837  
FXUS21 KWNC 141901  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT AUGUST 14 2020  
 
SYNOPSIS: AMPLIFIED MID-LEVEL HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE  
WESTERN TWO THIRDS OF THE LOWER 48-STATES AT THE BEGINNING OF THE PERIOD. BY  
THE MIDDLE OF WEEK-2, THE PATTERN IS ANTICIPATED TO BECOME LESS AMPLIFIED. THIS  
PATTERN IS PREDICTED TO CONTINUE THE ONGOING HEAT WAVE ACROSS THE SOUTHWEST  
QUADRANT OF THE CONTIGUOUS U.S. DOWNSTREAM, MANY MODELS INDICATE THE  
POSSIBILITY OF MID-LEVEL LOW PRESSURE FORMING ACROSS EASTERN PARTS OF THE LOWER  
48 STATES. HOWEVER, THERE ARE SIGNIFICANT MODEL DIFFERENCES REGARDING THE  
LOCATION AND ORIENTATION OF THIS FEATURE, RESULTING IN HIGH UNCERTAINTY  
REGARDING THE POTENTIAL OF MID-LEVEL HIGH PRESSURE BUILDING ACROSS PARTS OF NEW  
ENGLAND, DOWNSTREAM OF THIS MID-LEVEL LOW PRESSURE. OVER ALASKA, A SERIES OF  
SURFACE LOWS ARE ANTICIPATED TO FORM OVER THE BERING SEA AND GULF OF ALASKA,  
WHICH MAY BRING EPISODES OF WET WEATHER TO PARTS OF WESTERN MAINLAND ALASKA AND  
SOUTHEAST ALASKA, INCLUDING THE ALASKA PANHANDLE.  
 
HAZARDS  
 
MODERATE RISK OF EXCESSIVE HEAT FOR PARTS OF THE CALIFORNIA CENTRAL VALLEY,  
SOUTHERN CALIFORNIA, SOUTHWEST, AND CENTRAL GREAT BASIN, SAT-TUE, AUG 22-25.  
 
SLIGHT RISK OF EXCESSIVE HEAT FOR PARTS OF THE WESTERN U.S., SAT-FRI, AUG 22-28.  
 
DETAILED SUMMARY  
 
FOR MONDAY AUGUST 17 - FRIDAY AUGUST 21:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
FOR SATURDAY AUGUST 22 - FRIDAY AUGUST 28: THERE CONTINUES TO BE RELATIVELY  
GOOD MODEL AGREEMENT INDICATING AMPLIFIED RIDGING ACROSS THE WESTERN TWO-THIRDS  
CONTINUING FROM THE PRESENT INTO WEEK-2. THROUGHOUT WEEK-2, THE RIDGE IS  
ANTICIPATED TO SOMEWHAT DEAMPLIFY, WITH THE RIDGE AXIS SHIFTING WESTWARD.  
HOWEVER, THERE IS MORE MODEL UNCERTAINTY COMPARED TO YESTERDAY'S MODEL  
SOLUTIONS REGARDING THE EVOLUTION OF THIS RIDGE. THE 6Z GEFS ENSEMBLE MEAN  
SHOWS THIS RIDGE TO REMAIN MORE AMPLIFIED FOR A LONGER DURATION COMPARED TO THE  
0Z ECMWF AND 0Z CANADIAN ENSEMBLE MEANS. THESE MODEL DIFFERENCES RESULT IN SOME  
UNCERTAINTY OF THE TEMPORAL EXTENT OF THE HEAT WAVE ACROSS THE WEST. DESPITE  
THESE MODEL DIFFERENCES, THE MODERATE RISK OF EXCESSIVE HEAT IS SLIGHTLY  
EXPANDED COMPARED TO YESTERDAY'S FORECAST, AND INCLUDES PARTS OF THE CALIFORNIA  
CENTRAL VALLEY, SOUTHERN CALIFORNIA, SOUTHWEST, AND CENTRAL GREAT BASIN FOR  
AUGUST 22 TO 25 WHERE THE ECMWF AND GEFS REFORECAST TOOLS DEPICT AT LEAST A 40%  
CHANCE THAT MAXIMUM TEMPERATURES EXCEED THE 85TH PERCENTILE OVER PARTS OF THE  
REGION. THE SKILL WEIGHTED HEAT TOOL INDICATES PROBABILITIES GREATER THAN 40  
PERCENT IN SOME OF THE AREAS HIGHLIGHTED OF HEAT INDEX VALUES EXCEEDING THE  
95TH PERCENTILE. THE SLIGHT RISK COVERS A GREATER EXTENT OF THE WEST EXTENDING  
EASTWARD THROUGH THE FRONT RANGE OF THE ROCKIES, AND IS VALID THROUGH THE  
ENTIRETY OF WEEK-2. THE HOTTEST TEMPERATURES ARE FAVORED IN THE VALLEYS AND THE  
DESERT SOUTHWEST, WITH COOLER CONDITIONS MORE LIKELY ACROSS THE HIGHER  
ELEVATIONS. BOTH ANTECEDENT AND EXPECTED DRY, HOT CONDITIONS MAY SUPPORT  
ENHANCED WILDFIRE RISK ACROSS THE SOUTHWEST, WHERE THERE ARE CURRENTLY ACTIVE  
WILDFIRES. THIS PERSISTENT PATTERN MAY ALSO LEAD TO THE CONTINUATION OF POOR  
AIR QUALITY CURRENTLY OCCURRING ACROSS PARTS OF THE WESTERN U.S., ASSOCIATED  
WITH SMOKE AND ADDED OZONE FROM WILDFIRES. MANY WEEKS 3-4 MODEL TOOLS FAVOR  
ANOMALOUSLY WARM TEMPERATURES CONTINUING ACROSS THE WEST INTO THE WEEKS 3 TO 4  
PERIOD, WHICH MAY PROLONG THESE HAZARDOUS CONDITIONS.  
 
MID-LEVEL TROUGHING IS PREDICTED TO FORM ACROSS PARTS OF THE EASTERN THIRD OF  
THE CONUS THROUGHOUT WEEK-2. HOWEVER, THERE ARE SIGNIFICANT MODEL DIFFERENCES  
REGARDING THE AMPLITUDE AND LOCATION OF THIS TROUGH. FROM THE MIDDLE TO END OF  
THE PERIOD, THE 6Z GEFS ENSEMBLE MEAN FAVORS THE FORMATION OF TROUGHING CENTER  
ACROSS THE NORTHEAST WHEREAS THE 0Z ECMWF AND 0Z CANADIAN ENSEMBLE MEAN FAVORS  
A LOWER AMPLITUDE TROUGH AND RIDGING ACROSS NEW ENGLAND. THIS PATTERN RESULTS  
IN THE ECMWF HEAT TOOL INDICATING A SLIGHT RISK (>20% CHANCE) OF EXCESSIVE HEAT  
ACROSS PARTS OF NEW ENGLAND, WHEREAS THE GEFS HEAT TOOL DOES NOT. DUE TO THE  
LOW MODEL AGREEMENT REGARDING THIS FEATURE, NO ADDITIONAL HEAT HAZARDS FOR THIS  
AREA IS POSTED AT THIS TIME. THERE MAY BE FRONTAL ACTIVITY LEADING TO AN  
INCREASED LIKELIHOOD FOR PRECIPITATION ON THE FRONT SIDE OF THIS TROUGH, MAINLY  
AT THE BEGINNING OF WEEK-2, ACROSS PARTS OF THE EASTERN THIRDS OF THE CONUS.  
HOWEVER, RAINFALL AMOUNTS ARE NOT ANTICIPATED TO REACH HEAVY PRECIPITATION  
HAZARDS CRITERIA.  
 
A MAJOR CLIMATE-RELATED HAZARD IS THE EXPANDING AND INTENSIFYING DROUGHT ACROSS  
THE SOUTHWEST DUE TO A POOR NORTH AMERICAN MONSOON AND THE AFOREMENTIONED  
EXCESSIVE HEAT THAT IS FORECAST. ACCORDING TO THE U.S. DROUGHT MONITOR VALID ON  
AUGUST 4, MODERATE TO SEVERE DROUGHT COVERS 81.73% OF ARIZONA. BASED ON THE  
500-HPA HEIGHT PATTERN, BELOW NORMAL PRECIPITATION IS FAVORED FOR WEEK-2 AS  
WELL AS INTO WEEKS 3-4 ACROSS MANY PARTS OF THE REGION.  
 
TROPICAL CYCLONE ACTIVITY IS FORECAST TO INCREASE ACROSS THE EAST PACIFIC WITH  
MULTIPLE STORMS POSSIBLY FORMING OR ALREADY OCCURRING DURING WEEK-2. WHILE  
THESE SYSTEMS ARE NOT PREDICTED TO DIRECTLY IMPACT THE CONUS, INCREASED  
MOISTURE FLOW INTO PARTS OF THE DESERT SOUTHWEST IN ADDITION TO DEW POINTS NEAR  
70 DEG F COULD SUPPORT SOME ISOLATED THUNDERSTORMS AND LOCALIZED FLASH FLOODING  
ACROSS PARTS OF ARIZONA DURING THE PERIOD. ANY RAIN WOULD BRING WELCOME RELIEF  
TO THE ONGOING DROUGHT, BUT A WIDESPREAD EVENT IS UNLIKELY. MANY OF THE WEEK-2  
CPC MODEL GUIDANCE TOOLS INDICATE A DRIER PATTERN ACROSS THE SOUTHWEST COMPARED  
TO THE LAST FEW DAYS, SO THE POSSIBLE FLOOD RISK HAS BEEN REMOVED IN TODAY'S  
FORECAST. OVER THE ATLANTIC BASIN, THE NATIONAL HURRICANE CENTER IS CURRENTLY  
ISSUING ADVISORIES ON TROPICAL STORM JOSEPHINE, LOCATED IN THE CENTRAL  
ATLANTIC. THIS SYSTEM IS FORECAST TO ENCOUNTER SOME WEAKENING IN THE NEXT  
COUPLE DAYS AS IT APPROACHES A LESS FAVORABLE ENVIRONMENT. NO RELATED HAZARDS  
ARE POSTED ASSOCIATED WITH JOSEPHINE AT THIS TIME DURING WEEK-2.  
 
A SERIES OF SURFACE LOWS ARE PREDICTED TO FORM OVER THE GULF OF ALASKA AND  
BERING SEA DURING WEEK-2. ENHANCED RAINFALL ASSOCIATED WITH STORM ACTIVITY OVER  
THE GULF OF ALASKA COMBINED WITH SATURATED SOILS MAY RESULT IN INCREASED RISK  
OF EPISODES OF LOCALIZED FLOODING FOR PARTS OF SOUTHEAST ALASKA, INCLUDING THE  
ALASKA PANHANDLE DURING WEEK-2. THERE IS RELATIVELY GOOD MODEL AGREEMENT  
INDICATING THE POTENTIAL BUILDING OF MID-LEVEL HIGH PRESSURE ACROSS MUCH OF THE  
STATE AT THE BEGINNING OF THE PERIOD, WITH SIGNIFICANTLY MORE UNCERTAINTY IN  
THE EXPECTED PATTERN FOR THE REMAINDER OF WEEK-2. THIS PATTERN FAVORS  
ANOMALOUSLY WARM TEMPERATURES. HOWEVER, NO TEMPERATURE RELATED HAZARDS ARE  
POSTED AT THIS TIME.  
 
FORECASTER: MELISSA OU  
 

 
 
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