Threats Assessment Discussion

377 
FXUS21 KWNC 201926
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 20 2017
SYNOPSIS: A COLD FRONT IS FORECAST TO ADVANCE SOUTH ACROSS THE EASTERN U.S. 
LATER THIS WEEK. A WEAK SURFACE LOW IS EXPECTED TO DEVELOP OVER THE GULF OF 
MEXICO BY THANKSGIVING WITH A SUBSEQUENT TRACK ACROSS THE FLORIDA PENINSULA AND 
NEAR THE OUTER BANKS. ONSHORE FLOW IS EXPECTED TO CONTINUE TO AFFECT THE 
PACIFIC NORTHWEST THROUGH THE WEEKEND. SURFACE HIGH PRESSURE IS FORECAST TO 
PREVAIL OVER EASTERN MAINLAND ALASKA DURING THE NEXT WEEK, WHILE A COUPLE OF 
LOW PRESSURE SYSTEMS TRACK ACROSS THE BERING SEA. THE PERSISTENT AREA OF 
UPPER-LEVEL HIGH PRESSURE OVER THE BERING SEA IS EXPECTED TO BE REPLACED BY 
UPPER-LEVEL LOW PRESSURE BY THE BEGINNING OF WEEK-2. 
HAZARDS
HEAVY SNOW DOWNWIND OF LAKE ONTARIO, SUN, NOV 26.
MUCH BELOW-NORMAL TEMPERATURES ACROSS PARTS OF EASTERN MAINLAND ALASKA, 
THU-SAT, NOV 23-25.
HIGH WINDS FOR PARTS OF NORTHWEST ALASKA, SAT-SUN, NOV 25-26.
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF THE NORTHEAST, TUE, 
NOV 28.
FLOODING LIKELY OR POSSIBLE FOR PARTS OF WASHINGTON.
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT 
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.
DETAILED SUMMARY
FOR THURSDAY NOVEMBER 23 - MONDAY NOVEMBER 27: AN UPPER-LEVEL TROUGH IS 
EXPECTED TO REMAIN OFFSHORE OF THE WEST COAST THROUGH THE WEEKEND, MAINTAINING 
PERIODS OF RAIN AND HIGH-ELEVATION SNOW ACROSS THE PACIFIC NORTHWEST. ALTHOUGH 
RAINFALL AMOUNTS (GENERALLY LESS THAN 1.5 INCHES PER 24 HOURS) ARE NOT EXPECTED 
TO EXCEED HAZARDS CRITERIA, THE FREQUENT RAIN MAY PROLONG OR EXACERBATE RIVER 
FLOODING, ESPECIALLY IN NORTHERN WASHINGTON. FLOODING IS LIKELY ALONG PARTS OF 
THE SKOKOMISH, WHILE LEVELS ALONG THE FOLLOWING RIVERS MAY NEAR FLOOD STAGE: 
COWLITZ, DUNGENESS, NISQUALLY, NOOKSACK, SKAGIT, AND SNOQUALMIE. ADDITIONAL 
RAINFALL THIS WEEK ALSO LEADS TO AN INCREASING RISK OF LANDSLIDES. 
SOUTHWESTERLY FLOW IS FORECAST TO MAINTAIN RELATIVELY HIGH SNOW LEVELS (ABOVE 
7,000 FEET) ACROSS THE CASCADES. 
A COLD FRONT IS FORECAST TO USHER IN BELOW-NORMAL TEMPERATURES TO THE GREAT 
LAKES, NORTHEAST, AND MID-ATLANTIC THIS WEEKEND. COLD AIR ADVECTION IS FORECAST 
TO RESULT IN 850-HPA TEMPERATURES FALLING TO AROUND -15 DEGREES C ACROSS THE 
GREAT LAKES ON NOV 25 AND 26. THE MOST LIKELY AREA FOR HEAVY, LAKE-EFFECT SNOW 
(6 INCHES OR MORE) IS EXPECTED DOWNWIND OF LAKE ONTARIO DUE TO THE LONGEST 
DURATION OF COLD AIR ADVECTION ON NOV 26. 
DETERMINISTIC MODEL SOLUTIONS CONTINUE TO INDICATE THAT A SHORTWAVE TROUGH 
SEPARATES FROM THE WESTERLIES AND INDUCES SURFACE CYCLOGENESIS IN THE GULF OF 
MEXICO BY NOV 23. THIS SURFACE LOW IS THEN FORECAST TO TRACK EAST ACROSS THE 
FLORIDA PENINSULA, BUT RAINFALL AMOUNTS ASSOCIATED WITH THIS LOW PRESSURE 
SYSTEM ARE EXPECTED TO BE AROUND AN INCH OR LESS.
AN ARCTIC SURFACE HIGH OVER THE YUKON IS FORECAST TO RESULT IN MUCH BELOW 
NORMAL TEMPERATURES (NEGATIVE ANOMALIES OF 10 TO 20 DEGREES F) ACROSS SOUTHEAST 
MAINLAND ALASKA THROUGH NOV 25. THE GEOGRAPHIC EXTENT OF THIS HAZARD IS BASED 
ON GUIDANCE FROM THE 0Z ECMWF MODEL. AS THE HIGH AMPLITUDE RIDGE UPSTREAM OVER 
THE BERING SEA WEAKENS, ANOMALOUS OFFSHORE FLOW IS EXPECTED TO END WHICH WILL 
LEAD TO A RAPID MODERATION IN TEMPERATURES ACROSS EASTERN ALASKA. A SURFACE LOW 
IS FORECAST TO STRENGTHEN AS IT TRACKS EAST FROM THE KAMCHATKA PENINSULA AND 
BRING HIGH WINDS (SPEEDS OF 40 TO 50 KNOTS, OR HIGHER) TO PARTS OF NORTHWEST 
ALASKA ON NOV 25 AND 26.
FOR TUESDAY NOVEMBER 28 - MONDAY DECEMBER 04: THE ECMWF AND CANADIAN ENSEMBLE 
MEANS ARE IN GOOD AGREEMENT WITH AN UPPER-LEVEL TROUGH (RIDGE) OVER THE BERING 
SEA (WESTERN NORTH AMERICA). HOWEVER, THE GFS ENSEMBLE MEAN INDICATES AN 
UPPER-LEVEL RIDGE NEAR THE BERING SEA. HIGH UNCERTAINTY EXISTS IN THE EVOLVING 
LONGWAVE PATTERN ACROSS THE MIDDLE AND HIGH LATITUDES OF NORTH AMERICA DUE TO 
THESE DIFFERING MODEL SOLUTIONS. ALSO, SPREAD AMONG ENSEMBLE MEANS IS MODERATE 
TO LARGE. THE ENSEMBLE MEANS AGREE ON A LARGE POSITIVE 500-HPA HEIGHT ANOMALY 
CENTER OVER THE NORTH ATLANTIC, WHICH MAY MAINTAIN NEAR TO BELOW NORMAL 
TEMPERATURES ACROSS THE NORTHEASTERN U.S. INTO THE BEGINNING OF DECEMBER. A 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS POSTED FOR THE NORTHEAST WHERE 
THE GEFS TEMPERATURE TOOL INDICATES THAT MINIMUM TEMPERATURES HAVE CLOSE TO A 
20 PERCENT CHANCE OF FALLING BELOW THE 15TH PERCENTILE ON NOV 28.
THE U.S. DROUGHT MONITOR, VALID ON NOV 14, INDICATES THAT SEVERE TO EXCEPTIONAL 
(D2-D4) DROUGHT COVERS 3.52 PERCENT OF THE CONTINENTAL U.S., WHICH IS A SLIGHT 
INCREASE FROM 2.90 PERCENT LAST WEEK. SEVERE DROUGHT HAS EXPANDED THIS MONTH 
ACROSS ARKANSAS AND MISSOURI.
FORECASTER: BRAD PUGH 
$$