Prognostic Meteorological Discussion
Issued by NWS

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FXUS21 KWNC 231820
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 23 2017

SYNOPSIS: AT THE START OF THE PERIOD, AREAS OF SURFACE LOW PRESSURE AND ONSHORE
FLOW ARE EXPECTED TO AFFECT THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA. A
COLD FRONT IS FORECAST TO CROSS THE WESTERN U.S. NOVEMBER 26-27 AND BE OVER THE
CENTRAL PART OF THE NATION NOVEMBER 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 NEXT WEEK. AN AREA OF UPPER-LEVEL
HIGH PRESSURE IS FORECAST TO BUILD OVER THE NORTHEAST PACIFIC, EASTERN ALASKA,
AND WESTERN NORTH AMERICA BY THE END OF NOVEMBER/BEGINNING OF DECEMBER.

HAZARDS

HEAVY SNOW FOR PARTS OF THE NORTHERN CASCADES, COASTAL RANGE OF NORTHERN
CALIFORNIA, AND NORTHERN SIERRAS, SUN, NOV 26.

HEAVY SNOW FOR PARTS OF THE SOUTHERN CASCADES, BITTERROOTS, AND NORTHERN
ROCKIES, SUN-MON, NOV 26-27.

HIGH WINDS FOR PARTS OF THE WESTERN AND WEST-CENTRAL U.S., SUN-MON, NOV 26-27.

HIGH WINDS FOR SOUTHERN CALIFORNIA, MON-TUE, NOV 27-28.

PERIODS OF HIGH WINDS FOR THE ALEUTIANS, EXTREME SOUTHWESTERN MAINLAND ALASKA,
AND THE ALASKA PENINSULA, SUN-WED, NOV 26-29.

PERIODS OF HIGH SIGNIFICANT WAVE HEIGHTS FOR PARTS OF THE ALEUTIANS AND ALASKA
PENINSULA, SUN-WED, NOV 26-29.

FLOODING LIKELY, OCCURRING, OR IMMINENT ALONG MULTIPLE RIVES ACROSS WESTERN AND
WEST-CENTRAL WASHINGTON.

SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.

DETAILED SUMMARY

FOR SUNDAY NOVEMBER 26 - THURSDAY NOVEMBER 30: AN AREA OF SURFACE LOW PRESSURE
AND ITS ASSOCIATED UPPER-LEVEL TROUGH ARE EXPECTED TO CROSS THE PACIFIC
NORTHWEST AND NORTHERN CALIFORNIA NOV 26-27 MAINTAINING PERIODS OF RAIN AND
HIGH-ELEVATION SNOW ACROSS THOSE REGIONS. ALTHOUGH RAINFALL AMOUNTS (GENERALLY
1.5 INCHES OR LESS PER 24 HOURS) ARE NOT EXPECTED TO EXCEED HAZARDS CRITERIA ON
NOV 26 AND 27, THE ADDITIONAL RAINFALL MAY PROLONG OR EXACERBATE RIVER
FLOODING, ESPECIALLY IN WESTERN AND WEST-CENTRAL WASHINGTON. ADDITIONAL
RAINFALL ALSO LEADS TO AN INCREASING RISK OF LANDSLIDES. AS THE UPPER-LEVEL
TROUGH SHIFTS INLAND FROM THE EAST PACIFIC, SNOW LEVELS ARE EXPECTED TO
DECREASE BELOW 4,000 FEET (NEAR HIGHWAY PASS LEVELS) ACROSS PARTS OF THE
NORTHERN CASCADES, NORTHERN SIERRAS AND NORTHERN CALIFORNIA RANGE (NOV 26), AND
THE SOUTHERN CASCADES (NOV 26-27) WHERE HEAVY SNOW (6 INCHES OR MORE PER 24
HOURS) IS FORECAST. THE VIGOROUS SHORTWAVE TROUGH IS EXPECTED TO TRACK ACROSS
THE BITTERROOTS AND NORTHERN ROCKIES LEADING TO HEAVY SNOW (AMOUNTS IN EXCESS
OF 6 INCHES IN 24 HOURS) NOV 26-27. AS THE AREA OF SURFACE LOW PRESSURE AND
COLD FRONT ASSOCIATED WITH THE UPPER-LEVEL TROUGH MOVE INLAND, HIGH WINDS
(SPEEDS OF NEAR 40 MPH) ARE EXPECTED FOR PARTS OF THE WESTERN AND WEST-CENTRAL
CONUS NOV 26-27. FOR AREAS RECEIVING SNOWFALL, BLIZZARD CONDITIONS ARE POSSIBLE.



DETERMINISTIC MODEL SOLUTIONS ARE IN GOOD AGREEMENT THAT SURFACE HIGH PRESSURE
(AROUND 1036-HPA) BECOMES CENTERED OVER THE GREAT BASIN EARLY NEXT WEEK. 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 AND ALASKA PENINSULA FROM NOV 26 TO 29.

FOR FRIDAY DECEMBER 01 - THURSDAY DECEMBER 07: 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. THERE IS AN INCREASING RISK OF COLDER
TEMPERATURES RETURNING TO THE NORTHWESTERN U.S. DURING EARLY DECEMBER AS THE
UPPER-LEVEL RIDGE BUILDS UPSTREAM OVER ALASKA.



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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