497  
FXUS21 KWNC 131909  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 13 2017  
 
SYNOPSIS: AT THE START OF THE PERIOD, SURFACE HIGH PRESSURE IS FORECAST TO  
DOMINATE MOST OF THE LOWER 48 STATES. A COLD FRONT, AND AREAS OF LOW PRESSURE  
AT THE SURFACE AND MID-LEVELS ARE ANTICIPATED TO AFFECT THE NORTHWESTERN U.S.  
OCT 17-22. AREAS OF SURFACE LOW PRESSURE ARE EXPECTED TO MOVE ACROSS THE GULF  
OF ALASKA TOWARDS THE ALASKA PANHANDLE DURING MUCH OF THE ASSESSMENT PERIOD.  
 
HAZARDS  
 
HIGH WINDS FOR COASTAL SECTIONS OF WASHINGTON AND OREGON, WED-THU, OCT 18-19.  
 
HEAVY RAIN FOR THE COASTAL PACIFIC NORTHWEST AND NORTHERN CALIFORNIA, TUE-FRI,  
OCT 17-20.  
 
HEAVY SNOW FOR THE HIGHER ELEVATIONS OF THE CASCADES, WED-FRI, OCT 18-20.  
 
HEAVY SNOW FOR PARTS OF THE NORTHERN ROCKIES, BITTERROOTS, AND COLUMBIA PLATEAU  
THU-FRI, OCT 19-20.  
 
HEAVY RAIN FOR PARTS OF WESTERN WASHINGTON AND WESTERN OREGON, SAT-SUN, OCT  
21-22.  
 
HEAVY SNOW FOR PARTS OF THE CASCADES, BITTERROOTS, COLUMBIA PLATEAU, AND  
NORTHERN ROCKIES, SAT-SUN, OCT 21-22.  
 
FLOODING OCCURRING ALONG THE ST. JOHNS AND WITHLACOOCHEE RIVERS ACROSS CENTRAL  
FLORIDA.  
 
FLOODING POSSIBLE FOR PART OF SOUTHERN TEXAS.  
 
SEVERE DROUGHT ACROSS PARTS OF THE NORTHERN ROCKIES, NORTHERN AND CENTRAL GREAT  
PLAINS, MIDWEST, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR MONDAY OCTOBER 16 - FRIDAY OCTOBER 20: INCREASING ONSHORE FLOW AND AN  
AMPLIFYING UPPER-LEVEL TROUGH ARE EXPECTED TO BRING UNSETTLED WEATHER TO THE  
NORTHWESTERN CONUS. HEAVY RAINFALL (AMOUNTS OF 2-3 INCHES OR MORE) IS  
ANTICIPATED FOR THE COASTAL PACIFIC NORTHWEST AND NORTHERN CALIFORNIA OCT  
17-20. HEAVY SNOW (AMOUNTS IN EXCESS OF 6 INCHES IN 24 HOURS) IS EXPECTED FOR  
THE HIGHEST ELEVATIONS OF THE CASCADES OCT 18-20, AND PARTS OF THE NORTHERN  
ROCKIES, THE BITTERROOTS, AND THE COLUMBIA PLATEAU, OCT 19-20. HIGH WINDS  
(SPEEDS IN EXCESS OF 30 KNOTS) ARE FORECAST FOR COASTAL SECTIONS OF WASHINGTON  
AND OREGON OCT 18-19.  
 
AREAS OF LOW PRESSURE ARE FORECAST TO TRACK ACROSS THE GULF OF ALASKA ON OCT  
16-20. SINCE THE AREAS OF LOW PRESSURE ARE EXPECTED TO BE MODERATE IN STRENGTH,  
WIND SPEEDS AND WAVE HEIGHTS ARE MOST LIKELY TO REMAIN BELOW HAZARDS CRITERIA.  
 
HEAVY RAIN, ASSOCIATED WITH HURRICANE IRMA, AND SUBSEQUENT RAINFALL, TRIGGERED  
FLOODING ALONG THE ST. JOHNS AND WITHLACOOCHEE RIVERS ACROSS CENTRAL FLORIDA.  
MODERATE TO MAJOR FLOODING IS FORECAST TO CONTINUE ALONG THESE RIVERS THROUGH  
MID-OCTOBER. FLOODING IS ALSO POSSIBLE FOR THE NUECES RIVER NEAR BLUNTZER IN  
SOUTHERN TEXAS.  
 
FOR SATURDAY OCTOBER 21 - FRIDAY OCTOBER 27: THE GFS AND ECMWF ENSEMBLE MEANS  
REMAIN IN EXCELLENT AGREEMENT AND GOOD MODEL CONTINUITY EXISTS WITH THE  
LIKELIHOOD OF AN AMPLIFIED UPPER-LEVEL TROUGH AND ENHANCED ONSHORE FLOW  
AFFECTING THE NORTHWESTERN U.S. DURING THE EARLY PART OF WEEK-2. BASED ON THE  
GEFS PRECIPITATION TOOL AND PRECIPITATION FORECASTS FROM THE ENSEMBLE MEANS,  
HEAVY RAIN (AMOUNTS IN EXCESS OF 1-2 INCHES) IS ANTICIPATED FOR WESTERN  
WASHINGTON AND NORTHWESTERN OREGON OCT 21-22, WHILE HEAVY SNOW IS EXPECTED FOR  
HIGHER ELEVATIONS OF THE CASCADES, BITTERROOTS, COLUMBIA PLATEAU, AND NORTHERN  
ROCKIES DURING THE SAME PERIOD. THE HIGHEST ELEVATIONS OF THE CASCADES,  
BITTERROOTS, AND NORTHERN ROCKIES COULD SEE AMOUNTS IN EXCESS OF 8 INCHES).  
 
AN AMPLIFIED UPPER-LEVEL TROUGH IS FORECAST TO DOMINATE MUCH OF ALASKA DURING  
WEEK-2. THIS AMPLIFIED TROUGH ALONG WITH GUIDANCE FROM THE GEFS TEMPERATURE  
TOOLS FAVOR A HIGH LIKELIHOOD OF BELOW-NORMAL TEMPERATURES FOR MUCH OF THE  
STATE BUT TEMPERATURE ANOMALIES ARE NOT CURRENTLY EXPECTED TO REACH HAZARDOUS  
CRITERIA.  
 
THE U.S. DROUGHT MONITOR, VALID ON OCTOBER 10, INDICATES THAT SEVERE TO  
EXCEPTIONAL (D2-D4) DROUGHT COVERS 4.03 PERCENT OF THE CONTINENTAL U.S. WITH  
THE MOST INTENSE DROUGHT CONDITIONS ACROSS NORTHEASTERN MONTANA.  
 
FORECASTER: RANDY SCHECHTER  
 

 
 
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