NORTHERN ARIZONA FORECAST DISCUSSION
NWS FLAGSTAFF AZ
AFDFLG 315 AM MST SAT FEB 17 2001
SYNOPSIS...HIGH PRESSURE WILL REMAIN OVER ARIZONA DURING THE DAY
SATURDAY...BRINGING DRY CONDITIONS AND WARMER DAYTIME TEMPERATURES.
SATURDAY NIGHT AND SUNDAY...A WEAK WEATHER DISTURBANCE WILL AFFECT
THE NORTHLAND. THIS WILL BRING MOSTLY CLOUDY SKIES AND A CHANCE OF
LIGHT RAIN AND SNOW SHOWERS. HIGH PRESSURE WILL MOVE IN AGAIN FOR
THE START OF THE WORK WEEK...BRINGING PARTLY CLOUDY SKIES AND MILD
CONDITIONS.
DISCUSSION...TROF LOCATED IN THE EASTERN PACIFIC POSES AN
INTERESTING SITUATION NEXT FEW DAYS. CURRENT ACARS DATA SHOW A 110
KNOT JET DROPPING DOWN THE WEST SIDE OF THE SYSTEM. AT THE SAME TIME
THERE IS A 110 KNOT SUBTROPICAL JET JUST TO THE SOUTH. ALTHOUGH THE
SYSTEM IS CURRENTLY NEARLY EQUIVALENT BAROTROPIC EXPECT THAT THE
INFLUENCE OF THE JET WILL LEAD TO SOME BAROCLINITY REDEVELOPING IN
THE SYSTEM AS IT MOVES EASTWARD. IN ADDITION...THE STJ HAS TAPPED
INTO A MODERATE MOISTURE SOURCE THAT IS MOVING TOWARDS THE SW CONUS.
EXPECT THAT THE SYSTEM WILL INGEST SOME OF THIS AS IT MOVES ONSHORE
SATURDAY NIGHT AND SUNDAY. BECAUSE THIS SYSTEM WILL BE MOVING INTO A
STRONG RIDGE IT WILL QUICKLY DEAMPLIFY AS IT MOVES OVER THE REGION.
THE MAIN QUESTION THEN BECOME HOW MUCH PCPN IS LIKELY TO FALL FROM A
DYING SYSTEM. BOTH AVN AND ETA BREAK OUT PCPN SATURDAY NIGHT AND
INTO SUNDAY. SUSPECT THAT SOME OF THIS IS OVERDONE SINCE THE MODELS
TYPICALLY ARE SLOW TO DIMINISH PCPN WITH DECAYING SYSTEMS.
NONETHELESS THERE EXISTS ENOUGH MOISTURE AND OROGRAPHIC FLOW THAT
PCPN IS STILL A GOOD CHANCE. LAPSE RATES UNFAVORABLE FOR DEEP CLOUD
GROWTH SO EXPECT THAT PCPN WILL BE ON THE LIGHT SIDE. WILL BUMP UP
POPS OVER THE HIGHER TERRAIN FOR SATURDAY NIGHT. THIS IS A WARM
SYSTEM AND SNOW LEVELS SHOULD BE AROUND 7000 FEET OVERNIGHT AND A
BIT HIGHER DURING THE DAY ON SUNDAY.
EXTENDED...MRF AND ECMWF ARE MORE IN ALIGNMENT THAN THE PREVIOUS FEW
DAYS AND HAVE HIGHER CONFIDENCE IN SOLUTIONS. PROBLEM IS THAT THE
STORM TRACK IS ONLY A SHORT DISTANCE TO THE NORTH AND EACH S/W TROF
THAT PASSES BY COULD PRODUCE SOME SHOWER ACTIVITY. FOR NOW BELIEVE
THAT THE BEST CHANCE OF PCPN IS ON TUESDAY AND WILL MENTION IN THE
EXTENDED. DB
.FGZ...NONE.
NORTHEAST AND NORTH CENTRAL COLORADO AREA FORECAST DISCUSSION
NWS DENVER COLORADO
AFDDEN 310 AM MST SAT FEB 17 2001
SHORT TERM DISCUSSION: LOW STRATUS DECK AND FOG CONTINUE TO COVER
NORTHEAST CO ATTM WITH STRONG INVERSION JUST OFF THE DECK WITH TOP
OF INV NR 8000 FT MSL. LATEST ACARS SOUNDINGS DO INDICATE INVERSION
HAS WEAKENED A LITTLE DURING LAST 6 HOURS PLUS...INDICATIONS OF WK
INV LAYER FORMING AT MTN TOP LVL ALONG WITH INCREASED WESTERLY
WINDS. THERE HAS ALREADY BEEN SOME WINDS DEVELOPING ON NIWOT RIDGE
IN BOU COUNTY...WHERE EARLY TNT WIND GUSTS RECORDED UP TO 60 MPH.
FORECAST HINGES ON THESE WESTERLY WINDS TO BEGIN TO SCOUR OUT
SHALLOW COLD AIRMASS AS DAY WEARS ALONG. BELIEVE THIS WILL SPEAK
TRUE...WITH GRADUAL CLEARING FROM WEST TO EAST. PLATTEVILLE PROFILER
HAS JUST SEEN A SHIFT TO A MORE NW DIRECTION AROUND 700MB...THIS IN
EVIDENT WITH DEPARTING SHORTWAVE AND DEVELOPING SUBSIDENCE. SFC WNDS
OVR SRN WY ALSO SHIFTING W-NW AND CYS HAS MOSTLY CLEARED OUT AS
WELL. WITH CLEARING EXPECTED BY MID MORNING ALONG URBAN
COORIDOR...AND THIS AFTN OVER FAR NERN PLAINS...TEMPS SHOULD REBOUND
INTO THE 35 TO 45 DEG RANGE...WARMEST NEAR THE FOOTHILLS WITH
BIGGEST DOWNSLOPE COMPONENT. WILL NEED TO ADD SOME WINDS OVR
MTNS/HIR FOOTHILLS TDY WITH SOME GUSTS TO 50 MPH EXPECTED ABOVE
TIMBERLINE. ENTREKIN
LONG TERM DISCUSSION: MODELS CONTINUE TO TREND SLOWER WITH THE
SHORTWAVES EJECTING OUT OF THE WEST COAST LOW. THE ETA HAS BEEN
CONSISTENTLY STRONGER WITH THE RIDGING TO OUR WEST AND SLOWER WITH
THE SHORTWAVES COMING INTO IT...AND THIS LOOKS TO BE CORRECT. SO
THE SHORTWAVE WHICH WILL BRING MOISTURE BACK TO THE MOUNTAINS...
STRENGTHENING WEST WINDS ALOFT...AND CLOUDS NOW LOOKS LIKE IT WILL
PASS OVER DURING THE DAY SUN. AFTER THAT A STRING OF DISTURBANCES
PASSES THROUGH A BROAD RIDGE. WHILE THE AMOUNT OF FOG AND LOW
CLOUDS SHOULD BE MUCH LESS TONIGHT...THE COLD AIR WILL NOT BE
COMPLETELY SCOURED OUT UNTIL AFTER THE SUN SHORTWAVE. CLOUD COVER
WILL ALSO HOLD TEMPS BACK A BIT SUN AND MON SO GUIDANCE STILL LOOKS
SEVERAL DEGREES TOO WARM. MOUNTAINS SHOULD BE SNOWING AND BLOWING
STARTING SUN...BUT LAPSE RATES NOT TOO STEEP. PRELIMINARY RUN OF
THE SNOW MODEL GAVE 4-8 INCHES FOR THE SUN/SUN NIGHT PERIOD.
FOR THE EXTENDED PERIOD...LOTS OF COLD AIR IN CANADA BUT WITH FLOW
SPLIT AROUND A BLOCKY RIDGE FROM THE NWRN US THROUGH ALASKA THE
CONFLUENCE TO OUR EAST IS TENDING TO HOLD THE COLD AIR AT BAY.
THERE IS STILL POTENTIAL FOR A GLANCING BLOW AT MIDWEEK. MOST OF
THE MEDIUM RANGE MODELS...CANADIAN ENSEMBLES...ECMWF...NOGAPS...HOLD
THE NORTHERN STREAM SHORTWAVE AND THE REALLY COLD AIR FARTHER
NORTHEAST THAN THE OPERATIONAL MRF. HOWEVER...MOST OF THE CANADIAN
RUNS AND THE NOGAPS SWING THE CLOSED LOW FROM THE COAST TOWARD
COLORADO BY THE END OF THE WEEK. A NUMBER OF REASONS TO BELIEVE THE
MEAN TROUGH WILL STAY OFF THE COAST...BUT THERE COULD BE A STRONG
ENOUGH WAVE COMING THROUGH THE RIDGE OVER US TO PULL IN A PIECE OF
THE COLD AIR AT MIDWEEK...AND IF SO THERE COULD BE A QUICK SHOT AT
PRECIPITATION. IN GENERAL THOUGH THE WLY FLOW SHOULD DOMINATE.
GIMMESTAD
.DEN...NONE.