Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 141122

Area Forecast Discussion
National Weather Service Cheyenne WY
422 AM MST Wed Nov 14 2018

.SHORT TERM...(Today through Friday)
Issued at 320 AM MST Wed Nov 14 2018

Pleasant and benign weather is expected as an upper level ridge
axis will gradually expand eastward over the central Rockys
today. 700mb temperatures will climb above 0c this afternoon,
allowing surface temperatures to increase into the upper 50s and
low/mid 60s across the lower elevations of far eastern Wyoming and
western Nebraska. Increased max temperatures this afternoon, but
forecast temperatures are still likely a few degrees too low. Once
the ridge axis pushes east of the Front Range and weakens,
northwest flow will redevelop over the area for Thursday allowing
slightly lower temperatures with highs ranging in the mid 40s to
the mid 50s. Dry weather is expected over the next 36 to 48 hours
through early Friday morning.

Attention then shifts northward towards the Canada/US border for
Friday as the next strong cold front and snow event is looking
more probable over southeast Wyoming and western Nebraska
beginning late Friday afternoon/evening. Ahead of the front,
surface pressure will decrease across the high plains as a weak
lee side low develops and moves southward along the front. This
will result in an increasing pressure gradient along the mountains
and a low potential of strong gap winds across the wind prone
locations including portions of I-25 and I-80. Increased winds in
these areas with gusts up to 50-55 mph for now. May need High Wind
headlines since there is a 3 to 6 hour window early Friday morning
for gusts around 60 mph ahead of the main cold front. The front
and associated upper level trough are trending a little bit slower
compared previous runs, so kept any mention of rain/snow well
north of I-80 Friday afternoon.

.LONG TERM...(Friday night through Tuesday)
Issued at 225 AM MST Wed Nov 14 2018

Mid and long range models have consolidated on a winter weather
event that is expected Friday evening through early Sunday. A mid
level trough in Northwest flow will amplify as it moves over the
area beginning early Friday. 30 to 40 meter height falls will
overspread southeastern Wyoming as a surface low and cold front
move to the southeast. Winds, initially from the west, will
quickly change to north northeast bringing upslope flow in across
the central and southern Laramie Range. To the north, an Arctic
high will drive a cold airmass in from southern Canada. Aloft,
midlevel frontogenesis is quite apparent in the models with a
southward moving band developing over southern Wyoming and the
Nebraska Panhandle. Models show strong omega and a saturated
dendritic growth zone through early to mid day Saturday. The large
scale dynamic lift, in combination with the advancing cold air
mass, and local upslope flow will favor widespread areas of
moderate snowfall through the period. Several questions remain
about the heaviest snowfall. The progressive nature of systems in
northwest flow suggest that the areas of heaviest snowfall will be
short lived. Localized banding from mid level frontogenesis,
symmetric instability release, and steep low level lapse rates
from the strong cold advection could also locally augment snowfall
rates. M mesoscale details and the heaviest snowfall are unlikely
to be resolved this far out. A better picture is likely as the
CAMS and mesoscale models latch onto muddy details. For now, a
general 3 to 6 inches for the plains and foothill, and 5 to 10
inches with up to a foot of accumulation in the mountains seems
plausible. This storm should be watched closely over the coming
days as it has the potential for significant travel impacts.
Confidence is high for widespread snowfall but the exact magnitude
of impacts are uncertain at this time.

Behind the storm, the colder air mass should keep things quite
chilly with overnight lows in the teens through the weekend. High
pressure will keep the remainder of the weekend and the first part
of next week mostly uneventful. A few stray snow showers are
possible as the upper level jet starts to work back over the area
Tuesday. There is a signal for some stronger winds on Monday and
Tuesday but details remain too hazy for now. Beyond day 6, models
diverge on the upper level jet position and the overall flow
pattern. Some snow potential could re appear by mid week, but
model agreement is low. For now, expect a wet and cold start to
the long term with break late weekend and early next week.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 420 AM MST Wed Nov 14 2018

VFR conditions will continue as high pressure remains dominate. A
few stronger gusts will be possible this afternoon but weather
hazards are negligible.


Issued at 225 AM MST Wed Nov 14 2018

No Fire Weather concerns through the weekend with much colder
temperatures and a good chance for snow late Friday through most
of Saturday. Humidities will be the lowest Today and Thursday
across the area, but values should be near or above 25 percent
each afternoon.




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