Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160806
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
206 AM MDT Fri Jun 16 2017

.SHORT TERM...Friday through Sunday night

All is quiet right now. However, things will turn somewhat more
active as a cold front drops down across the area from the north and
west. This will result in some showers and isolated thunderstorms to
fire across the northern two thirds of the area this afternoon and
tonight. As for any stronger storms, things are not overly
impressive. CAPE is rather limited and lifted indices do not drop
below minus 2. In addition, the best upper level dynamics are not in
the right spot. The area is in the right front quad of the jet,
which usually produced downward motion. However, it does promote
windy conditions. And with a tightening pressure gradient and 700
millibar winds rising to around 40 knots, it will become breezy
to windy across much of the area this afternoon and evening. At
this point, we look to fall short of high wind criteria but it
will definitely be a toupee alert day for many areas. And with the
strong winds at 700 millibars, we kept gusty winds in
thunderstorm criteria.

The models have come into decent agreement in placing a decent
amount of QPF across portions of the west this evening. As a result,
some thunderstorms could have some heavy rain. Precipitable waters
do rise to around 0.8 inches as well. However, we held off on any
flash flood watches for now. The soil is not as saturated as earlier
this spring. In addition, with vegetation now growing and the ground
unfrozen, the ground should be able to absorb some of the water. So,
although there could be some localized flooding, it does not look
widespread enough at this time for a watch. The chance of showers
and thunderstorms will also continue across areas East of the Divide
as well through the night.

We did make some changes for Saturday however. The models have
trended with less coverage of showers as ridging builds across the
area and moisture drops. The NAM has all precipitation out of the
area by Saturday afternoon. The GFS does linger a few showers
however. For now, we half stepped and removed POPS from many areas
and kept them in the isolated category for all areas. Expect high
temperatures to average 5 to 10 degrees cooler then normal for
Saturday. Sunday at this point looks mainly dry with near normal
temperatures with dry northwesterly flow dominating the state.

.LONG TERM...Monday Through Friday

Desert Southwest ridge expands northward Monday into Tuesday for a
brief preview of summer before weakening and allowing the northern
stream to take over once again for the second half of the week.
Temperatures peak Tuesday although Wednesday will be close behind,
especially in the south. Cooling off Thursday into Friday (to
near normal for most areas or possibly cooler in the north) as a
potentially significant shortwave passes just to the north. The
GFS is most bullish attm (further south) than the GEM and Euro.
GFS 5-wave transitions from a ridge over the west to a trough
centered over the central part of the country. This pattern would
favor a stronger system in the nw flow like the OP GFS but we`ll
see. Will lean that way for now and let the next several shifts
look at this some more over time. Backing up to early in the week,
we have a dry day Monday with warming mid level temps and
heights. On Tuesday, we`re still quite warm in the mid levels but
models are starting to show some weak disturbances riding the nrn
periphery of the low. Weak instability Tuesday afternoon over the
mountains will aid in isold gusty storms with a couple trying to
move off the mountains/foothills late in the day or evening but
overall still dry across the lower elevations and even most
mountain locations. More ripples comes across Tuesday night and
Wednesday for more late day convection mainly around the mountains
and a few near the foothills by evening. Could see our morning
accas Wednesday morning with virga around depending on where these
little disturbances are at the time. There is a weak front that
may sneak into the far north Wednesday behind a srn Canadian
disturbance that could provide a little upslope and forcing.
Increasing wind across the mountains, wind corridor and south
ahead of our stronger shortwave with a better chance of
showers/storms in at least the north and turning cooler Thursday
into Friday. Much more if the stronger GFS is right with 0C to -2C
readings briefly Friday morning. Although even the Euro has H7
temps down to around 0C Friday morning briefly. So a sneak peak at
some fairly warm/hot temps for a day or two early next week
before we cool back off significantly by Friday as much as 10 to
20 degrees. Most in the north.

&&

.AVIATION...12Z Issuance

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

VFR conditions will prevail across the area through 18z Friday.
Isolated to scattered showers will be found mainly over Yellowstone
National Park.  A cold front will then spread widespread rain
showers and mountain obscurations south to vicinity KJAC by 20z
Friday with westerly flow aloft strengthening and areas of LLWS
developing.  MVFR conditions should prevail KJAC from around 20z
Friday and overnight.  These rain showers are not expected to make
it much further south than KBPI-KPNA Friday night.  Surface winds
will gradually diminish Friday evening with some areas of LLWS
remaining through early Saturday morning.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail through Friday afternoon.  Surface west
winds will increase ahead of a cold front that will push into the
north, vicinity KCOD-KBYG by 00z Saturday.  Scattered to numerous
rain showers will develop behind this cold front as it pushes
further south Friday evening.  Areas of lower MVFR ceilings are
expected to develop after 03z Saturday behind the cold front.  The
front will push south to a vicinity KRIW-KCPR line by 06z Saturday
and further south to near Interstate 80 by 12z Saturday.  Lower
MVFR, local IFR ceilings will be most probable along the Interstate
25 corridor, KBYG-KCPR and vicinity KCOD Friday night.
&&

.FIRE WEATHER...

A cold front will drop across the state today and bring scattered
showers and isolated thunderstorms, mainly in the afternoon and with
the best coverage across areas West of the Divide. Any thunderstorm
could have gusty, erratic winds. In addition, a strong west to
southwest wind will develop across much of the area this afternoon
and evening. Relative humidity may fall to around 20 percent in some
areas but should remain above critical levels. Mixing and smoke
dispersal will be generally good to excellent. Cooler weather with
less wind will move in for Saturday.

&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...Meunier
FIRE WEATHER...Hattings



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