Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201751 AAA

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1151 AM MDT Fri Apr 20 2018

.SHORT TERM...(Today - Saturday)
Issued at 425 AM MDT Fri Apr 20 2018

A very challenging forecast scenario for today & tonight w/respect
to precipitation types and amounts across the CWA. Vigorous upper-
level low was evident on WV satellite over southeast UT early this
AM, and is expected to continue to drift east across the 4 Corners
through 12z Saturday. A strongly difluent upper pattern coupled w/
moist/southeast low-level upslope will certainly yield an increase
in precipitation coverage across the entire region later today and
tonight. Some travel impacts appear to be a possibility.

Precipitation will be slow to get going over western areas this AM
due to some mid-level drying, despite the presence of good low-lvl
convergence associated with an inverted surface trough extending N
across Carbon county. The main area of interest during the morning
and afternoon will be over the high plains, especially the western
NE Panhandle. Southeasterly winds will increase over the plains in
the next few hours, along with excellent isentropic ascent helping
to promote precipitation development. We may see some decent rates
set up with some of these bands, but confidence in rain vs snow is
low. The NAM remains strongly consistent with the coldest solution
of all models concerning the thermal profile, and BUFKIT soundings
do suggest snow for Sidney. However, GFS profiles are considerably
warmer and suggest predominantly rain or a very wet snow. We opted
to split the difference here, as the NAM was a bit too cold on sfc
temperatures overnight. A rain/snow mix seems most probable, but a
quick shot of moderate/heavy snow is not impossible. The day shift
will need to monitor this closely. Fortunately, mid-level RH progs
suggest a dry slot will end this precipitation fairly quickly.

Precipitation should become fairly widespread during the evening &
overnight on the back side of the system w/easterly low-level flow
across the entire area. Rates are in question as moisture profiles
in the H7-H3 layer suggests quite a few dry pockets seen on pretty
much all models. Believe this will keep overall QPF low. Now, snow
levels will come into play. The NAM remains cold tonight while GFS
and other model soundings suggest mainly rain. Some of the higher-
res guidance is also a bit colder like the NAM, so our methodology
was to split the difference and hedge more toward the NAM. Believe
we will see a complete change over to snow for most areas, but for
elevations below about 5500 feet it seems like any accums would be
mainly on grassy surfaces with minimal impacts. It has been on the
warm side recently, so we will need to see some heavy rates to get
significant road impacts in a warmer environment, and that appears
unlikely at this time for lower elevations. Above 7000 feet, could
see some travel issues develop as early as mid afternoon on the I-
80 Summit as rain changes to snow there. In the mountains, a solid
6 to 12 inches with locally higher amounts appears likely. Did not
make any changes to inherited Winter headlines this AM. Precip may
linger into Saturday afternoon, but should generally end by 18z or

.LONG TERM...(Saturday Night - Thursday)
Issued at 425 AM MDT Fri Apr 20 2018

The remainder of the weekend will be quiet w/ dry zonal flow aloft
and a lack of any notable mid-level disturbances. The GFS/ECM both
suggest a fast-moving short wave tracking across the Intermountain
West on Monday/Tuesday, likely bringing our next chance for rain &
snow to the area. Trending warmer and drier afterwards.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1136 AM MDT Fri Apr 20 2018

Main focus on aviation impacts will remain on the lowering MVFR
to IFR ceilings through the late afternoon and overnight along
with -RA and -SN coverage increasing over the TAF sites during
this period. -SN has already been observed at KSNY that briefly
brought visibilities down to 1/2-1 SM. Mountain TAF sites will
initially have -RASN mix this afternoon and then convert over to
all SN overnight. For KCYS, a -RA to -RASN and then brief -SN is
the expected precip type evolution early evening into Saturday
morning. For western NE sites, -RA is expected this afternoon with
a transition to -RASN overnight. Brief change over to all -SN
will be possible for NE sites but any accumulations are expected
to be low. MVFR to IFR ceilings and visibilities are likely during
this period. Patchy LIFR visibilities could occur in heavier snow
showers. Conditions will improve late morning Saturday as
precipitation ends from west to east and ceilings increase to MVFR
and VFR.


Issued at 425 AM MDT Fri Apr 20 2018

No fire weather concerns. A cool/unsettled weekend ahead with good
chances for measurable rain/snow across all of southeast Wyoming &
the western Nebraska Panhandle.


WY...Winter Storm Warning until 9 AM MDT Saturday for WYZ114.

     Winter Weather Advisory until 9 AM MDT Saturday for WYZ112.

     Winter Weather Advisory until noon MDT Saturday for WYZ116.



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