682  
FXUS21 KWNC 121901  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 12 2017  
 
SYNOPSIS: AT THE START OF THE PERIOD, DEEP SURFACE LOW PRESSURE IS FORECAST TO  
MOVE FROM THE NORTHERN GREAT LAKES INTO EASTERN CANADA. COLD SURFACE HIGH  
PRESSURE IS FORECAST TO BUILD FROM THE GREAT BASIN INTO THE SOUTHERN PLAINS OCT  
15-16 ALLOWING OFFSHORE WINDS OVER CALIFORNIA TO GRADUALLY WEAKEN. AREAS OF LOW  
PRESSURE AT THE SURFACE AND MID-LEVELS ARE ANTICIPATED TO AFFECT THE  
NORTHWESTERN U.S. OCT 17-21. AREAS OF SURFACE LOW PRESSURE ARE EXPECTED TO MOVE  
ACROSS THE GULF OF ALASKA TOWARDS THE ALASKA PANHANDLE DURING MUCH OF THE  
ASSESSMENT PERIOD.  
 
HAZARDS  
 
HIGH WINDS FOR PARTS OF THE GREAT LAKES, SUN, OCT 15.  
 
MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF THE CENTRAL GREAT BASIN, SUN, OCT  
15.  
 
HIGH WINDS FOR COASTAL SECTIONS OF NORTHERN CALIFORNIA AND PARTS OF SOUTHERN  
CALIFORNIA, SUN, OCT 15.  
 
HIGH WINDS FOR COASTAL SECTIONS OF WASHINGTON, OREGON, AND NORTHERN CALIFORNIA,  
WED-THU, OCT 18-19.  
 
HEAVY RAIN FOR THE COASTAL PACIFIC NORTHWEST AND NORTHERN CALIFORNIA, TUE-THU,  
OCT 17-19.  
 
HEAVY SNOW FOR THE CASCADES AND PARTS OF THE NORTHERN ROCKIES AND BITTERROOTS,  
WED-THU, OCT 18-19.  
 
HEAVY SNOW FOR PARTS OF THE NORTHERN ROCKIES, THU, OCT 19.  
 
HEAVY RAIN FOR WESTERN WASHINGTON AND NORTHWESTERN OREGON, FRI-SAT, OCT 20-21.  
 
HEAVY SNOW FOR PARTS OF THE CASCADES, BITTERROOTS, COLUMBIA PLATEAU, AND  
NORTHERN ROCKIES, FRI-SAT, OCT 20-21.  
 
FLOODING OCCURRING ALONG THE ST. JOHNS AND WITHLACOOCHEE RIVERS ACROSS CENTRAL  
FLORIDA.  
 
FLOODING POSSIBLE FOR PART OF SOUTHERN TEXAS.  
 
SEVERE DROUGHT ACROSS PARTS OF THE NORTHERN ROCKIES, NORTHERN AND CENTRAL GREAT  
PLAINS, MIDWEST, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SUNDAY OCTOBER 15 - THURSDAY OCTOBER 19: AN AREA OF STRONG LOW PRESSURE  
OVER THE GREAT LAKES IS FORECAST TO MOVE NORTHEASTWARD INTO EASTERN CANADA.  
RAINFALL IS EXPECTED OVER PARTS OF NORTHERN NEW ENGLAND BUT THE HEAVIEST  
RAINFALL IS EXPECTED OVER PARTS OF EASTERN CANADA SO A HAZARD SHAPE WAS NOT  
INDICATED. THE TIGHT PRESSURE GRADIENT ASSOCIATED WITH THIS AREA OF LOW  
PRESSURE LEADS TO HIGH WINDS (SPEEDS IN EXCESS OF 30 KNOTS) FOR PARTS OF THE  
GREAT LAKES OCT 15.  
 
SURFACE HIGH PRESSURE (1032-HPA) IS EXPECTED TO BUILD FROM THE GREAT BASIN INTO  
THE SOUTHERN PLAINS FROM OCT 15 TO 16, RESULTING IN MUCH BELOW-NORMAL  
TEMPERATURES (NEGATIVE ANOMALIES EXCEEDING 10 DEGREES F) ACROSS PARTS OF THE  
CENTRAL GREAT BASIN OCT 15. THIS SURFACE PATTERN IS FAVORABLE FOR A SANTA ANA  
WIND EVENT TO AFFECT SOUTHERN CALIFORNIA ON OCT 15. HIGH WINDS (NORTHEAST GUSTS  
ABOVE 35 KNOTS) ARE MOST LIKELY FOR THE COASTAL FOOTHILLS AND CANYONS OF  
SOUTHERN CALIFORNIA. MULTIPLE WILDFIRES RECENTLY INCREASED IN SIZE ACROSS NAPA  
AND SONOMA COUNTIES OF NORTHERN CALIFORNIA. LOW RELATIVE HUMIDITY VALUES THIS  
WEEK WILL MAINTAIN FIRE WEATHER CONCERNS ACROSS THIS REGION. HIGH WINDS ARE  
ALSO ANTICIPATED FOR PARTS OF COASTAL NORTHERN CALIFORNIA OCT 15.  
 
INCREASING ONSHORE FLOW AND AN AMPLIFYING UPPER-LEVEL TROUGH ARE EXPECTED TO  
BRING UNSETTLED WEATHER TO THE NORTHWESTERN CONUS. HEAVY RAINFALL (AMOUNTS OF  
2-4 INCHES OR MORE) IS ANTICIPATED FOR THE COASTAL PACIFIC NORTHWEST AND  
NORTHERN CALIFORNIA OCT 17-19. HEAVY SNOW (AMOUNTS IN EXCESS OF 6 INCHES IN 24  
HOURS) IS EXPECTED FOR THE CASCADES AND PARTS OF THE NORTHERN ROCKIES OF  
MONTANA AND THE BITTERROOTS IN IDAHO OCT 18-19, AND FOR THE NORTHERN ROCKIES IN  
WYOMING OCT 19. HIGH WINDS (SPEEDS IN EXCESS OF 30 KNOTS) ARE FORECAST FOR  
COASTAL SECTIONS OF WASHINGTON, OREGON, AND NORTHERN CALIFORNIA OCT 18-19.  
 
AREAS OF LOW PRESSURE ARE FORECAST TO TRACK ACROSS THE GULF OF ALASKA ON OCT  
15-19. SINCE THE AREAS OF LOW PRESSURE ARE EXPECTED TO BE MODERATE (984-HPA OR  
HIGHER) IN STRENGTH, WIND SPEEDS AND WAVE HEIGHTS ARE MOST LIKELY TO REMAIN  
BELOW HAZARDS CRITERIA.  
 
HEAVY RAIN, ASSOCIATED WITH HURRICANE IRMA, AND SUBSEQUENT RAINFALL, TRIGGERED  
FLOODING ALONG THE ST. JOHNS AND WITHLACOOCHEE RIVERS ACROSS CENTRAL FLORIDA.  
MODERATE TO MAJOR FLOODING IS FORECAST TO CONTINUE ALONG THESE RIVERS THROUGH  
MID-OCTOBER.  
 
FOR FRIDAY OCTOBER 20 - THURSDAY OCTOBER 26: THE GFS AND ECMWF ENSEMBLE MEANS  
REMAIN IN EXCELLENT AGREEMENT AND GOOD MODEL CONTINUITY EXISTS WITH THE  
LIKELIHOOD OF AN AMPLIFIED UPPER-LEVEL TROUGH AND ENHANCED ONSHORE FLOW  
AFFECTING THE NORTHWESTERN U.S. DURING THE EARLY PART OF WEEK-2. BASED ON THE  
GEFS PRECIPITATION TOOL AND PRECIPITATION FORECASTS FROM THE ENSEMBLE MEANS,  
HEAVY RAIN (AMOUNTS IN EXCESS OF 1-3 INCHES) IS ANTICIPATED FOR WESTERN  
WASHINGTON AND NORTHWESTERN OREGON OCT 20-21, WHILE HEAVY SNOW IS EXPECTED FOR  
HIGHER ELEVATIONS OF THE CASCADES, BITTERROOTS, COLUMBIA PLATEAU, AND NORTHERN  
ROCKIES DURING THE SAME PERIOD. THE HIGHEST ELEVATIONS OF THE CASCADES,  
BITTERROOTS, AND NORTHERN ROCKIES COULD SEE AMOUNTS IN EXCESS OF 8 INCHES).  
 
AN AMPLIFIED UPPER-LEVEL TROUGH IS FORECAST TO DOMINATE MUCH OF ALASKA DURING  
WEEK-2. THIS AMPLIFIED TROUGH ALONG WITH GUIDANCE FROM THE GEFS TEMPERATURE  
TOOLS FAVOR A HIGH LIKELIHOOD OF BELOW-NORMAL TEMPERATURES FOR MUCH OF THE  
STATE BUT TEMPERATURE ANOMALIES ARE NOT CURRENTLY EXPECTED TO REACH HAZARDOUS  
CRITERIA.  
 
THE U.S. DROUGHT MONITOR, VALID ON OCTOBER 10, INDICATES THAT SEVERE TO  
EXCEPTIONAL (D2-D4) DROUGHT COVERS 4.03 PERCENT OF THE CONTINENTAL U.S. WITH  
THE MOST INTENSE DROUGHT CONDITIONS ACROSS NORTHEASTERN MONTANA.  
 
FORECASTER: RANDY SCHECHTER  
 

 
 
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