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Area forecast discussion 
National Weather Service Cheyenne Wyoming
400 PM MST Wednesday Dec 24 2014

Short term...(this evening through Christmas night)
issued at 359 PM MST Wednesday Dec 24 2014

Main impact in the short term will be strong winds over the wind
prone areas through this evening...and the strong winter storm on
Christmas.

Strong winds will continue through the early overnight period in
the wind prone areas along and west of the Laramie range. The
cag-cpr gradient will remain tight through the early evening
before weakening overnight as the strong upper trough continues to
dig southeast across the Great Basin and induces surface pressure
falls across The Rockies. Therefore...will likely see sustained
winds of 35 to 50 miles per hour with gusts 55 to 65 miles per hour continue through at
least midnight before tapering off slowly through the night. Will
keep the High Wind Warning as is to account for the slow weakening
of winds overnight...although there is a chance the warning could
be cancelled early.

As for the Christmas storm...models remain consistent with timing
and placement of the upper low as it tracks across the Great
Basin tonight and into the 4 corners region of Utah/co/AZ/nm on
Christmas. So...confidence remains high for a widespread moderate
to heavy snow event for our forecast area on Christmas. Broadscale
lift will increase over the County Warning Area late tonight and tomorrow morning
as the low digs toward the 4 corners with increasing positive vorticity advection ahead of
the main vorticity maximum. The cold front still looks to move from
northwest to southeast across all counties starting shortly after
sunrise...likely reaching the Colorado border around 1 PM. WV
imagery this afternoon shows plentiful Pacific moisture streaming
in with this system...and the llvls will saturate quickly as the
front rolls through. So not expecting much time between frontal
passage and onset of snow...with perhaps even a few light snow
showers developing out ahead of the front as surface cyclogenesis
occurs over northeast Colorado and induces localized convergence
over the Wyo/Nebraska plains in the late morning. Surface winds will
quickly become north-northeast behind the front and will
strengthen as the surface low deepens and shifts south. So looking at
a fairly strong upslope component to the overall forcing
compilation...especially along east-northeast slopes of the Laramie range and
Snowy Range. The upper level jet will strengthen through the day
over the southern and Central Plains while the deformation zone on
the northern quadrant of the upper low approaches from the northwest.
Seems that in general...upper level forcing is stronger than
previous model runs...and in combination with the upslope forcing
and low level instability over the mountains...will be sufficient to
support widespread moderate to heavy snow rates developing through
the afternoon and evening over much of the forecast area.

Went ahead and upgraded the Winter Storm Watch to a warning for
these areas...and included Converse County as the locations
along I-25 will be in favorable upslope regime along the northern
Laramie range so will see heavier snow rates here.
Similarly...expanded the warning include eastern Laramie County
for enhanced upslope component. For Niobrara...Goshen...and all of
the Panhandle...went ahead with an advisory as a good 2 to 5
inches of snow is looking likely for these areas under good
broadscale dynamics. Surface winds will out of the north-northeast gusting 25 to
30 miles per hour or so and will create areas of blowing and drifting of snow
and reduced visibilities for all areas...and will perhaps produce
locally higher amounts of snow along north-northeast faces of any terrain
features. Concerned especially for areas from Cheyenne to The
Summit along I-80...and from Laramie to Rawlins for enhanced
snowfall as good upslope combined with some instability prognosticated
over these areas by the models will enhance snow rates in the late
afternoon and evening timeframe.

Light snow will continue through much of Christmas night with the
deformation zone overhead....although the surface pressure gradient
will relax as the surface low continues to trek south and thus the
upslope winds will decrease.

Bottom line is that travel conditions will quickly deteriorate
across southeast Wyoming and western Nebraska Christmas afternoon
and evening from northwest to southeast. So if you must
travel...do it early on Christmas and keep your winter weather kit
handy in case of an emergency.

Long term...(friday through wednesday)
issued at 359 PM MST Wednesday Dec 24 2014

Lingering light snow/flurries will come to an end Friday as the
main upper trough axis passes across the area and upslope flow
ends. Quite cold on Friday as the surface high center drops down
the High Plains. Mainly dry and milder Saturday and Sunday but
still on the cool side as return flow sets up and heights rise in
advance of the next upper shortwave that will be moving over the
Pacific northwest late Saturday. Conditions look to turn wintry again
Sunday night as a strong push of Arctic air drops south out of the
western Canadian prairies under a deepening upper trough over the
northern and central Rocky Mountain region. Should see another round of
upslope-induced snowfall spread across the County Warning Area Sunday night as
the colder air builds in ahead of a 1050mb surface high. Looking
bitterly cold for Monday night through Tuesday night as the upper
trough moves slowly across the region and the surface high settles
down the High Plains. Mins well below zero looking a decent bet
Tuesday and Wednesday. Went below guidance for this time period and it
may still be too warm. Otherwise some more light snow should occur
over the area in association with this system Sunday night into
Monday.

&&

Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 359 PM MST Wednesday Dec 24 2014

Some MVFR ceilings expected to hang around krwl into this evening with
VFR elsewhere overnight. Gusty winds will continue into this
evening around the southeast Wyoming mountains conditions will deteriorate
Thursday morning from northwest to southeast as a cold front moves across the
area with ceilings lowering to MVFR/IFR. Snow will also increase
across the area Christmas day.

&&

Fire weather...
issued at 359 PM MST Wednesday Dec 24 2014

Strong winds will continue over wind prone areas of southeast
Wyoming this evening. Elsewhere winds will be light to breezy. A
winter storm system will move across the area Christmas day and
night with several inches of snow expected. Lingering snow showers
and flurries Friday then dry Saturday. Another weather system to
impact the are early next week.

&&

Cys watches/warnings/advisories...
Wyoming...Winter Weather Advisory from 11 am Thursday to 5 am MST Friday
for wyz102-103-108.

High Wind Warning until 5 am MST Thursday for wyz106-110-116-117.

Winter Storm Warning from 11 am Thursday to 5 am MST Friday for
wyz106-107-115>119.

Winter Storm Warning from 8 am Thursday to 5 am MST Friday for
wyz101-104-105-109>114.

NE...Winter Weather Advisory from 11 am Thursday to 5 am MST Friday
for nez002-003-019>021-054-055-095-096.

&&

$$

Short term...rjm
long term...re
aviation...re
fire weather...rjm

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