Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 052330
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
530 PM MDT TUE MAY 5 2015

AVIATION SECTION UPDATED

.DISCUSSION...

TONIGHT THROUGH THURSDAY...AN UPPER LEVEL TROUGH OF LOW PRESSURE
OVER THE PACIFIC NORTHWEST IS KEEPING THE FLOW ALOFT MOIST AND
UNSTABLE OVER MONTANA THIS AFTERNOON. AS A RESULT, SCATTERED
SHOWERS AND THUNDERSTORMS HAVE ALREADY DEVELOPED ALONG A COLD
FRONT THAT IS CURRENTLY CENTERED OVER NORTH CENTRAL AND SOUTHWEST
MONTANA. SOME OF THE STRONGER STORMS HAVE PRODUCED WIND GUSTS UP
TO 40 MPH, BRIEF HEAVY RAIN, AND PEA SIZED HAIL. STORMS WILL
INCREASE IN COVERAGE THIS AFTERNOON AND EVENING ALONG AND EAST OF
A LINE FROM CHESTER TO HELENA TO DILLON AS THE COLD FRONT MOVES
EAST INTO MORE MOIST AND UNSTABLE AIR FROM THE GULF OF MEXICO.
THE MAIN THREATS FROM THESE STORMS WILL BE SIMILAR TO STORMS
MENTIONED ABOVE. GUSTY WESTERLY WINDS HAVE ALREADY DEVELOPED
BEHIND THE FRONT IN THE CUT BANK AREA, AND THEY SHOULD SPREAD EAST
WITH THE FRONT. THE UPPER LEVEL TROUGH WILL THEN APPROACH MONTANA
OVERNIGHT, BRINGING DRIER AIR AND A DECREASING CHANCE OF SHOWERS
OVERNIGHT INTO WEDNESDAY MORNING. HOWEVER, THE TROUGH WILL BEGIN
TO SPLIT WEDNESDAY AFTERNOON, ALLOWING THE MAIN ENERGY TO DIVE
INTO THE SOUTHWESTERN UNITED STATES WEDNESDAY NIGHT INTO THURSDAY.
AS A RESULT, MOISTURE FROM OFF THE SOUTHWEST COAST WILL BE DRAWN
NORTH INTO MONTANA, KEEPING THE MAIN FOCUS OF PRECIPITATION ACROSS
SOUTHWEST MONTANA WEDNESDAY THROUGH THURSDAY. SURFACE HIGH
PRESSURE WILL ALSO MOVE SOUTH OUT OF CANADA THROUGH THIS PERIOD,
SHIFTING WINDS MORE NORTHERLY AND COOLING TEMPERATURES TO BETWEEN
5 AND 10 DEGREES BELOW NORMAL. THIS WILL LOWER SNOW LEVELS ACROSS
THE AREA, CAUSING LIGHT MEASURABLE SNOW ABOVE MOUNTAIN PASS LEVEL,
ESPECIALLY WEDNESDAY NIGHT.  COULSTON

THURSDAY NIGHT THROUGH TUESDAY...FORECAST MODELS CONTINUE TO SHOW
A LOT OF VARIABILITY AND REMAIN RATHER UNHELPFUL DURING THE MEDIUM
RANGE TIME FRAME. LATEST RUNS START OFF THE PERIOD SHOWING SPLIT
FLOW ALOFT WITH A CLOSED LOW ALONG THE CENTRAL CALIFORNIA COAST
AND AN OPEN WAVE EXTENDING NORTH-NORTHEASTWARD THROUGH MONTANA.
MAIN ENERGY WITH THIS OPEN WAVE REMAINS IN CENTRAL CANADA SO THAT
ONLY A SLIGHT CHANCE TO CHANCE FOR SHOWERS CAN BE EXPECTED IN
MONTANA WITH THE BEST CHANCES EXPECTED IN SOUTHWEST PORTIONS OF
THE STATE. IT SHOULD ALSO BE NOTED THAT CONFIDENCE IN THIS
FORECAST REMAINS EXTREMELY LOW. MODELS ARE CURRENTLY SUGGESTING
THAT THE CLOSED LOW OVER CALIFORNIA WILL SWEEP ACROSS THE FOUR
CORNERS REGION BY SATURDAY BUT DIVERGE IN THEIR SOLUTIONS FOR THE
REMAINDER OF THE FORECAST PERIOD. HAVE OPTED TO TREND THE FORECAST
TOWARD CLIMATOLOGICAL HIGHS AND LOWS WHILE KEEPING POPS GENERALLY
BELOW CLIMATOLOGY. MPJ

&&

.AVIATION...
UPDATED 2330Z.

BROAD AREA OF -SHRA WITH SOME SHORT-LIVED -TSRA WILL CONTINUE OVER
MOST OF CENTRAL AND SOUTHWEST MT UNTIL ABOUT 03-04Z.  CONVECTIVE
ACTIVITY IS AHEAD OF A SURFACE COLD FRONT THAT WAS JUST WEST OF KCTB
AND KHLN AT 23Z.  FRONT IS FORECAST TO MAKE STEADY EASTWARD PROGRESS
THIS EVENING WITH PRECIPITATION TAPERING OFF WEST-TO-EAST TONIGHT,
THOUGH -SHRA WILL LINGER VCNTY KHVR/KLWT UNTIL 09-12Z. CONDITIONS
SHOULD REMAIN MOSTLY VFR WITH BRIEF PERIODS OF MVFR CEILINGS NEAR
ANY STRONGER SHOWERS OR THUNDERSTORMS.  WINDS WILL SHIFT TO WEST-
NORTHWEST AND GUST TO 20-30 KTS BEHIND THE FRONT BEFORE DECREASING
LATER TONIGHT.
WARANAUSKAS

&&

.HYDROLOGY...
UPDATED 05/2135Z.

SNOWPACK HAS BECOME QUITE RIPE IN THE MOUNTAINS OF LATE. WITH THE
POTENTIAL FOR BRIEF MODERATE RAIN AT TIMES IN THE MOUNTAINS OVER
THE NEXT WEEK OR SO (ESPECIALLY WITH THUNDERSTORMS TODAY AND
TONIGHT), RIVERS, CREEKS, AND STREAMS WILL RISE AT TIMES. STREAM
FLOWS WILL BECOME QUITE BRISK AND LEVELS COULD APPROACH BANKFULL.
HOWEVER, WIDESPREAD FLOODING IS NOT ANTICIPATED AT THIS TIME.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  59  33  54 /  70  10  10  10
CTB  33  55  28  55 /  20  10  10  10
HLN  40  63  37  57 /  60  10  20  30
BZN  40  60  36  54 /  70  20  30  40
WEY  32  58  30  54 /  60  50  40  60
DLN  39  60  38  55 /  60  30  50  50
HVR  43  62  31  57 /  70  10  10  10
LWT  42  58  31  53 /  80  20  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

WEATHER.GOV/GREATFALLS


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