727  
FXUS66 KMFR 242154  
AFDMFR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MEDFORD OR  
254 PM PDT MON SEP 24 2018  
   
DISCUSSION
 
 
ALREADY DRY AIR ACROSS THE FORECAST AREA THIS AFTERNOON IS  
BECOMING DRIER, AND WILL CONTINUE TO DO SO THROUGH WEDNESDAY AS  
GENERALLY SUBSIDING EASTERLY WINDS CONTINUE ACROSS THE FORECAST  
AREA. THIS DRY AIR WILL SUPPORT LARGE DIURNAL TEMPERATURE RANGES  
IN AREAS SHELTERED FROM THE EAST WINDS, WHILE AREAS IN THE EAST  
WINDS WILL EXPERIENCE CONTINUING WARM CONDITIONS. IT'S CURRENTLY  
82 DEGREES IN BROOKINGS AS OF THE 2PM PDT OBSERVATION.  
TEMPERATURES THERE ARE EXPECTED TO TOP OUT IN THE MID 80S TODAY,  
AND THE UPPER 80S TOMORROW AS THE EASTERLY WINDS KNOWN AS THE  
CHETCO EFFECT CONTINUES.  
 
INLAND LOCATIONS ARE ALSO INDICATING LOW HUMIDITIES THIS  
AFTERNOON, WHICH IS ALLOWING TEMPERATURES TO CLIMB RAPIDLY.  
TEMPERATURE RANGES FOR VALLEY LOCATIONS THIS WEEK, THROUGH  
FRIDAY, ARE LIKELY TO BE ON THE ORDER OF 40 TO AS MUCH AS 60  
DEGREES BETWEEN HIGHS AND LOWS. THE LARGER END OF THE RANGE WILL  
BE ACROSS LOWER ELEVATION LOCATIONS EAST OF THE CASCADES.  
SPECIFICALLY, FOR MEDFORD, WHILE OVERNIGHT LOWS WILL THROUGH  
FRIDAY ARE EXPECTED TO BE 5 TO 10 DEGREES ABOVE THE 1981-2010  
NORMALS, DAYTIME HIGHS ARE EXPECTED TO BE 15 TO 20 DEGREES ABOVE  
NORMAL. CURRENTLY, THE WARMEST DAY IS EXPECTED TO BE THURSDAY,  
WITH A HIGH OF 96F. WHILE SUCH TEMPERATURES ARE WELL ABOVE NORMAL  
FOR THIS TIME OF THE YEAR, NOW RECORDS ARE EXPECTED TO BE BROKEN.  
 
THE EAST WIND PATTERN BRINGING THE DRYING CONTINENTAL AIR MASS TO  
THE AREA THIS WEEK IS EXPECTED TO GRADUALLY WEAKEN, SUCH THAT  
WINDS WILL BE MORE DIURNALLY DRIVEN IN THE AFTERNOON AND EVENING  
HOURS DURING THE LATTER HALF OF THE WEEK. HOWEVER, IT IS LIKELY TO  
TAKE UNTIL FRIDAY FOR US TO SEE EVEN SOME SLIGHT COOLING AND  
MOISTENING OF THE AIR MASS EVEN FROM THE CASCADES WESTWARD AS THE  
WARM DRY AIR OF THE THERMAL TROUGH HOVERS OVER THE AREA.  
 
NUMERICAL MODELS CONTINUE TO BE CONSISTENT IN INDICATING THAT THE  
UPPER LOW THAT HAS BEEN PART OF THE CURRENT REX, OR HIGH OVER LOW,  
BLOCK IN THE EASTERN PACIFIC TO PUSH TOWARD THE NORTHERN  
CALIFORNIA COAST FRIDAY, AND THEN OVER THE FORECAST AREA SATURDAY.  
AS THEY OFTEN DO WITH UPPER LEVEL LOWS, EACH MODEL AND MODEL RUN  
HAS INDICATED A SLIGHTLY DIFFERENT POSITION FOR THE LOW MOVING  
THROUGH, AS WELL AS DIFFERENCES IN TIMING. HOWEVER, FOR AT LEAST A  
FEW DAYS NOW (SINCE ABOUT WEDNESDAY) THERE HAS BEEN GENERAL  
AGREEMENT AMONG THE MODELS IN SHOWING THIS LOW EJECTING EASTWARD.  
IN SUMMARY, THE LATEST MODEL GUIDANCE SHOWS THE POTENTIAL OF SOME  
SHOWERS AND THUNDERSTORMS DEVELOPING SATURDAY AFTERNOON AND  
EVENING. INSTABILITY IS NOT TREMENDOUS, BUT DOES LOOK TO BE ENOUGH  
TO GENERATE SOME RAINFALL. THUS SOME SLIGHT CHANCES TO CHANCES OF  
SHOWERS HAVE BEEN ADDED TO THE FORECAST FOR SATURDAY AFTERNOON AND  
EVENING. A SLIGHT CHANCE OF THUNDERSTORMS FOR THE AFTERNOON AND  
EVENING HOURS HAS BEEN ADDED FOR THE NORTHERN CASCADES FROM ABOUT  
CRATER LAKE NATIONAL PARK NORTHWARD DUE TO REASONABLE MODEL  
AGREEMENT ON NECESSARY DYNAMICS THERE, AS WELL AS THAT BEING A  
CLIMATOLOGICALLY FAVORED AREA FOR SUCH ACTIVITY IN THIS PATTERN.  
 
AS WINDS BECOME MORE WESTERLY LATER THIS WEEK, WE EXPECT ANOTHER  
ROUND OF SMOKE TO PUSH EASTWARD ACROSS THE AREA FROM THE KLONDIKE  
AND NATCHES FIRES INTO THE ILLINOIS, APPLEGATE, AND ROGUE  
VALLEYS. THE TIME OF MOST CONCERN IS FROM THURSDAY EVENING  
THROUGH SATURDAY EVENING. THUS, THIS EXPECTATION HAS BEEN ADDED TO  
THE FORECAST.  
 
THE MODELS THEN DIVERGE FOR SUNDAY INTO EARLY NEXT WEEK. THE ECMWF  
BRINGS IN WHAT LOOKS TO BE A SEASON ENDING RAINFALL EVENT, WHILE  
THE GFS HAS TRENDED DRIER. ALL IN ALL, GIVEN THE WIDE SPREAD IN  
THE MODELS AND GREAT UNCERTAINTY IN THE FORECAST, THE OFFICIAL  
FORECAST IS A CONSENSUS BLEND OF THE GUIDANCE, INDICATING SLIGHT  
CHANCES TO CHANCES OF RAIN ACROSS THE FORECAST AREA EARLY NEXT  
WEEK. THE WEATHER PREDICTION CENTER AND CLIMATE PREDICTION CENTER  
HAVE SOME HELPFUL DISCUSSIONS PROVIDING MORE INFORMATION ON WHAT  
IS DRIVING THE MODEL DIFFERENCES IN THE EXTENDED PORTION OF THE  
FORECAST. -LUTZ  
 

 
   
AVIATION...FOR THE 24/18Z TAFS
 
VFR WILL PREVAIL ACROSS THE AREA  
THIS AFTERNOON AND EVENING WITH GUSTY NORTH WINDS UP TO 30KT ALONG  
THE COAST AND SOME BREEZY NNE WINDS DUE TO OFFSHORE FLOW OVER SW  
OREGON AND NW CALIFORNIA, INCLUDING THE COASTAL MOUNTAINS. WINDS  
EASE FOR ALL BUT THE COASTAL MOUNTAINS TONIGHT, AND WITH A VERY DRY  
AIR MASS IN PLACE, IT WILL REMAIN VFR. COASTAL RIVER VALLEY FOG  
(COQUILLE/UMPQUA) WILL BE PATCHY AT BEST. -SPILDE  
 

 
   
MARINE...UPDATED 230 AM PDT MONDAY 24 SEPTEMBER 2018
 
THE THERMAL  
TROUGH WILL STRENGTHEN TODAY AS INLAND CONDITIONS WARM, AND ANOTHER  
ROUND OF GALES AND HAZARDOUS SEAS ARE EXPECTED THIS AFTERNOON AND  
EVENING. THE THERMAL TROUGH WILL GRADUALLY WEAKEN STARTING TUESDAY  
AND WINDS WILL EASE SOME, BUT REMAIN HAZARDOUS TO SMALL CRAFT  
THROUGH MID-WEEK. SEAS WILL REMAIN STEEP AND HAZARDOUS FROM FRESH  
SWELL THROUGH AT LEAST TUESDAY NIGHT, GRADUALLY SUBSIDING  
BELOW SMALL CRAFT ADVISORY LEVELS BEYOND WEDNESDAY NIGHT.  
 
LATE WEEK AND INTO THE WEEKEND, MODELS ARE SHOWING A CHANGE IN THE  
PATTERN WITH AN AREA OF LOW PRESSURE APPROACHING FROM THE SOUTHWEST,  
AND DISRUPTING THE THERMAL TROUGH SETUP. CONFIDENCE IS LOW IN THE  
DETAILS AT THIS POINT IN THE FORECAST, BUT THERE IS THE POTENTIAL  
FOR A SHIFT TO STRONGER SOUTH WINDS BY THE WEEKEND. /BR-Y  
 

 
   
FIRE WEATHER...UPDATED 150 PM PDT MONDAY, 23 SEPTEMBER 2018
 
 
LOW RELATIVE HUMIDITIES AND NORTHEAST TO EAST WINDS GUSTING UP TO 35  
MPH ARE EXPECTED OVER THE COASTAL MOUNTAINS THROUGH TUESDAY MORNING.  
THE 06Z NAM12 925MB AND 850 MB WINDS INDICATED THE STRONGEST  
WINDS TO OCCUR THIS MORNING, AND SLIGHTLY WEAKER WINDS, BUT  
POORER HUMIDITY RECOVERY, FOR TUESDAY MORNING. WITH THE THERMAL  
TROUGH GRADIENT TIGHTENING FARTHER EASTWARD THE NEXT TWO NIGHTS,  
GUSTY WINDS WILL ALSO SHIFT TO BE OVER THE CASCADES. WITH THE VERY  
LOW HUMIDITIES THIS AREA LOOKS STRONGLY POSSIBLE TO REACH WARNING  
CRITERIA, AND HAVE ISSUED A WATCH FOR TOMORROW NIGHT INTO  
WEDNESDAY MORNING IN THIS AREA.  
 
OTHERWISE, DIURNAL TEMPERATURE RANGES WILL BE WIDE IN VALLEYS WITH  
HOT AND DRY CONDITIONS THROUGH FRIDAY AND OVERNIGHT INVERSIONS IN  
PLACE. THE STUBBORN BLOCKING PATTERN IS FORECAST TO FINALLY BREAK  
DOWN FRIDAY NIGHT WITH A CLOSED UPPER LOW SWINGING IN CLOSE TO THE  
COAST SATURDAY. THIS WOULD BRING HIGHER HUMIDITY AND COOLER  
TEMPERATURES FOR THE WEEKEND INTO EARLY NEXT WEEK. THE STRENGTH AND  
TRACK OF THE LOW ARE VERY MUCH IN QUESTION, WITH THE HIGHEST  
UNCERTAINTY REGARDING THE PROBABILITY OF RAIN/SHOWERS AND ANY RISK  
OF THUNDERSTORMS. THERE IS GENERAL MODEL AGREEMENT THAT THE  
PROBABILITY WILL ONLY BE IN THE SLIGHT CHANCE TO CHANCE CATEGORY  
THROUGH THE WEEKEND. THERE IS HIGHER UNCERTAINTY BEYOND THE WEEKEND,  
WHICH INCLUDES SOME POSSIBILITY OF A MORE WIDESPREAD RAIN EVENT.  
-SVEN  
 

 
   
MFR WATCHES/WARNINGS/ADVISORIES
 
 
OR...RED FLAG WARNING UNTIL 11 AM PDT TUESDAY FOR ORZ619-620.  
RED FLAG WARNING UNTIL 11 AM PDT TUESDAY FOR ORZ618.  
FIRE WEATHER WATCH FROM LATE TUESDAY NIGHT THROUGH WEDNESDAY  
MORNING FOR ORZ617-621>623.  
 
CA...RED FLAG WARNING UNTIL 11 AM PDT TUESDAY FOR CAZ280.  
 
PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY UNTIL 5 AM PDT WEDNESDAY FOR  
PZZ350-356-370-376.  
GALE WARNING UNTIL 5 AM PDT TUESDAY FOR PZZ356-376.  
HAZARDOUS SEAS WARNING FROM 5 AM TUESDAY TO 5 AM PDT WEDNESDAY  
FOR PZZ370-376.  
 

 
 
BTL/MAS/SBN  
 
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