Area Forecast Discussion
Issued by NWS Riverton, WY

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660
FXUS65 KRIW 171954
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
154 PM MDT Mon Jul 17 2017

.SHORT TERM...(This evening through Thursday) Issued at 142 PM
MDT Mon Jul 17 2017

Imagery shows flat troffing across the wrn half of the CONUS with
Wyoming under modest swrly flow aloft. Embedded sw  moving through
the flow over ern ID...due to arrive across WY this afternoon. SFC
has generally weak pressure fields across the forecast region with
weak lee side troffing into the cntral Basins of WY and another weak
SFC boundary (left over front) across swrn WY. Light precip...with
occasional thunder...falling across portions of swrn WY ATTM.

Rest of today/tonight, weak monsoonal flow continues to rotate
across the FA. this along with some modest upper level forcing from
a weak shortwave moving through the flow over WY this afternoon will
spark isolated to widely scattered showers/storms once again.
Increased cloud cover over srn to portions of cntrl WY should keep
temperatures and CAPE values from rising too high across that
region. A few stronger storms of the day will generally be across
nrn WY...especially ne Wyoming where ML CAPE will range from 750 to
1000 J/Kg late this afternoon and early evening and effective shear
maximizes at above 30 kts as a connectively reinforced frontal
boundary bursts through the area. Low level convergence along the
srn/cntrl WY boundary (left over front) will also allow some
sustained convective showers/storms to persist into the evening
from Sweetwater County...through ern Fremont County and across
Natrona County. Storm movement will generally be to the east or
northeast at 20 to 35 mph with a few stronger showers/storms
producing gusty outflow winds 30 to 50 mph. Very small hail also
possible. Showers/storms will mostly have left the area by around
midnight.

Tuesday, upper level flow becomes nearly zonal over WY while the
atmosphere dries out across the FA as the monsoon gets held back
over the srn/cntrl Rockies and Desert Southwest. A little moisture
may be left across the ern CWA where a few chances for an isolated
afternoon/evening storm may be possible...with the best chance for
convection over Johnson County in the evening.

Wednesday and Thursday, slightly better but modest ridging returns
to the region with monsoonal flow looking for another surge nwd
through the srn cntrl Rockies and into portions of srn WY Wednesday
afternoon and evening. A seasonally strong lee side low is also
scheduled to form vicinity/sern WY/nern CO by wed night which may also aid
in precip production across srn and scntrl WY including ern
Sweetwater and srn Natrona Counties as a very complex flow pattern
evolves under the area of weak flow aloft. Very heavy orographically
forced and possible MCV induced rainfall expected across the
mountains of NM and CO will limit the amount of moisture that is
able to get into this FA...so hard to pinpoint possible localized
heavy rainfall in this FA ATTM. Thursday, overnight activity exits
into the Plains along with the stronger SFC/LL forcing while the
monsoon will begin to pump a little better moisture into the
FA...especially into/through swrn/scntrl WY. (Very) Heavy rain with
slow shower/storm movement will be possible with what looks like
possible localized flooding situation over the Wyoming/Salt
Ranges...and even over nearby lower elevations over the desert and
Basins of southern WY...late Thursday afternoon (and into the
evening). This will tend to be a (regionally) high CAPE/low(er)
shear day. Small hail possible...and maybe a wet microburst...not
sure about large hail ATTM. All this a portent of coming attractions
over the next month or so as the monsoon settles in.

.LONG TERM...(Thursday night through Monday) Issued at 142 PM MDT
Mon Jul 17 2017

Synopsis...Upper level ridge flattens for a zonal flow pattern at
the beginning of the period. The ridge reestablishes itself through
the weekend and into early next week. A cold front will move through
the state on Friday, slightly cooling temperatures over the weekend.
Drier air will also proceed the cold front, shunting moisture well
to the south. Some return flow could develop over eastern portions
by Monday and Tuesday.

Discussion...Summer-time weather pattern will temporarily be halted,
due to the aforementioned cold front. However, warm and hot daytime
temperatures will persist with mild temperatures overnight. Model
differences begin around 00Z Sunday, with timing the return of the
upper level ridge. Sided with the ECMWF, as the previous shift kept
precip chances confined over Johnson/Natrona counties. The GFS is
slower with the supporting shortwave trough and thus, slower
building the ridge back by early next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued
at 142 PM MDT Mon Jul 17 2017

VFR conditions to prevail through the period. Showers and
thunderstorms continue to develop this afternoon, with coverage
being more widespread than Saturday. Outflow wind gusts of 30 to 40
knots will continue to be the main threat, along with locally heavy
rain possible with some storms. Local MVFR visibilities can be
expected in these areas of heavy rain. Storms will transition to
showers after 03Z and remain widespread across northern and central
portions through 08Z Monday. Isolated showers will be possible near
KRKS-KBPI-KPNA during this time. Mountains may be obscured at times,
especially from 06Z-18Z Monday west of the Divide, as additional
clouds and possible showers move in during this time.

&&

.FIRE WEATHER...
Issued AT 142 PM MDT Mon Jul 17 2017

Fuels are now at critical levels for most forecast zones east of the
Divide (Please see the Fire Weather Forecast product for more
information). Today through Thursday, fire danger low to moderate
for most mountain locations...elevated across Yellowstone due to
low RH values. Elevated for all other lower elevations due to very
warm to hot daytime temperatures and low humidity levels. Winds,
however, will continue relatively light across the entire forecast
area today through Thursday with typically gustier afternoon winds
found west of the Divide. Exceptions will occur near any
shower/storm today where winds could briefly gust 25 to 40 mph or
more...especially over the northern portion of the forecast area.
Isolated to widely scattered showers/storms today around the entire
area...strongest over Johnson County this evening. A few isolated
storms possible over Johnson and Natrona counties on Tuesday.
Isolated to widely scattered showers/storms over southern Wyoming on
Wednesday followed by possible localized heavy rain across
southwestern Wyoming on Thursday.

&&

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

$$

SHORT TERM...Braun
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Braun



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