128  
FXUS21 KWNC 171839  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 17 2017  
 
SYNOPSIS: A SURFACE LOW IS EXPECTED TO TRACK FROM WESTERN TO NORTHEASTERN  
CANADA OVER WEEK-1. ITS TRAILING COLD FRONT IS ANTICIPATED TO SWEEP ACROSS THE  
LOWER 48 STATES THROUGHOUT THE DAY 3 TO 7 PERIOD. A SECONDARY COLD FRONT IS  
ANTICIPATED TO ENTER THE PACIFIC NORTHWEST BY OCT 22 CAND MOVE ACROSS PARTS OF  
THE U.S. BEHIND THE INITIAL COLD FRONT. SURFACE HIGH PRESSURE IS FORECAST TO  
BUILD ACROSS MUCH OF THE CONTIGUOUS U.S. AHEAD OF, AND IN THE WAKE OF, THE COLD  
FRONTS. A SERIES OF SURFACE LOW PRESSURE SYSTEMS ARE PREDICTED TO DEVELOP OVER  
COASTAL PARTS OF MAINLAND ALASKA AS WELL AS THE GULF OF ALASKA DURING THE 3 TO  
7 DAY PERIOD. SOME MODELS FAVOR AMPLIFIED MID-LEVEL TROUGHING THROUGHOUT WEEK-2  
ACROSS THE EASTERN HALF OF THE U.S. WITH THE POTENTIAL FOR A SIGNIFICANT STORM  
APPROACHING THE ALEUTIANS, SOUTH COAST OF ALASKA, AND THE ALASKA PANHANDLE BY  
THE BEGINNING OF WEEK-2.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF CASCADES, SAT, OCT 21.  
 
HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST, SAT, OCT 21.  
 
HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT  
BASIN, FRI-SAT, OCT 20-OCT 21.  
 
HIGH WINDS ACROSS COASTAL PORTIONS OF NORTHERN CALIFORNIA AND THE PACIFIC  
NORTHWEST, SAT, OCT 21.  
 
HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE PACIFIC NORTHWEST,  
FRI, OCT 20.  
 
HEAVY RAIN ACROSS PORTIONS OF THE SOUTHERN PLAINS, SAT, OCT 21.  
 
HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST, THE MIDDLE MISSISSIPPI VALLEY, THE  
LOWER MISSISSIPPI VALLEY, THE OHIO VALLEY, AND THE TENNESSEE VALLEY, SUN, OCT  
22.  
 
HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, TUE, OCT 24.  
 
HIGH WINDS FOR PARTS OF SOUTHWESTERN CALIFORNIA, SAT, OCT 21.  
 
HIGH WINDS ACROSS PORTIONS OF THE CENTRAL ROCKIES, THE CENTRAL PLAINS, THE  
NORTHERN PLAINS, AND THE NORTHERN ROCKIES, SUN-MON, OCT 22-OCT 23.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE, KENAI PENINSULA,  
AND KODIAK ISLAND, WED-FRI, OCT 25-OCT 27.  
 
HIGH WINDS ACROSS PORTIONS OF THE ALASKA PENINSULA AND THE ALEUTIANS, WED-THU,  
OCT 25-OCT 26.  
 
HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE ALASKA PENINSULA AND  
THE ALEUTIANS, WED-THU, OCT 25-OCT 26.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE EASTERN HALF  
OF THE CONUS, WED-THU, OCT 25-OCT 26.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF NORTHERN MAINLAND  
ALASKA, WED-THU, OCT 25-OCT 26.  
 
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF CALIFORNIA AND  
THE SOUTHWEST, WED-THU, OCT 25-OCT 26.  
 
FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST, THE GREAT  
LAKES, AND THE SOUTHERN PLAINS.  
 
SEVERE DROUGHT ACROSS THE MIDDLE MISSISSIPPI VALLEY, THE NORTHERN PLAINS, THE  
NORTHERN ROCKIES, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR FRIDAY OCTOBER 20 - TUESDAY OCTOBER 24: A SURFACE LOW IS PREDICTED TO FORM  
OVER ALBERTA AT THE START OF THE PERIOD AND TRACK EASTWARD ACROSS CANADA  
THROUGHOUT THE 3 TO 7 DAY PERIOD. ITS TRAILING COLD FRONT OVER THE CONUS IS  
EXPECTED TO SWEEP ACROSS THE LOWER 48 STATES THROUGHOUT WEEK-1, BRINGING HEAVY  
PRECIPITATION TO PARTS OF THE COUNTRY. AT THE BEGINNING OF THE PERIOD, AS THE  
COLD FRONT STRETCHES FROM THE NORTHERN PLAINS SOUTHWESTWARD TO CALIFORNIA,  
PARTS OF THE PACIFIC NORTHWEST MAY RECEIVE HEAVY RAIN ON OCT 21 AND HEAVY RAIN  
AND HIGH ELEVATION SNOW ACROSS PARTS OF THE CASCADES OCT 21. SOME MODELS  
INDICATE THE POSSIBILITY OF LOCALIZED AREAS RECEIVING 2 TO 3 INCHES OR GREATER  
OF LIQUID EQUIVALENT IN A 24-HOUR PERIOD. ADDITIONALLY, PARTS OF THE NORTHERN  
ROCKIES AND NORTHERN GREAT BASIN MAY RECEIVE HEAVY SNOW OCT 20 TO 21. SEVERAL  
INCHES OF SNOW IS POSSIBLE ACROSS HIGH ELEVATION AREAS.  
 
ANOTHER SURFACE LOW IS ANTICIPATED TO FORM IN THE EASTERN PACIFIC AND APPROACH  
THE PACIFIC NORTHWEST COAST. THIS SURFACE LOW AS WELL AS ANOTHER LOW IN THE  
GULF OF ALASKA, SUPPORTS STRONG WESTERLY ONSHORE FLOW TO THE PACIFIC NORTHWEST  
COAST, BRINGING INCREASED CHANCES FOR HIGH SIGNIFICANT WAVE HEIGHTS (21 FEET OR  
GREATER) TO COASTAL PARTS OF THE PACIFIC NORTHWEST. ON OCT 21, COASTAL PORTIONS  
OF THE PACIFIC NORTHWEST AND NORTHWESTERN CALIFORNIA MAY EXPERIENCE HIGH WINDS  
(40 KNOTS OR GREATER).  
 
AS THE AFOREMENTIONED COLD FRONT TRAVERSES THE CONUS, PARTS OF THE SOUTHERN  
PLAINS MAY RECEIVE HEAVY RAINFALL ON OCT 21, FOLLOWED BY THE SOUTHEAST, THE  
MIDDLE MISSISSIPPI VALLEY, THE LOWER MISSISSIPPI VALLEY, THE OHIO VALLEY, AND  
THE TENNESSEE VALLEY ON OCT 22. SOME MODELS INDICATE THE POTENTIAL FOR  
ATMOSPHERIC CONDITIONS TO BE CONDUCIVE FOR SEVERE THUNDERSTORMS ALONG THE COLD  
FRONT, MOST LIKELY ON OCT 21 ACROSS PARTS OF KANSAS, OKLAHOMA, AND NORTH TEXAS.  
PARTS OF THE NORTHEAST MAY ALSO RECEIVE HEAVY RAIN OCT 24 AS THE COLD FRONT  
APPROACHES THE EAST COAST. IT SHOULD BE NOTED THAT THERE ARE SIGNIFICANT  
DIFFERENCES AMONGST MODELS REGARDING THE SPATIAL COVERAGE AND RAINFALL TOTALS.  
THE 6Z DETERMINISTIC GFS INDICATES A SMALLER REGION, LIMITED TO MAINLY THE  
NORTHEAST AS INDICATED, WHEREAS THE 0Z DETERMINISTIC ECMWF SHOWS A LARGER  
REGION INCLUDING EASTERN NEW YORK, PENNSYLVANIA, AND MARYLAND. THE ENSEMBLE  
MEANS OF EACH OF THESE MODELS FAVOR LOWER RAINFALL ACCUMULATIONS AND SMALLER  
COVERAGE SIMILAR TO THE 6Z DETERMINISTIC GFS. LOCALIZED AREAS IN THE REGIONS  
MENTIONED MAY RECEIVE UP TO 2 INCHES OR GREATER OF RAINFALL IN A 24-HOUR  
PERIOD.  
 
STRONG SURFACE HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE WESTERN CONUS  
BEHIND THE COLD FRONT BY OCT 21. THIS PATTERN MAY SUPPORT MODERATE TO STRONG  
OFFSHORE WINDS ACROSS PARTS OF SOUTHERN CALIFORNIA. THESE ELEVATED WIND SPEEDS  
MAY LEAD TO INCREASED CHANCES FOR WILDFIRE RISK THROUGHOUT THE WEEKEND.  
SUSTAINED WIND SPEEDS MAY REACH 30 KNOTS OR GREATER.  
 
A SURFACE TROUGH IS EXPECTED TO FORM ACROSS THE NORTHERN PLAINS BY OCT 22. A  
TIGHT PRESSURE GRADIENT ASSOCIATED WITH THIS TROUGH MAY BRING HIGH WINDS TO  
PARTS OF THE CENTRAL ROCKIES, THE CENTRAL PLAINS, THE NORTHERN PLAINS, AND THE  
NORTHERN ROCKIES OCT 22 TO 23. SOME AREAS MAY EXPERIENCE SUSTAINED WIND SPEEDS  
REACHING 40 KNOTS OR GREATER.  
 
A SURFACE LOW IS PREDICTED TO FORM OVER THE SOUTH COAST OF ALASKA BY OCT 21  
SUPPORTING STRONG ONSHORE FLOW FROM THE GULF OF ALASKA TO PARTS OF THE SOUTH  
COAST OF ALASKA AND THE ALASKA PANHANDLE. THIS STORM MAY BRING HEAVY SNOW TO  
PARTS OF THE KENAI PENINSULA ON OCT 21. HOWEVER, DUE TO SIGNIFICANT MODEL  
DIFFERENCES REGARDING ANTICIPATED PRECIPITATION TOTALS, A SPECIFIC RELATED  
HAZARDS AREA IS NOT INDICATED AT THIS TIME.  
 
FOR WEDNESDAY OCTOBER 25 - TUESDAY OCTOBER 31: TODAY THE 6Z GEFS AND 0Z ECMWF  
ENSEMBLE HAVE COME INTO BETTER AGREEMENT REGARDING THE DEVELOPMENT OF A  
MID-LEVEL TROUGH ACROSS THE EASTERN HALF OF THE CONUS THROUGHOUT WEEK-2. THIS  
PATTERN MAY TRANSLATE TO MUCH BELOW NORMAL TEMPERATURES ACROSS THE EASTERN HALF  
OF THE CONUS OCT 25 TO 26. THE 6Z GEFS SHOWS THIS TROUGH REMAINING IN PLACE  
UNTIL OCT 28, WHEREAS THE 0Z ECMWF ENSEMBLE SHOWS A BRIEF RETURN TO MORE ZONAL  
FLOW ON OCT 27 BEFORE REBUILDING A TROUGH THE FOLLOWING DAY. THERE IS A  
POSSIBILITY OF MORE DAYS IN THE WEEK-2 PERIOD BEING INCLUDED FOR A MUCH BELOW  
NORMAL TEMPERATURE HAZARD DEPENDING ON HOW THE MODELS EVOLVE IN THE NEXT FEW  
DAYS. THE GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OF TEMPERATURES  
FOR HIGHLIGHTED AREAS REACHING THE LOWEST 15TH PERCENTILE, WITH THE MINIMUM  
TEMPERATURES OF SOME AREAS REACHING SUB-FREEZING TEMPERATURES. THIS COLD EVENT  
COULD BE THE FIRST FREEZE FOR THE NORTHERN PART OF THE CORN BELT.  
 
AN AMPLIFIED MID-LEVEL RIDGE IS PREDICTED TO BUILD ACROSS THE WESTERN HALF OF  
THE CONUS AT THE BEGINNING OF WEEK-2. THIS PATTERN FAVORS A SLIGHT CHANCE OF  
MUCH ABOVE NORMAL TEMPERATURES ACROSS PARTS OF CALIFORNIA AND THE SOUTHWEST OCT  
25 TO 26. THE GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OR GREATER OF  
MAXIMUM TEMPERATURES IN THE HIGHLIGHTED AREAS REACHING THE 85TH PERCENTILE AND  
LOCALIZED AREAS REACHING UP TO 90 DEG F.  
 
A STRONG SURFACE LOW PRESSURE SYSTEM IS FORECAST TO DEVELOP OVER THE NORTHERN  
PACIFIC AND REACH THE WESTERN ALEUTIANS BY THE END OF WEEK-1. AS THIS STORM  
TRACKS EASTWARD ALONG THE ALEUTIANS TO THE GULF OF ALASKA BY THE MIDDLE OF  
WEEK-2, THERE IS AN INCREASED CHANCE OF HIGH WINDS (40 KNOTS OR GREATER) AND  
SIGNIFICANT WAVE HEIGHTS (24 FEET OR GREATER), INITIALLY ACROSS THE WESTERN AND  
CENTRAL ALEUTIANS OCT 25, SHIFTING EASTWARD TO THE ALASKA PENINSULA OCT 26.  
STRONG ONSHORE FLOW AHEAD OF THIS STORM MAY BRING HEAVY RAIN AND SNOW TO PARTS  
OF THE ALASKA PANHANDLE, KENAI PENINSULA, AND KODIAK ISLAND OCT 25 TO 27. THE  
GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OR GREATER OF THESE REGIONS  
RECEIVING 3-DAY PRECIPITATION ACCUMULATIONS REACHING OR EXCEEDING THE 85TH  
PERCENTILE.  
 
SURFACE HIGH PRESSURE IS ANTICIPATED TO DEVELOP OVER THE BEAUFORT SEA AT THE  
BEGINNING OF WEEK-2. THIS PATTERN MAY SUPPORT A SLIGHT CHANCE OF MUCH BELOW  
NORMAL TEMPERATURES ACROSS NORTHERN PARTS OF MAINLAND ALASKA OCT 25 TO 26. THE  
GEFS REFORECAST TOOL INDICATES A SLIGHT CHANCE OF MINIMUM TEMPERATURES IN THIS  
REGION REACHING THE LOWEST 15TH PERCENTILE, WITH TEMPERATURES REACHING 0 DEG F.  
 
FORECASTER: MELISSA OU  
 

 
 
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