Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 271737
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1135 AM MDT Tue Jun 27 2017

.SHORT TERM...(Today through Thursday night)

Imagery shows ridge/trof pattern continuing from west to east across
the CONUS with ridge axis passing into the High/Cntrl Plains and an
upstream embedded SW trof now moving through OR/WA and headed toward
WY. As the ridge breaks down and is pushed out of the way, the storm
pattern will morph into a period of seasonally strong upper lvl
northwesterly flow aloft (generally not a good pattern for nice
weather in WY). SFC has high P breaking down as a lee trof forms
over nrn/cntrl WY. A few showers/storms moving across the far wrn
border with ID.

Today and tonight, the upper ridge continues to translate eastward
into the Central Plains with lower P over wrn WY redirecting SFC
flow into the FA from the e and se. Deeper moisture return now on
its way into nwrn WY as the ridge axis passes through the ern CWA
and into the cntrl High Plains while flow begins to flatten ahead
of/with the approach of the embedded SW trof/jet streak.
Additionally, lee side low P develops across the SFC over cntrl WY
while low to mid lvl moisture continues to slowly fill ern WY from
the se. Through this morning, the embedded upper level SW will ride
through the flow w of the ridge axis, across nwrn WY along with a
SFC front. Scattered showers and thunderstorms will quickly move
into/through the Yellowstone area (more isolated to the south) at 30
to 40 kts with relatively light rainfall...small hail and very gusty
winds that could border on severe at times from mid-day on. With
seasonally warm/hot temperatures continuing early on, dpt
depressions will be in the 50 to 60 deg range with DCAPE running
1000-1500 in the early afternoon across nrn WY...supporting some
strong to severe wind gusts from the convection...particularly
across the Big Horn Basin and adjacent foothills/mountains. Later
this afternoon and through the evening period, increasing moisture
from the west along with decreasing post frontal temps over nrn WY
will increase RH values and lower LCLs. Heavy rain and hail will
then become more likely as evening nears while MLCAPE ranges from
800 to 1200 J/KG. Somewhat less wind threat can then be expected
over Yellowstone...then eastward across the Big Horn Basin through
the late afternoon/evening period. Heaviest rain of the day/night,
with a few possible severe storms, may occur this evening across nrn
WY...with the storm environment over Johnson County providing better
conditions conducive to marginally severe hail and strong (wet)
downburst winds. Possible moderate to heavy rain to continue with
the passage of the upper SW trof through at least midnight across
nern WY. Otherwise a few terrain induced showers will persist
overnight across the Yellowstone area.

Wednesday, post upper trof/sfc frontal early with fast moving nearly
zonal flow aloft and a seasonally strong e/w jet across srn WY. As
lowering pressure to the south and mesoscale high P over the nrn FA
forms through the morning/afternoon, LL flow will turn back out of
the s to sw. Brisk flow and cooling aloft along with with an
approaching embedded SW/jet streak will keep decent instability,
forcing and dynamics over the nrn half of WY. With some low to mid
level moisture still retained across nrn WY from the previous
day`s/night`s precipitation, showers and thunderstorms will have
another chance for initiation, by mid-day, for a few fast moving
wind producers across the Big Horn and possibly the Wind River Basin
(later after a front passes through. Possible isolated strong/severe
from Big Horn County through Johnson County. Showers and isolated
thunder continue through the overnight period until the SW trof axis
moves east of the FA near sunrise Thursday morning.

Thursday, will be post frontal and under good nw flow aloft with yet
another SW trof on its way into/across the FA from the north by mid-
day. Frontogenesis will be ongoing across MT early before crossing
through WY late in the afternoon. Cooler conditions from top to
bottom will limit CAPE and overall instability, however shear along
and ahead of the front will be high. Isolated to widely scattered
storms can be expect to move quickly across WY...pushing strong wind
gusts out ahead of the thunderstorms along with brief heavy rain and
hail within the storms. Strongest storms of the day look to be across
Fremont/Natrona and perhaps srn Johnson county. Again, with the
upper SW trof still moving through the FA Thursday night along with
cooling temps aloft, showers/isolated storms will continue through
the overnight period.

.LONG TERM...(Friday through Independence Day)

Overview...Cool conditions with slight chances of showers and
thunderstorms will linger across the north and central on Friday,
while dry and warmer weather prevails across the southwest and far
west.  Warmer conditions, slight chances of showers and
thunderstorms across the northern third of Wyoming will prevail over
the weekend.  Mainly dry conditions with above normal temperatures
(highs in the 90s in the lower central basins) will prevail Monday
and Independence Day.

Discussion...Friday morning begins with upper trough over the
northern and central plains, upper ridge along roughly 120W, NW flow
across Wyoming.  Surface high pressure over the northern High Plains
will keep stationary frontal boundary banked up against the east
slopes of the Continental Divide, with upslope N-NE winds.  This
will result in some afternoon thunderstorms developing over or near
the high terrain with SE storm motion.  Flow over the west and
central U.S. deamplifies over the weekend with both GFS and Ecmwf
showing a Pacific shortwave in westerly flow translating the area
Saturday night-Sunday morning, the GFS faster than the Ecmwf. Higher
CAPES, and more organized, stronger storms are expected to remain
east of the area on both Saturday and Sunday, with mainly some high-
based gusty wind producers expected over northern Wyoming mainly
late Saturday and Saturday night.  Upper ridge translates east
across area on Monday for dry weather, with upper flow backing on
Independence Day as ridge strengthens over the southern/central
Plains and upper trough deepens across western Canada.  Both GFS and
Ecmwf show 1000-500mb thicknesses increasing to 586 dam Tuesday.
Forecast brings temperatures closer to MEX and ECE MOS numbers
during this period with the model blend output still a bit too cool.
The MOS numbers put the lower basin highs in the mid 90s both
Monday and Independence Day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)

A low pressure system is currently moving through southern MT and
northern WY this afternoon.  An initial line of weak showers and
isolated thunderstorms with some very gusty W to NW wind, currently
from about KBYG SW to KRKS, is moving eastward through the area.
Stronger thunderstorms have now entered NW WY associated with the
colder core of the system, and they are moving eastward as well.
These storms are producing heavy rain, some small hail, frequent
lightning, and outflow wind over 40 kts.  Expect the higher terrain
to be obscured at times with the lowered ceilings and precipitation.
 As the afternoon continues, the storms are expected to strengthen
across the northern half of the area, with the primary threat being
moderate size hail around 1 inch and wind gusts potentially over 50
kts.  Breezy to windy conditions can be expected this afternoon and
evening, mainly from the west, then switching to the north around
00Z at some locations due to outflow from storms and the progression
of the overall system.  Expect brief MVFR/IFR conditions at many
terminals due to the storminess.  VFR conditions then expected after
03Z except for MVFR with lingering showers.  Another disturbance
will impact the area Wednesday afternoon and evening.

&&

.FIRE WEATHER...
Issued AT 104 AM MDT Tue Jun 27 2017

All fuels are currently in green-up and below critical levels for
all locations. Fire danger remains low across all mountain
areas...but will continue somewhat elevated in the afternoons across
the lower elevations east of the Divide and over portions of
southern WY. These areas with elevated fire concerns will generally
coincide with low afternoon RH values and/or gusty winds...today
through Thursday. Winds will become west to northwest 15 to 30 mph
today...west to northwest 10 to 25 on Wednesday...and north to
northwest 5 to 20 on Thursday...all days with higher gusts in the
afternoons and especially near stronger showers/storms...especially
today. Showers and isolated thunderstorms move into/through the area
early this morning and continue across the area through tonight.
These isolated to scattered storms will be quickly moving across the
northern half of Wyoming from west to east, producing little
rainfall through the mid-day hours with strong gusty outflow winds
of 40 to 60 mph possible at times. Then, by late this afternoon and
through the evening, potential brief heavy rainfall and large hail
will be possible...especially across portions of north central and
northeastern Wyoming. This unstable pattern will continue through
Thursday night. Smoke dispersion will be good to excellent each
afternoon and evening.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...McDonald
FIRE WEATHER...Braun



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