Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 252030
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
230 PM MDT Wed Apr 25 2018

.SHORT TERM...(This evening through Saturday)
Issued at 205 PM MDT Wed Apr 25 2018

Warm,dry and windy conditions will be over and
near the east slopes of the mountains and through
the wind corridor from northern Sweetwater, southern
Fremont and Natrona counties producing red flag
conditions until 7 pm mdt.  A cold front in central
Montana will then sweep south across the north and east
spreading some showers, most numerous in the northern
mountains, with mainly a rain snow mix in the lower
elevations.  A sharp wind shift to the northeast along
with much colder temperatures will accompany the cold
front.  The showers will likely make it into central
Wyoming into the Wind River basin before hanging up
on the Continental divide along the peaks of the Wind
River mountains.  Thursday and Friday will see clearing
with cooler temperatures Thursday, warming on Friday.
Southwest winds will return with warming temperatures
and low relative humidity returning to central and
eastern areas on Saturday.

.LONG TERM...(Saturday night through Wednesday)
Issued at 205 PM MDT Wed Apr 25 2018

Final discussion (umteenth platitude) written on this virtual
sheepskin parchment:

Beginning of forecast: Upper ridge getting pushed east over the
eastern Rockie Mountains and into Plains states. Upstream, a large
closed/nearly cut-off low (COL) is finally getting kicked inland
across the western CONUS with southwest flow aloft beginning to
overflow WY. At the surface, lee side troughing will be ongoing.

Overall, this is the best agreement the forecast has seen with
regard to the MR models and this forecast period. The first portion
of the prognosticate through Monday has some semblance of order
between the models insofar as dealing with the persistent and
ubiquitous cut-off (now closed) low that has been hanging around the
west coast for many (many) days. This gives better confidence, both
spatially and temporally, of where and when and how this upper level
phenom will affect the forecast area over the entire forecast
period. The upstream kicker shortwave, now for a few model runs,
will pinch-off and form a far upstream COL well out over the central
Pacific. Now (finally) both the GFS and EC keep a portion of the
original long distance shortwave (before being cut-off) strong and
far enough south to still act on the closer west coast/EPAC COL
without being completely obliterated by a growing ridge between the
two COLs. All models now kick the west coast COL eastward by
Saturday night with the evolution of the kicked system (trajectories
and dispersion) now coming in line and ejecting the low across the
northern Great Basin/central-northern Rockies and into the northern
High Plains/Northern Plains by end of the day Monday. First impact
will arrive Saturday night across the south and west portions of the
CWA. With available moisture modest at best under this scenario
(briefly pulling from a long distance from MX over the southern
Rockies + what still left in the low itself), both QPF and snowfall
(accumulating mainly above 10k ft) will also be modest and mainly
confined to the western mountains Saturday night/Sunday morning
(just stray precipitation east of the Divide and across the south).
Both showers and isolated thunderstorms will be possible.

Next, Sunday afternoon/Sunday evening will then see better chances
for precipitation west of the Divide as PVA and embedded vorticity
lobes within the closed rotating upper low swing over the region.
The best period for overall precip chances will land across western
WY later Monday/Monday night as the closed system opens,
cyclogenesis proceeds over the northern High/Northern Plains while
an associated cold front moves through the forecast area by Monday
evening. Positioning of the surface development/evolution of this
whole system through Tuesday/Tuesday night will mean everything with
regard to possible moisture return and subsequent precipitation/QPF
amounts. Currently, the GFS is faster (as has been the case all
week), putting itself in the a better position with respect to
precipitation chances and amounts on Tuesday. However, the EC
follows on Wednesday with the better numbers. In a nut-shell,
seasonally cool, unstable and possibly wet for much of the forecast
area from Monday night through the end of the forecast period.

SIS...BOOM...BAH!

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday)
Issued at 205 PM MDT Wed Apr 25 2018

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

VFR conditions to prevail for most terminal sites, except KCOD and
perhaps KWRL, until after 06Z. KCOD followed by KWRL, will see the
effects of a passing cold front with a shift in the winds to the
north to northwest this evening while CIGs begin to drop to low
VFR/MVFR by/after 02Z this evening. The other terminal sites will
follow suit sometime after 06Z tonight. As this system will be a
quick hitter, conditions should begin to improve after 06Z - first
across northern WY, heading south to central/southern WY 11Z-15Z.
Mountains of the area will also be occasionally obscured through
around 15Z. Ahead of the front early this evening, winds may gust
out of the west G20-30kt.


West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

VFR conditions to prevail west of the Divide through the forecast
period. Some gusty west to northwest winds (G20-30kt) expected early
this evening - mainly along/east of a KPNA-KBPI_KRKS line.

&&

.FIRE WEATHER...
Issued AT 205 PM MDT Wed Apr 25 2018

Warm,dry and windy conditions will be over and
near the east slopes of the mountains and through
the wind corridor from northern Sweetwater, southern
Fremont and Natrona counties producing red flag
conditions until 7 pm mdt.  A cold front in central
Montana will then sweep south across the north and east
spreading some showers, most numerous in the northern
mountains, with mainly a rain snow mix in the lower
elevations.  A sharp wind shift to the northeast along
with much colder temperatures will accompany the cold
front.  The showers will likely make it into central
Wyoming into the Wind River basin before hanging up
on the Continental divide along the peaks of the Wind
River mountains.  Thursday and Friday will see clearing
with cooler temperatures Thursday, warming on Friday.
Southwest winds will return with warming temperatures
and low relative humidity returning to central and
eastern areas on Saturday.  With colder temperatures
and a frontal inversion limiting mixing north and east,
fair to low end good smoke dispersal will occur over
the north and east, with good or better smoke dispersal
to the south and west on Thursday.  With warming most
of the area should see good or better smoke dispersal
on Friday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT this evening for WYZ280-289.

&&

$$

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



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