Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 151504

Area Forecast Discussion
National Weather Service Cheyenne WY
904 AM MDT Thu Mar 15 2018

Issued at 853 AM MDT Thu Mar 15 2018

Complicated weather forecast over the next 24 to 30 hours. 12z
model data is starting to come in and continues the trend of
showing high QPF further west towards the Laramie Range and the
I-25 corridor. In addition, the HRRR and NAM show a good amount of
dynamic cooling with impressive dynamic lift as rapid cyclogenesis
occurs this evening over eastern Colorado and northwest Kansas.
Will likely need to extend Winter Weather Advisories across the
high plains by this afternoon with a few locations, such as the
I-80 summit, upgraded to Winter Storm Warnings. Confidence is
trending higher for 0.50 to 1.00 inch of liquid, but confidence
remains low to moderate for the exact timing of rain changing over
to snow and how much cooling occurs. Although the pavement is
relatively warm, snowfall rates may be 1 to 2 inches per hour for
a time. High confidence in the forecast further north where a
Winter Storm Warning remains in effect. Even if locations such as
Chadron and Alliance struggle to see snow accumulations on the
roads, believe the higher ridges will see slick and snow packed
roadways. Also, model soundings are showing some potential for
freezing rain late this evening from Alliance and Chadron.

In addition, isolated thunderstorms look like a good bet across
most of the southern half of the forecast area, south of a line
from Rawlins to Alliance Nebraska. With impressive upper level
diffluence and some llvl instability and frontogenesis, may see
some small hail as well this afternoon and into early this


.SHORT TERM...(Today through Saturday)
Issued at 250 AM MDT Thu Mar 15 2018

Tricky forecast with lots of forecast challenges over the next
24-36 hours. Snow amounts, turnover temperatures and winds are all
challenges in the short term.

Currently...Surface analysis showing a low pressure system in
northwestern Colorado this morning. Radar mosaic showing showers
moving over western Carbon County. Temperatures quite warm across
southeast Wyoming and Nebraska Panhandle this morning with mid 30s
across the Panhandle and low 40s west of the Laramie Range. Any
precip falling is likely liquid.

For today...All short term model guidance in good agreement with
precip developing across the CWFA as the day wears on. Best
chances for rainfall will be late afternoon after 21Z or so. GFS
and NAM forecasting lifted indices of -1 to -2C as well, so a few
thunderstorms are possible. Mountains likely to see a quick turn
over to snow as 700mb temperatures fall to -2 to -4C by 00Z.
Getting predominantly snow over the I-80 Summit and the
SNowy/Sierra Madre Ranges today, so decided to issue Snow
Advisories for these areas.

Challenge tonight will be turnover times for snow. GFS soundings
for the northern Panhandle showing a definite possibility of
temperature wet bulbing to freezing by 06Z. Model QPF quite high across
Laramie, southern Goshen and Platte Counties this evening, with
near 3/4ths of an inch after 00Z. Temperatures are right on the
cusp of rain Vs snow. If temperatures stay warm, then a lot of
this precip will fall as rain. Should the snow begin sooner than
what I have forecast, we are going to need to expand winter
headlines across areas east of the Laramie Range. So Day shift and
evening shift could be pretty busy today/tonight.

Other concern for tonight into Friday was winds. GFS has backed
off on the stronger winds a little, now forecasting 40-45kts
instead of 50-55kts it was showing yesterday. So kept speeds just
under High Wind Warning criteria over the Summit and out by
Arlington. Would not be surprised if we do hit at Arlington and
over the Summit a couple times Friday morning.

.LONG TERM...(Saturday night through Wednesday)
Issued at 250 AM MDT Thu Mar 15 2018

Models in better agreement with the next system that will be
moving across the area later on Sunday, closing off a small upper
low as it moves across southern Colorado and Kansas through Sunday
night. his will bring another round of scattered mainly snow
showers to the CWA Sunday afternoon and evening as well as the
next push of cooler air. Pcpn should move out of the CWA Sunday
night/early Monday but a few snow showers should still hold on
through the day Monday primarily over the mtns as another weak
impulse drops southeast across the region. Tuesday and Wednesday
look to be mainly dry with warming temperatures as upper ridging
builds into the region.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 250 AM MDT Thu Mar 15 2018

Mainly VFR conditions for all terminals today though some periods
of MVFR cigs may still affect KRWL this morning as some showers
move by. More widespread MVFR/IFR conditions expected tonight as
an upper low moves over Colorado and moist upslope flow occurs
over se Wy and the Panhandle. Rain and snow will also increase
later today with even a few tstms possible mainly near the
Colorado border. Pcpn becoming all snow tonight.


Issued at 250 AM MDT Thu Mar 15 2018

Low pressure expected to track into northeastern Colorado today
and intensify over the next 24 to 36 hours. This low will bring
widespread wetting rains to all locations today before turning
over to snow tonight on the back side of the low. Some isolated
thunderstorms could be seen today with the widespread rain along
with strong west to northwest winds. Given the widespread rain and
snow forecast with this system, fire weather concerns are expected
to be minimal. Another low pressure system expected to bring rain
and snow showers Sunday, so fire weather concerns continue to be
minimal into the early part of next week.


WY...Winter Weather Advisory from noon today to noon MDT Friday for

     Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for WYZ102.

NE...Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for NEZ002-003-095-096.



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