FXUS65 KBOI 220356
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
856 PM MST MON DEC 21 2009
.DISCUSSION...THE 18Z NAM WAS MORE MOIST IN ITS FORECAST THAN
WHAT ACTUALLY OCCURRED. THE NEW 00Z NAM...WHICH STARTED DRIER...
DEVELOPS A TIGHT PRECIPITATION BAND WITH THE FRONT AS IT
APPROACHES SW IDAHO TONIGHT. THAT SEEMS TO BE WHAT THE RADAR IS
SHOWING AS THE UPPER FLOW OVER SW IDAHO BACKS AROUND TO MORE
SOUTHERLY AND THE ECHOES ARE BECOMING MORE CONCENTRATED IN
PROXIMITY TO THE FRONT WHICH WAS FROM JUST EAST OF BAKER CITY TO
JUST EAST OF BURNS AT 03Z. THIS BOILS DOWN TO AN EXPECTATION THAT
THERE WILL BE LESS PRECIPITATION AHEAD OF THE FRONT WHERE IT WOULD
FALL AS RAIN IN THE SOUTHERN VALLEYS AND MORE WITH AND BEHIND THE
FRONT WHERE IT WOULD BE AS SNOW. BELIEVE THE NARROW CONCENTRATED
BAND AND TIMING OF THE NEW NAM IS REASONABLE BUT THE AMOUNTS SEEM
OVERDONE. CURRENT ADVISORY FOR AROUND 3 INCHES OF SNOW IN THE
WEISER RIVER AREA...4 TO 6 INCHES IN THE BOISE MOUNTAINS...AND 1
TO 2 INCHES IN THE TREASURE VALLEY LOOKS GOOD. BELIEVE THE MAGIC
VALLEY AND SOUTHERN TWIN FALLS COUNTY AREAS WILL SEE SIMILAR 1 TO
2 INCHES OF SNOW DURING THE MORNING TRAFFIC PERIOD AS WELL SO WILL
EXTEND THE ADVISORY TO THOSE AREAS FROM 6 AM TO 1 PM MST.
&&
.AVIATION...AREAS OF MVFR/IFR RAIN AND SNOW ARE SPREADING ACROSS THE
AREA THIS EVENING. THIS IS EXPECTED TO CONTINUE AND INTENSIFY
OVERNIGHT AS RAIN CHANGES OVER TO SNOW. A COLD FRONT IS HELPING
DRIVE THIS WEATHER...AND IT WAS LOCATED FROM JUST EAST OF KBKE TO
JUST EAST OF KBNO AT 03Z. AS THIS MOVES EAST OVERNIGHT...CONDITIONS
WILL DETERIORATE. ON TUESDAY...CONDITIONS WILL IMPROVE FROM THE WEST
...BUT SCATTERED SNOW SHOWERS WILL PERSIST FROM KMYL THROUGH KTWF.
MODERATE SURFACE WINDS WILL SHIFT FROM SOUTH AND EAST TO NORTH AND
WEST FOLLOWING THE COLD FRONT...WITH MODERATE NORTHWEST WINDS ON
TUESDAY. WINDS ALOFT WILL BE LIGHT TO MODERATE SOUTHWEST...AND VARY
IN TIME MUCH MORE THAN USUAL DUE TO A CLOSED LOW MOVING NEARLY
OVERHEAD.
&&
.PREV DISCUSSION...
SHORT TERM...UPPER TROUGH DEVELOPING RAPIDLY OFF THE NORTHWEST
COAST THIS AFTERNOON. TROUGH WILL COME INLAND THIS EVENING AND
COOLING ALOFT WILL CONDENSE MOISTURE OVER EASTERN OREGON...THEN
IN WESTERN IDAHO TONIGHT. MILD AIR INITIALLY WILL RESULT IN RAIN
AS HIGH AS 5000 FEET MSL BUT SNOW WILL LEVE WILL FALL RAPIDLY
BEHIND A COLD FRONT DUE ACROSS ERN OREGON 04Z-06Z AND WRN IDAHO
06Z-15Z TUESDAY. ENOUGH QPF FOR WINTER WX ADVISORY FOR SNOW IN
IDAHO MOUNTAINS NORTH OF THE SNAKE BASIN OVERNIGHT THROUGH SUNRISE
TUESDAY. RAIN WILL CHANGE TO SNOW IN THE TREASURE VALLEY AROUND 3
AM MST AND SLUSH ON ROADS WILL FREEZE THEN BECOME COVERED BY SNOW
JUST BEFORE COMMUTING PERIOD TUESDAY. WE THEREFORE HAVE ALSO ISSUED
AN ADVISORY FOR THESE CONDITIONS. SNOW WILL PROGRESS EASTWARD INTO
THE MAGIC VALLEY MID-MORNING TUESDAY POSSIBLY CAUSING TRAVEL PROBLEMS
THERE. EVENING CREW WILL DECIDE IF AN ADVISORY IS NEEDED THERE.
TURNING COLDER TUESDAY UNDER NNWLY FLOW ALOFT...AND POST-FRONTAL
WEST-NORTHWEST SURFACE WINDS 15 TO 25 MPH IN THE SNAKE BASIN...10-15
MPH ELSEWHERE. CAMAS PRAIRIE WILL HAVE AREAS OF BLOWING SNOW AND
REDUCED VISIBILITY TUESDAY DUE TO 2-3 INCHES OF NEW SNOW AND WEST
WINDS GUSTING TO 25 MPH.
SNOW SHOWERS WILL END FROM WEST TO EAST TUESDAY...FINALLY ENDING IN
SOUTHERN TWIN FALLS COUNTY TUESDAY NIGHT. COLD ADVECTION WILL
CONTINUE INTO WEDNESDAY BUT WITH LESS WIND.
LONG TERM...WEDNESDAY NIGHT THROUGH MONDAY...MODELS SHOW GOOD
CONSISTENCY AND CONTINUITY WITH THE WEST COAST RIDGE AND NORTHERLY
FLOW OVER OUR AREA WEDNESDAY NIGHT THROUGH FRIDAY...FOLLOWED BY
INLAND MOVEMENT OF THE RIDGE OVER OUR AREA FOR SATURDAY AND SUNDAY.
THIS UPPER LEVEL PATTERN IS EXPECTED TO CONTINUE TO SUPPORT THE
SOUTHWARD MIGRATION OF ARCTIC AIR...WHICH SETTLES OVER IDAHO ON
CHRISTMAS DAY. BY FRIDAY HIGHEST SURFACE PRESSURES WILL BE OVER
EASTERN IDAHO. COLDEST TEMPERATURES WILL BE WEDNESDAY NIGHT AND
THURSDAY NIGHT. THEREAFTER TEMPERATURES WILL BEGIN TO GRADUALLY
MODERATE ACROSS SOUTHWEST IDAHO AND SOUTHEAST OREGON WITH WARMING
TEMPERATURES ALOFT AND DOWNSLOPE SURFACE FLOW. HOWEVER WITH THE
UPPER LEVEL RIDGE OVER THE AREA TRAPPING COLD AIR IN THE
VALLEYS...WARMING WILL BE SLOWER AT LOWER ELEVATIONS. WE ARE
CURRENTLY FORECASTING HIGHS AND LOWS ABOUT 10 DEGREES BELOW
NORMAL...MODERATING TO ABOUT 5 DEGREES BELOW NORMAL ON MONDAY.
TEMPERATURES WILL BE COLDER IF THERE IS VALLEY SNOW COVER...BUT THIS
IS STILL UNCERTAIN.
&&
.BOI WATCHES/WARNINGS/ADVISORIES...
ID...WINTER WEATHER ADVISORY UNTIL 7 AM MST TUESDAY FOR IDZ013-IDZ033.
WINTER WEATHER ADVISORY FROM 6 AM TO 1 PM MST TUESDAY FOR IDZ016-
IDZ030.
WINTER WEATHER ADVISORY FROM 3 AM TO 9 AM MST TUESDAY FOR IDZ012-
IDZ014.
OR...WINTER WEATHER ADVISORY FROM 3 AM TO 9 AM MST TUESDAY FOR ORZ064.
&&
$$
WEATHER.GOV/BOISE
DISCUSSION...JJ
AVIATION.....SP
PREV SHORT TERM...LC
PREV LONG TERM....JT/BB
