Winter Storm Warning 28, NWS Missoula, MT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE MISSOULA MT
647 PM MDT SAT MAY 22 2010

.UPDATE...LOWERED THE CHANCE OF PRECIPITATION IN NORTHWEST MONTANA
FOR TONIGHT.

&&

.DISCUSSION...RADAR HAS SHOWN MOSTLY CONVECTIVE DRIVEN SHOWERS
ACROSS NORTHWEST MONTANA THIS EVENING WHICH SHOULD DISSIPATE
SHORTLY AFTER SUNSET. BEFORE THEY DISSIPATE GUSTY WINDS CAN BE
EXPECTED NEAR THESE SHOWERS. A NARROW BAND OF SHOWERS MOVING
THROUGH EASTERN WASHINGTON MAY HOLD TOGETHER LONG ENOUGH TO AFFECT
CENTRAL IDAHO SO LEFT THE POTENTIAL FOR SHOWERS IN THIS AREA FOR
TONIGHT. A BAND OF RAIN AND SNOW STRETCHING FROM SOUTHERN IDAHO TO
BUTTE WILL LIKELY PERSIST FOR MUCH OF THE NIGHT SO SNOW
ACCUMULATIONS ARE STILL LIKELY ACROSS LEMHI COUNTY...SILVER BOW
AND DEER LODGE COUNTIES.

&&

.AVIATION...THE ATMOSPHERE REMAINS QUITE MOIST AND UNSTABLE UNDER
A BROAD UPPER LEVEL TROUGH OF LOW PRESSURE. CLOUDS AND SHOWERS
WILL PERIODICALLY OBSCURE THE MOUNTAINS AND PASSES THROUGH THE NEXT
24 HOURS. A FEW THUNDERSTORMS ARE EXPECTED OVER NORTH-CENTRAL
IDAHO AND PORTIONS OF WESTERN MONTANA THROUGH THIS EVENING. THESE
THUNDERSTORM AND EVEN SOME OF THE HEAVIER SHOWERS MAY PRODUCE
SMALL HAIL AND GUSTY WINDS. VISIBILITY AND CEILINGS WILL BE
REDUCED AT TIMES OVER TERMINALS KMSN AND KBTM AS SHOWERS PASS
OVER THESE AREAS TONIGHT.

&&

.PREV DISCUSSION... /ISSUED 250 PM MDT SAT MAY 22 2010/

DISCUSSION...
TONIGHT THROUGH SUNDAY NIGHT...

RADAR AND SATELLITE IMAGERY SHOWING AN INCREASING TREND IN
CONVECTIVE SHOWER DEVELOPMENT THIS AFTERNOON. THE COOL MOIST
UNSTABLE ATMOSPHERE WILL RESIDE OVER THE NORTHERN ROCKIES REGION
THROUGH THIS PERIOD. THERE ARE TWO DISTINCT AREAS WHERE THE BEST
DYNAMICS AND INSTABILITY EXIST THIS AFTERNOON/EVENING FOR
THUNDERSTORM DEVELOPMENT: 1) CENTERED OVER THE CLEARWATER
MOUNTAINS OF NORTH CENTRAL IDAHO AND THE NORTHWEST CORNER OF
MONTANA...AND 2) ACROSS EASTERN LEMHI COUNTY IN NORTH CENTRAL
IDAHO. BRIEF GUSTY WINDS AND SMALL HAIL (PEA-SIZED OR SMALLER) MAY
ACCOMPANY ANY STORM THAT DEVELOPS. SNOW LEVELS WILL REMAIN LOW
ENOUGH TO IMPACT MANY MOUNTAIN PASSES WITH THE GREATEST POTENTIAL
FOR SIGNIFICANT SNOW OVER THE MOUNTAIN PASSES BETWEEN EASTERN
LEMHI AND BEAVERHEAD COUNTIES...AND THE GEORGETOWN LAKE AREA
TONIGHT. ANOTHER COOL SHOWERY DAY IS EXPECTED FOR SUNDAY.

MONDAY...THE NEXT WEATHER FEATURE OF INTEREST WILL BE SPREADING
CONSIDERABLE MOISTURE INTO LEMHI COUNTY AND THE BETTER PORTION
OF WESTERN MONTANA. THIS FEATURE MAY TAP INTO SOME ADDITIONAL SUB-
TROPICAL MOISTURE AS IT APPROACHES FROM THE SOUTH...RESULTING IN
MODERATE PRECIPITATION OVER THE MOUNTAINS ALONG THE CONTINENTAL
DIVIDE.

MONDAY NIGHT THROUGH TUESDAY...THE LATEST MODEL RUNS SHOW THIS
FEATURE TRACKING INTO WYOMING MONDAY NIGHT RATHER THAN THROUGH
SOUTHEASTERN IDAHO...AND UP INTO SASKATCHEWAN CANADA ON TUESDAY.
AT THIS TIME...THE HEAVIEST PRECIPITATION IS STILL EXPECTED TO
OCCUR MAINLY OVER SOUTHWEST MONTANA AND ALONG THE CONTINENTAL DIVIDE
MONDAY NIGHT AND QUICKLY MOVE NORTHWARD ALONG THE DIVIDE AND OUT
OF THE REGION TUESDAY. THE POTENTIAL STILL EXISTS FOR THIS SYSTEM
TO TAP INTO GULF OF MEXICO MOISTURE WHICH COULD FURTHER ENHANCE
PRECIPITATION IN FAVORED UPSLOPE AREAS ALONG THE DIVIDE...
PARTICULARLY IN THE GLACIER PARK REGION. FORECAST MODELS ARE
KEEPING SNOW LEVELS FAIRLY HIGH WITH THIS SYSTEM WHICH COULD
RESULT IN TRAVEL IMPACTS OVER MOUNTAIN PASSES AND HIGHER
ELEVATIONS DUE TO LIGHT SNOW ACCUMULATIONS. ELSEWHERE IN THE
NORTHERN ROCKIES...MOST LOCATIONS SHOULD RECEIVE MEASURABLE
RAINFALL MONDAY NIGHT AS THE REGION REMAINS UNDER AN UNSTABLE
NORTHWEST FLOW ALOFT. OVERALL THERE REMAINS A LEVEL OF UNCERTAINTY
REGARDING TIMING AND PLACEMENT OF THIS SYSTEM SINCE FORECAST
MODELS ARE TRENDING TOWARD THE BULK OF MOISTURE AND ENERGY
POTENTIALLY STAYING FURTHER EAST OF THE DIVIDE DURING THIS
TIMEFRAME. WILL CONTINUE TO MONITOR THIS SYSTEM FOR MORE
DEFINITIVE DETAILS.

TUESDAY NIGHT THROUGH FRIDAY...A BRIEF BREAK IN PRECIPITATION IS
POSSIBLE TUESDAY NIGHT AS YET ANOTHER CLOSED LOW PRESSURE SYSTEM
BEGINS TO IMPACT THE INTERMOUNTAIN WEST WEDNESDAY THROUGH FRIDAY.
INCREASED SOUTHERLY FLOW ON WEDNESDAY SHOULD ADVECT WARMER AIR INTO
THE REGION...POTENTIALLY MAKING WEDNESDAY THE WARMEST DAY OF THE
WEEK. WITH DIFFLUENT FLOW ALOFT...THE POTENTIAL FOR AFTERNOON
THUNDERSTORM DEVELOPMENT EXISTS...HOWEVER MODEL INCONSISTENCIES
WITH THE MONDAY SYSTEM ARE ADVERSELY IMPACTING THE TIMING OF THE
WEDNESDAY SYSTEM AND HAVE OPTED TO KEEP THUNDERSTORM MENTION OUT
OF THE FORECAST FOR THE TIME BEING. THE REMAINDER OF THE WORK WEEK
WILL FALL UNDER THE INFLUENCE OF THIS LOW...WHICH WILL RESULT IN
CONTINUED WET CONDITIONS AND DAYTIME TEMPERATURES COOLER THAN
NORMAL.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY UNTIL 6 AM MDT SUNDAY ABOVE 6000 FEET
     FOR THE BUTTE/BLACKFOOT REGION.

ID...WINTER WEATHER ADVISORY UNTIL 6 AM MDT SUNDAY ABOVE 6000 FEET
     FOR THE EASTERN LEMHI COUNTY.

&&

UPDATE/AVIATION...LOEFFELBEIN
PREVIOUS DISCUSSION....MSJ/KITSMILLER