Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 242016
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
216 PM MDT Mon Sep 24 2018

.SHORT TERM...This evening through Thursday

Overview...A reinforcing trough from Canada will quickly follow on
the heels of Sunday`s cold front today and impact the area with
another cold front this afternoon through the overnight hours.
Isolated to scattered showers will occur along and behind this front
across areas east of the Divide, including Yellowstone, late this
afternoon through the overnight hours. A few inches of snow will
also occur in the northern mountains tonight, especially in the
Bighorns. The rest of the period will be the tale of two divides,
with warm, dry conditions with gusty winds west of the Divide and
cool and dry conditions east of the Divide.

Synopsis...Red Flag Warnings will continue this afternoon across
southern and southwestern zones, with west to northwest winds with
gusts of 30 to 40 mph. RH values will also remain critically low
over these areas. Otherwise, dry conditions continue. Elsewhere,
showers remain very isolated across the north early this afternoon
with increasing chances through the rest of the afternoon. As
mentioned above, these showers will occur for areas east of the
Divide, including Yellowstone. Rain will also quickly change to snow
in the higher elevations, as snow levels range between 7000 and 8000
feet. These showers will end from northwest to southeast tonight,
with a few lingering showers over the north Tuesday morning.
A Freeze Warning will be in effect early Tuesday morning for the
Wind River Basin and Sweetwater County, as skies clear behind the
showers. A Freeze Watch is also now in effect for Natrona County
for Wednesday morning. Otherwise, dry conditions are expected
across the forecast area Tuesday through Thursday. Fire weather
conditions will be elevated west of the Divide Tuesday, including
Teton County, however west- northwest winds will thankfully be
much lighter with isolated gusts up to 20 mph. Unfortunately, Red
Flag conditions will return by Wednesday with low RH values
continuing and west- northwest winds with gusts of 30 to 35 mph
returning. These conditions will continue for these areas on
Thursday.

.LONG TERM...Thursday night through Monday night

Persistent upper level northwest flow will trend more cyclonic to
begin the forecast period Thursday night. The upper flow will
continue to back to a more westerly direction Friday with the
250mb jet becoming oriented west-to-east near our northern border.
Shortwaves and favorable jet position in this flow pattern will
increase precipitation chances across the north through Friday.
Temperatures will be well below normal Friday across at least the
northern-third of the forecast area. The southwest will remain
warm and dry with breezy conditions Friday, which could lead to
another elevated to critical weather day across the southwest
zones. The upper flow will back more to the southwest Saturday as
a trough moves onshore from the Pacific. Warmer mid-level
temperatures will return Saturday to the central and continue
across the south. The north is likely to remain mired in the
cooler air although temperatures will moderate. This increasing
southwest flow will draw moisture north from the Desert Southwest.
Precipitation chances improve late Saturday through Sunday as the
Pacific trough approaches and the moisture from the south spreads
across the region. Have increased precipitation chances across
northwest Wyoming, including Yellowstone National Park, Saturday
night and Sunday. Confidence is not high for any kind of
widespread precipitation across the central and south.
Temperatures are likely to be near or slightly above normal for
the central and south Sunday ahead of the trough. Medium-range
forecast models diverge more as we head into next week. The
overall pattern may end up being more progressive and unsettled
with the coolest air in the north, the warmest in the south.

&&

.AVIATION...For the 00Z TAFS through 00Z Wednesday

West of the Continental Divide...(KJAC, KRKS, KBPI, KPNA)

Conditions will be VFR through Tuesday with the exception of
occasional MVFR visibility at KPNA due to wildfire smoke. The
gusty west-northwest wind will bring smoke from the Roosevelt
Fire over KPNA Monday afternoon and evening. The gusty west to
northwest surface wind of 18-28kts will diminish at KBPI, KPNA,
and KRKS between 00Z-02Z/Tuesday. Wind speeds will be 12-20kts at
KJAC before decreasing shortly after 00Z/Tuesday. Early evening
mountain showers will end by 06Z/Tuesday with a clearing sky at
all terminals after that time. Westerly wind will be lower Tuesday
afternoon with mainly 10-12kt speeds anticipated.

East of the Continental Divide...(KCPR, KCOD, KRIW, KLND, KWRL)

Scattered showers associated with a shortwave and a southward moving
cold front will persist this evening. The showers will spread from
the Big Horn Basin Monday afternoon toward the southeast Monday
evening. These showers could have an embedded thunderstorm or two.
Conditions will be primarily VFR although low end VFR or MVFR
ceilings are possible at KCOD, KWRL, and KCPR through 06Z/Tuesday as
the showers pass. A cold front will continue a southward push late
Monday afternoon and evening. This front will produce northerly wind
12-22kts for a few hours until about 02Z-04Z/Tuesday. All terminals
will be VFR by 12Z/Tuesday, although patchy early morning fog cannot
be ruled out. This fog would depend on if there is late evening
rain, enough clearing, and if the wind dies off. KCPR seems the best
possibility if fog was to form. There could be a few lingering
showers across the Big Horn Basin Tuesday afternoon. Otherwise, less
wind and VFR conditions will be the rule Tuesday.

&&

.FIRE WEATHER...

...RED FLAG WARNINGS IN EFFECT THROUGH 8PM MONDAY EVENING FOR
ZONES 277, 278, 279, 289, 414 AND 416...

Isolated to scattered showers, and possibly a thunderstorm, will
occur for areas east of the Divide, including Yellowstone, this
afternoon and tonight. These showers will develop along and behind
another cold front that will move through the area today and
tonight. A few inches of snow will also fall with these showers
tonight, especially in the Bighorns. Apart from lingering showers
over the northern mountains Tuesday, conditions will be dry across
the area. Red flag conditions will also continue across the south
today, before a temporary reprieve Tuesday. Critical RH values will
continue west of the Divide Tuesday, but winds will be light across
the forecast area. Expect Red flag conditions to return for zones
west of the Divide Wednesday and, more than likely, the rest of the
week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for WYZ277>279-288-
289-414-416.

Freeze Warning from 3 AM to 9 AM MDT Tuesday for WYZ017-018-
028>030.

Freeze Watch from late Tuesday night through Wednesday morning
for WYZ020.

&&

$$

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


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