FXUS65 KBYZ 112235
AFDBYZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BILLINGS MT
335 PM MST FRI DEC 11 2009
.SHORT TERM...VALID FOR SAT AND SUN...
MAJOR WEATHER CHANGE THIS WEEKEND AS A MILDER SATURDAY WILL BE
RUDELY REPLACED BY ANOTHER BLAST OF ARCTIC AIR INTO MONTANA AND
NORTH CENTRAL WYOMING. SURFACE TROUGH LAYING ACROSS SOUTHEASTERN
MONTANA WITH FLOW ALOFT CONTINUING TO BACK IN RESPONSE TO AN UPPER
LOW OVER NORTHERN CANADA ELONGATING WESTWARD. RELATIVELY WARMER
WEATHER TO CONTINUE TONIGHT AS MIXING AND DOWNSLOPE FLOW KEEPS
BREEZY WINDS OVER THE WESTERN ZONES...AND EVEN SOUTHEASTERLY FLOW
OVER THE EASTERN ZONES WILL KEEP TEMPERATURES FROM FALLING TOO FAR.
MOISTURE SPREADING INTO SOUTHWEST MONTANA FROM A SYSTEM OVER
CALIFORNIA WILL START GENERATING INCREASED MOUNTAIN PRECIPITATION.
SATURDAY THE SURFACE TROUGH BEGINS TO SHIFT SOUTHWARD AS PRESSURE
FALLS OCCUR OVER WYOMING AND THIS WILL SHIFT THE WARMER AIR A BIT
WESTWARD BUT STILL WELL INTO THE 20S FOR MOST AREAS. PRECIPITATION
CONTINUES TO BE MAINLY OROGRAPHICALLY DRIVEN INTO THE MOUNTAINS AS
THE WESTERLY FLOW ALOFT WILL BE MAINLY DOWNSLOPISH FOR THE PLAINS.
SATURDAY NIGHT AN ARCTIC SURGE MOVES THROUGH NORTHERN MONTANA
HEADING TOWARDS THE AREA WITH SURFACE PRESSURE FALLS CONTINUING
OVER WYOMING. DOWNSLOPE WILL BEGIN TO LOOSE ITS INFLUENCE SATURDAY
NIGHT AND A FEW MORE SHOWERS WILL REACH THE PLAINS WITH A CHANCE
FOR THE NORTHERN ZONES TO FEEL THE INFLUENCE OF THE ARCTIC FRONT
BY EARLY SUNDAY MORNING. BY SUNDAY MORNING A WINTER WEATHER
EPISODE IS IN PLACE AS THE ARCTIC FRONT WILL SURGE THROUGH THE
AREA WHILE THE AIRMASS HAS BECOME FAIRLY MOIST. MODELS PROGGING
HIGHER THETA E VALUES ALONG AN AXIS TO THE LEE OF THE BIG HORNS TO
AROUND THE NORTH SIDE OF THE CRAZY MOUNTAINS. WHILE NOT CONFIDENT
WHERE SNOWFALL ACCUMULATIONS WILL BE GREATEST...MODELS ARE HINTING
AT BETTER BANDS OF PRECIPITATION DEVELOPING...WITH SOME UPSLOPE
AREAS BEING FAVORED AND OTHERS FAVORED AS UPPER LEVEL SUPPORT
EXISTS FROM WEAK WAVE CROSSING THE AREA. MAIN THINKING THAT THIS
SYSTEM WILL HAVE A CHANCE TO CAUSE MORE DIFFICULTY THAN PREVIOUS
ARCTIC STEM FROM THREE IDEAS...1. ITS A SHALLOWER ARCTIC SO FAVORED
SNOW GROWTH REGION WILL REMAIN OVER THE AREA LONGER. 2. PRESSURE
GRADIENT LINGERING OVER THE AREA WILL CAUSE STRONGER WINDS TO
PERSIST CAUSING BLOWING AND DRIFTING AND DRIVING WIND CHILLS. 3.
THE AIRMASS KEEPS DEEPER MOISTURE OVER THE AREA SO PRECIPITATION
SHOULD BE HEAVIER THAN PREVIOUS ARCTIC. ONE IMPORTANT DIFFERENCE
IS THAT ROAD TEMPERATURES SHOULD BE BELOW FREEZING THROUGH THE
EPISODE AND SNOW SHOULD STAY SNOW AND NOT MELT INTO ICE. HAVE
ISSUEE A WINTER STORM WATCH TO COVER THESE THREATS AND LATER
SHIFTS WILL CLARIFY THE THREATS FOR SPECIFIC AREAS AS DETAILS
BECOME CLEARER. DO THINK THE DRAMATIC CHANGE FROM A PLEASANT
SATURDAY TO A HAZARDOUS WEATHER SUNDAY IS WORTH HIGHLIGHTING GIVEN
THAT TRAVEL IS BEGINNING AN UPSWING IN ADVANCE OF THE HOLIDAYS.
SUNDAY NIGHT THE WINTRY WEATHER WILL CONTINUE AS TEMPERATURES
PLUNGE BELOW ZERO. BORSUM
.LONG TERM...VALID FOR MON...TUE...WED...THU...FRI...
MODELS WERE VERY CONSISTENT ON HAVING MONDAY BEING THE COLDEST
DAY WITH THE NEXT ARCTIC INVASION. THE MODELS WERE SETTING UP A
BAROCLINIC ZONE OVER NORTHERN LOCATIONS. KEPT THE IDEA OF LIKELY
POPS FOR MONDAY. RAISED POPS SOME FOR MONDAY NIGHT AS UPSLOPE
FLOW AND MOISTURE OVER RUNNING THE ARCTIC BOUNDARY CONTINUES. THE
ARCTIC LIFTS OUT VERY QUICKLY ON TUESDAY. TUESDAY WILL BE A VERY
TRICKY DAY FOR TEMPERATURES AS FRESH SNOW COVER WILL BE
COUNTERACTED BY STRONG WARM AIR ADVECTION. SHOULD BE ABLE TO
REACH INTO THE 20S OVER WESTERN AND CENTRAL ZONES...AND MAY EVEN
BE A BIT WARMER. THE EAST WILL BE SLOWER TO REBOUND WITH HIGHS
HELD IN THE TEENS. RIDGING TAKES OVER FOR WEDNESDAY THROUGH
SATURDAY AND THIS WILL PROVIDE A PERIOD OF MILD AND MAINLY DRY
WEATHER. HUMPHREY
&&
.AVIATION...
STRONG SOUTHWEST WINDS AT LIVINGSTON WILL GUST TO 40KTS EARLY
THIS EVENING BEFORE DECREASING BY LATE EVENING. VFR FLYING
WEATHER CAN BE EXPECTED TONIGHT AND AGAIN SATURDAY. SOME MID
LEVEL MOISTURE WILL WORK INTO THE MOUNTAINS THROUGH THE PERIOD
AND CREATE SCATTERED SNOW SHOWERS. MOUNTAIN TOPS WILL
OCCASIONALLY BE OBSCURED IN THE SNOW SHOWERS. HUMPHREY
&&
.PRELIMINARY POINT TEMP/POPS...
SAT SUN MON TUE WED THU FRI
-----------------------------------------------------------
BIL 014/031 012/014 905/000 907/024 020/035 026/042 027/044
01/E 37/S 66/S 31/B 11/B 11/N 11/B
LVM 017/034 017/022 002/010 007/031 024/037 027/042 031/042
02/J 37/S 66/S 31/E 11/N 11/N 11/N
HDN 007/029 006/016 905/000 908/022 009/035 014/040 017/045
01/B 26/S 66/S 31/B 11/B 11/B 11/B
MLS 001/026 902/007 910/903 916/015 005/028 012/033 016/041
11/E 35/S 53/S 21/B 11/B 11/B 11/B
4BQ 005/029 003/010 906/000 916/018 006/032 015/037 016/045
01/B 24/S 65/S 21/B 11/B 11/B 11/B
BHK 000/025 902/008 910/903 920/013 003/026 010/030 012/039
12/J 24/S 43/S 11/B 11/B 11/B 11/B
SHR 009/034 013/020 901/008 908/025 010/037 015/040 016/044
01/B 24/S 66/S 31/B 11/B 11/B 11/B
&&
.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH IN EFFECT FROM LATE SATURDAY NIGHT THROUGH
MONDAY AFTERNOON FOR ZONES 28>42-56>58-63>66.
WY...WINTER STORM WATCH IN EFFECT FROM LATE SATURDAY NIGHT THROUGH
MONDAY AFTERNOON FOR ZONE 99.
&&
$$
WEATHER.GOV/BILLINGS
