Area Forecast Discussion
Issued by NWS Riverton, WY

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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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