Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 021805
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1205 PM MDT SAT MAY 2 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

IMAGERY SHOWING NEAR ZONAL FLOW ALOFT OVR WY...SANDWICHED BETWEEN
UPR LOW TO THE N...UPR HIGH TO THE S. SFC HAS WEAK SFC HIGH PRESSURE
ACROSS MUCH OF THE WRN CONUS...WITH A WEAK TROF CUTTING WY IN HALF
FROM N TO S.

TODAY...MODEST MID/UPR MOISTURE WILL INVADE THE FA FROM BOTH THE NW
AND SW ALONG WITH A COUPLE OF WEAK UPR LVL DISTURBANCES MOVING INTO
THE REGION THIS AFTERNOON/EVENING.  THIS WILL INITIATE SOME SHOWERS
AND ISOLATED THUNDER ACROSS THE SRN HALF AND NRN PORTIONS OF THE FA
AS A WEAK WARM FRONTAL BOUNDARY FORMS AND MOVES NORTH INTO CNTRL WY
FROM THE SOUTH WHILE A WEAK COLD FRONT MOVES SOUTH FROM THE NORTH. A
LITTLE STRONGER UPR LVL FORCING ACROSS THE SOUTH WILL ALLOW FOR
SOMEWHAT BETTER COVERAGE LATER THIS AFTERNOON THAN ACROSS THE NORTH
THRU THE EVENING HRS. BEST COVERAGE EXPECTED WEST OF THE DIVIDE IN
GENERAL. THIS TIME PERIOD WILL BE CHARACTERIZED BY A DRY
SFC...ELEVATED CONVECTION MOVING E AT 25 TO 35 KTS UNDER STRONG
LAPSE RATES BUT LITTLE BUOYANCY...WITH 1 TO 6 KM SHEAR RUNNING
BETWEEN 20 AND 25 KTS...ENOUGH TO KEEP STORMS SOMEWHAT
ORGANIZED...AND DCAPE RUNNING BETWEEN 500 AND 700 J/KG. DEFINITELY A
GUST FEST WITH TALLER STORMS THAT DO FORM WITH THEIR CLOUD BASES
RUNNING AROUND 8K FT OR HIGHER AND LOTS OF EVAPORATION GOING ON
BELOW. THAT SAID...MOST SHOWERS/STORMS SHOULD KEEP GUSTS BELOW
SEVERE...THE EXCEPTION BEING A COUPLE OF STORMS THAT MOVE ACROSS
MOUNTAIN RIDGE LINES WITH THE AREA IMMEDIATELY TO THE LEE OF THE
BARRIER BEING SOME WHAT SUSCEPTIBLE TO NEAR SVR GUSTS. SMALL HAIL
WILL BE POSSIBLE...BUT MAINLY ACROSS THE SRN FA WHERE THE
DYNAMICS ARE MORE FAVORABLE. VIL OF THE DAY (NON DIGITAL) FOR 1
INCH HAIL WOULD BE AROUND 45...ALTHOUGH THAT IS A LONG WAY TO
TRAVEL THROUGH RELATIVELY DRY AIR AND STILL REMAIN LARGE ENOUGH. THE
ACTION WILL SUBSIDE IN THE EVENING AS DARKNESS APPROACHES.

SUNDAY...FRONT THRU THE FA E OF THE DIVIDE WITH SHALLOW UPSLOPE
UNDER WESTERLIES ALOFT. ANOTHER UPR DISTURBANCE IS STILL SCHEDULED
TO ROTATE INTO THE REGION OUT OF THE SW CONUS DURING THE DAY...ALONG
W/ INCREASED MID LVL MOISTURE. BOUNDARY/FRONT TO THE S MAY STILL BE
LIFTED NWD DURING THE DAY WITH THE BEST AREA OF CONVECTIVE PRECIP
AND STRONGER FORCING OCCURRING BETWEEN ROCK SPRINGS...TO THE NE THRU
MOST OF NATRONA COUNTY. THIS BOUNDARY WILL BE THE AREA TO WATCH FOR
STRONGER BRIEF STORMS OF THE DAY WITH A BETTER CHANCES FOR SMALL
HAIL...BUT LESS CHANCES FOR STRONG WIND GUSTS. FURTHER NORTH...SHEAR
WILL BE MUCH STRONGER...BUT THERMODYNAMICS WILL BE MUCH
WEAKER...WITH GENERALLY TOO MUCH SHEAR FOR THE AMOUNT OF CAPE...WITH
FAST MOVING SHOWERS COMING AND GOING. HOWEVER...WITH LCLS
SIGNIFICANTLY LOWER ACROSS THE NORTH AND ALONG THE BOUNDARY...NOT
OUT OF THE QUESTION TO GET A BRIEF SPIN-UP OF SOME KIND.

MONDAY...SOMEWHAT LESS MOISTURE THAN SUNDAY SHOULD MAKE IT AROUND
THE SRN CONUS HIGH AS INFLUENCES FROM UPSTREAM TROF BEGIN TO PUSH
MOISTURE AND THE UPR HIGH TO THE EAST A BIT. WITH STATIONARY FRONT
HANGING AROUND/THRU CNTRL WY UP AGAIN THE WIND RIVER AND ABSAROKA
MOUNTAINS AND GENERAL DEEP HIGH PRESSURE BUILDING OVR THE CNTRL
PLAINS...MOISTURE WILL GET SHUNTED WWD...WITH MOST OF THE DAY`S
ACTION WEST OF THE DIVIDE. STRONGER STORM OF THE DAY WILL RESIDE
ACROSS THE WRN MOUNTAINS...WITH GUSTY WINDS AND SMALL HAIL
POSSIBLE...BUT SHOULD BE BELOW SEVERE LIMITS.

.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...A PACIFIC STORM SYSTEM WILL DIG ACROSS THE NORTHWEST U.S.
INTO THE GREAT BASIN DURING THIS PERIOD BRINGING CHANCES OF SHOWERS
AND THUNDERSTORMS EACH DAY ALONG WITH AN OVERALL COOLING TREND.

DISCUSSION...MEDIUM RANGE GLOBAL MODELS HAVE COME INTO BETTER
AGREEMENT OVER THE LAST 24 HOURS WITH TWO PACIFIC SYSTEMS THAT WILL
IMPACT THE AREA DURING THIS PERIOD.  THE FIRST IS A SUB-TROPICAL LOW
NEAR 28N/137W EARLY THIS MORNING WHICH WILL BE KICKED EASTWARD BY
THE SECOND NORTHERN STREAM UPPER LOW MOVING EAST ACROSS THE GULF OF
ALASKA.   REMNANTS OF THE SUB-TROPICAL LOW ARE EXPECTED TO MOVE INTO
THE SW U.S. AND THEN LIFT NNE ACROSS THE SOUTHERN ROCKIES AND
CENTRAL HIGH PLAINS ON TUESDAY AS IT ENCOUNTERS BACKSIDE OF STRONG
RIDGE OVER THE PLAINS/SE U.S.  THIS SYSTEM WILL SPREAD SHOWERS AND
THUNDERSTORMS NORTH ACROSS THE AREA TUESDAY.  BOTH THE ECMWF AND GFS
HAVE SHIFTED SFC TROUGH AXIS A BIT FURTHER EAST TUESDAY WITH THE
HEAVIEST PRECIPITATION EXPECTED TO PUSH NORTHWARD ACROSS FAR EAST
WYOMING.  HOWEVER...MOST OF THE WEST AND CENTRAL WYOMING CAN EXPECT
AT LEAST SCATTERED SHOWER ACTIVITY THROUGH WEDNESDAY.  THERE MAY BE
A BREAK IN SHOWERS WEDNESDAY NIGHT INTO THURSDAY BETWEEN THE FIRST
SYSTEM LIFTING INTO THE NORTHERN PLAINS AND THE SECOND SYSTEM
DIGGING SOUTH ACROSS THE INTERMOUNTAIN WEST.  THIS LATTER SYSTEM IS
EXPECTED TO CUTOFF THURSDAY NIGHT OVER THE WESTERN GREAT
BASIN...GRADUALLY FILLING AND FRAGMENTING OUT AS IT WOBBLES EAST
ACROSS THE GREAT BASIN INTO THE CENTRAL ROCKIES FRIDAY AND SATURDAY.
MORE WIDESPREAD AND HEAVIER WCB PRECIPITATION IS POSSIBLE OUT AHEAD
OF THIS SYSTEM ACROSS THE AREA FRIDAY AND SATURDAY.  SNOW LEVELS ARE
EXPECTED TO REMAIN MOSTLY ABOVE 7.5KFT ON FRIDAY...POSSIBLY LOWERING
TO BETWEEN 6-7KFT ON SATURDAY.

.AVIATION.../18Z ISSUANCE/

VFR CONDITIONS ARE EXPECTED THROUGH THE PERIOD AT THE AIRPORTS. A
PACIFIC TROUGH WILL ENTER NORTHWEST WYOMING AFTER 18Z AND HELP
PRODUCE SCATTERED RAIN SHOWERS...POSSIBLE SNOW ABOVE 9500 FEET...
AND ISOLATED THUNDERSTORMS OVER THE SOUTHWEST AND WESTERN
MOUNTAINS WITH WIND GUSTS TO 45KTS THROUGH 03Z SUNDAY. THE HIGHER
MOUNTAINS WILL BE OBSCURED LOCALLY WITH THE HEAVIER SHOWERS AND
THUNDERSTORMS. THE COVERAGE IS NOT EXPECTED TO BE WIDESPREAD
ENOUGH TO SINGLE OUT ANY AIRPORT FOR LOWER CEILINGS AND VISIBILITY
SATURDAY AFTERNOON. ANOTHER PACIFIC TROUGH WITH MORE MOISTURE
COMING UP FROM THE GREAT BASIN WILL PRODUCE INCREASED CLOUDS AND
SHOWER ACTIVITY IN THE SOUTHWEST AND WESTERN MOUNTAINS FROM 15Z TO
18Z SUNDAY.

.FIRE WEATHER...

FIRE DANGER ENHANCED BUT BELOW CRITICAL TODAY AND SUNDAY AS MOISTURE
SLOWLY BEGINS TO INFILTRATE THE REGION. ELEVATED SHOWERS AND
ISOLATED THUNDERSTORMS WILL INCREASE MAINLY WEST OF THE DIVIDE BY
LATE AFTERNOON...WITH GUSTY WINDS...OCCASIONAL LIGHTNING AND SMALL
HAIL WILL BE POSSIBLE. MINIMUM RH PERCENT VALUES WILL DROP INTO
THE TEENS ACROSS THE LOWER ELEVATIONS OF CENTRAL AND NORTHERN
WYOMING...TO THE 20S TO NEAR 30 IN THE MOUNTAINS AND ACROSS THE
SOUTHERN DISPATCH ZONES. WINDS WILL INCREASE OUT OF THE WEST AT 10
TO 20 MPH FOR MANY LOCATIONS EXCEPT THE BIGHORN BASIN WHICH WILL
REMAIN ON THE LIGHTER SIDE. SUNDAY CAN BE CHARACTERIZED AS COOLER
AND MORE HUMID REGION WIDE...WITH BETTER CHANCES FOR PRECIPITATION
ACROSS THE ENTIRE FORECAST AREA. LESS WIND OVERALL THAN
TODAY...EXCEPT FOR ERRATIC WINDS UNDER/NEAR SHOWERS AND STORMS.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...C.BAKER
FIRE WEATHER...BRAUN






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