast which painted a swath of 1-3"
across the forecast area with the highest amounts north and west of the Tri-
Less than 2" Plainville Kansas-Hebron Nebraska-York
2-4" Beaver City-Lexington/Kearney-Ord/Greeley
Main axis: multi-model consensus and the past several runs of the
sref mean have the axis of greatest snowfall/quantitative precipitation forecast from Imperial-
Broken Bow-O'Neill. The southern fringe of this should brush Dawson/
Snow ratio: used 13-17:1 00z-06z with the higher values north and west of
the Tri-Cities. This is a 6-hour average which should handle the
decreasing lift in the ideal snow growth zone...and what lift is
forecast primarily above that level. After 06z used 8-10:1 as the
quality of the flakes will be poor for accumulate.
The middle shift will need to monitor trends in vertical motion for
possible changes to slr's and possible adjustments to accumulate. Would
not be surprised to see 20:1 on the northwest fringe of the forecast area. The
temperature profile is fairly cold /near -10c in the lowest 12k ft/.
Sat: clearing. High temperatures below average confidence as fresh snow
will be on the ground. Models do not handle this well and it is
tough to account for. We currently have 23-30f but believe we could
be 3-5f too cold.
Sun: multi-layered middle-high clouds. Believe it ends up M/cloudy and
we are probably not pessimistic enough in the forecast just yet. Again
highs are below average confidence.
Mon: cold air advection. Blustery/cloudy/much colder with north winds
gusting 25-30 miles per hour. Possibly some flurries or brief snow showers.
Tue: bitterly cold. We continue to see potential for highs in the single
digits /over S-central Nebraska/ in the models that handle extreme cold the
best. The forecast continues to trend colder.
Wed: quiet and dry. Still very cold.
Thu: quiet and dry. Temperatures should temporarily return to near normal
Lots of uncertainty in when the next Arctic cold frontal passage
occurs...but the pattern favors more Arctic intrusions.&&
Aviation...(for the 18z kgri/kear tafs through 18z Friday afternoon)
issued at 1136 am CST Thursday Dec 25 2014
Expect VFR conditions through middle to late afternoon today. A cold
front will transition through the region later today...resulting
in a northerly wind speed behind the front around 15 knots...and
an increase in cloud cover as well. Ceilings will begin to
degrade late this afternoon and will transition to MVFR
conditions for at least the reminder of this taf period. Although
too early to enter into this taf issuance...confidence for IFR
conditions and snowfall are looking likely by at least Friday
afternoon...perhaps as early as late Friday morning. During this
time visibility may be around 1 mile and ceilings look to be
around 1000 feet. Again...too early for this taf...but anticipate
these changes or something similar in the coming terminal
NE...Winter Weather Advisory from 9 am Friday to midnight CST Friday
night for nez039-040-046-047-060-061-072-073-082.