National Weather Service Raw Text Product

429 
FXUS65 KCYS 112206
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
406 PM MDT Sun Jun 11 2017

.SHORT TERM...(This evening through Monday Night) 
Issued at 154 PM MDT Sun Jun 11 2017

...Widespread Severe Weather Outbreak Likely on Monday...Including 
Potential for Several Tornadoes...Very Large Hail...and Damaging 
Winds... 

The main focus over the next 24-36 hours will be a higher-end threat 
for severe weather on Monday. However, there is some potential for a 
few severe storms this evening as a mid-level shortwave lifts to the 
northeast across the CWA. Increasing low-level moisture, combined w/ 
considerable warm advection aloft should yield the development of at 
least isolated thunderstorms in the vicinity of the Laramie Range by 
21z-00z, and perhaps the Snowy Range and adjacent Laramie Valley too 
with enhanced low-level convergence in that area. If a storm were to 
form near Laramie, RAP soundings suggest decent shear w/good veering 
and increasing winds with height and around 1000 J/kg CAPE. A threat 
for severe hail/wind, or possibly a tornado is there. Elsewhere, SPC 
mesoanalysis shows excellent deep layer vertical shear and 1000-2000 
J/kg CAPE which may support supercells with storms that develop. Not 
much of a window for development though as soundings show only an hr 
or two where the low-level inversion weakens sufficiently. Will need 
to watch areas along/se of a line from CYS-AIA between 03z-09z w/low 
level warm advection and a strong mid-level vort max progged to lift 
across northeast CO and the southern NE Panhandle overnight.

Eventually, surface winds are expected to veer to the southeast from 
their current northeasterly direction over the high plains. However, 
any convective activity could impact low-level wind fields and delay 
this until close to 12z Monday. That said, fog and low-level stratus 
may not be as significant on Monday AM as earlier thought, but opted 
to keep patchy fog in the grids between 09z-15z. Models are in quite 
good agreement w/forecast soundings showing rapid erosion of low-lvl 
stratus by mid-morning. 

Our attention then shifts to what very well could be one of the most 
significant severe weather episodes we have seen in recent years for 
southeast WY and the western NE Panhandle. Models have been in great 
agreement over the last several days w/ a significant upper low over 
the Great Basin/Intermountain West at 00z Tuesday. The net result is 
impressive upper-level difluence over the entire area along with sfc 
cyclogenesis over northern CO. This 992 mb low then proceeds to lift 
NNE across southeast WY and western NE through the day, which should 
be a classic track for a potential tornado event. Thunderstorms will 
become numerous along/e of the Laramie Range by 00z with the help of 
low-level upslope, excellent convergence along w/steadily increasing 
large scale ascent. Biggest remaining question is storm mode as some 
models have trended toward a weak, almost negligable cap with no CIN 
to be had which could be messy with too much forcing. However, GFS & 
NAM soundings both show at least a shallow low-level inversion at 5k 
feet AGL or so which could promote discrete cells early. It also may 
help that the stronger dynamic support does not arrive until late in 
the afternoon which could keep early storms from merging too quickly 
and limiting storm intensity to some degree. By mid/late evening, we 
expect storms to merge into one or more clusters across east central 
WY or the northern NE Panhandle. 

This convection, especially if discrete, will be associated w/a high 
threat for severe weather. Steep mid-level lapse rates, coupled with 
strong surface heating is expected to yield a moderately to strongly 
unstable air mass characterized by MLCAPE over 2000 J/kg. Impressive 
forecast soundings remain with 0-6 km shear over 50 knots across all 
of our high plains zones. Very large, curved, and even a few sickle-
like hodographs are common along/n of the warm front between 00z-03z 
as the low-level jet rapidly strengthens with 50 knot flow at 850 mb 
and resulting 0-1 km/effective SRH over 400 m2/s2. This will support 
intense supercells with low-level mesocyclones, likely being capable 
of all facets of severe weather including tornadoes, extremely large 
and damaging hail, flash flooding, and destructive winds. Any storms 
which can become rooted in the boundary layer by evening will almost 
certainly have the potential to produce a strong/violent, long-track 
tornado given 0-1 km EHI of 2-4. SREF and NCAR Ensembles suggest STP 
values of 3+ as well, which is impressive. Dew points in the 60s may 
help promote low LCLs under 1000 meters, perhaps under 500 meters in 
the early evening. Expect this to support a threat for tornadoes, as 
long as the storm mode can at least stay quasi-discrete. 

.LONG TERM...(Tuesday through Sunday) 
Issued at 230 PM MDT Sun Jun 11 2017

All models show a much cooler day on Tuesday behind the strong
cold front with high temperatures mainly in the 60's to mid 70's,
warmest over western Nebraska. Some lower elevation locations 
west of the Laramie Range may remain in the 50's throughout the
day. Of more concern is the strong winds. A strong low to midlevel
pressure gradient will set up across southeast Wyoming as the main
upper level low slowly moves across northern Wyoming and eastern
Montana. May see High Wind criteria across portions of southeast
Wyoming, but not confident enough to add gusts 60+ mph due to it
being the time of the year where strong synoptic winds are
difficult to observe. Kept gusts between 50 to 55 MPH for now over
most of southeast Wyoming with gusts between 25 to 35 MPH across
western Nebraska through Tuesday evening. Any rain showers and
isolated thunder will be mainly confined to Carbon, Albany, and
Converse counties. Some light snow is possible above 9000 feet
across the Snowy range and Sierra Madre range as the upper level
low and southern extent of the cold pool push across the area
through Tuesday evening. 

Expect drier conditions Wednesday through the end of the week with
zonal flow aloft across the area and the absence of any fronts or
llvl boundaries across the area. Expect temperatures to gradually
increase back into the mid 70's (west) to mid 80's (east) Friday
afternoon. Extended models are hinting at the next cold front
moving in from the north during the weekend, which will enhance
the threat for thunderstorms by Saturday afternoon. 

&&

.AVIATION...(For the 18Z TAFS through Monday morning)
Issued at 537 AM MDT Sun Jun 11 2017

VFR conditions will prevail through late this evening. However, 
there is chance for late afternoon and evening thunderstorms near 
KCYS and KLAR today, with maybe some activity pushing into KSNY 
tonight. MVFR CIGS and VIS are possible with this activity.  Low 
stratus and IFR CIGS are expected tonight along and east of 
Interstate 25 including most of the western Nebraska terminals after 
midnight tonight. Fog is also possible around KCYS by early Monday 
morning. 


&&

.FIRE WEATHER...
Issued at 457 AM MDT Sun Jun 11 2017

A windy pattern shaping up across the districts for the first half 
of the week as a strong area of low pressure spins through the 
northern Rockies. Wind gusts of 35-50 mph will be a possibility 
especially across southeast Wyoming Monday night and Tuesday as a 
strong Pacific cold front moves through. Otherwise, expect two days 
of possible severe weather, beginning today for areas in Wyoming 
along and east of the Laramie Range. Monday afternoon/evening looks 
to feature a better chance for severe weather for most areas along 
and east of the Laramie Range. Temperatures cool on Tuesday, with a 
warming and drying trend anticipated through the rest of the week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...TJT
AVIATION...TJT 
FIRE WEATHER...CAH