Issued by NWS Sacramento, CA
000 FXUS66 KSTO 220615 AFDSTO AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SACRAMENTO CA 1010 PM PST SAT NOV 21 2009 .UPDATED DISCUSSION... WARM FRONTAL/WAA CLOUDS HAVE ALREADY SPREAD ACROSS THE NRN ZONES WITH THE LEADING EDGE TO ABOUT THE I-80 CORRIDOR AT 930 PM PST. THE CLOUD COVER WILL PRECLUDE THE FORMATION OF FOG IN THE VLY. WARM FRONTAL PCPN HAS SPREAD INLAND ALONG THE N COAST...AND IS NOW NEARING OUR WRN SHASTA/TEHAMA COUNTY AREAS. BY 15Z SUN...THE 22/00Z WRF MODEL SPREADS PCPN ACROSS OUR ENTIRE MTN ZONES...AND THEN ENDS OVER THE COASTAL RANGE BY LATE AFTN AND THEN OVER THE W SLOPE SIERNEV BY EARLY EVENING. VERY LITTLE...IF ANY...PCPN IS FORECAST FOR THE INTERIOR VLY...AND LOOKING AT THE OMEGA/UVM PROGS...THE BEST CHANCE OF ANY PCPN IN THE NRN HALF OF THE SAC VLY MAY BE IN THE MORNING...AS BY AFTN...WEAK SUBSIDENCE IS FORECAST OVER THE AREA. THIS IS IN PART DUE TO WEAKENING WAA...BUT ALSO DUE TO UPPER JET DYNAMICS AS THEY ARE STRONGER IN THE MORNING THAN IN THE AFTERNOON AS THE INDIRECT CIRCULATION OF THE UPPER JET IS FORECAST TO MOVE ACROSS THE ORE BORDER 06Z-18Z SUN...AND THEN INTO NV BY AND AFT 18Z. THIS WILL WEAKEN THE UVM OVER NORCAL DURING THE AFTN. WITH THE UPPER JET PASSING ALONG THE CA/ORE BORDER...THE DYNAMICS FROM THE SHORT WAVE WILL REMAIN WELL N OF OUR FORECAST AREA...THUS WE EXPECT A HIGH POP BUT LOW QPF SYSTEM FOR INTERIOR NORCAL. THE SHORT WAVE WILL PASS QUICKLY TO THE E...AND THE FLOW WILL TURN NWLY. DRIER NWLY FLOW WILL PREVAIL OVER INTERIOR NORCAL INTO EARLY NEXT WEEK WITH SEASONABLE TEMPS. JHM .PREVIOUS DISCUSSION... MOSTLY CLEAR SKIES ACROSS MUCH OF INTERIOR NORTHERN CALIFORNIA. MODERATE NORTHWESTERLY WINDS DEVELOPED DURING THE DAY WHICH INTRIGUINGLY ENOUGH INITIALLY LIFTED THIS MORNINGS FOG TO A LOW STRATUS CLOUD...THEN ADVECTED THE STRATUS CLOUD SOUTHWARD BEFORE IT FINALLY DISSIPATED. INFRARED SATELLITE SHOWS A BAROCLINIC LEAF ASSOCIATED WITH A RAPIDLY ADVANCING STORM SYSTEM CURRENTLY APPROACHING THE PACIFIC NORTHWEST. THE TAIL END OF THIS SYSTEM HAS SPREAD INCREASING CLOUDS TOWARD THE NORTHERN END OF THE VALLEY AND SURROUNDING MOUNTAINS. WITH ABUNDANT SUNSHINE TODAY...TEMPERATURES ACROSS THE FORECAST ARE RUNNING A FEW DEGREES WARMER THAN 24 HOURS AGO. THE REPRIEVE THAT WE ARE CURRENTLY SEEING WILL BE BRIEF BECAUSE OF THE ENCROACHING SYSTEM. THIS WAVE IS A FAIR BIT QUICKER THAN THE LAST AND MOST OF ITS ENERGY WILL REMAIN NORTH TOWARDS OREGON. HOWEVER...WARM ADVECTION PROCESSES WILL ALLOW SOME LIGHT PRECIPITATION TO DEVELOP ACROSS THE REGION MAINLY TO THE NORTH OF I-80 LATE TONIGHT AND CONTINUE INTO MIDDAY SUNDAY. NOT A WHOLE LOT OF MOISTURE WITH THIS SYSTEM AND SNOW LEVELS SHOULD REMAIN RELATIVELY HIGH...SO ONLY LIGHT SNOW ACCUMULATIONS ARE EXPECTED OVER HIGHER ELEVATIONS. WHILE THE BULK OF THE PRECIPITATION IS EXPECTED TO REMAIN NORTH OF INTERSTATE 80...CANNOT RULE OUT A FEW STRAY SHOWERS SOUTH OF THAT LINE. AFTER THIS SYSTEM MOVES THROUGH TOMORROW...THE EASTERN PACIFIC RIDGE WILL BEGIN TO AMPLIFY ALONG THE WEST COAST RESULTING IN A WARMING AND DRY TREND FOR SEVERAL DAYS. SURFACE HIGH PRESSURE BUILDING INTO THE GREAT BASIN WILL RESULT IN LOCALLY NORTHEASTERLY BREEZES DURING THE OVERNIGHT HOURS THROUGH FAVORED SIERRA CANYONS. EXPECT TEMPERATURES ON MONDAY TO WARM UP TO NEAR NORMAL LEVELS IN THE LOWER 60S THROUGH THE VALLEY AND HIGHS ON TUESDAY TO BE A FEW DEGREES ABOVE NORMAL. DANG .EXTENDED DISCUSSION (WEDNESDAY THROUGH SATURDAY)... RIDGE STILL DOMINATES WED. OTHER THAN SOME PATCHY MORNING FOG EXPECT A GENERALLY CLEAR SKY WED MORN. THE MODELS ARE IN GOOD AGREEMENT WITH A TROUGH IMPACTING THE NRN CAL ON THU AND INTO FRI... BUT THE GFS AND EC ARE OFF JUST A BIT ON THE TIMING WITH EC BEING MORE PROGRESSIVE. THIS IS WHEN THE MODELS DIVERGE FROM ONE ANOTHER. THE GFS ONLY DEVELOPS A WEAK RIDGE OVER THE ERN PAC AND NRN CAL WHILE THE EC HAS A MORE AMPLIFIED RIDGE WITH TROUGH SLIDING DOWN FROM BC AND INTO THE WRN GREAT BASIN. THE HPC SOLUTION USED A BLEND OF THE TWO...BUT FAVORED MORE TOWARD THE EC. SO WENT WITH A SLIGHT CHANCE OF RAIN OVER THE SHASTA AND PLUMAS CO MTNS SAT. JMC && .AVIATION... CURRENT VFR CONDITIONS WILL GIVE WAY TO INCREASING CLOUDS AND SHOWERS FROM NORTH TO SOUTH AS A SYSTEM APPROACHES LATE TONIGHT. RADAR SHOWS RAIN SHOWERS SPREADING INTO NORTHWESTERN CALIFORNIA AND ARE EXPECTED INTO THE NORTHERN SACRAMENTO VALLEY AFTER 06Z...SPREADING SOUTHWARD THROUGH SACRAMENTO BY AROUND 12Z. THIS WILL BRING AREAS OF MVFR CIGS WITH LOCAL IFR VSBYS IN HEAVIER RAIN. BR BRINGING MVFR/LIFR VSBYS IS POSSIBLE AFTER 12Z OVER THE NORTHERN SAN JOAQUIN VALLEY AND PORTIONS OF THE SOUTHERN SACRAMENTO VALLEY WHERE CLOUD COVER WILL BE THINNER OVERNIGHT. SNOW SHOWERS WILL BRING LOCAL LIFR IN THE MOUNTAINS...ABOVE 5000 FEET IN THE EVENING AND DROPPING TO 3000-4000 BY SUNDAY MORNING. && .STO WATCHES/WARNINGS/ADVISORIES...NONE. && $$
