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000
FXUS65 KBYZ 191544
AFDBYZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BILLINGS MT
944 AM MDT SUN MAY 19 2013

.UPDATE...
DEEP UPPER TROUGH REMAINS OVER THE REGION TODAY AND IS BECOMING
NEGATIVELY TILTED. THE UPPER TROUGH IS ALSO CLOSING OFF AN UPPER.
CIRCULATION AROUND THE UPPER LOW IS WRAPPING PRECIPITATION INTO
OUR AREA FROM BAKER TO LIVINGSTON THIS MORNING. HAVE INCREASED
POPS ACROSS SOUTH CENTRAL AND SOUTHEAST MONTANA THIS MORNING TO
100`S AS VERY MOIST FLOW MOVING INTO THE AREA. UPPER AIR SOUNDINGS
AT GGW AND UNR SHOW PW`S NEAR 1 INCH FOR TODAY. STRONG UPPER LEVEL
FORCING ALONG WITH ISENTROPIC LIFT/BAND OF FRONTOGENESIS AND DEEP
UPSLOPE FLOW...WILL ALL COMBINE TO BRING PERIODS OF LIGHT TO
MODERATED RAINFALL ACROSS THE FORECAST AREA THE ENTIRE DAY. THE
MAJOR CONCERN CONTINUES TO BE THE BURN SCARS FROM LAST YEAR AND
WILL KEEP CURRENT FLASH FLOOD WATCH FOR THESE AREAS GOING. NO
OTHER CHANGES TO THE FORECAST FOR THIS MORNING. RICHMOND

&&

.SHORT TERM...VALID FOR TDY AND MON...

CONFIDENCE IN A SIGNIFICANT RAINFALL EVENT ACROSS MUCH OF THE AREA
OVER THE NEXT 36 TO 48 HOURS IS GROWING. ANOTHER ONE TO TWO INCHES
OF RAIN LOOKS LIKELY...AND REALISTICALLY-SPEAKING TOTALS MAY BE IN
THE 2 TO 4 INCH RANGE FROM THE BIG HORN MOUNTAINS NORTHEAST ACROSS
THE PLAINS OF FAR SOUTH CENTRAL AND MUCH OF SOUTHEASTERN MT.

AT 09 UTC...REGIONAL RADAR AND MOISTURE CHANNEL IMAGES SUGGEST THE
MID-LEVEL TROUGH IS BEGINNING TO CLOSE OFF OVER WESTERN SD...WHICH
IS WHAT ALL OF THE 00 UTC MODEL SOLUTIONS SUGGESTED WILL HAPPEN BY
MID MORNING. THOSE 00 UTC MODELS ALSO HAD A NOTABLE WESTWARD SHIFT
IN THE PLACEMENT OF THE MID- AND UPPER-LEVEL LOW WHICH IS EXPECTED
TO WOBBLE ACROSS THE DAKOTAS TODAY AND MON...AND THAT PUTS MUCH OF
THE AREA WITHIN FAVORABLE DYNAMIC FORCING FOR RAINFALL. THE 00 UTC
ECMWF ALSO MADE A SHIFT WEST AND NORTH...BUT IT IS NOT AS WET OVER
MT AS THE REST OF THE GUIDANCE HEADING INTO MON...SO WE CAUTIOUSLY
WEIGHTED THE FORECAST AWAY FROM THAT IDEA FOR NOW SINCE IT ENDS UP
AS A BIT OF AN OUTLIER.

TODAY...SHOWERS WHICH FORMED ACROSS SOUTH CENTRAL MT OVERNIGHT ARE
EXPECTED TO CONTINUE AND FILL IN FURTHER. MEANWHILE...BOTH FORCING
AND MOISTURE ADVECTION WILL INCREASE ALOFT OVER SOUTHEASTERN MT BY
MIDDAY AS THE 700-HPA LOW DEEPENS OVER WESTERN SD. CONVECTION SEEN
IN THE DAKOTAS EARLY THIS MORNING SHOULD ROTATE WESTWARD...AND NEW
PRECIPITATION WILL LIKELY FILL IN AS WELL. THE 00 UTC GFS AND GFS-
FED RAP RUNS ARE ESPECIALLY STRONG WITH THE 700-HPA LOW...AND THEY
ARE ALSO THE FARTHEST NORTHWEST MODEL SOLUTIONS WITH THIS FEATURE.
THAT IS NOT OUT OF THE QUESTION...AND IF IT ENDS UP BEING RIGHT WE
MAY EVEN HAVE OUTRIGHT HEAVY STRATIFORM RAIN BY AFTERNOON ACROSS A
GOOD PART OF THE AREA. NEEDLESS TO SAY...CATEGORICAL POPS NEAR 100
PERCENT ARE JUSTIFIED NEARLY EVERYWHERE TODAY.

TONIGHT...MOISTURE-LADEN AND DYNAMIC FORCING WILL CONTINUE FEEDING
PRECIPITATION OVER THE AREA...ESPECIALLY OVER SOUTHEASTERN MT NEAR
THE MID-LEVEL LOW...AND WHERE THE 500-300 HPA Q-VECTOR CONVERGENCE
IS STRONGEST. POPS NEAR 100 PERCENT THUS CONTINUE FOR MOST PLACES.
THE 03 UTC SREF HAS A 70 TO 90 PERCENT PROBABILITY FOR AT LEAST AN
INCH OF RAIN IN SOUTHEASTERN MT OVERNIGHT...WHICH IS NOTEWORTHY OF
AN ENSEMBLE SOLUTION. PRECIPITABLE WATER VALUES ARE EXPECTED TO BE
FROM 0.80 INCHES IN SOUTH CENTRAL MT TO AN INCH OR MORE ACROSS THE
SOUTHEASTERN PART OF MT...AND GIVEN 12 HOURS OF FORCING...RAINFALL
WILL PROBABLY BE AT LEAST NEAR THOSE VALUES IN MOST AREAS. OUR QPF
WAS INCREASED SIGNIFICANTLY OVER PREVIOUS FORECASTS...BUT IT STILL
LAGS THE 00 UTC MULTI-MODEL CONSENSUS IN SOME CASES...AND SO THERE
IS A CHANCE WE ARE STILL UNDERPLAYING TOTAL RAINFALL. THAT WILL BE
PARTICULARLY TRUE IF THE 00 UTC ECMWF ENDS UP BEING AN INCORRECTLY
DRY OUTLIER.

MON...MOISTURE SHOULD CONTINUE WRAPPING AROUND THE MID- AND UPPER-
LEVEL CIRCULATION AND BACK INTO SOUTHEASTERN MT THROUGH THE DAY. A
DRYING TREND COULD BEGIN IN SOUTH CENTRAL MT BY AFTERNOON...MAINLY
AWAY FROM UPSLOPE AREAS...AS FORECAST SOUNDINGS IN LOCALES SUCH AS
BILLINGS START SHOWING LESS SATURATION AFTER 18 UTC. IT COULD TAKE
UNTIL VERY LATE MON NIGHT OR EVEN INTO TUE THOUGH BEFORE DRYING IS
ABLE TO TAKE HOLD IN SOUTHEASTERN MT. LARGE LOWS LIKE THIS ONE ARE
OFTEN SLOWER TO MOVE THAN EXPECTED INITIALLY. THE RIDGE THAT FORMS
AT 500 HPA TO THE NORTH OF THE LOW IN SOUTHERN CANADA WILL PLAY A
BIG ROLE IN ITS SPEED SINCE IT WILL BE A BLOCKING MECHANISM.

SO...IN SUMMARY...THIS LOOKS INCREASINGLY LIKE A BIG RAIN EVENT IN
MUCH OF THE AREA. WHILE CONVECTIVE ELEMENTS WILL BE LESS LIKELY AS
WE GO FORWARD...THERE IS STILL ENSEMBLE SUPPORT FOR THREE AND SIX-
HOUR RAIN TOTALS OF A HALF INCH OR MORE IN MANY AREAS. WE CHOSE TO
ISSUE A FLASH FLOOD WATCH FOR THE BURN SCAR AREAS OF SOUTH CENTRAL
AND SOUTHEASTERN MT THROUGH 12 UTC TUE SINCE AMOUNTS LIKE THAT MAY
BE ENOUGH TO CAUSE ISSUES IN SOME OF THOSE AREAS. SCHULTZ

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

CONTINUE TO SLOW THE TRANSITION TO DRIER WEATHER. MODELS
STRUGGLING WITH THE EXIT SPEED OF THE DEEP UPPER LOW CURRENTLY
PROVIDING GENEROUS PRECIPITATION TO THE FORECAST AREA. MODELS WERE
FAIRLY CONSISTENT NOW IN VERY SLOWLY SLIDING THE UPPER LOW
EASTWARD. DIFFERENCES CENTER ON THE POSITION OF THE BLOCKING HIGH
IN SOUTHERN CANADA. THE GFS WAS FARTHER SOUTH AND THUS CLEARS THE
FORECAST AREA WITH DYNAMICS SOONER THAN THE ECMWF ON TUESDAY. THE
ECMWF HAS THE BLOCKING HIGH FARTHER NORTH AND THUS LINGERS ENERGY
INTO TUESDAY OVER THE SOUTHEAST. WILL KEEP SCATTERED POPS IN THE
EAST ON TUESDAY WHILE THE WEST SHOULD EFFECTIVELY DRY OUT. LOWERED
HIGH TEMPERATURES AS THE UPPER RIDGE WAS NOT NEARLY AS AMPLIFIED
ON 00Z RUNS.

THE NEXT UPSTREAM...AND VERY DEEP...UPPER LOW OVER THE NORTHWEST
SLIDES INTO PLACE AND TURNS THE FLOW SOUTHERLY OVER SOUTHERN
MONTANA AND NORTHERN WYOMING BY WEDNESDAY. THIS WILL ESTABLISH LOW
LEVEL EASTERLY FLOW WITH LOWER SURFACE PRESSURES OVER THE GREAT
BASIN ALLOWING DEWPOINTS TO STAY UP. MODELS CONTINUE TO BACK OFF
WITH THE DRIER AIR ALOFT AND LINGER 700MB MOISTURE INTO FRIDAY NOW
AS THE UPPER LOW INCHES EASTWARD. WILL KEEP POPS MENTIONED FROM
WEDNESDAY ONWARD WITH THE BEST CHANCE OVER THE WEST CLOSER TOO THE
UPPER LOW AND UNDER THE INFLUENCE OF SLIGHTLY COOLER AIR ALOFT.
THE ECMWF WAS A LITTLE FASTER IN KICKING THE UPPER LOW NORTHEAST
ON FRIDAY BUT BOTH MODELS HAVE THE STRONGEST DYNAMICS HEADED NORTH
OF THE FORECAST AREA...SO DID NOT GO THAT HIGH WITH POPS FRIDAY
AND SATURDAY. OVERALL...COOLED THINGS DOWN A LITTLE BIT THROUGHOUT
THE EXTENDED WITH A LESS AMPLIFIED RIDGE NOW ADVERTISED BETWEEN
THE TWO DEEP UPPER LOWS. TWH
&&

.AVIATION...

ACTIVE WEATHER WILL CONTINUE TODAY AS AN AREA OF LOW PRESSURE
ALOFT MOVES SLOWLY THROUGH THE REGION. EXPECT SHOWERS AND ISOLATED
THUNDERSTORMS TODAY...WITH AREAS OF MVFR TO IFR CONDITIONS. SOME
FOG IS ALSO POSSIBLE EARLY THIS MORNING DUE TO THE ABUNDANT
SURFACE MOISTURE IN PLACE. FOG MAY REDUCE FLIGHT CONDITIONS TO IFR
OR LOWER IF IT DEVELOPS...AND THIS IS MOST LIKELY TO OCCUR IS IN
EASTERN PARTS NEAR KMLS AND KBHK. MOUNTAINS WILL BE OBSCURED. TWH

&&

.PRELIMINARY POINT TEMP/POPS...

    TDY MON     TUE     WED     THU     FRI     SAT
-------------------------------------------------------
BIL 058 049/061 045/069 046/069 049/072 049/074 050/075
    +/T +8/R    41/B    12/T    42/T    22/T    22/T
LVM 056 046/058 038/065 040/067 040/067 041/069 042/069
    9/T 86/R    41/B    14/T    44/T    43/T    33/T
HDN 059 050/061 045/070 044/071 049/074 048/077 050/077
    +/T ++/R    51/E    12/T    32/T    22/T    22/T
MLS 059 051/060 046/064 046/068 050/071 050/074 052/076
    +/T ++/R    53/W    12/T    22/T    22/T    22/T
4BQ 059 049/057 044/062 044/068 048/072 049/074 051/076
    +/T ++/R    64/W    22/T    22/T    22/T    22/T
BHK 059 049/057 043/058 044/064 047/067 048/070 051/074
    +/T ++/R    84/W    22/T    22/T    22/T    22/T
SHR 055 047/056 042/062 040/069 046/072 046/075 048/075
    +/T ++/R    52/W    12/T    32/T    22/T    22/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...FLASH FLOOD WATCH IN EFFECT THROUGH LATE MONDAY NIGHT FOR
      ZONES 29>31-36>38-57-58.
WY...NONE.

&&

$$
WEATHER.GOV/BILLINGS