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000
FXUS65 KRIW 281636
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1036 AM MDT SAT MAY 28 2016

.SHORT TERM...Today through Monday night

Imagery shows large trof across the western CONUS with a ridge
across the east and an embedded sw trof over the cntrl plains.
Another small embedded sw currently located ovr s cntrl MT. Surface
has general weak low pressure across WY with a weakening slow moving
front off to the west and a slightly more mobile portion of the same
front moving through cntrl MT. Tonight/this morning, isolated
showers/thunder from north to south across cntrl Wyoming.

Today, surface low develops weakly across southeastern WY with
boundary stretching back across the southern portion of the state
while the front currently across MT swings through northern/eastern
WY mainly as a clipper. Flow aloft becomes nearly zonal to slightly
southwesterly. Most precipitation will occur around the front or the
mountains of northern Wyoming and the boundary/weak front across the
south. Showers/storms will be of low CAPE, modest shear, low QPF,
very small hail (if any) and gusty wind variety with little chance
for any severe parameters to be met. Tonight, skies clear quickly
after sunset.

Sunday, flow remains modest and rather flat aloft while some weak
moisture tries to infiltrate the southern portion of WY. Most
convective precipitation will be isolated to widely scattered
afternoon based, centered around the area of moisture across south
and southwest WY. The western/southwestern mountains will stand the
best chance of seeing measurable precipitation - with a few spots in
the Salt and Wyoming ranges to perhaps the southern Tetons getting
up to a tenth of an inch of rainfall. Very small hail and gusty
winds will be the usual form of accompaniment to the relatively
sparse rainfall. Stronger storms of the day will be near the western
border areas with ID/UT. Sunday night into Monday, another weak sw
trof will move across WY from west to east with a few spurious light
showers continuing overnight into/through the Monday morning hours.

Monday, lee side troffing across eastern Wyoming will be quickly
followed by the formation of a modest surface cyclone across the
high/cntrl plains with rapid frontogenesis occuring across
southern/southeastern MT. A couple of clipper fronts will then move
to the south across nrn/cntrl/ern Wyoming in the afternoon and
evening. Another area of low pressure will occur further south ovr
nern NM/sern CO. This will give a maximum in convergence between the
two systems across sern WY/nern CO and to the east of this area.
This will also be the region that will see most of the severe
stormsof the day. The second clipper will push through the nrn and
ern portions of the forecast area through the late afternoon/evening
period. Showers/storms will accompany this front with gusty winds
and small hail a possibility for the most part. However, a severe
storm or two will be possible near this front in the afternoon/early
evening across Johnson county and points eastward, with large hail
and strong wind the main threat within these rotating storms. There
will also be a low chance for a brief tornado with this(these)
storm(s) near to just behind the front late in the afternoon/early
evening as LCLs drop quickly and these storms have their best
chance at becoming surface based.

.LONG TERM...Tuesday Through Friday

Synopsis...an upper level low pressure system will drift east across
the northern high plains Tuesday with some rain and mountain snow
showers wrapping back across northern Wyoming in cool northerly
flow.  A ridge of high pressure will build into the area Wednesday
and Thursday bringing a warming trend with summer-like temperatures
5-10F above normal expected Friday into next weekend.

Discussion...GFS and ECMWF in better agreement with track of upper
low into the northern plains on Tuesday with both placing the H5 low
center near the MT/ND/SD triple point 12z Tuesday, drifting east
across southern North Dakota during the day.  An overall trend of
placing wraparound precipitation further to the north and east will
place mainly the Bighorn Range in line for the best chances of
receiving accumulating snowfall on order of 3 to 5 inches from
Powder River Pass and northward Tuesday morning with snow levels
near 7500 feet. The best chances for valley rain showers will be
along and north of a Cody to Buffalo line.  A brisk northerly wind
will develop Tuesday afternoon east of the divide with highs mostly
in the 50s to lower 60s.

A ridge of high pressure will then bring a more summer-like pattern
to the area Thursday through Saturday with a few afternoon
thunderstorms developing along mountain/valley breeze boundaries,
mainly along eastern slopes.  Warming mountain temperatures will
accelerate the melting of remaining mountain snowpack above 9000
feet with corresponding rises in creeks and rivers expected.

&&

.AVIATION.../18Z ISSUANCE/

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

Isolated showers and storms over northwest WY through early
evening. Local mvfr cigs/vsbys are possible with storms. In
southern WY this afternoon isolated showers and storms will occur
into the early evening. KRKS airport will be close to this
activity and VCSH will be included in the TAF. Otherwise VFR
conditions to prevail over the region through tonight. Isolated to
widely scattered showers and thunderstorms will move back into the
KBPI, KPNA, KJAC and KRKS area Sunday afternoon.

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

Isolated showers and thunderstorms will continue into the evening
hours along and east of a line from near KCOD down to KCPR.
MVFR cigs/vsbys will be likely with storms thru 02z. Otherwise,
VFR conditions will prevail over the rest of the area today and
tonight. Isolated mainly mountain showers or thunderstorms are
possible Sunday afternoon.

&&

.FIRE WEATHER...

Fire danger low today and Sunday as green-up and seasonally elevated
humidity levels continue across Wyoming. Isolated to widely
scattered showers and storms possible by mid-day through the early
evening period across mainly northern, eastern and southern Wyoming
with little overall rainfall expected with any of this activity and
less than a tenth of an inch of precipitation at the best locations
in the Bighorn Mountains. Winds will flow out of the west to
southwest west of the Divide at 10 to 20 mph, but will be light and
variable to the east. Sunday will see the warmest temperatures of
the weekend with a few locations in the Wind River and Bighorn
Basins seeing RH values drop into the the upper teens (percent).
Winds across the entire forecast area will be on the light side.

&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Braun
LONG TERM...AEM
AVIATION...PS
FIRE WEATHER...Braun





000
FXUS65 KRIW 280505
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1105 PM MDT FRI MAY 27 2016

.SHORT TERM...TONIGHT THROUGH MONDAY (issued at 359 PM MDT)

AN EXITING WELL DEVELOPED COLD LOW OVER EASTERN COLORADO WILL
CONTINUE TO EXIT TO THE EAST. THE NEXT SIGNIFICANT UPPER LOW
EXISTS OVER SOUTHERN BRITISH COLUMBIA WITH A SLIGHTLY NEGATIVELY
TILTED TROUGH EXTENDING TO THE SOUTH OF THERE. WESTERN AND CENTRAL
WYOMING LIES BETWEEN THESE TWO LOW CENTERS WITH THE
AFOREMENTIONED TROUGH DRIVING A WEAK VORT EASTWARD NOW SITUATED
ALONG THE MONTANA IDAHO BORDER. WITHOUT MUCH FORCING FROM THESE
TWO MAIN FEATURES... WE ARE RELYING Solely ON WEAK CAPE/LIFTED
INDICES AND WEAK WESTERLY DIFLUENT FLOW ALOFT TO PRODUCE MOSTLY
CONTINUED WEAK CONVECTION FOR THE REMAINDER OF THIS AFTERNOON AND
EVENING. MOST OF THESE CELLS HAVE HAD A TOUGH TIME BREAKING AWAY
FROM THE MOUNTAINS WHERE THEY FORMED DUE TO FORCED ASCENT. SOME OF
THE TINY MORE ENHANCED CELLS HAVE MANAGED TO PRODUCE SMALL HAIL.
THE AFOREMENTIONED VORT LOBE IS HELPING TO A DEGREE. ANOTHER
TROUGH WILL SWING BY ON SATURDAY WITH SIMILAR RESULTS, WITH THE
BEST INSTABILITY WEST OF THE DIVIDE. SUNDAY WILL BE THE WARMEST
AND DRIEST DAY OF THE HOLIDAY WEEKEND AS HEIGHTS RISE AHEAD OF THE
APPROACHING CLOSED LOW THAT WILL GET DISLODGED FROM SOUTHERN
BRITISH COLUMBIA AND WILL COME SCOOTING ACROSS MONTANA MEMORIAL
DAY/MONDAY. THE ONLY CONVECTION EXPECTED SUNDAY WILL BE MOSTLY IN
THE SOUTHWEST AND NORTHEAST ZONES. THIS CLOSED LOW WILL SWING A
PROMINENT TROUGH ACROSS THE STATE FROM WEST TO EAST ACROSS
WYOMING, COMPLETE WITH A TIGHT THERMAL GRADIENT. DRIER AIR WILL
BE PUNCHING BEHIND THE TROUGH WITH MUCH HIGHER CAPES OVER THE
EASTERN ZONES WHERE THE BEST CHANCE OF ANY STRONG THUNDERSTORMS
WILL BE. HOWEVER...A BETTER CHANCE OF THUNDERSTORMS EXISTS
THROUGHOUT THE AREA...ESPECIALLY UP NORTH. THE SNOW LEVEL WILL
LIKELY DROP TO 8K IN THE NORTHERN MTNS BY THE LATTER PORTION OF
MEMORIAL DAY.


.LONG TERM...MONDAY NIGHT THROUGH FRIDAY

Main issue with the forecast is generally at the beginning of the
period in regards to where our next upper low will be Monday night
and Tuesday. The GFS is furthest south with a majority of the trough
swinging across the north half of the forecast area. The ECMWF is
furthest north in Nrn MT/Southern Canada (but trending south again)
and the GEM is about in the middle. Longer wave analysis would
support a further south solution so will keep the going forecast
unchanged. With that said, there is enough cold air to bring a quick
shot of snow to the nrn mtns Monday night. After that, the mean
ridge begins to build into the region with warming temps. Models are
trying to keep the ridge somewhat dirty with moisture spilling se
down the front side of the ridge, mainly Thursday into Friday. Going
forecast already had that handled with some isold storms in the nrn
mtns trying to spill se of the foothills late in the day. No reason
to change this.

&&

.AVIATION.../06Z ISSUANCE/

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

Isolated showers over the northwest through Saturday morning. Then
isolated showers and storms over far north WY Saturday afternoon
into early evening. Local mvfr cigs/vsbys are possible with storms.
In southern WY Saturday afternoon isolated showers and storms will
occur into the early evening. KRKS airport will be close to this
activity and VCSH will be included in the TAF. Otherwise VFR
conditions to prevail over the region through Saturday evening.

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

Isolated showers will end by 12z in the eastern part of the area,
mainly from KBYG to KCPR. New showers and storms will develop by 18z
over the Bighorn Mountains southward to Casper. This activity will
occur through 00z Sunday and then diminish by 02z. MVFR cigs/vsbys
will be likely with storms from 18z Sat to 01z Sun. Otherwise, VFR
conditions will prevail over the rest of the area Saturday and
Saturday evening.

&&

.FIRE WEATHER...

A slow drying and warming trend will take place through the weekend.
Temperatures will slowly warm with relative humidity lowering each
day. There will still be some mainly afternoon and evening showers
and thunderstorms around, especially in and adjacent to the
mountains. Any thunderstorm could have erratic winds. Otherwise,
winds should remain light to moderate outside of showers or
thunderstorms. Coverage of thunderstorms will shrink each day with
many of the lower elevations mainly dry on Saturday and Sunday.
MORE Unsettled WEATHER AND SOMEWHAT COOLER WEATHER WILL RETURN
EARLY NEXT WEEK.

&&

.RIW Watches/Warnings/Advisories...None.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...AR
FIRE WEATHER...Lipson





000
FXUS65 KRIW 262045
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
245 PM MDT THU MAY 26 2016

.SHORT TERM...TONIGHT THROUGH SUNDAY

THE FIRST THUNDERSTORMS OF THE DAY HAVE ERUPTED AT THE TIME OF THIS
WRITING ACROSS SOUTHERN JOHNSON AND NORTHERN NATRONA COUNTIES AND
ARE TRACKING EAST. OTHER SMALL THUNDER CELLS ARE TRACKING SOUTHEAST
ACROSS FAR NORTHERN WY. IN ADDITION...CONVECTION IS FORMING OVER OUR
FAVORED CONVERGENT ZONES OVER SOUTH PASS AND WASHAKIE NEEDLES IN THE
OWL CREEK MTNS. INSTABILITY CONTINUES TO SLOWLY INCREASE ACROSS THE
CWA. THE WET BULB ZERO WILL LIKELY BE LOW ENOUGH TO SUPPORT SMALL
HAIL FROM ANY THUNDERSTORMS THIS AFTERNOON AND EVENING. THE
SOUTHEAST BOUND CELLS TO THE NORTH APPEAR TO BE MORE DIRECTLY
ASSOCIATED WITH THE CIRCULATION OVER THE NORTH DAKOTA MONTANA
BORDER. IN FACT, IT APPEARS THAT THE BEST CHANCE OF T STORM ACTIVITY
WILL BE OVER THE NORTHERN PORTION OF THE CWA BUT THEY COULD OCCUR
ANYWHERE WITH LOW GRADE INSTABILITY BECOMING ESTABLISHED. HIGH AND
MIDDLE DEBRIS CLOUDS STREAMING NORTH FROM THE TIGHTLY WOUND
CIRCULATION OVER THE FOUR CORNERS REGION WILL PROBABLY NOT BE ENOUGH
TO RETARD ANY CONVECTION OVER OUR SOUTHEAST ZONES THAT DO FORM. SO
FAR IT APPEARS THAT THE NORTHERN STREAM STORMS ARE PACKING AN
ABUNDANCE OF WIND AS NOTED BY THE UNIMPRESSIVE T STORMS TRACKING
SOUTHEAST ACROSS LOVELL WHERE A 50 MPH WIND HAS ALREADY BEEN NOTED.
WILL KEEP SCT POPS THROUGH THIS EVENING. THE SNOW LEVEL SHOULD DROP
AS LOW AS 7500 FEET IN THE NORTHERN MTNS THROUGH THIS EVENING AS
COOLER AIR CONTINUES TO CIRCULATE AROUND THE NORTHERN PLAINS LOW.
THEN ON FRIDAY WILL KEEP MAINLY SCT POPS AROUND THE CWA ALTHOUGH ANY
AFTERNOON CONVECTION SHOULD REMAIN RATHER TAME CONSIDERING THE WEAK
RIDGING AND RESULTANT WARMING ALOFT SO WILL NOT MENTION ANY SMALL
HAIL FOR NOW. ON SATURDAY THE NEXT UPSTREAM CLOSED LOW WHICH WILL BE
OVER BRITISH COLUMBIA WILL SWING ANOTHER SHORTWAVE TROUGH ACROSS THE
AREA FOR CONTINUED MAINLY SCT POPS MAINLY IN THE PM. SNOW LEVELS
WILL BRIEFLY REACH 7500 FEET IN NW WY LATER SATURDAY AS THIS TROUGH
BRINGS IN SOME COOLER AIR BEHIND IT. THEN SUNDAY SHOULD STAND OUT AS
THE WARMEST AND DRIEST DAY OF THE NEXT SEVERAL DAYS AS HEIGHTS RISE
AHEAD OF THE APPROACHING BRITISH COLUMBIA TROUGH WHICH COULD
POTENTIALLY BE OUR NEXT ANNUAL MEMORIAL DAY WEEKEND COOLING AND
WETTING EVENT THAT WILL APPROACH FROM THE NORTHWEST. MORE ON THIS
FROM OUR LONG TERM FORECAST.

.LONG TERM...SUNDAY NIGHT THROUGH THROUGH THURSDAY

Synopsis...Sunday will be the least active and mildest day over
the holiday weekend with mainly isolated afternoon and evening
thunderstorms across the south and west. A cold front will push
across the area on Memorial Day spreading more numerous showers
and thunderstorms across the area with isolated strong to severe
thunderstorms possible across the north and central. A cool and
unsettled northwest flow will follow this system on Tuesday with
several inches of snow possible across the northern mountains and
Yellowstone National Park Monday night and Tuesday morning. A
ridge of high pressure will bring drier and warmer weather to the
area by Thursday.

Discussion...Omega-type block with upper high over Alaska and
trough across western Canada and the northwest U.S. is expected to
breakdown Sunday and Memorial Day, dislodging the trough across
the northern Rockies on Memorial Day. GFS and ECMWF still show
differences for Memorial Day. However, the GFS has trended toward
the ECMWF solution of moving the upper low through Montana Memorial
Day and night and into the Northern Plains by late Tuesday.

After a relatively pleasant day Sunday, upper trough is dislodged
from western Canada and translates the Northern Rockies Memorial
Day. Showers and thunderstorms are likely to spread into the west
during the morning. Depending on timing, if this strong upper
trough crosses the area during maximum heating Monday afternoon, we
could see some strong to severe thunderstorms develop, especially
across north and central Wyoming. At this time, it would appear
there will be enough surface heating across the lower elevations
east of the Divide to generate more robust and widespread convection.

ECMWF has trended colder behind the trough passage Monday night and
Tuesday with H7 temperatures of 0 to -2C across the north, with the
GFS showing the coldest air of -3C across Yellowstone National Park.
This would bring snow levels down to near 7000 feet with accumulating
snow possible in the northern and northwest mountains.

Upper ridge is expected to slowly progress into the area Wednesday
with drier and warmer air pushing into southwest Wyoming. Coolest air
and best chance for convection will be across north-central Wyoming
in the vicinity of the upper low over the Dakotas. Temperatures
across the region will be continue to rise and conditions turn
drier Thursday and Friday as ridge sets-up over the forecast area.
Temperatures 5-10F above normal are anticipated both days.

&&

.AVIATION.../00Z ISSUANCE/

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

Cyclonic flow around upper low in western South Dakota will keep
west-northwest flow across the region Thursday afternoon and tonight.
While the best moisture and instability will be east of the Divide,
there is still enough residual moisture to generate scattered
showers and thunderstorms through 03Z/Friday. Convection will be
primarily diurnal in nature. VFR conditions will prEVAIL through
at least 06Z/Friday. Other than surface wind gusts in excess of
30kts with convection, surface winds will be less than 12kts from
the west-to-northwest. Weak frontal boundary will slide southwest
across KRKS this evening. This will produce lowering ceilings,
still VFR, and a chance of light showers. Expect areas of IFR/MVFR
ceilings in the far west valleys again tonight between 10Z-
15Z/Friday. This will produce localized obscurations of the
foothills at KJAC and in the Star Valley. All terminals will be
VFR after 15Z/Friday. Gusty west-northwest wind 14-22kts expected
after 19Z/Friday with drier air working into western Wyoming. KJAC
will see southerly wind of 10-20kts. Convection should be less
widespread.

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

Cyclonic flow around upper low in western South Dakota will keep
west-northwest flow across the region Thursday afternoon and tonight.
Already ongoing convection north of a KLND-KCPR with the bulk over
the Big Horn Basin. Weak frontal boundary has also pushed south to
around this KLND-KCPR line with light north wind in its wake. Expect
most robust and widespread convection within this region through
03Z/Friday. Best insTAiability will be along an axis from KCOD-
KWRL- KCPR. Convection will weaken but continue through 06Z-
09Z/Friday, particularly across northern Wyoming. Terminals are
expected to remain VFR, with the best chance of MVFR VISIBILITY
with convection at KCOD-KWRL-KCPR. Occasional mountain top
obscurations are likely later Thursday afternoon into the evening
hours. Much of the same diurnal convection is expected after
19Z/Friday. Weak northeast flow will aid convective development in
the vicinity of KCPR, while KCOD and KLND will be assisted because
of proximity to the foothills. Convection will be least likely at
KWRL and KRIW.

&&

.FIRE WEATHER...

SCATTERED AFTERNOON AND EVENING THUNDERSTORMS WILL TAKE PLACE
MAINLY OVER THE NORTHERN HALF OF WYOMING THROUGH THIS EVENING.
THESE STORMS COULD PRODUCE BRIEF GUSTS OF OVER 50 MPH. SNOW LEVELS
COULD BE AS LOW AS 7500 FEET LATER THIS EVENING IN THE NORTHERN
MOUNTAINS. THERE IS LESS OF A CHANCE OF LATE DAY THUNDERSTORMS
ACTIVITY FRIDAY AS A WEAK RIDGE BUILDS INTO THE AREA AND THE LOW
PRESSURE CENTER OVER THE NORTHERN PLAINS CONTINUES TO MOVE EAST
AND AWAY FROM THE AREA. TEMPERATURES WILL WARM FRIDAY THROUGH
SUNDAY. ONLY A BRIEF RESPITE FROM THE WARM UP IS EXPECTED LATER SATURDAY
WHEN ANOTHER TROUGH WILL SWING THROUGH THE AREA, PRODUCING MORE
SCATTERED LATE DAY THUNDERSTORMS ALONG WITH A SHORT LIVED PERIOD
OF SNOW LEVELS POSSIBLY REACHING 7500 FEET IN THE NORTHWEST
MOUNTAINS LATER IN THE  EVENING. SUNDAY SHOULD BE THE WARMEST AND
DRIEST DAY OF THE WEEK BEFORE A MORE SIGNIFICANT COOL DOWN AND
AN INCREASE IN MOISTURE OCCURS DURING THE BEGINNING OF NEXT WEEK.
Mixing and smoke dispersal should be fair to good outside of any
precipitation.

&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

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





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