265  
FXUS21 KWNC 301904  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 30 2017  
 
SYNOPSIS: A PAIR OF COLD FRONTS ARE FORECAST TO CROSS THE LOWER-48 STATES  
DURING THE COURSE OF WEEK-1. MID-LEVEL LOW PRESSURE IS ANTICIPATED OVER THE  
PACIFIC COAST DURING THE MIDDLE OF WEEK-1, BRINGING ONSHORE FLOW OF PACIFIC  
MOISTURE INTO MUCH OF THE WEST. MID-LEVEL HIGH PRESSURE IS FORECAST TO BUILD  
ACROSS THE SOUTHERN PLAINS AND GULF OF MEXICO DURING WEEK-1. SURFACE HIGH  
PRESSURE IS FORECAST OVER ALASKA THROUGHOUT WEEK-1. DURING WEEK-2, MID-LEVEL  
HIGH PRESSURE IS FORECAST OVER ALASKA AND THE EASTERN U.S., WITH LOW PRESSURE  
OVER THE NORTHERN ROCKIES.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA, OREGON, WASHINGTON STATE,  
AND THE NORTHERN ROCKIES, THU-SUN, NOV 2-5.  
 
HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN AND CENTRAL ROCKIES AND GREAT BASIN,  
FRI-SUN, NOV 3-5.  
 
MUCH BELOW-NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHERN PLAINS, THE  
NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN, SUN-MON, NOV 5-6.  
 
HEAVY RAIN FROM THE MIDDLE MISSISSIPPI VALLEY THROUGH THE GREAT LAKES, MON, NOV  
6.  
 
HEAVY RAIN FROM THE SOUTHERN PLAINS THROUGH THE OHIO VALLEY AND GREAT LAKES,  
TUE, NOV 7.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE NORTHERN  
GREAT BASIN, TUE, NOV 7.  
 
FLOODING POSSIBLE ACROSS PORTIONS OF THE MID-ATLANTIC, THE NORTHEAST, AND THE  
GREAT LAKES.  
 
FLOODING OCCURRING ACROSS THE ST JOHNS RIVER IN FLORIDA.  
 
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN  
PLAINS, THE NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN, TUE-SAT, NOV 7-11.  
 
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF CALIFORNIA AND  
OREGON, TUE-WED, NOV 7-8.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MUCH OF THE INTERIOR WEST  
THROUGH THE UPPER MISSISSIPPI VALLEY, TUE-MON, NOV 7-13.  
 
SEVERE DROUGHT ACROSS THE NORTHERN PLAINS, THE NORTHERN ROCKIES, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR THURSDAY NOVEMBER 02 - MONDAY NOVEMBER 06: MID-LEVEL TROUGHING IS FORECAST  
TO BUILD ACROSS THE PACIFIC NORTHWEST THROUGHOUT THE FIRST HALF OF WEEK-1. THIS  
SCENARIO SUPPORTS AMPLIFIED FLOW OF MOISTURE OFF THE PACIFIC OCEAN INTO MUCH OF  
THE WEST. HEAVY PRECIPITATION (1 INCH OR GREATER LIQUID EQUIVALENT OF  
PRECIPITATION IN 24 HOURS, WITH SNOW AT HIGHER ELEVATIONS) IS FORECAST TO BEGIN  
ACROSS THE PACIFIC NORTHWEST ON NOVEMBER 2ND AND BUILD SOUTHWARD THROUGH  
SOUTHERN CALIFORNIA AND EASTWARD INTO THE NORTHERN ROCKIES BY THE 5TH. PEAK  
LIQUID PRECIPITATION TOTALS THROUGHOUT THE EVENT COULD REACH 6 INCHES OR  
GREATER FOR LOCALIZED AREAS. WHILE MOST AREAS ARE ANTICIPATED TO RECEIVE RAIN  
FROM THE EVENT, SNOW LEVELS SHOULD DROP WITH TIME AS MORE COLD AIR IS BROUGHT  
SOUTHWARD INTO THE REGION. TEMPERATURES IN THE WILLAMETTE VALLEY CURRENTLY  
APPEAR WARM ENOUGH TO NOT SUPPORT AN ICING EVENT, BUT ANY SHIFT TOWARDS A  
COLDER FORECAST COULD INTRODUCE ICING CONCERNS FOR THAT AREA. EVENTUALLY, MUCH  
BELOW-NORMAL TEMPERATURES ARE FORECAST FOR THE NORTHERN PLAINS AND NORTHERN  
ROCKIES ON NOVEMBER 5TH AND 6TH WHERE MINIMUM TEMPERATURE ANOMALIES OF -12 TO  
-16 DEGREES F ARE FORECAST. UPSLOPE ENHANCED HEAVY SNOWFALL (6 INCHES OR  
GREATER IN 24 HOURS) IS ALSO POSSIBLE FOR WESTWARD FACING SLOPES IN THE  
VICINITY OF YELLOWSTONE AND THE TETONS FROM THE 3RD THROUGH THE 5TH.  
 
A SURFACE LOW PRESSURE CENTER IS FORECAST TO AMPLIFY DOWNSTREAM OF THE  
AFOREMENTIONED 500-HPA TROUGH ANTICIPATED OVER THE WEST. THIS DISTURBANCE IS  
FORECAST TO LIFT NORTHWARD INTO THE NORTHERN PLAINS AND TOWARDS HUDSON BAY BY  
LATE IN WEEK-1, BUT THE TRAILING COLD FRONT FROM THIS SYSTEM WILL PROGRESS  
ACROSS THE EASTERN TWO-THIRDS OF THE CONUS. WITH SURFACE HIGH PRESSURE FORECAST  
OVER THE SOUTHEASTERN U.S., AMPLE MOISTURE FROM THE GULF OF MEXICO IS  
ANTICIPATED TO BE LIFTED NORTHWARD FOR THE COLD FRONT TO WORK WITH, YIELDING A  
REGION OF HEAVY RAIN (EXCEEDING 1 INCH IN 24 HOURS) STRETCHING FROM THE MIDDLE  
MISSISSIPPI VALLEY THROUGH THE GREAT LAKES ON THE 6TH. THIS BOUNDARY IS  
FORECAST TO PROGRESS SLOWLY AND FLATTEN OUT SOMEWHAT, LINGERING INTO EARLY IN  
WEEK-2 FOR SIMILAR AREAS. ECMWF FORECASTS ARE THE WETTEST WITH THIS SYSTEM IN  
BOTH DETERMINISTIC AND ENSEMBLE FRAMEWORKS, WHILE THE CANADIAN GUIDANCE IS WET  
BUT MORE PROGRESSIVE AND THE GFS GUIDANCE IS SLOWER AND SUBSTANTIALLY DRIER.  
 
ANOMALOUS RIDGING IS FORECAST TO BUILD ACROSS THE GULF STATES THROUGHOUT  
WEEK-1, WITH THE WEATHER PREDICTION CENTER FORECASTING DAILY HIGH TEMPERATURE  
ANOMALIES OF UP TO 16 DEGREES F ABOVE NORMAL FOR PORTIONS OF THE PLAINS FOR THE  
WEEK. WHILE THIS WOULD ONLY BRING DAILY HIGH TEMPERATURES INTO THE UPPER 80S,  
AND THUS NOT HAZARDOUS, SOME HIGH TEMPERATURE RECORDS COULD FALL FOR PLACES  
SUCH AS ABILENE, TEXAS, WHERE RECORD VALUES ARE ONLY IN THE MID 80S.  
 
ANTECEDENT RAINS GIVE RISE TO FLOODING POSSIBLE ACROSS PORTIONS OF THE  
NORTHEAST FROM THE SYSTEM THAT IMPACTED THE REGION OVER THE PAST WEEKEND, WHILE  
PROLONGED FLOODING CONTINUES ALONG THE ST. JOHNS RIVER IN FLORIDA.  
 
ALASKA IS FORECAST TO BE IMPACTED BY SURFACE HIGH PRESSURE ACROSS THE STATE  
THROUGHOUT WEEK-1. THIS SCENARIO SUPPORTS WARMER THAN USUAL CONDITIONS AND  
GENERALLY MOIST WEATHER, BUT THERE ARE NO SIGNS OF ANY SUBSTANTIVE DISTURBANCES  
APPROACHING THE STATE THAT WOULD BRING HEAVY PRECIPITATION. AS SUCH, NO HAZARDS  
ARE FORECAST FOR ALASKA.  
 
FOR TUESDAY NOVEMBER 07 - MONDAY NOVEMBER 13: THE HEAVY RAIN FROM THE COLD  
FRONT PROGRESSING ACROSS THE EASTERN CONUS LATE IN WEEK-1 IS FORECAST TO LINGER  
THROUGH NOV 7, GIVING REGIONS FROM THE SOUTHERN PLAINS THROUGH GREAT LAKES  
ANOTHER SHOT AT HEAVY RAIN. THE GEFS PROBABILISTIC EXTREMES PRECIPITATION TOOL  
INDICATES A 60 TO 80% CHANCE OF EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE  
DURING THE 7TH THROUGH 9TH, WITH ENSEMBLE MEANS OF AT LEAST THREE QUARTERS OF  
AN INCH OF PRECIPITATION FOR THE HIGHLIGHTED AREA ON THE 7TH. BEYOND THE 7TH,  
ENSEMBLE MEAN PRECIPITATION TOTALS DECAY IN BOTH THE GEFS AND ECMWF ENSEMBLES.  
 
THE DETERMINISTIC AND ENSEMBLE ECMWF SYSTEMS FORECAST A SURGE OF PACIFIC  
MOISTURE INTO THE PACIFIC NORTHWEST ON THE 7TH OF NOVEMBER, THAT COULD BRING  
HEAVY PRECIPITATION TO THE REGION (SNOW POSSIBLE IN THE CASCADES AND HIGHER  
ELEVATIONS OF THE OLYMPIC PENINSULA, RAIN ELSEWHERE). THE ENSEMBLE MEAN  
PROJECTS UP TO 1.25 INCHES OF PRECIPITATION FOR THE REGION, WHILE THE  
DETERMINISTIC RUN INDICATES UP TO 3 INCHES OF PRECIPITATION. THE DETERMINISTIC  
AND ENSEMBLE GFS FORECASTS ARE MUCH WEAKER WITH PRECIPITATION FOR THIS REGION,  
HOWEVER, THE PROBABILISTIC EXTREMES TOOL DOES GIVE A BETTER THAN 40% CHANCE FOR  
PARTS OF THE REGION TO EXCEED THE 85TH CLIMATOLOGICAL PRECIPITATION TOTALS  
DURING THE 7TH THROUGH 9TH.  
 
DURING WEEK-2 ANOMALOUS 500-HPA TROUGHING IS INITIALLY FORECAST TO EXTEND FROM  
THE WESTERN U.S. THROUGH THE NORTHERN PLAINS BEFORE BECOMING CONFINED TO THE  
NORTHERN PLAINS LATER IN THE WEEK. THIS IS ANTICIPATED TO GIVE RISE TO A SLIGHT  
RISK OF MUCH BELOW-NORMAL TEMPERATURES THROUGHOUT WEEK-2 FOR THE WESTERN U.S.  
THROUGH THE NORTHERN PLAINS, ALTHOUGH THE HAZARD IS EXPECTED TO BE CONFINED TO  
THE NORTHERN TIER BEYOND THE 8TH OF NOVEMBER. THE GEFS PROBABILISTIC EXTREMES  
TOOL INDICATES AT LEAST A 40% CHANCE OF MINIMUM TEMPERATURES BEING BELOW THE  
15TH CLIMATOLOGICAL PERCENTILE FOR PORTIONS OF CALIFORNIA AND OREGON ON THE 7TH  
AND 8TH AND FROM EASTERN WASHINGTON THROUGH WESTERN NORTH DAKOTA FROM THE 7TH  
THROUGH 11TH, RESPECTIVELY.  
 
THE U.S. DROUGHT MONITOR, VALID ON OCTOBER 24, INDICATES THAT SEVERE TO  
EXCEPTIONAL (D2-D4) DROUGHT COVERS 2.58 PERCENT OF THE CONTINENTAL U.S., WHICH  
IS THE LOWEST COVERAGE SINCE JUNE. DUE TO RECENT RAINFALL, SEVERE TO  
EXCEPTIONAL DROUGHT IS LIMITED TO THE NORTHERN GREAT PLAINS, AND NORTHERN  
ROCKIES AND HAWAII.  
 
FORECASTER: CPC FORECASTER  
 

 
 
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