281  
FXUS21 KWNC 241929  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 24 2017  
 
SYNOPSIS: AT THE START OF THE PERIOD, ONSHORE FLOW AND AN APPROACHING FRONTAL  
SYSTEM ARE EXPECTED TO AFFECT THE PACIFIC NORTHWEST. A COLD FRONT IS FORECAST  
TO CROSS THE INTERIOR WESTERN U.S. AND MOVE TOWARD THE CENTRAL PART OF THE  
NATION NOVEMBER 27-28. SURFACE HIGH PRESSURE IS ANTICIPATED TO BUILD OVER THE  
CENTRAL GREAT BASIN NOVEMBER 27-28 LEADING TO OFFSHORE WINDS OVER SOUTHERN  
CALIFORNIA. MULTIPLE SURFACE LOW PRESSURE SYSTEMS ARE EXPECTED TO TRACK ACROSS  
THE BERING SEA AND ALEUTIANS NOVEMBER 27-DECEMBER 1. AN AREA OF UPPER-LEVEL  
HIGH PRESSURE IS FORECAST TO BUILD OVER THE NORTHEAST PACIFIC AND EASTERN  
ALASKA FOR THE BEGINNING OF DECEMBER. DOWNSTREAM OF THIS AREA OF UPPER-LEVEL  
HIGH PRESSURE, UPPER-LEVEL LOW PRESSURE IS ANTICIPATED OVER THE WESTERN PART OF  
THE NATION.  
 
HAZARDS  
 
HEAVY SNOW FOR PARTS OF THE NORTHERN CASCADES, MON-TUE, NOV 27-28.  
 
HEAVY SNOW FOR PARTS OF THE NORTHERN AND CENTRAL ROCKIES, MON, NOV 27.  
 
HEAVY PRECIPITATION FOR PARTS OF SOUTH COASTAL MAINLAND ALASKA, TUE-WED, NOV  
28-29.  
 
HIGH WINDS FOR PARTS OF THE INTERIOR WESTERN U.S., MON, NOV 27.  
 
HIGH WINDS FOR SOUTHERN CALIFORNIA, MON-TUE, NOV 27-28.  
 
PERIODS OF HIGH WINDS FOR THE ALEUTIANS, EXTREME SOUTHWESTERN MAINLAND ALASKA,  
ALASKA PENINSULA, AND SOUTH COASTAL MAINLAND ALASKA, MON-FRI, NOV 27-DEC 1.  
 
PERIODS OF HIGH SIGNIFICANT WAVE HEIGHTS FOR PARTS OF THE ALEUTIANS, ALASKA  
PENINSULA, AND SOUTH COASTAL MAINLAND ALASKA, MON-FRI, NOV 27-DEC 1.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF NORTHERN AND CENTRAL  
CALIFORNIA, MON-TUE, DEC 4-5.  
 
FLOODING OCCURRING OR IMMINENT ALONG MULTIPLE RIVES ACROSS WEST-CENTRAL  
WASHINGTON.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR MONDAY NOVEMBER 27 - FRIDAY DECEMBER 01: LOW PRESSURE AND ITS ASSOCIATED  
FRONTAL SYSTEM ARE FORECAST TO BRING HEAVY SNOW (AMOUNTS IN EXCESS OF 6 INCHES  
IN 24 HOURS) TO PARTS OF THE NORTHERN AND CENTRAL ROCKIES NOV 27. ONSHORE FLOW  
AHEAD OF AN APPROACHING FRONTAL SYSTEM IS EXPECTED TO AFFECT THE PACIFIC  
NORTHWEST. ALTHOUGH RAINFALL AMOUNTS (GENERALLY 1.5 INCHES OR LESS PER 24  
HOURS) ARE NOT EXPECTED TO EXCEED HAZARDS CRITERIA ON NOV 27-28, THE ADDITIONAL  
RAINFALL MAY PROLONG OR EXACERBATE RIVER FLOODING, ESPECIALLY IN WEST-CENTRAL  
WASHINGTON. ADDITIONAL RAINFALL ALSO LEADS TO AN INCREASING RISK OF LANDSLIDES.  
SNOW LEVELS ARE EXPECTED TO DECREASE BELOW 4,000 FEET (NEAR HIGHWAY PASS  
LEVELS) ACROSS PARTS OF THE REGION AND HEAVY SNOWFALL (24 HOUR AMOUNTS IN  
EXCESS OF 6 INCHES) IS EXPECTED FOR PARTS OF THE NORTHERN CASCADES NOV 27-28.  
AS THE AREA OF SURFACE LOW PRESSURE AND COLD FRONT MOVE INLAND, HIGH WINDS  
(SPEEDS OF NEAR 40 MPH) ARE EXPECTED FOR PARTS OF THE INTERIOR WESTERN CONUS  
NOV 27. FOR AREAS RECEIVING SNOWFALL, BLIZZARD CONDITIONS ARE POSSIBLE.  
 
DETERMINISTIC MODEL SOLUTIONS ARE IN GOOD AGREEMENT THAT SURFACE HIGH PRESSURE  
(AROUND 1030-HPA) BECOMES CENTERED OVER THE GREAT BASIN NOV 27-28. THIS SURFACE  
PATTERN IS EXPECTED TO PROMOTE A SANTA ANA WIND EVENT (WIND GUSTS ABOVE 50 MPH)  
FOR SOUTHERN CALIFORNIA ON NOV 27-28.  
 
A COUPLE OF INTENSE LOW PRESSURE SYSTEMS ARE FORECAST TO BRING PERIODS OF HIGH  
WINDS (SPEEDS ABOVE 50 MPH) AND HIGH SIGNIFICANT WAVES (PEAK HEIGHTS OF MORE  
THAN 36 FEET) TO THE ALEUTIANS, ALASKA PENINSULA, AND COASTAL SOUTHERN MAINLAND  
ALASKA FROM NOV 27 TO DEC 1. IN ADDITION, HEAVY PRECIPITATION (LOW LEVEL  
RAINFALL OF 2 INCHES IN 24 HOURS, HIGHER ELEVATION SNOWFALL OF 8 INCHES IN 24  
HOURS) IS ANTICIPATED FOR PARTS OF SOUTH COASTAL MAINLAND ALASKA NOV 28-29.  
 
FOR SATURDAY DECEMBER 02 - FRIDAY DECEMBER 08: THE ENSEMBLE MEANS DURING WEEK-2  
FEATURE AN AMPLIFIED UPPER-LEVEL RIDGE EXTENDING FROM THE NORTHEAST PACIFIC TO  
ALASKA WITH ITS AXIS CENTERED AT 140 TO 150 DEGREES WEST LONGITUDE. THIS  
UPPER-LEVEL RIDGE IS LIKELY TO RESULT IN A MUCH DRIER PATTERN FOR THE PACIFIC  
NORTHWEST AT THE START OF DECEMBER. IN ADDITION, A DOWNSTREAM TROUGH FORECAST  
OVER THE WESTERN CONUS LEADS TO A SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES  
FOR PARTS OF NORTHERN AND CENTRAL CALIFORNIA DEC 4-5.  
 
THE U.S. DROUGHT MONITOR, VALID ON NOV 21, INDICATES THAT SEVERE TO EXCEPTIONAL  
(D2-D4) DROUGHT COVERS 4.14 PERCENT OF THE CONTINENTAL U.S., WHICH IS A SLIGHT  
INCREASE FROM 3.52 PERCENT LAST WEEK. SEVERE DROUGHT HAS EXPANDED THIS MONTH  
ACROSS ARKANSAS AND MISSOURI.  
 
FORECASTER: RANDY SCHECHTER  
 

 
 
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