Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200836
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
236 AM MDT Sun Aug 20 2017

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

..Eclipse Forecast For Portions Of Central Wyoming Currently
Looking A Bit Better...

Imagery continues to show trof/ridge/trof pattern across the CONUS
with split flow to the west of WY...around the main center of the
upper ridge and from the northern branch of the flow associated with
the wrn CONUS trof. This results in a difficult cloud forecast for
some of the eclipse path with small deviations north or south of the
upstream split causing downstream (over WY) deviations in possible
cloud location. The broad center of the ridge/upper high is located
over TX with the upstream trof axis down through nwrn MT/cntrl ID
and through California where a slowly developing upper low
circulation is beginning. This area between the high and low will
modestly (re)direct mid level moisture into swrn WY today. A weak
upper vort max is also current circulating over nern NV/nwrn UT this
morning. The SFC has an elongated trof of low pressure and a cold
front lying nearly under the weak upper ridge down through the
nrn/cntrl Plains. A portion of the cold front is now draped through
nrn/nern WY with areas of smoke following the front into this
region...particularly across Johnson and nrn Natrona Counties.
Smoke, however, is also making its way into wrn and cntrl WY from
fires in California/Oregon and srn ID. A couple of light showers
currently present across far northern and northeastern Wyoming.

The northern portion of the split flow pattern has currently
backed off and receded slightly more northward over the last 24
hrs. This may allow for even less morning cloud over portions of
central WY within the eclipse path.

Today, slightly better moisture feed into WY from the southwest from
modest mid level moisture circulating between the upper high and
developing low - along with the approach of the upstream trof
(mentioned above) will increase upper/mid level clouds across swrn
WY early this morning...with some decrease in this cloud through the
rest of the morning hours and into the afternoon. Meanwhile,
northern WY will see another upper level induced front push
into/through nrn WY and into cntrl WY through the afternoon...headed
south and toward somewhat of an inverted trof over srn WY. By this
evening, the boundary between these two air-masses will be a minor
focal point for some light isolated convective activity across
swrn/srn WY as the upstream vort max rolls overhead. Additionally,
some serly flow into and through sern/ern WY around a SFC low ovr
nern CO will also produce some mid/upper level cloud across ern
WY...as far west as portions of ern Johnson and ern Natrona Counties
late tonight into Monday morning. Bottom line - improving cloud fcst
along the eclipse path with what looks like only a very few
mid/upper clouds over western and most of central WY to start Monday
morning...perhaps partly cloudy after sun-up over Natrona
County...but clearing through the morning hours. More mid/upper
cloud will be present on either side of the path (to the north and
south of totality)...over northern and southern WY. However, even
over those locations, cloud cover should not be continuous or
opaque. There is a side note to all this that has to do with smoke
from wild fires across the west, including California, Oregon, Idaho
and Montana. As the area saw today, weak surface frontal/trof
passages can increase smoke coverage from Montana fires to the
north. The forecast area will see a frontal push later today/tonight
into portions of northern/central WY...followed by another weak
front Monday morning. This may cause visibility to decrease, keeping
the sky on the hazy side. Additionally, with the flow aloft generally
running west to east over the next couple of days, smoke from fires
further to the west may also get into the region. This will not
keep the eclipse from being viewed, but may limit how it/what can
be seen around the eclipse itself. On the other hand, the colors
associated with this kind of filtering could be quite dramatic.

Tuesday, post trof passage with nw flow aloft transitioning to upper
ridging and drying once again, expect most clear/sunny (smokey?)
skies with the only moisture/cloud intrusion into the FA coming
into/near swrn/srn WY. However, there are some hints that weak
monsoonal moisture will make it back into Wyoming by mid-week.


.LONG TERM...(Wednesday through Sunday)

Overview...Upper level disturbances in southwest flow will bring
slight chances of showers and thunderstorms Wednesday and Thursday
along with above normal temperatures. A cold front will move
across the area Friday resulting in continued chances of showers
and thunderstorms, breezy conditions and slightly cooler
temperatures. A ridge of high pressure will build in next weekend
for mainly dry weather with near or slightly above normal
temperatures.

Discussion...Upper pattern at the outset on Wednesday features
ridge across central Canada flanked by upper lows over Quebec and
off the coast of British Columbia. Weak upper low will be near Los
Angeles. ECMWF and GFS in good overall agreement with the
progression of this pattern through next weekend. Upper low of the
B.C. coast is expected to bottom out near Vancouver Thursday
morning before lifting NE into downstream ridge. This system will
kick out the weak upper low off near L.A., shearing it out across
the Four Corners region Thursday/Thursday night. This latter
system is not expected to affect Wyoming; however, numerous other
ripples in the flow are expected to lift NE from the Great Basin
across Wyoming Wednesday and Thursday. Chances of afternoon
thunderstorms confined to mostly west of the Divide on Wednesday,
spreading across north central area on Thursday. Upper low in
northern stream is forecast to lift NE across Alberta/Saskatchewan
on Friday, dragging a cold front across Wyoming. Best moisture and
instability is expected near the MT/WY border and across eastern
Wyoming ahead of the cold front, where environment may be
favorable for a few stronger - marginally severe storms. All
medium=range models in good agreement that an upper ridge will re-
establish across the Rockies next weekend.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)

Terminals to remain VFR through 12Z/Mon. Wildfire smoke has been
filtering into the state from the north and west since Saturday.
Heaviest smoke will be across far northern Wyoming and KBYG, but
northerly surface flow has brought low end VFR visibility of 6SM FU
to KCPR and KCOD. Expect the visibility to improve mid-morning to
late morning Sunday at these two terminals. There could be
occasional obscurations of the Bighorn and eastern Absaroka ranges.
Additional smoke has been wafting into western Wyoming. This smoke
may begin to settle in some of the western valleys this morning. As
for clouds and convection, mid-level moisture will continue to work
slowly north into the southwest quadrant of Wyoming this morning.
This moisture along with CAPE 300-500J/kg will help set the stage
for some late day convection Sunday across southwest Wyoming. Wind
shear is not impressive, so don`t anticipate convection to be too
strong. However, with temperature-dew point spreads of 40-50F in
this region outflow wind of 45kts will be the likely hazard. This
may even be possible from weak convection. Best chance for
convection appears to be from 22Z/Sun to 04Z/Mon. Clouds will
gradually dissipate after this time with mainly scattered cirrus
clouds remaining by 12Z/Mon. Gusty westerly wind 10-20kts expected
at the southwest terminals along with KRIW. KCPR looks to stay
around 10-12kts through the day. The winds will be diurnally driven,
so expect decreasing speeds around 01Z/Mon.

&&

.FIRE WEATHER...

Fuels at critical levels for most of the low elevation forecast
zones in addition to a few mountain zones above 8000 feet across
western Wyoming. (Please see the Fire Weather Forecast product for
more information). Elevated fire weather concerns continue for the
entire forecast area today due to low RH values expected by this
afternoon along with wind gusts between 15 and 25 mph at times along
and west of the Continental Divide. Very dry conditions continue
with minimum RH values dropping into the low to mid teens across the
lower elevations, upper teens to lower 20s in the foothills, and 20
to 35 percent in the mountains. Monday will see some improvement
with slightly cooler conditions, slightly higher RH values and a
little less wind in the afternoon.

&&

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

$$

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


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