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000
FXUS65 KCYS 260003
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
603 PM MDT WED MAY 25 2016

.SHORT TERM...(This evening through Friday Night)
Issued at 300 PM MDT Wed May 25 2016

Unsettled weather to continue through the short term. Challenges
deal with shower coverage and severe thunderstorm possibilities.

Currently...Water vapor imagery showing longwave trough across the
northern Rockies to a closed off low in southern California.
Southeast Wyoming remains in southwest upper flow with pieces of
energy being ejected northeast from the southern California low.
One such piece of energy is moving through the area this afternoon
and it has kicked off scattered showers and thunderstorms. Did
increase pops this afternoon in an earlier update.

Latest SPC Mesoanalyst CAPE showing highest CAPE values over
southwest Nebraska this afternoon...with 2500J/KG over Ogallala.
Further west...CAPEs are on the order of 500J/KG. Convection will
ease by early evening with lack of surface heating.

Process to return Thursday with maybe a little more coverage and
intensity. Both the GFS and ECMWF bring some energy into
southeast Wyoming from the southwest. Looks like the severe threat
should be to our southeast where SPC has an Enhanced area
identified.

Friday shows a closed upper low tracking into southwestern
Nebraska from central Colorado. Should see fairly widespread
shower coverage across our southern zones and especially out west
over the mountains. Fortunately...freezing levels will be quite
low...so most of the precip over the mountains will fall as snow.

.LONG TERM...(Saturday through Wednesday)
Issued at 300 PM MDT Wed May 25 2016

The active pattern persists with daily afternoon/evening shower
and thunderstorm chances continuing thru the period. Low pressure
area centered over western Canada/PacNW over the weekend along
with weak afternoon instability (progged sbcapes of 200-500 j/kg)
will produce isolated showers and thunderstorms Saturday and
Sunday. The upstream system will split as it shifts east on
Memorial Day sending shortwave energy southeast into the
intermountain west. Latest forecast information suggests that
Monday will bring better convective organization and coverage to
the CWA. Otherwise, expect temperatures to slowly warm, but even
so just back to seasonal averages. Models diverge towards the
middle/end of next week as the ECMWF ushers in a drier/warmer
pattern, while the GFS remains seasonally unsettled.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 600 PM MDT Wed May 25 2016

Isolated showers will be possible at CYS over the next few hours.
Vfr conditions will prevail through the taf period. Showers and
thunderstorms will be more widespread on Thursday afternoon from
LAR to SNY.

&&

.FIRE WEATHER...
Issued at 300 PM MDT Wed May 25 2016

Cool and moist conditions will preclude fire weather concerns
through the end of the week. Scattered showers and thunderstorms
will provide wetting rains, with snow showers for elevations above
9000 feet.

&&

.HYDROLOGY...
Issued at 300 PM MDT Wed May 25 2016

Cool temperatures will persist through the end of the week with
morning lows in the upper 20s-lower 30s and afternoon highs in the
mid 40s-lower 50s for the mountains. These cool temperatures will
slow the snowmelt runoff into the streams and tributaries of the
Upper North Platte and Laramie rivers. A low pressure system moving
across Colorado Thursday and Friday will produce a good coverage
of showers and thunderstorms, with precipitation amounts from a
quarter to half inch. Locally higher amounts will be possible with
thunderstorms. The higher peaks of the southeast Wyoming mountains
may receive a couple inches of snow. Will be monitoring the impacts
of rain on snow for area rivers, creeks and streams this weekend.

&&

.CYS Watches/Warnings/Advisories...
WY...None.
NE...None.
&&

$$

SHORT TERM...GCC
LONG TERM...CAH
AVIATION...ZF
FIRE WEATHER...GCC
HYDROLOGY...GCC





000
FXUS65 KCYS 251745
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1145 AM MDT WED MAY 25 2016

.UPDATE...
Issued at 1121 AM MDT Wed May 25 2016

Latest radar trends showing isolated showers developing across
southeast Wyoming where we had dry conditions in the forecast.
Went ahead and added 20 percent pops in the forecast for this
late this morning and afternoon. Updates have been sent.|

&&

.SHORT TERM...(Today through Friday)
Issued at 330 AM MDT Wed May 25 2016

Predawn water vapor loop depicted a positively-tilted upper trough
from south central Canada across the northern Rockies to a low off
the central California coast. An active southwest flow aloft was
transporting weak shortwave disturbances northeast across the Four
Corners...central Rockies and plains. A fast 250 mb jet max and
shortwave aided in the development of several convective clusters
over the Great Plains. Converging outflow boundaries from convection
over northeast Colorado generated a narrow line of thunderstorms
over the southern Nebraska Panhandle shortly after midnight. This
activity will soon enter western Nebraska. A quasi-stationary
surface front extended from eastern South Dakota into western
Nebraska then south through the high plains. A moist boundary and
light winds over the Nebraska Panhandle resulted in a growing area
of stratus.

Good run-to-run consistency with the numerical models as the upper
trough to the west slowly translates east across the Rockies and
high plains through Friday. The front draped across the high plains
will also shift east with its moist unstable air mass today. The
next shortwave lifting northeast from the Four Corners combined
with the weak surface trough will aid in the development of widely
scattered afternoon showers and thunderstorms. The HRRR and RAP
models indicate cell motion to the east-southeast. There may be a
few stronger cells producing small hail and gusty winds, especially
east of the Laramie Range. High temperatures today will be near
seasonal normals with 60s to mid 70s valleys and plains, and 40s
and 50s for the high country.

The coverage of showers and thunderstorms will increase Thursday
and Friday as the upper low over southern California tracks east-
northeast across the Four Corners, Colorado and central High Plains.
Model QPF has been consistent showing between a quarter and a half
inch of liquid equivalent precipitation. Locally heavier amounts
will be possible with thunderstorms. Snow levels will drop to
8500 feet Thursday night, with scattered showers producing a few
inches of snow over the higher peaks of the Snowy and Sierra Madre
mountains. The greatest severe potential Thursday and Friday will
be focused over the Great Plains, with a marginal risk for a few
strong to possibly severe thunderstorms south and east of a line
from Chadron to Laramie. High temperatures for the high plains
will cool back into the 60s on Friday, with 40s and 50s for higher
elevations.

.LONG TERM...(Friday night through Tuesday)
Issued at 1124 PM MDT Tue May 24 2016

The main weather story through the long term forecast period will
be near normal temperatures with isolated to scattered showers and
thunderstorms. Generally looking at an upper level trough across
the western conus through early next week. Chances for showers and
thunderstorms will occur nearly each day with the main chances in
the afternoon. Several shortwaves will move across the forecast
area this weekend and into early next week but timing is somewhat
in question. Placement and timing will one of the main drivers as
to how much precipitation falls. Currently the expectation is for
weak upper forcing with any given shortwave and limited moisture
availability. generally not looking for widespread or heavy
precipitation through early next week, of course with thunderstorm
activity a few localized places may see some heavier rains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1142 AM MDT Wed May 25 2016

The main concern to aviation ops this afternoon will be the
isolated shower/thunderstorm activity that is already developing
across southeast WY & west NE. Coverage and intensity will be
limited so sufficient to just include mention of VCTS at most TAF
sites. Will of course monitor these trends and amend if
thunderstorms become more imminent. Thunderstorm activity will
dissipate quickly this evening, giving way to just some passing
mid/high-level clouds overnight.

&&

.FIRE WEATHER...
Issued at 245 AM MDT Wed May 25 2016

Cool and moist conditions will preclude fire weather concerns
through the end of the week. Scattered showers and thunderstorms
will provide wetting rains, with snow showers for elevations above
9000 feet.

&&

.HYDROLOGY...
Issued at 245 AM MDT Wed May 25 2016

Cool temperatures will persist through the end of the week with
morning lows in the upper 20s-lower 30s and afternoon highs in the
mid 40s-lower 50s for the mountains. These cool temperatures will
slow the snowmelt runoff into the streams and tributaries of the
Upper North Platte and Laramie rivers. A low pressure system moving
across Colorado Thursday and Friday will produce a good coverage
of showers and thunderstorms, with precipitation amounts from a
quarter to half inch. Locally higher amounts will be possible with
thunderstorms. The higher peaks of the southeast Wyoming mountains
may receive a couple inches of snow. Will be monitoring the impacts
of rain on snow for area rivers, creeks and streams this weekend.

&&

.CYS Watches/Warnings/Advisories...
WY...None.
NE...None.
&&

$$

UPDATE...GCC
SHORT TERM...MAJ
LONG TERM...SML
AVIATION...CAH
FIRE WEATHER...MAJ
HYDROLOGY...MAJ





000
FXUS65 KCYS 250532
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1132 PM MDT TUE MAY 24 2016

.UPDATE...
Issued at 1006 AM MDT Tue May 24 2016

Ongoing forecast looks to be in good shape. did pull severe
thunderstorm wording back west a little bit more to include
Cheyenne. Current SPC Mesoanalyst page showing a slight cap still
right along the east slopes of the Laramie Range that breaks
around 17Z. Latest EMCWRF shows convection beginning shortly after
that time. Could be looking at severe thunderstorms initiating
around 20-21Z along the I-25 corridor (Chugwater) with storms
congealing into a possible squall line late afternoon into the
central and northern Panhandle towards 00Z.

&&

.SHORT TERM...(Today through Thursday)
Issued at 145 AM MDT Tue May 24 2016

The main concern with this portion of the forecast is where the
convection will initiate today and where the best chance of severe
storms will be.

Today-Tonight:
Latest water vapor loop continues to show 60 to 70 kt of jet energy
moving out of the southwest with broad scale upper level diffluence
developing over the Rockies. It appears the leading edge of the
potential vorticity anomaly (pva) is in Northern Utah. The main
challenge for today will be where this pva ends up this afternoon
and where the convection will initiate. Latest NAM solution is
showing this anomaly affecting the northern Laramie Range early
this afternoon. Interestingly enough the experimental HRRR/local
WRF solutions are showing the initial development of the
convection this afternoon taking place in the northern Laramie
Range as well around 18-19z. If this ends up panning out, then we
may have an early threat for strong to severe storms further north
around Lusk and the Northern Panhandle. However, I am not as
confident that the low level moisture will get that far north with
these storms possibly having higher bases. However, the low level
shear (0-1km) is around 20 to 25kts at Chadron near a boundary
which may favor some brief high based tornados. However, the main
threat still appears to be large hail and damaging wind.

Further south, it is a matter of timing of when the second piece
of pva moves into northern Colorado and southern Wyoming. The
local WRF/HRRR are both showing convection developing by 21z.
These storms will certainly have more moisture to work with which
will tend to favor lower cloud bases especially in the southern
Panhandle. Effective bulk shear values will generally range from 45
to 50 kt, which will also favor rotating updrafts. A couple of
these storms may also have a threat of becoming supercellular with
even a possible brief tornado.

Most of this convection should move northeast at 30 to 35 mph, but
the right moving supercells will generally move east at 30-40 mph.

All of this convection may merge into a broken squall line between
00z-03z, but most of it should exit the forecast area by around
03z.

Tuesday night-Thursday:
We are a bit worried about some gusty winds following the line of
convection this evening. Models are showing good downward vertical
motion which may allow for some good mixing before sunset. Beyond
Tuesday night more upper level energy is progged to dive southeast
from Montana on Wednesday. Ahead of this upper level energy we will
see good downslope flow which will favor rather mild temperatures
before the next wave moves through the Rockies on Thursday and
brings cooler and wetter conditions.

.LONG TERM...(Thursday night through Monday)
Issued at 330 AM MDT Tue May 24 2016

The medium range models have been consistent with the timing and
track of the upper low affecting the central Rockies and plains
Thursday night and Friday. This system will widespread showers and
thunderstorms, with snow showers as snow levels drop to 9000 feet.
Refer to the Hydrology section for details on precipitation amounts.
It will be cooler than normal with highs in the 60s to near 70,
with 40s and 50s for the high country.

Precipitation coverage decreases somewhat this weekend and early
next week as a short wave ridge develops along the Front Range
ahead of the next upper trough moving into the Northwest CONUS.
The best chances for convection will focus over the higher terrain,
with more isolated coverage across the plains. The southwest flow
aloft between a upper ridge over the central CONUS and upper trough
axis to over the Northwest CONUS will allow temperatures to warm
to seasonal normals with highs in the mid 60s-mid 70s, and mid
50s-mid 60s for the high country.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1124 PM MDT Tue May 24 2016

Isolated showers and thunderstorms over southeast Wyoming and the
western Nebraksa Panhandle as of 0530Z should end by 0800Z.
Outside of showers and thunderstorms, light winds and vfr can be
expected through the taf period. Isolated showers and
thunderstorms are expected Wednesday afternoon.

&&

.FIRE WEATHER...
Issued at 145 AM MDT Tue May 24 2016

Fire weather concerns will be minimal during the next several days
due to chances of precipitation through the period and mostly wet
fuels from greenup.

&&

.HYDROLOGY...
Issued at 300 AM MDT Tue May 24 2016

Cool temperatures will persist through the end of the week with
morning lows in the upper 20s-lower 30s and and afternoon highs
in the mid 40s-lower 50s for the mountains. These cool temperatures
should slow the snowmelt into the streams and tributaries of the
Upper North Platte and Laramie rivers. A low pressure system will
be moving across the central Rockies Thursday and Friday, with the
potential for widespread precipitation on the order of a quarter
to half inch. Locally higher amounts will be possible with
thunderstorms. The southeast Wyoming mountains may receive one to
three inches of snow, especially above 9000 feet. Will continue to
monitor the impacts of rain on snow on area rivers, creeks and streams
this weekend.

&&

.CYS Watches/Warnings/Advisories...

WY...None.
NE...None.

&&

$$

UPDATE...GCC
SHORT TERM...REC
LONG TERM...MAJ
AVIATION...SML
FIRE WEATHER...REC
HYDROLOGY...MAJ





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