Area Forecast Discussion
Issued by NWS Billings, MT

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000
FXUS65 KBYZ 050322
AFDBYZ

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BILLINGS MT
922 PM MDT MON MAY 4 2015

.UPDATE...
WEAKENING DYNAMICS AND LOSS OF DIURNAL HEATING HAVE CAUSED A
GENERAL DECREASE IN ACTIVITY WITH MAIN SHOWERS ON RADAR THIS
EVENING IN AN AREA OF UPSLOPE ALONG THE SNOWY MOUNTAINS. WATER
VAPOR IMAGERY DOES SHOW INCREASING MOISTURE PUSHING IN FROM THE
SOUTH SO EXPECT SOME LIGHT SHOWER DEVELOPMENT OVER THE MOUNTAINS.
UPDATED FORECAST TO DECREASE POPS FOR MOST AREAS. BORSUM

&&

.SHORT TERM...VALID FOR TUE AND WED...

A FEW SHOWERS AND THUNDERSTORMS HAD DEVELOPED OVER THE MOUNTAINS
THIS AFTERNOON AS AIRMASS HAS DESTABILIZED WITH MESOANALYSIS
SHOWING 500 J/KG TO 1000 J/KG OF SURFACE CAPE OVER THE AREA. SHEAR
WAS MINIMAL. WILL HAVE POPS OVER AND NEAR THE MOUNTAINS THIS
EVENING...AIDED BY SURFACE CONVERGENCE...WITH THE BEST CHANCE OF
THUNDER THROUGH 03Z PER THE SREF. KEPT THE OVERNIGHT DRY DUE TO
LACK OF LIFT AND THE RETURN OF STABILITY. THE GUSTY SE WINDS
SHOULD CONTINUE OVER SE MT THROUGH THE NIGHT...BUT DECOUPLE AT
KSHR.

ON TUE...THE SOUTHERN PACIFIC WAVE OVER THE FOUR CORNERS REGION WILL
ROTATE NE...SENDING A FEW SHORTWAVES INTO THE FORECAST AREA.
SURFACE CONVERGENCE WILL AGAIN DEVELOP OVER WESTERN AREAS. THE
SREF SHOWED SURFACE CAPES OF 500 TO 1000 J/KG DEVELOPING ON TUE
WITH MINIMAL SHEAR. WILL CARRY POPS OVER THE MOUNTAINS TUE MORNING
AND WILL MENTION THUNDER AS AIRMASS DESTABILIZES EARLY. KEPT POPS
ACROSS CENTRAL AND WESTERN AREAS TUE AFTERNOON FOR SHOWERS AND
THUNDERSTORMS.

THE WAVE WILL CONTINUE ROTATING NE TUE NIGHT AND WED...REACHING THE
CENTRAL PLAINS BY 00Z THU. Q-VECTOR CONVERGENCE INCREASES
BEGINNING TUE NIGHT DUE TO THE APPROACH OF THE WAVE. IN
ADDITION...AREAS OF JET DIVERGENCE BEGIN MOVING THROUGH THE REGION
ON TUE AHEAD OF THE JET IN THE PACIFIC NW UPPER LOW. THE S TO SE
850 MB JET WILL CONTINUE TO ADVECT MOISTURE INTO THE AREA INTO WED
AND PRECIPITABLE WATERS ARE EXPECTED TO CLIMB TO A HALF INCH W TO
AROUND AN INCH IN THE FAR E BY WED AFTERNOON. THE PACIFIC SYSTEM/S
COLD FRONT PUSHES E INTO THE AREA BY 12Z WED...A LITTLE SLOWER
THAN PREVIOUS MODEL RUNS. FOR TUE NIGHT...HAD SCATTERED POPS
OVERSPREADING THE AREA FROM W TO E WITH A MENTION OF THUNDER IN
THE EVENING. THE COLD FRONT WAS FORECAST TO REACH THE MT/DAKOTAS
BORDER BY 00Z THU. THE 12Z DETERMINISTIC MODELS HAD BANDS OF
HEAVIER RAINFALL SETTING UP OVER SE MT WED AFTERNOON WHILE THE
SREF WAS MORE EVENLY DISTRIBUTED WITH THE HEAVY RAINFALL ACROSS
THE MT/DAKOTAS BORDER. THIS PRECIPITATION SHIFT TO THE W BY THE
MODELS WAS A NEW TREND. GIVEN THE ABOVE...HAD THE MAIN
PRECIPITATION FOCUS E OF KBIL ON WED WITH LOWER POPS CENTRAL AND
W. GIVEN THE NEW MODEL TREND...THERE WAS SOME UNCERTAINTY AS TO
HOW THIS SCENARIO WILL PLAY OUT.

THE WAVE WILL MOVE NE THROUGH THE DAKOTAS WED NIGHT. THERE WERE
INCREASING MODEL DIFFERENCES IN HOW FAST THE PRECIPITATION WOULD
EXIT THE AREA...WITH THE WRF DRYING OUT THE AREA FASTER THAN THE
OTHER MODELS. HAVE GONE WITH A MODEL COMPROMISE FOR POPS WED
NIGHT.

AS FOR TEMPERATURES...TUE WILL BE A WARM PREFRONTAL DAY WITH HIGHS
IN THE 70S TO AROUND 80 DEGREES. IT WILL BE MUCH COOLER ON WED
WITH HIGHS IN THE UPPER 50S TO LOWER 60S. ARTHUR


.LONG TERM...VALID FOR THU...FRI...SAT...SUN...MON...

COOL AND POTENTIALLY WET WEATHER IS THE THEME IN THE LONG TERM AS
AN UPPER-LEVEL TROUGH DIGS INTO THE GREAT BASIN LATE THIS WEEK AND
THEN EJECTS INTO THE PLAINS BY LATE IN THE WEEKEND. WE ARE NOT YET
CONFIDENT IN THE TRACK OF THE MIDDLE- AND UPPER-LEVEL LOW WHICH IS
EXPECTED TO ACCOMPANY THIS DYNAMIC TROUGH...AND IT/S THE TRACK OF
THAT FEATURE THAT WILL ULTIMATELY DICTATE HOW MUCH MOISTURE WE GET
IN SOUTHERN MT AND NORTH CENTRAL WY. REGARDLESS...DIFFLUENCE ALOFT
SHOULD YIELD AT LEAST A PERIOD OF LIGHT PRECIPITATION AND THERE IS
A CONSENSUS AMONG MODELS FOR THAT TO BE MOST PREVALENT FROM LATE
FRIDAY THROUGH SATURDAY NIGHT. IT IS HOW LONG THE PRECIPITATION
CONTINUES AND HOW MUCH FALLS THAT/S UNCERTAIN. THE 12 UTC GFS AND
ECMWF DID TREND LOWER WITH POTENTIAL RAINFALL TOTALS THIS WEEKEND
SINCE THEY SIMULATED A FURTHER-SOUTH TRACK TO THEIR 500-MB LOWS...
BUT RUN-TO-RUN CHANGES IN THE POSITION OF THAT FEATURE ARE APT TO
CONTINUE FOR SEVERAL MORE MODEL CYCLES.

OTHERWISE...EAST-NORTHEAST LOW-LEVEL FLOW AND CLOUDS WILL MAKE FOR
A COOL PERIOD...WITH HIGHS MAINLY IN THE 50S F MOST DAYS. SCHULTZ

&&

.AVIATION...

MAINLY VFR CONDITIONS WILL PREVAIL TONIGHT AS SHOWERS AND
THUNDERSTORMS DISSIPATE...WITH LOSS OF DIURNAL HEATING.  LOW-
LEVEL WIND SHEAR IS LIKELY IN SOUTHEASTERN MT OVERNIGHT BECAUSE
SOUTHEAST WINDS AT THE GROUND WILL EASE WITH SUNSET WHILE THE
WINDS BETWEEN 1000 AND 3000 FT AGL WILL STRENGTHEN NOCTURNALLY. SHOWERS
AND THUNDERSTORMS ARE EXPECTED TO DEVELOP IN THE WESTERN MOUNTAINS
TUESDAY MORNING...AND SPREAD ACROSS THE REGION FROM WEST TO
EAST THROUGH THE DAY. AAG/SCHULTZ

&&

.PRELIMINARY POINT TEMP/POPS...

    TUE     WED     THU     FRI     SAT     SUN     MON
-----------------------------------------------------------
BIL 048/079 050/061 043/055 039/049 039/050 036/061 041/064
    12/T    42/W    22/W    33/W    55/W    54/W    32/W
LVM 042/075 043/061 038/050 034/048 035/046 032/059 037/063
    23/T    54/W    34/W    44/W    55/W    53/W    33/W
HDN 045/082 050/061 042/057 038/055 039/052 034/062 038/067
    11/B    44/W    22/W    23/W    55/W    54/W    32/W
MLS 047/079 051/062 041/058 038/055 038/053 036/059 038/065
    01/B    36/W    42/W    22/W    44/W    44/W    42/W
4BQ 046/075 051/061 043/056 037/054 038/050 035/054 038/063
    01/B    36/W    43/W    22/W    55/W    55/W    52/W
BHK 043/072 049/063 041/055 035/053 034/051 034/053 033/061
    01/N    38/W    63/W    22/W    44/W    44/W    42/W
SHR 043/073 047/055 039/054 036/053 037/048 033/055 035/063
    12/T    35/W    43/W    33/W    55/W    54/W    43/W

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...NONE.
WY...NONE.

&&

$$
WEATHER.GOV/BILLINGS



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