Area Forecast Discussion
Issued by NWS Billings, MT

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000
FXUS65 KBYZ 050920
AFDBYZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BILLINGS MT
320 AM MDT SAT SEP 5 2015

.SHORT TERM...VALID FOR TDY AND SUN...

A POTENTIALLY ACTIVE WEATHER DAY AHEAD...AND A COMPLICATED ONE AT
THAT. A DEEP UPPER TROUGH OVER THE PACIFIC NORTHWEST WAS SLOWLY
MOVING EAST. THIS SYSTEM WILL BE THE WEATHER MAKER THIS WEEKEND AS
IT MOVES THROUGH THE NORTHERN ROCKIES. MODELS HAVE SLOWED THE
SYSTEM DOWN A BIT WITH TROUGH PASSAGE OVERNIGHT AND STRONG
SHORTWAVE ENERGY RIPPING THROUGH THIS EVENING. THIS MAKES TOTAL
SENSE BASED ON WATER VAPOR IMAGERY.

SOUTHWEST FLOW ON THE FRONT SIDE OF THE TROUGH WAS OVER THE
FORECAST AREA AND HAS PRODUCED PERIODIC SHOWERS AND THUNDERSTORMS
THROUGH THE NIGHT. TWO MAIN CORRIDORS OF SHOWERS...ONE OVER THE
TWO EASTERN COUNTIES...AND THE OTHER FROM COLUMBUS WESTWARD. HI
RESOLUTION MODELS WEAKEN THIS THROUGH THE MORNING...BUT THAT TREND
HAS BEEN SLOW. THE SLOWER THESE SHOWERS WEAKEN...THE MORE THAT
WILL IMPACT STRONG THUNDERSTORM DEVELOPMENT. ANOTHER PARAMETER TO
CONSIDER IS AN AREA OF LOW CLOUDS AND STRATUS DRAPED OVER MUCH OF
THE AREA. HI RESOLUTION MODELS LIFT THIS AREA OUT OF WESTERN AND
CENTRAL ZONES BY LATE MORNING. THIS SEEMS REASONABLE AS SOUTHWEST
FLOW ALOFT PERSISTS AND DRIER AIR WORKING UP IN THE SOUTHWEST FLOW
SHOULD SCOUR OUT THE MOISTURE. A SLOWER LIFTING OF THIS STRATUS
WOULD LIMIT THE STRONG CONVECTION POTENTIAL.

WILL GO ALONG WITH THE STRATUS LIFTING BY LATE MORNING IDEA AND
ALLOWING FOR DIFFERENTIAL HEATING OVER SOUTHERN ZONES...NEAR THE
FOOTHILLS. AN INVERTED SURFACE TROUGH IS PROGGED TO BE POSITIONED
BASICALLY FROM JOLIET TO BIG TIMBER EARLY THIS AFTERNOON. THIS
WOULD BE A POTENTIAL FOCUS POINT FOR CONVECTION. THIS ALONG WITH
DIFFERENTIAL HEATING AND DIFFLUENT FLOW ALOFT WITH THE ADVANCING
TROUGH...SHOULD GET CONVECTION TO FORM. HI RESOLUTION MODELS
DEVELOP A CLUSTER OF STORMS RIGHT AROUND BILLINGS AND A BIT
SOUTHWEST OF TOWN...THEN STRENGTHEN THEM AS THEY MOVE NORTHEAST
TOWARD HYSHAM. GFS PROJECTED CAPE WAS AROUND 1200J/KG OVER CENTRAL
ZONES WITH BUFKIT SHOWING 800J/KG ABOVE THE BOUNDARY LAYER. STRONG
STRAIGHT LINE SHEAR PRESENT WITH EFFECTIVE BULK SHEAR AROUND
45KTS. PRECIPITABLE WATER VALUES PROJECTED AT AN INCH OVER CENTRAL
ZONES. THUS STORMS WILL HAVE THE CAPABILITY TO PRODUCE TORRENTIAL
RAIN AND COULD BECOME SEVERE. SSEO PROJECTED UPDRAFT HELICITY WAS
RIGHT AROUND 40KTS OVER YELLOWSTONE COUNTY FOR A STRONG SIGNAL FOR
SEVERE POTENTIAL. ANTICIPATE STRONG STORMS TO DEVELOP AROUND
BILLINGS OR SLIGHTLY SOUTHWEST OF TOWN THIS AFTERNOON. THESE
STORMS SHOULD STRENGTHEN QUICKLY AND EXPECT A FEW SEVERE CELLS AS
THEY MOVE NORTHEAST SWIFTLY. WILL LEAVE MENTION OF SEVERE IN THE
ZONES. STORMS SHOULD MOVE FAST ENOUGH TO MINIMIZE FLASH FLOOD
THREAT BUT VERY HEAVY RAIN TOTALLY EXPECTED WILL THE STORMS.

LINGERED POPS INTO THE OVERNIGHT HOURS WITH SLOWER TROUGH
MOVEMENT WITH THE HIGHEST POPS BEING OVER THE EAST. WINDS WILL
INCREASE TONIGHT AS THE SURFACE LOW CONTINUES TO DEEPEN OVER THE
DAKOTAS. THE GRADIENT DOES NOT LOOK QUITE AS TIGHT FOR SUNDAY WITH
THE CUT OFF LOW WITHIN THE TROUGH FARTHER NORTH IN CANADA THAN
PREVIOUS MODEL RUNS. GUIDANCE HAS BACKED OFF QUITE A BIT ON WINDS
AND 850MB WINDS HAVE BEEN GREATLY REDUCED SUNDAY AFTERNOON AS
WELL. WILL MAINTAIN SOME GUSTINESS IN THE GRIDS...BUT NOT
EXPECTING A TREMENDOUSLY WINDY DAY NOW. WILL FOCUS OUR WEATHER
STORY ON THE SEVERE POTENTIAL TODAY AND LESS ON THE WIND FOR
SUNDAY. SUNDAY SHOULD BE DRY WITH ZONAL FLOW AND PRECIPITABLE
WATER VALUES DROPPING BELOW FOUR TENTHS OF AN INCH. TWH

.LONG TERM...VALID FOR MON...TUE...WED...THU...FRI...

QUIET AND MAINLY DRY WEATHER IS EXPECTED NEXT WEEK. MORNINGS WILL
BE COOL WITH OVERNIGHT LOWS IN THE 40S F MOST NIGHTS...WHILE HIGHS
WILL GENERALLY BE BETWEEN 65 AND 75 F MOST DAYS. THE FORECAST WAS
ALMOST ENTIRELY DERIVED FROM THE 00 UTC MULTI-MODEL CONSENSUS WITH
BROAD AGREEMENT ON THE LARGER-SCALE PATTERN IN GLOBAL MODELS. FLOW
ALOFT WILL BE CHARACTERIZED BY A WEAK TROUGH ACROSS THE REGION FOR
MUCH OF THE WEEK...AND WEAK IMPULSES RIDING THROUGH THE FLOW COULD
YIELD ISOLATED SHOWERS. HOWEVER...OUR CONFIDENCE WAS NOT YET HIGH
ENOUGH TO INCLUDE A CHANCE OF THEM IN THE FORECAST ON THE PLAINS.

ONE QUESTION MARK NEXT WEEK IS HOW...AND WHEN...THE REMNANTS OF
POST-TROPICAL CYCLONE IGNACIO IN THE NORTHEAST PACIFIC OCEAN WILL
AMPLIFY THE WEAK TROUGH OVER THE REGION. THE 00 UTC GFS AND ECMWF
HAD SUBTLE DIFFERENCES IN THEIR DEPICTION OF THIS...BUT THEY BOTH
SUGGESTED THE TROUGH AND COOLER WEATHER WILL LIKELY LINGER LONGER
THAN PREVIOUSLY THOUGHT...THROUGH THURSDAY. THEY THEN SUGGESTED A
MORE DEFINITIVE WARM-UP TAKING SHAPE NEXT FRIDAY. SCHULTZ

&&

.AVIATION...

MVFR AND POSSIBLY LOCAL IFR CEILINGS WILL OCCUR THIS MORNING OVER
PARTS OF SOUTHERN MT...WITH SOME IMPROVEMENT /RISING CEILINGS/ BY
AFTERNOON. A FEW SHOWERS OR WEAK STORMS ARE POSSIBLE THROUGH MID
AFTERNOON...BUT STRONGER CELLS WILL LIKELY FORM IN SOUTH CENTRAL
MT BY 21 UTC AND MOVE EAST-NORTHEAST INTO SOUTHEASTERN MT BY THIS
EVENING. MVFR TO IFR CONDITIONS WITH HEAVY RAIN AND STRONG WINDS
ARE POSSIBLE WITH THE STRONGEST STORMS. FINALLY...AREA MOUNTAINS
WILL BE FREQUENTLY OBSCURED THROUGH THIS EVENING. SCHULTZ

&&

.PRELIMINARY POINT TEMP/POPS...

    TDY SUN     MON     TUE     WED     THU     FRI
-------------------------------------------------------
BIL 073 050/072 044/069 044/076 048/071 046/072 046/078
    4/T 80/N    00/B    11/B    11/B    11/U    11/U
LVM 069 043/067 035/067 039/074 043/072 042/074 044/079
    6/T 61/N    00/B    01/B    11/B    11/U    11/U
HDN 078 049/073 042/072 043/077 047/073 045/073 045/079
    4/T 70/N    00/B    01/B    11/B    11/B    11/U
MLS 079 053/072 044/070 045/076 049/071 044/070 045/076
    2/T 70/N    01/B    01/B    11/B    11/B    10/U
4BQ 085 054/071 043/070 047/076 050/072 047/071 046/075
    2/T 30/N    00/B    00/B    11/B    11/B    10/U
BHK 085 054/070 043/070 043/074 047/070 043/068 041/072
    2/T 30/N    11/B    11/B    11/B    11/U    10/U
SHR 081 047/071 038/071 041/076 044/073 042/071 042/076
    3/T 41/B    00/U    00/B    11/B    11/U    11/U

&&

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

&&

$$
WEATHER.GOV/BILLINGS


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