969  
FXUS66 KMFR 172325  
AFDMFR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MEDFORD OR  
325 PM PST SUN DEC 17 2017  
   
SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT
 
CURRENT SATELLITE  
OBSERVATIONS ARE SHOWING HIGH CLOUDS ACROSS MOST OF THE PACIFIC  
NORTHWEST. RADAR AND SURFACE OBSERVATIONS INDICATE THAT WE ARE  
STILL UNDER THE INFLUENCE OF HIGH PRESSURE, MEANING THAT THE AIR  
STAGNATION ADVISORY WILL CONTINUE THROUGH TOMORROW BEFORE THE NEXT  
FRONT COMES THROUGH ON TUESDAY.  
 
A FEW IMPORTANT CHANGES HAVE OCCURRED IN THIS FORECAST CYCLE  
REGARDING THE ARRIVAL OF THE FRONTAL SYSTEM TUESDAY. THE BIGGEST  
OF THOSE CHANGES ARE THAT MODELS ARE NOW INDICATING THAT THIS  
FRONT WILL ARRIVE A FEW HOURS EARLIER THAN PREVIOUSLY EXPECTED,  
AND THAT THIS WILL SPREAD THE TIMING OF THE BULK OF THE  
PRECIPITATION. THAT BEING SAID, TEMPERATURES ON TUESDAY WILL START  
OUT A BIT WARMER BEFORE THE FRONT MOVES THROUGH. THEN THE  
TEMPERATURES WILL PLUMMET QUICKLY BEHIND THE FRONT. THE  
PRECIPITATION WILL TRANSITION TO SHOWERS QUICKLY AFTER THE FRONT  
PASSES, AND ALL OF THE MOUNTAIN PASSES WITHIN THE AREA WILL SEE  
SNOW FALL. HAVE ISSUED A WINTER STORM WATCH (WSWMFR) FOR AREAS  
ABOVE 2000 FEET WHERE CONFIDENCE IS HIGHER THAT IMPACTFUL SNOW  
WILL OCCUR. HAVE ISSUED A SPECIAL WEATHER STATEMENT (SPSMFR) FOR  
MOST OF THE OTHER PASSES ABOVE 1000 FEET. SEE THESE PRODUCTS FOR  
DETAILS, OR OUR WEATHER STORY ON THE FRONT PAGE FOR DETAILS AND  
POSSIBLE SNOW AMOUNTS.  
 
THIS SYSTEM IS EVEN POISED TO BRING SNOW TO THE VALLEY FLOOR WITH  
LESS THAN A HALF AN INCH FORECAST FOR MEDFORD. THIS IS DUE TO A  
SECONDARY SHORTWAVE THAT EJECTS THROUGH THE AREA WHILE MOISTURE IS  
HIGH AND SNOW LEVELS ARE ALREADY BELOW THE ROGUE VALLEY FLOOR.  
CONFIDENCE IS NOT AS HIGH AS PASS SNOW, BUT STILL HIGH ENOUGH TO  
ADD TO THE FORECAST. OTHER THAN IMPACTFUL LOW ELEVATION SNOW,  
THIS STORM IS TYPICAL FOR THIS TIME OF YEAR. BEYOND THAT,  
TEMPERATURES REMAIN COLD, ESPECIALLY WITH SNOW ON THE GROUND AND  
AS HIGH PRESSURE SETS UP YET AGAIN (EVEN IF BRIEF). -SCHAAF  
   
LONG TERM...THURSDAY THROUGH SUNDAY NIGHT
 
BIG CHANGES HAVE  
CROPPED UP IN THE MODELS SINCE YESTERDAY. WHERE MOST GUIDANCE  
SHOWED A DOMINATING RIDGE AND OFFSHORE FLOW JUST 24 HOURS AGO,  
SOME ENSEMBLE MEMBERS FROM THE GFS SUGGESTED THIS RIDGE WOULD  
SHIFT A BIT TO THE WEST, AND SWIFT NORTHERLY FLOW WOULD SET UP  
OVER THE FORECAST AREA INSTEAD. TODAY, NOT ONLY HAS THE  
OPERATIONAL GFS AND THE BULK OF ITS ENSEMBLE MEMBERS TRENDED  
TOWARDS THIS SOLUTION, BUT THE CANADIAN, THE ECMWF, AND THE BULK  
OF THE EC ENSEMBLE MEMBERS HAVE DONE SO AS WELL.  
 
THIS SLIGHT SHIFT IN THE RIDGE RESULTS IN A SIGNIFICANT CHANGE TO  
THE FORECAST. NOW A SHORTWAVE WITHIN THE NORTHERLY FLOW IS EXPECTED  
TO DROP SOUTH OVER OUR AREA, PRODUCING A CHANCE OF LIGHT  
PRECIPITATION FRIDAY INTO SATURDAY MORNING. COLD AIR ALREADY IN  
PLACE WILL PRODUCE SNOW LEVELS OF ROUGHLY 2000 TO 3000 FEET, SO SOME  
LIGHT SNOW IS NOT OUT OF THE QUESTION FOR MOST OF THE PASSES AND ALL  
BUT THE COAST AND WEST SIDE VALLEYS. BUT BEHIND THIS WAVE, A SHOT OF  
VERY COLD AIR WILL QUICKLY BRING TEMPERATURES DOWN TO SOME OF THE  
COLDEST OF THE SEASON. SNOW ON THE GROUND AND CLEAR SKIES WILL ONLY  
MAKE IT COLDER, ESPECIALLY ON THE EAST SIDE WHERE LOW TEMPERATURES  
SATURDAY MORNING COULD EASILY DROP TO WELL BELOW ZERO. HIGH  
TEMPERATURES ON SATURDAY MAY NOT EVEN MAKE IT ABOVE FREEZING FOR ALL  
BUT THE COAST.  
 
OF COURSE, ALL OF THIS HINGES ON THE LOCATION OF THE RIDGE. AS OF  
THIS WRITING, CONFIDENCE ON THE COLD FORECAST SOLUTION IS GOOD GIVEN  
THE SHEAR BULK OF THE GUIDANCE SHOWING THE INFLUX OF ARCTIC AIR. A  
LATE LOOK AT SOME ADDITIONAL ENSEMBLE AND LONG TERM GUIDANCE HAS  
REINFORCED THIS CONFIDENCE, AND QUITE FRANKLY, THESE NEW COLDER  
FORECAST TEMPERATURES ARE LIKELY TOO HIGH FOR ANY LOCATION THAT  
STILL HAS SNOW ON THE GROUND WHEN THE COLD AIR ARRIVES. -BPN  
 

 
   
AVIATION...17/18Z TAF CYCLE
 
VFR CONDITIONS AT ALL LOCATIONS THIS  
AFTERNOON ARE AGAIN EXPECTED TO DETERIORATE ACROSS MANY OF THE WEST  
SIDE VALLEYS FROM ABOUT MEDFORD WESTWARD THIS EVENING THROUGH MONDAY  
MORNING. HOWEVER, THE SITUATION WILL BE A BIT DIFFERENT THAN THE  
CONSISTENT PATTERN THAN WE'VE BEEN SEEING, AS A WEAK WARM FRONT WILL  
PUSH TOWARD THE COOS AND DOUGLAS COUNTY COASTLINES LATE TONIGHT  
THROUGH MONDAY MORNING. MVFR TO IFR LOW CLOUDS ARE EXPECTED WITH  
THIS WARM FRONT NORTH OF CAPE BLANCO ALONG AND NEAR THE COAST DURING  
THAT TIME PERIOD. FURTHER INLAND, FROM ROSEBURG TO MEDFORD TO GRANTS  
PASS MVFR TO LIFR FOG AND LOW CLOUDS ARE EXPECTED, BUT CONFIDENCE IS  
ONLY LOW TO MODERATE IN THE DETAILS BECAUSE OF THE INFLUENCE OF HIGH  
CLOUDS AND TEMP/DEWPOINTS SUPPORTIVE OF FROST FORMATION. BTL  
 

 
   
MARINE...UPDATED 300 PM PST SUNDAY, 17 DEC 2017
 
 
HIGH PRESSURE CENTERED OFFSHORE AND A COASTAL THERMAL TROUGH WILL  
CONTINUE TO SUPPORT SOMEWHAT GUSTY NORTH WINDS AND CHOPPY SEAS SOUTH  
OF CAPE FERRELO INTO THIS EVENING. A WEAK WARM FRONT IS EXPECTED TO  
TURN WINDS SOUTHERLY NORTH OF CAPE BLANCO THIS EVENING. WINDS WILL  
BECOME SOUTHERLY ACROSS ALL AREAS MONDAY EVENING AND THEN INCREASE  
AHEAD OF A STRONG COLD FRONT. THE FRONT WILL SWEEP THROUGH TUESDAY  
INTO TUESDAY EVENING WITH HIGH SEAS AND ADVISORY TO GALE FORCE WINDS  
LIKELY. HIGH AND STEEP SWELL WILL FOLLOW TUESDAY NIGHT INTO  
WEDNESDAY NIGHT. A PROLONGED PERIOD OF NORTH WINDS LIKELY, BUT IT  
NOW APPEARS IT WILL TAKE UNTIL THE WEEKEND FOR THESE NORTH WINDS TO  
REACH ADVISORY LEVELS. BTL  
 

 
   
MFR WATCHES/WARNINGS/ADVISORIES
 
 
OR...AIR STAGNATION ADVISORY UNTIL 8 AM PST TUESDAY FOR  
ORZ023-024-026.  
AIR STAGNATION ADVISORY UNTIL 8 AM PST TUESDAY FOR ORZ029>031.  
 
CA...NONE.  
 
PACIFIC COASTAL WATERS...GALE WATCH FROM TUESDAY MORNING THROUGH TUESDAY AFTERNOON FOR  
PZZ350-370.  
 

 
 
BMS/BPN/BTL  
 
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