588
FXUS65 KREV 071047
AFDREV
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
247 AM PST SAT NOV 7 2009
SHORT TERM
MAIN UPPER LOW CONTINUES SPINNING IN THE GULF OF ALASKA WITH COLD
AND UNSTABLE AIRMASS OVER THE PACIFIC NW AND EMBEDDED VORT ENERGY
PASSING OVERHEAD. AN UPPER LEVEL FRONT ASSOCIATED WITH THE UPPER
JET MAX SHOWS UP WELL ON WATER VAPOR IMAGERY CURRENTLY OVER NRN
CA. THIS FEATURE IS RESPONSIBLE FOR THE VERY LIGHT RAIN THAT
CONTINUES OVER THE SIERRA FROM LAKE TAHOE NORTH. IR SATELLITE AND
RADAR IMAGERY STRUGGLE AT SEEING THE LIGHT PRECIP AS IT IS QUITE
SHALLOW...WHICH IS EVIDENT BY THE 07/00Z KREV SOUNDING INDICATING
VERY DRY AIR ABOVE 700 MB. LATEST MODELS SHOW PV ADVECTION
CONTINUING IN THE SAME REGION UNTIL A LITTLE AFTER 12Z...SO KEPT
SHOWERS GOING FROM TAHOE NORTH UNTIL AFTER SUNRISE. SOME VIRGA MAY
REACH THE LEESIDE VALLEYS BUT NOT EXPECTING ENOUGH VERTICAL
VELOCITY FOR RAIN TO REACH THE SURFACE.
WEAK TROUGHINESS CONTINUES OVER THE WEST COAST THROUGH THE WEEKEND
WITH EMBEDDED VORT ENERGY AND UPPER LEVEL MOISTURE BRINGING
VARIABLE CLOUDINESS. NORTH LOW LEVEL FLOW WILL BRING COLD AIR
ADVECTION OVER WRN NV THROUGH SUNDAY KEEPING TEMPERATURES RIGHT
AROUND NORMAL...IN THE MID TO UPPER 50S FOR VALLEY LOCATIONS.
A STRONGER UPPER SHORTWAVE APPROACHES THE PACIFIC NW COAST ON
MONDAY AMPLIFYING A RIDGE OVER THE WEST COAST AND INCREASING
SOUTHWEST FLOW ALOFT. THIS WILL BRING WARMING ALOFT AS WELL AS
BETTER LOW LEVEL MIXING AND ALLOW VALLEY TEMPS TO CLIMB BACK NEAR
THE 60 DEGREE MARK. THE INITIAL SHORTWAVE MOVES ONSHORE MONDAY
NIGHT WITH A PRECIP BAND MOVING OVER NRN CA THAT QUICKLY
DISSIPATES AS IT DOES SO. KEPT WITH SLIGHT CHANCE POPS OVER THE
FAR NRN AND WRN ZONES BUT MOST PRECIP ACTIVITY IS NOT EXPECTED
UNTIL THE LONG TERM. JORDAN
LONG TERM...TUESDAY THROUGH FRIDAY
MEDIUM RANGE MODELS IN GENERAL AGREEMENT WITH OVERALL PATTERN EARLY
IN THE EXTENDED FORECAST THIS CYCLE...BUT JUST LIKE YESTERDAY THE
MODELS BEGIN TO DIVERGE AFTER ABOUT 36 HOURS...AND TODAY THIS
DIVERGENCE IS GREATER THAN YESTERDAY. MODELS SHOW A GENERAL TREND
TOWARD SLOWING THE EASTWARD PROGRESSION OF THE POSITIVELY TILTED
TROUGH INTO AND THROUGH THE RGN STARTING TUESDAY/WEDNESDAY. THIS
WOULD TEND TO SUPPORT LOWER POPS FOR TUESDAY AND HIGHER POPS FOR
WEDNESDAY THAN CURRENTLY IN THE FORECAST. FOR NOW HAVE OPTED TO
LEAVE TUESDAY AS IS AND JUST SHOW A SLIGHT INCREASE IN POPS FOR
WEDNESDAY. AM NOT INCLINED TO JUMP TOO HEAVILY INTO WEDNESDAY AT
THIS POINT GIVEN THE EVER CHANGING MODEL GUIDANCE.
BY LATE WEDNESDAY THE OPERATIONAL GFS IS FASTEST WITH PUSHING THE
TROUGH INTO THE RGN WHILE BOTH THE OPERATIONAL ECMWF AND CANADIAN
MODELS HOLD THE BULK OF THE TROUGH OFF SHORE. THE CANADIAN IS NOW
THE FIRST TO SPLIT A CLOSED LOW OFF OF THE SOUTHERN END OF THE
TROUGH AND DROP IT INTO CA ON THURSDAY. MOST GEFS ENSEMBLE MEMBERS
AREA A BIT SLOWER THAN THE OPERATIONAL GFS AT THIS POINT AS WELL.
DIVERGENCE INCREASES MORE BY THURSDAY. AS MENTIONED THE CANADIAN HAS
SPLIT THE SYSTEM BY THEN WITH A LOW DROPPING SOUTH INTO CA. THIS HAS
SOME SUPPORT FROM GEFS ENSEMBLE MEMBERS. MEANWHILE THE ECMWF IS
STARTING TO CLOSE A LOW OFF OVER SE OREGON INTO NW NV. LASTLY THE
PROGRESSIVE GFS IS PUSHING THE TROUGH WELL EAST AND TRYING TO
DEVELOP A SHORT WAVE RIDGE ALONG THE COAST. THE LOW DEVELOPS IN THE
ECMWF AND REMAINS IN PLACE OVER NE NV/SE IDAHO ON FRIDAY. BY THEN
THE GFS IS DEVELOPING A WEAK LOW OVER NE NV AS WELL BUT DOES NOT
CLOSE IT OFF. THIS LOW IS MUCH STRONGER/COLDER IN THE ECMWF
SOLUTION. GIVEN THE VAST DIFFERENCES IN THE MODEL SOLUTIONS AFTER
DAY 5 HAVE OPTED TO MAKE ONLY A FEW VERY MINOR CHANGES TO THE
FORECAST AFTER WEDNESDAY.
SUBSEQUENT SHIFTS WILL LIKELY NEED TO PUSH POPS A LITTLE MORE INTO
WEDNESDAY AND POSSIBLY EVEN THURSDAY GIVEN CURRENT TRENDS. GFS TENDS
TO BE A BIT MORE PROGRESSIVE THAN OTHER SOLUTIONS AND GIVEN THAT
ECMWF IS TYPICALLY A LEADING MODEL THINK ITS SOLUTION OF SLOWING THE
UPPER TROUGH AND DEVELOPING A CLOSED LOW COULD WIN OUT. THIS WOULD
BE A GOOD COMPROMISE BETWEEN CANADIAN/GFS/GEFS ENSEMBLE MEMBERS.
HOWEVER WOULD LIKE TO SEE SOME SUPPORT FROM OTHER MODELS IN
SUBSEQUENT RUNS. MLF
AVIATION
SCATTERED MVFR CONDITIONS IN SHOWERS LIKELY THROUGH THE EARLY MRNG
FOR AREAS OF THE SIERRA FROM TAHOE NORTH AS CIGS WILL OCCASIONALLY
DROP BELOW 3K FT. THROUGH THE DAY THE LOW LVL MSTR WILL BEGIN TO DRY
OUT AND VFR CONDITIONS SHOULD RETURN FOR ALL AREAS BY EARLY
AFTERNOON. EAST OF THE SIERRA EXPECT VFR CONDITIONS THROUGH THE DAY.
LOW LVL WINDS BECOME EASTERLY LATE TONIGHT SO LIKELIHOOD OF ANY FOG
DEVELOPING IN VCNTY KTRK IS LIMITED AND WILL NOT MENTION FOR
TONIGHT. MLF
REV WATCHES/WARNINGS/ADVISORIES
NV...NONE.
CA...NONE.
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