000
FXUS65 KBYZ 211512
AFDBYZ

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BILLINGS MT
912 AM MDT TUE MAY 21 2013

.UPDATE...

LARGE UPPER LOW OVER THE EASTERN DAKOTAS WILL CONTINUE TO ROTATE
CLOUDS WESTWARD ACROSS EASTERN MONTANA TODAY. RAIN SHOWERS WILL
CONTINUE TO BE POSSIBLE MAINLY EAST OF A MILES CITY TO BROADUS
LINE TODAY BUT THESE WILL BE SHIFTING EAST BY LATE AFTERNOON.
EXPECT A MOSTLY SUNNY AND MILDER DAY TODAY FOR THE REMAINDER OF
THE FORECAST AREA. PATCHY FOG WILL CONTINUE FOR THIS MORNING FOR
THE LIVINGSTON AREA BUT THIS HAS BEEN BREAKING UP DURING THE PAST
HOUR. CURRENT FORECAST IN GOOD SHAPE WITH NO UPDATES PLANNED THIS
MORNING. HOOLEY

&&

.SHORT TERM...VALID FOR TDY AND WED...

THE WEEKEND RAIN-MAKER WILL CONTINUE EXITING STAGE RIGHT...LEAVING
EASTERLY LOW-LEVEL FLOW IN PLAY THE NEXT SEVERAL DAYS OUT AHEAD OF
A 500-HPA LOW WHICH MOVES INTO THE PACIFIC NORTHWEST. THIS PATTERN
WILL YIELD SEASONABLE TEMPERATURES...AND SOME CHANCE OF STORMS FOR
SOUTH CENTRAL MT...ESPECIALLY LATER ON WED AND WED NIGHT.

TODAY...EARLY-MORNING RADAR TRENDS SUGGEST THAT LIKELY POPS REMAIN
NECESSARY AROUND BAKER AND EKALAKA INTO THE MORNING TO ACCOUNT FOR
RESIDUAL BANDS OF SHOWERS ROTATING WESTWARD INTO THAT AREA. THE 00
UTC GUIDANCE IS CLOSELY CLUSTERED IN ITS QPF OUTPUT...AND IT SHOWS
A SHARP WESTERN GRADIENT TO THE SHOWERS DICTATED BY A WEDGE OF DRY
AIR OVER CENTRAL MT. SURFACE OBSERVATIONS AS OF 09 UTC SUGGEST THE
LOW- AND MID-LEVEL DRYING IS BEGINNING TO INFLUENCE THE MILES CITY
AREA...AND MOST 00 UTC GUIDANCE KEEPS SHOWERS EAST OF THERE TODAY.
EVEN RECENT RAP RUNS AND THE 06 UTC NAM KEEP SHOWERS EAST OF MILES
CITY SO WE WENT WITH A DRY FORECAST THERE THIS MORNING. POPS OVER
THE BAKER AND EKALAKA AREAS DROP BACK TO THE CHANCE CATEGORY AFTER
18 UTC AS WELL SINCE MID-LEVEL FORCING AND MOISTURE ADVECTION WILL
BE WEAKENING BY THAT TIME. OTHERWISE...THE EASTERLY LOW-LEVEL FLOW
WILL BECOME ESTABLISHED OVER MUCH OF THE AREA BY AFTERNOON...WHICH
COULD HELP ASSIST WITH SOME CUMULUS DEVELOPMENT GIVEN THE RESIDUAL
NEAR-SURFACE MOISTURE AFTER WEEKEND RAINFALL. THERE IS JUST ENOUGH
INSTABILITY FORECAST BY THE 00 UTC GUIDANCE OVER THE MOUNTAINS FOR
US TO INCLUDE SLIGHT CHANCES OF CONVECTION AFTER 18 UTC THERE TOO.
HIGHS TODAY WERE LARGELY BASED ON A CONSENSUS OF 00 UTC MOS AS THE
EASTERLY FLOW WILL TEND TO LIMIT MIXING...AND USING BIAS-CORRECTED
FLAVORS OF GUIDANCE SEEMS A BIT DANGEROUS GIVEN NEW SOIL MOISTURE.

TONIGHT...QUIET CONDITIONS SHOULD PREVAIL...AND IN FACT BASED ON A
BLEND OF 00 UTC MOS OUTPUT...LOWS WILL PROBABLY BE BELOW NORMAL IN
MANY SPOTS. WE HAVE A DRY FORECAST IN PLAY AFTER 06 UTC EXCEPT FOR
EXTREME SOUTHEASTERN MT. WE WILL NEED TO BE MINDFULL FOR POTENTIAL
FOG DEVELOPMENT...BUT IT SEEMS THAT NEAR-SURFACE FLOW MIGHT END UP
STAYING JUST STRONG ENOUGH TO PREVENT ITS FORMATION...SO IT IS NOT
IN THE FORECAST AT THIS POINT.

WED INTO WED NIGHT...A BETTER CHANCE OF CONVECTION WILL EXIST OVER
SOUTH CENTRAL MT AS MID-LEVEL LAPSE RATES STEEPEN TO AROUND 7 C/KM
ON STRENGTHENING SOUTHERLY FLOW ALOFT. MOST 00 UTC MODEL SOLUTIONS
GENERATE CONVECTION TOWARD 00 UTC...SO OUR GOING CHANCE-STYLE POPS
ARE CERTAINLY WARRANTED. THE ONLY QUESTION MARK MIGHT BE TIMING OR
EASTWARD EXTENT OF THE WEAK SHORT WAVE THAT GUIDANCE KEYS IN ON TO
YIELD CONVECTION...ESPECIALLY SINCE THE MERIDIONAL FLOW MIGHT TAKE
IT FURTHER WEST THAN EXPECTED. THERE IS PROBABLY A NON-ZERO THREAT
OF SEVERE WEATHER IF STORMS DEVELOP...BUT IT WILL BE KEPT IN CHECK
BY A LACK OF BUOYANCY /MUCAPE 500 J/KG OR LESS/. OVER SOUTHEASTERN
MT...STORM CHANCES ARE MINIMAL OWING TO MID-LEVEL RIDGING. SCHULTZ

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

MEDIUM RANGE MODELS WERE PRETTY CONSISTENT THROUGH THE PERIOD
WITH SOME TIMING DIFFERENCES OF SHORTWAVES PROGGED TO GET KICKED
OUT OF A DEEP UPPER TROUGH OVER THE WEST MOVING INLAND.

THURSDAY COULD BE AN ACTIVE DAY FOR THUNDERSTORMS...MAINLY
CENTRAL AND WESTERN ZONES. THE DEEP WESTERN TROUGH ADVANCES EAST
SLOWLY AND LOWERS HEIGHTS THROUGH THE DAY. LOW LEVEL EAST TO
SOUTHEAST WINDS WILL ALLOW FOR DEWPOINTS TO HANG AROUND 50 DEGREES
OVER CENTRAL ZONES...WITH SLIGHTLY LOWER VALUES OVER THE WEST.
EFFECTIVE BULK SHEAR INCREASES TO AROUND 35KTS WITH CAPES REACHING
AS HIGH AS 1000 J/KG. STRONG SURFACE CONVERGENCE WITH SHORTWAVE
ENERGY WILL IGNITE THUNDERSTORMS OVER WESTERN ZONES IN THE
AFTERNOON. STORMS WILL MOVE NORTH TO NORTHEAST AND KICK OUT OF THE
FORECAST FAIRLY QUICKLY IN THE EVENING. SHEAR WOULD BE STRONG
ENOUGH TO PRODUCE SOME STRONG STORMS WITH THE MAIN THREAT BEING
HAIL. WILL HAVE TO WATCH THIS EVENT FOR A POSSIBLE SEVERE WEATHER
DAY. WILL MENTION STRONG STORMS IN THE HWO.

FRIDAY IS A DAY A BIT IN QUESTION AS MODELS DIFFER ON TIMING AND
STRENGTH OF A SHORTWAVE KICKING OFF THE MAIN LOW TO THE WEST. THE
ECMWF WAS STRONGEST AND SLOWEST WITH THIS WAVE AS IT CROSSES THE
FORECAST AREA DURING THE DAY. THE GFS WAS WEAKER AND TAKES IT
ACROSS LATE THURSDAY NIGHT. THE ECMWF WOULD BE WETTER. INHERITED
SCATTERED POPS FOR FRIDAY...BUT IF THE GFS WAS RIGHT...IT WILL BE
DRIER.

DRIER MID LEVEL AIR WILL RUSH IN BEHIND THE SHORTWAVE REGARDLESS
OF THE TIMING. THIS WILL SET UP A MAINLY DRY PERIOD FOR THE
WEEKEND WITH THE MAIN ENERGY STAYING NORTH. HIGH BASED SHOWERS OR
THUNDERSTORMS CAN NOT BE RULED OUT OVER THE WEEKEND THOUGH AS
MODELS KICK DISTURBANCES ACROSS IN SOUTHWEST FLOW. DEWPOINTS
SHOULD BE MUCH LOWER AND THESE SHOWERS WILL NOT CONTAIN A GREAT
DEAL OF PRECIPITATION AND BE MORE WIND PRODUCERS. WILL KEEP POPS
LOW. TEMPERATURES SHOULD SLIGHTLY ABOVE SEASONAL AVERAGES THROUGH
PERIOD. TWH
&&

.AVIATION...

SOME LINGERING SHOWERS OVER KBHK SHOULD DISSIPATE BY THE EARLY
AFTERNOON. FOG IS SLOWLY LIFTING NEAR KLVM...BUT STILL MAY CAUSE
SOME IMPACTS...SO UPDATED THE KLVM TAF TO INCLUDE ONE MORE HOUR OF
LOWERED VISIBILITIES DUE TO FOG. THE REST OF THE AREA SHOULD
REMAIN UNDER VFR CONDITIONS TODAY...HOWEVER THERE IS A CHANCE FOR
AN ISOLATED SHOWER OR THUNDERSTORM...MOSTLY NEAR THE HIGHER
TERRAIN. SINGER
&&

.PRELIMINARY POINT TEMP/POPS...

    TDY WED     THU     FRI     SAT     SUN     MON
-------------------------------------------------------
BIL 067 043/068 047/074 050/075 048/077 051/077 051/073
    1/B 12/T    33/T    43/T    22/T    22/T    23/T
LVM 069 039/067 044/065 041/068 040/075 041/076 042/070
    1/F 14/T    45/T    43/T    22/T    22/T    23/T
HDN 069 043/070 048/077 050/078 048/079 051/080 051/076
    1/B 12/T    33/T    33/T    22/T    22/T    23/T
MLS 059 044/067 047/073 053/075 051/078 055/078 054/076
    1/E 11/B    11/N    23/T    22/T    23/T    33/T
4BQ 056 040/065 046/074 052/076 050/079 053/080 052/077
    3/W 11/B    11/N    13/T    22/T    23/T    33/T
BHK 050 040/061 043/067 050/070 050/075 053/073 052/073
    7/W 21/N    11/N    13/T    22/T    33/T    33/T
SHR 061 037/068 045/075 049/075 046/078 048/079 048/075
    1/B 12/T    32/T    23/T    22/T    22/T    23/T

&&

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

&&

$$
WEATHER.GOV/BILLINGS