Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210848

Area Forecast Discussion
National Weather Service Riverton WY
248 AM MDT Mon Aug 21 2017

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

...Final Eclipse Forecast for Portions of Western and Central

Imagery shows general broad longwave trof across the northern half
of the CONUS with part of the Bermuda High sticking into and across
the southeastern/south central USA,  a slowly developing near cut-
off low over southern California, and a weak trof/axis from nwrn MT
into cntrl OR. This pattern continues to produce a tricky split flow
situation which under normal conditions would barely make a
difference to anyone. However, with the Total Eclipse on the way
today, clouds of any sort do make a difference. As such, an
embedded modest (50-70 kt) jet is wandering eastward from Oregon
through eastern Montana this morning producing areas of mainly
cirrus (various) under this same region. The SFC has a broad area
of low pressure over the sw quad of the CONUS with high pressure
across the PAC NW building in behind a weak front that extends
from srn OR to ecntrl WY. Current light precipitation is now all
east of the FA and over far ern WY.

The forecast area continues to be located/bisected under the the
split flow pattern interface of the northern and southern branches
of flow - the southern branch essentially being directed
around/between the upper high and upper low across the south
central/southwestern CONUS - and the northern branch associated with
the embedded jet across the northern tier of states. Additionally,
the northern portion of the split flow pattern has now pushed back
to the south a bit from where it was 24 hrs ago. The jet streak
embedded in the northern branches flow is currently producing upper
level clouds across much of the southern half of Montana, central
Idaho, and eastern Oregon. As the upstream trof over the PAC NW
continues to head toward Wyoming and the jet moves east southeast,
the area of clouds will sink somewhat southward with the northern
branch of the split, allowing for an intrusion of some morning cloud
cover across portions of northern, western, and central Wyoming. The
good thing is, however, that as the upper jet pulls away to the east
through the day, the area of associated cloud will essentially be
stretching and thinning through the morning and afternoon. So, this
morning near the beginning of the eclipse period, some upper level
cloud will lay from west central Wyoming to and through northeast
Wyoming...moving and thinning to the southeast through the rest of
the eclipse period. The progression of the skies (clouds) across the
eclipse path of totality will be as follows (as best as can be
figured at the moment): Some high cloud to start the morning off
across west central Wyoming...including Jackson...with generally
clearing skies through the rest of the eclipse period. After a
mostly clear early start to the day, there will be some increase in
high cloud cover across portions of central Wyoming including the
Wind River Basin and Mountains with some slowly improving conditions
from west to east after noon. Even with the possible cloud cover,
skies/clouds should not be opaque to the eclipse, with the event
still being seen, although perhaps filtered to some degree. Natrona
county, including Casper, will lie within the other good area to
view the eclipse as it will likely be mostly clear and sunny to
begin the day with high clouds not making it into the County until
after totality, through the afternoon. There is also one additional
caveat to this astronomical event - smoke cover. The forecast area
will see another frontal push through the area later this morning
perhaps bringing in more wildfire smoke and causing or continuing
some visibility decrease (keeping the sky a bit hazy side even
without the clouds). Again, none of these factors will keep the
eclipse from being viewed - but may somewhat limit how it/what can
be seen around the eclipse itself...especially near/at totality. 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
swrn/srn WY with low POP chances across this area in the
afternoon/evening. Wednesday will have the ridge axis directly over
WY with some weak monsoonal flow rotating into and across WY from
the south. Lee side troffing and the passage of a weak upper level
disturbance will allow for isolated (low elevations and foothills)
to widely scattered (mountains) showers/thunder to blossom in the
afternoon and evening across much of the FA except for the Big Horn
Basin. Brief heavy rain, small hail and gusty winds will all be what`s new.

.LONG TERM...(Thursday through Monday)

Overview...Isolated to scattered thunderstorms will develop
Thursday afternoon and evening in moist and slightly unstable
southwest flow. A cold front will move through the area Friday
bringing isolated to scattered thunderstorms mainly across the
north and west. Upper level high pressure will then strengthen
across the Great Basin over the weekend into early next week,
resulting in dry weather and above normal temperatures Sunday and

Discussion...Upper flow on Thursday morning features ridge over
central Canada flanked by upper lows over eastern Canada and near
Vancouver, B.C.. Surface high pressure will be centered over
western Ontario with return E-SE flow east of a lee side surface
trough across eastern Wyoming, with cold front progressing
eastward across the NW U.S. Upper low near Vancouver is expected
to continue to kick out some ripples from the Great Basin across
Wyoming during the day Thursday, with both the GFS and ECMWF
lifting most of this energy into the high plains Thursday night.
The entire area will stand at least a slight chance of a
thunderstorm Thursday afternoon with the better instability
expected across the north and east. Upper low near Vancouver is
expected to lift across central Canada on Friday and drag a cold
front across Wyoming. Gusty west winds ahead of the front will
likely elevate the fire danger across much of southern and central
Wyoming. A few strong storms will be possible along the front
across mainly Johnson County.

ECMWF is a bit quicker to build up ridge across the Great Basin
Saturday with the GFS keeping a disturbed NW flow across the area.
Forecast gives the ECMWF more weight here, which also aligns
better with the ensemble means. Closed upper high is expected to
strengthen over the Great Basin Sunday and Monday with NW flow
aloft across Wyoming, mean surface high pressure over the High
Plains - a dry and warm pattern with generally light wind.


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

VFR across the forecast area through Monday night. Despite weak
northerly flow across northern and central Wyoming, smoke has not
been as prevalent as Saturday night. Therefore, don`t expect any
reductions in visibility Monday morning due to smoke. Cirrus clouds
continue to filter into Wyoming from the north. Trend continues to
be for these clouds to thin with warming cloud tops seen in IR
imagery. Still looks like the tail end of these clouds will lay west-
to-east across central Wyoming by late Monday morning, but that this
cloud cover will be thin and scattered. Diurnally driven wind
west-northwest wind between 20Z/Mon to 01Z/Tue at KBPI, KPNA, and
KRKS with speeds mainly 10-12kts. North-to-northeast surface winds of
7-10kts will be found east of the Continental Divide. Very limited
chance for convection only exists in far southwest Sweetwater County
near the Uinta Mountains late Monday afternoon.


Issued AT 140 AM MDT Mon Aug 21 2017

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
much of the forecast area today, mainly due to low RH values
expected by this afternoon along with occasional diurnal mixed out
wind gusts between 15 and 25 mph at times along and west of the
Continental Divide. Dry conditions continue with minimum RH values
dropping into the mid teens across the lower elevations, lower to
mid 20s in the foothills, and 25 to 35 percent in the mountains.
Tuesday, slightly warmer and drier but overall very similar
conditions to those of today with only slight chances for an
isolated shower/storm across southern WY.




LONG TERM...Meunier
FIRE WEATHER...Braun is the U.S. government's official web portal to all federal, state and local government web resources and services.