Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 161203 CCA

Area Forecast Discussion...Corrected
National Weather Service Cheyenne WY
446 AM MST Sat Dec 16 2017

.SHORT TERM...(Today through Sunday)
Issued at 259 AM MST Sat Dec 16 2017

Winter weather is the main forecast concern over the next 24 to 48
hours. Surface cold front is moving slowly southward this morning,
and was analyzed from W-E across northern Converse/Niobrara county
into southwestern SD at 10z. This front is still expected to stall
sometime soon, providing a focus for snow development through this
morning and afternoon. Recent radar imagery shows snow showers now
developing over northern Carbon county. This is expected to expand
in coverage/intensity across our central/east central Wyo zones w/
dynamic support in the right entrance region of a 100 knot H25 jet
streak. Mid-level energy is expected to split to the north & south
later today, so would anticipate precip to be focused to primarily
northern areas through this afternoon closer to the upper jet, and
stronger mid-level PVA. Models remain in good agreement w/ a broad
area of light/moderate snow north of a line extending from Rawlins
to Wheatland to Chadron. Main change to the inherited grids was to
inch higher snow totals a bit further north and west. We gave some
consideration to adding Niobrara county to the WSY, but most areas
there should see less than an inch w/ the exception of the extreme
northwest corner. Shirley basin should be shadowed by the northern
Laramie Range. Winter Weather Advisory looks good w/ 2-5 inches of
snow accumulation likely with QPF progs of 0.15 to 0.3 inch.

Things become more interesting for the remainder of southeast WY &
the western NE Panhandle after 00z. GFS/NAM forecast soundings are
showing classic FZDZ profiles over much of the plains. Deep (1500-
2000 ft) saturation between 0C and -10C, coupled w/moist low-level
upsloping post-fropa is concerning for ice accumulations along & E
of I-25 from Cheyenne to Wheatland into western NE. Probability of
ice present in cloud is expected to be near zero, so expect the P-
type to be predominantly freezing drizzle between 0-12z before low
level profiles cool enough to support a FZDZ/SN mix. There is good
potential for travel impacts from this, and later shifts will most
likely need to consider additional headlines for ice accumulations
of 0.05 to 0.1 inch if this comes to fruition. Areas of fog should
accompany the FZDZ, especially over I-80 between LAR-CYS.

.LONG TERM...(Sunday Night through Friday)
Issued at 259 AM MST Sat Dec 16 2017

Late Sunday into Monday looks to be a favorable period for
increased gap winds in the Arlington area west of Laramie,
otherwise fairly quiet until early Thursday when a strong cold
front sweeps southward across the forecast area bringing with it
an increase in snow chances. Right now the models are fairly
progressive with a digging upper trough moving southeast across
the area, but there are indications that we could see a deepening
low over Colorado Thursday, and if that happens a more significant
snow event may evolve across southeast Wyoming and the western
Nebraska Panhandle. Confidence in higher snow totals not overly
high this far out, but this is something that we will have to
monitor into next week as Thursday will be when Christmas travel
will be on the uptick. The other story for the next week will be
the cold air. In fact, models are showing 700 mb temps around
-20C, so this is our first real shot at sub-zero temps for many
 areas by late next week. Stay tuned.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 446 AM MST Sat Dec 16 2017

Ceilings and visibilities will trend downward across the Chadron
and Rawlins areas this morning as the cold front move south
bringing areas of mixed precipitation turning to snow. HRRR
guidance shows ceilings at Scottsbluff and Alliance falling to IFR
this afternoon as the coldest shallow air with the front
continues a southward push down the east side of the Laramie
Range, with ceilings falling to LIFR to IFR at Cheyenne and Sidney
early this evening. Guidance is mixed on dropping ceilings and
visibility at Laramie this afternoon, but given moisture coming up
from the southwest we also dropped ceilings there to less than
1000 ft agl by this evening.


Issued at 259 AM MST Sat Dec 16 2017

We continue with no fire weather headlines this weekend. Getting
low RH and gusty winds 30 to 40 mph across the areas in SE Wyoming
that have had snow cover and some moisture over the past 36
hours...and in those areas expect the winds to gradually diminish
this morning. Otherwise, cool turning downright cold through the
course of the next week with a chance of precipitation late
today, then again with what would appear to be a greater chance of
a more widespread wetting snow on Thursday.


WY...Winter Weather Advisory until 6 AM MST Sunday for WYZ101-103-104.



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