Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 160924
Area Forecast Discussion
National Weather Service Riverton WY
324 AM MDT Thu Mar 16 2017
.SHORT TERM...(Today through Saturday night)
THE BASE OF THE SHORTWAVE APPEARS TO BE DIGGING AND TRACKING EAST
ACROSS THE CALIFORNIA/NEVADA BORDER ATTM. MOISTURE OUT AHEAD OF THE
TROUGH IS AIDING IN SHOWER ACTIVITY ACROSS IDAHO AND IS CURRENTLY
ENTERING WESTERN WYOMING. THIS SYSTEM IS MOVING FAIRLY QUICKLY SO
ONLY 1 TO 3 INCHES OF SNOWFALL ARE EXPECTED IN THE WESTERN MOUNTAINS
AND LESS THAN AN INCH IN THE WESTERN VALLEYS WHERE THE SNOW WILL BE
MIXED WITH RAIN. THE SNOW LEVEL SHOULD HOVER AROUND 7500 FEET OR SO.
PERIODS OF LIGHT SNOW AND RAIN WILL LINGER THROUGH THIS AFTERNOON
OUT WEST TAPERING OFF TO FLURRIES AND SPRINKLES TONIGHT WITH SOME OF
THIS PRECIP SPREADING EAST TO THE NORTHERN MOUNTAINS EAST OF THE
DIVIDE TONIGHT AS AN ASCT WEAK COOL FRONT SAGS SOUTH ACROSS WYOMING,
MAINLY EAST OF THE DIVIDE, WITH THE BOUNDARY LEVEL MOISTURE BEHIND
THE FRONT RELEGATED TO THE NORTHERN TIER ZONES AND JOHNSON
COUNTY. AHEAD OF THIS TROUGH AND COOL FRONT. THE PRESSURE GRADIENT
WILL TIGHTEN AS IT FLATTENS THE RIDGE AND THE DOWNWARD FORCING OF
THE RIGHT FRONT QUAD OF THE JET WILL PUSH THE 700MB 45 TO 50 KNOT
WINDS TO THE SURFACE FOR INCREASED WEST SOUTHWEST WINDS OF 30 TO
40 MPH IN SOME AREAS ALONG WITH OCCASIONAL GUSTS OF 50 MPH FROM
THE RED DESERT TO CASPER TODAY. THESE WINDS WILL COMBINE WITH AN
RH IN THE UPPER TEENS TO MID 20S FOR ELEVATED FIRE CONCERNS
INVOLVING FINE DORMANT FUELS. EXPECT INCREASED WINDS ALONG THE
EAST SLOPE OF THE CONTINENTAL DIVIDE TODAY AS WELL. SNOWMELT WILL
REMAIN ELEVATED TODAY WITH MOST HIGHS IN THE 50S AND 60S TODAY.
FRIDAY WILL FEATURE COOLER WEATHER WITH TEMPS AROUND 10 DEGREES
COOLER EAST OF THE DIVIDE BUT ONLY A COUPLE OF DEGREES COOLER WEST.
WARMER AIR WILL QUICKLY ADVECT BACK INTO THE AREA ALOFT FROM THE
SOUTHWEST LATE FRIDAY.
ON SATURDAY, THE NEXT RIDGE AXIS WILL EXIT TO THE EAST WITH
INCREASING WARMING SOUTHWEST WINDS AHEAD OF THE NEXT PACIFIC TROUGH.
SATURDAY COULD VERY WELL BE THE WARMEST DAY OF THE YEAR SO FAR FOR
MANY AREAS WITH THE LOWER ELEVATIONS OF JOHNSON AND NATRONA COUNTIES
SEEING TEMPS WELL UP INTO THE 70S. THIS WILL INCREASE SNOWMELT AND
ELEVATE THE FIRE DANGER SOMEWHAT ONCE AGAIN, ESPECIALLY IN THE
EASTERN ZONES. RAIN TURNING TO SNOW WILL OCCUR IN FAR WESTERN
WYOMING SATURDAY NIGHT AS THIS NEXT PACIFIC DRAWS CLOSER WITH ADDED
LIFTED FROM THE RIGHT REAR QUAD OF THE NEXT ASCT JET MAX.
.LONG TERM...(Sunday through Thursday)
On Sunday, global models show the Cowboy State at the base of a
trough pulling east across the Canadian Prairies and northern
High Plains. The associated Pacific cold front is expected to
push across the forecast area Sunday/Sunday evening. At the same
time, a Canadian cold front will drop south, east of the Divide,
but the westerly flow in the mid-levels (700mb and above) will
retard its southwest advancement, and depth of the true Canadian
airmass. A quasi-stationary cold front is expected to remain
across the area Monday, before retreating northward on Tuesday.
The combination of cold front, disturbances in the flow and some
upper jet dynamics at times will allow for chances of
precipitation at times Sunday and Monday across much of the area.
Right now it look like there will be one round moving across the
area Sunday with the best shot over the west/northwest, then a
brief break with another round with the main emphasis a bit
farther south across west/southwest Wyoming. Models are also
showing the possibility of jet couplet between the
polar/subtropical jets both days with an emphasis late Sunday
night/Monday. Although precipitation amounts still look on the
light side, the fact that models show the potential jet couplet
makes the systems Sunday and Monday more interesting to watch.
Snow levels will vary greatly with time and location due to the
cold front, but in general will be mountain snow/valley rain with
far western valleys and even perhaps far northern lower elevations
changing to snow Sunday night. On Monday, snow levels look to be
between 6500 and 8000 feet in most areas, but the far northern
sections depending upon the intrusion of the Canadian airmass
could be much lower.
Late Monday night into Tuesday, models are showing a shortwave
ridging with warming and mainly dry conditions. However this will
be short-lived as models show a moist southwest flow developing
late Tuesday into Tuesday evening as deep trough pushes into the
West Coast. This will would result in precipitation breaking out
across the west/southwest again. Models then show this trough
splitting with the main energy shifting east across the
central/southern Rockies Wednesday/Wednesday night, and then into
the central/southern Plains on Thursday. The associated cold front
is expected to push across the area Wednesday into Wednesday
night with the possibility of precip spilling east of the Divide.
.AVIATION...(For the 12Z TAFS through 12Z Thursday night)
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
VFR conditions should prevail through most of the forecast period.
The ridge axis is finally progged to move east through the area
during the beginning of the forecast period. The models are
continuing to indicate that a low level flow will result in the
lower atmosphere gradually moistening. Will maintain the predominant
6sm -SN from 09z-15z at KJAC and expect VCSH downstream at KBPI and
KPNA between 12Z and 18Z today. Mountain top obscurations will be
widespread from 09Z-18Z today along and west of the Continental
Divide. Substantial drying is expected in the wake of a shortwave
and cold frontal passage by late this morning, so all terminals will
be VFR by 18Z this afternoon. Surface winds will increase around 18Z-
19Z today in the wake of this weather system. Speeds will generally
be 15-25kts during this afternoon. Will gradually lessen wind speeds
across the TAF sites after the 02z Friday time frame toward the end
of the forecast period.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
VFR conditions will continue through the forecast period. Will
continue with gusty west-southwest winds at 15-25kts at KCPR through
about 00Z/Thu. Still looks like late afternoon mixing will allow for
westerly wind of 10-18kts at KRIW and KLND. West-northwest down
valley flow will favor wind continuing through this evening at
KRIW. KCOD will see off-again, on-again west wind of 10-20kts
throughout the period.
Shortwave and cold front will sweep east toward midday today with
surface wind speeds increasing from the west toward 18Z today.
Afternoon winds will continue to increase between 18Z-20Z today with
widespread 15-25kts. Higher speeds are expected at KRIW where west-
northwest flow is favored and at KCPR where pressure gradient will
keep winds up. Will eventually lessen winds toward the 02z-03z time
frame as the pressure gradient begins to relax somewhat.
Fire Danger low west of the Divide...low to high, depending on
elevation, east of the Divide through Saturday. Today...winds
increase out of the west to southwest east of the Divide and across
southern WY...gusting 25 to 40 mph at times along with isolated
gusts of 50 mph. However, as low to mid level moisture and clouds
feed into the area, minimum RH values will also increase to the 20
to 30 percent range, keeping fire danger elevated...but not extreme.
Otherwise, light areas of precipitation will be possible west of the
Divide and across the north with the approach and passage of the
next weather disturbance and associated front. Temperatures will be
around 10 degrees cooler Friday east of the Divide but only a couple
of degrees cooler west. Saturday afternoon in particular could see
Red Flag conditions for a few hours over both Natrona and Johnson
counties as warm temperatures, low relative humidities and gusty
winds combine. The likelihood of precipitation will increase early