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Area Forecast Discussion (AFD)

Issued by Cheyenne, WY (CYS)

                            
000
FXUS65 KCYS 270410
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1010 PM MDT Wed Jun 26 2019

.UPDATE...
Issued at 835 PM MDT Wed Jun 26 2019

Thunderstorms have weakened and/or exited our eastern CWA and
cancelled Severe Thunderstorm Watch 450. Isolated showers and 
thunderstorms are possible along and east of a Chadron to Sidney 
line through late this evening.

UPDATE Issued at 452 PM MDT Wed Jun 26 2019

Updated for Severe Thunderstorm Watch #450 valid until 11 PM MDT
for Niobrara/Platte WY, and all of the western NE Panhandle.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 143 PM MDT Wed Jun 26 2019

Biggest talking point for the forecast short term today is the
severe weather potential and the warm temperatures. The nearly 
stationary west coast trough is continuing to provide strong 
southwest flow aloft across the Northern and Central Plains. 
Subtle ripples in the flow aloft from WV and VIS sat indicate 
several shortwave disturbances in the height field moving over the
mountains. This will help initiate isolated to widely scattered 
convection amid the southerly and southeasterly low level flow. A
well defined surface trough was located along the lee of the 
laramie range with a stalled front orthogonal to the through
through the northern nebraska Panhandle. Higher moisture content 
with dews in the lower 60s was pooling along the the front across 
the northern Nebraska Panhandle as evident by manual analysis. 
This front and immediate proximity will be the focus for a 
localized greater severe weather threat over the next 2 to 6 
hours. Hi-Res guidance is over mixing the boundary layer in 
proximity to the front reducing forecasted MLCAPES and raising 
LFC/LCLs. Manual adjustment of latest HRRR soundings results in 
2000 to 3000 MLCAPE and 30 to 40 KTS effective shear. LCLs and 
LFCs under 2 km are also forecast which is very favorable for 
severe weather. Low level hodographs are supportive of updraft 
rotation with 100-200 ESRH as analyzed by mesoanalysis. Low level 
shear is likely enhanced in proximity to the front thanks to a 
greater easterly component of the low level wind fields. Very 
steep mid level lapse rates of 8.5 to 9.5 C/km and hail growth 
cape near 1000 j/kg are present indicating ripe conditions for 
large to very large hail growth. Several analogs highlight very 
large hail potential with some of the modified forecast soundings.
 
Current thinking based on observations and CAM guidance is that 
initial updrafts will evolve near the surface trough in the lee of
the laramie range. These updrafts will advect eastward into 
better moisture and gradually intensify. Isolated supercells with 
all hazards appear possible in the parameter space. The greatest 
threat area appears to be along the Nebraska Wyoming South Dakota 
border where low level shear and CAPE are maximized. Hail to 
baseball size and a tornado or two are possible if a sustained 
storm can tap into some of the extra energy. Storms will likely 
persist into the evening as they move further east into the 
Panhandle. 

Tonight high clouds will clear as shortwave ridging develops and 
moves in. Some fog could develop as the low level wind field dies
down and residual moisture sticks around. Temperatures will be
very mild overnight with lows in the 50s and 60s. Dry conditions
are expected with little lift and most of the moisture pushing 
off to the east. 

Thursday will be a very warm day with highs in the 80-s and 90s
for most. Ridging aloft should keep shower and thunderstorm
chances to a minimum. A few clouds should develop but expect 
mostly sunny skies and generally warm and pleasant weather for 
thursday. 

.LONG TERM...(Thursday night through Wednesday)
Issued at 143 PM MDT Wed Jun 26 2019

West coast troughing will continue through the remainder of the
week as the trough elongates and ridging builds in fro the south 
and east. Warm temperatures are expected with the southwest flow 
and increasing height fields. Friday and Saturday look to continue
the recent trend of very warm temperatures. Highs should be near 
peak summer performance through the first part of the weekend in 
the 80 to 90 degree range. Overnight lows should be very pleasant 
in the upper 50s and low to mid 60s. Across the mountains warm 
temperatures in the 60s and 70s can be expected. A cool front will
develop Saturday evening and dive south across the plains. This 
should bring cooler weather and the chance for showers and 
thunderstorms from early Sunday through Tuesday. Thunderstorm 
activity will continue through mid week as the remains of the once
stationary west coast trough break down and eject to the east 
Wednesday and Thursday. Mostly zonal westerly flow should develop 
behind the passing storm system keeping at least some threat of 
rain in the forecast fro the time being. Model diverge on the 
upper air pattern by day 8 but climate trends continue to suggest 
above average temperatures through the extended. 

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday evening)
Issued at 1010 PM MDT Wed Jun 26 2019

VFR. Skies will clear over western Nebraska overnight. Winds will be 
light/variable for western Nebraska terminals. Across southeast 
Wyoming, light winds will prevail through late Thursday morning, 
then become south-southwest 10-20 kt with gusts to 30 kt.

&&

.FIRE WEATHER...
Issued at 143 PM MDT Wed Jun 26 2019

Marginal elevated fire weather condiitons will be possible across
the interior valleys over the next few days. Warm temperatures 
and low humidity will bring atmospheric conditions to elevated but
not critical values. Area fuels remain unreceptive and little 
threat is expected. 

&&

.HYDROLOGY...
Issued at 143 PM MDT Wed Jun 26 2019

Overall trend of elevated but slowly decreasing flows will
continue for the next few days as snowmelt slowly continues to
contribute to baseflow. Canceled the flood warnings and will 
maintain an RVS for now as insurance given that flows will remain 
just below flood stage. Dont expect much in the way of precip for 
a few days but next weekend could trigger more hydro concerns. 

...PREVIOUS DISCUSSION...

Trends on headwaters gauges coming out of the Snowy and Sierra
Madre Ranges have shown river levels coming down slowly. Still
need to wait to see effects of increased snowmelt over the next
several days before we can say for certain that flooding threat is
over. Warmer temperatures over the next several days will let us
know that impact. Rain still forecast for late weekend into next
week and a rain on snow event is still not out of the question.

&&

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

$$

UPDATE...MAJ
SHORT TERM...Lyons
LONG TERM...Lyons
AVIATION...MAJ
FIRE WEATHER...Lyons
HYDROLOGY...Lyons/GCC