279  
FXUS21 KWNC 111946  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 11 2017  
 
SYNOPSIS: A FRONTAL SYSTEM IS FORECAST TO STRETCH FROM THE GREAT LAKES  
SOUTHWESTWARD ACROSS THE MIDDLE MISSISSIPPI VALLEY TO THE CENTRAL AND SOUTHERN  
PLAINS. A SURFACE LOW IS EXPECTED TO DEVELOP ON THE FRONT AND MOVE ACROSS THE  
MIDDLE MISSISSIPPI VALLEY TO THE GREAT LAKES, OCT 14-15. COLD SURFACE HIGH  
PRESSURE IS FORECAST TO BUILD ACROSS THE GREAT BASIN DURING OCT 14-15 WITH  
OFFSHORE WINDS BECOMING ESTABLISHED FOR PARTS OF CALIFORNIA. A FRONTAL SYSTEM  
AND ONSHORE FLOW IS ANTICIPATED NEAR THE NORTHWEST U.S. OCT 16-18. A STRONG  
AREA OF MID-LEVEL LOW PRESSURE IS LIKELY TO DEVELOP OVER ALASKA AND THE GULF OF  
ALASKA BY WEEK-2.  
 
HAZARDS  
 
SEVERE WEATHER FOR PARTS OF THE CENTRAL AND SOUTHERN GREAT PLAINS AND MIDDLE  
MISSISSIPPI VALLEY, SAT, OCT 14.  
 
HEAVY RAIN FOR PARTS OF THE MIDDLE AND UPPER MISSISSIPPI VALLEY, GREAT LAKES,  
AND EXTREME NORTHERN PORTIONS OF THE NORTHEAST, SAT-SUN, OCT 14-15.  
 
HEAVY RAIN FOR PARTS OF THE PACIFIC NORTHWEST, MON-WED, OCT 16-18.  
 
MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF THE INTERIOR WESTERN U.S., SAT-SUN,  
OCT 14-15.  
 
HIGH WINDS FOR COASTAL SECTIONS OF NORTHERN CALIFORNIA AND PARTS OF SOUTHERN  
CALIFORNIA, SAT-SUN, OCT 14-15.  
 
HEAVY RAIN FOR THE COASTAL PACIFIC NORTHWEST, THU-FRI, OCT 19-20.  
 
HEAVY SNOW FOR THE CASCADES AND PARTS OF THE NORTHERN ROCKIES AND BITTERROOTS,  
THU-FRI, OCT 19-20.  
 
FLOODING OCCURRING ALONG THE ST. JOHNS AND WITHLACOOCHEE RIVERS ACROSS CENTRAL  
FLORIDA.  
 
SEVERE DROUGHT ACROSS PARTS OF THE NORTHERN ROCKIES, NORTHERN AND CENTRAL GREAT  
PLAINS, MIDWEST, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SATURDAY OCTOBER 14 - WEDNESDAY OCTOBER 18: A VIGOROUS SHORTWAVE TROUGH IS  
FORECAST TO EJECT OUT OF THE INTERIOR WESTERN CONUS PROMOTING SURFACE LOW  
DEVELOPMENT ON AN ASSOCIATED FRONTAL SYSTEM ACROSS THE MIDDLE MISSISSIPPI  
VALLEY OCT 14. THIS AREA OF LOW PRESSURE IS FORECAST TO MOVE NORTHEASTWARD TO  
THE GREAT LAKES, AND THEN NORTHERN NEW ENGLAND. MODEL SOLUTIONS ARE VERY  
CONSISTENT WITH A SWATH OF HEAVY RAINFALL (1 TO 2.5 INCHES) FOR PARTS OF THE  
MIDDLE AND UPPER MISSISSIPPI VALLEY, GREAT LAKES, AND EXTREME NORTHERN PORTIONS  
OF THE NORTHEAST OCT 14-15. INSTABILITY AND LOW-LEVEL MOISTURE ARE FORECAST TO  
BE SUFFICIENT TO SUPPORT SEVERE THUNDERSTORMS ALONG THE FRONT AS IT ADVANCES  
SOUTH ACROSS THE SOUTHERN AND CENTRAL GREAT PLAINS AND MIDDLE MISSISSIPPI  
VALLEY ON OCT 14.  
 
SURFACE HIGH PRESSURE (1032-HPA) IS EXPECTED TO BUILD OVER THE GREAT BASIN FROM  
OCT 14 TO 15, RESULTING IN MUCH BELOW-NORMAL TEMPERATURES (NEGATIVE ANOMALIES  
EXCEEDING 10 DEGREES F) ACROSS PARTS OF THE INTERIOR WEST. THIS SURFACE PATTERN  
IS FAVORABLE FOR A SANTA ANA WIND EVENT TO AFFECT SOUTHERN CALIFORNIA ON OCT 14  
AND 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.  
 
INCREASING ONSHORE FLOW AND AN AMPLIFYING UPPER-LEVEL TROUGH ARE EXPECTED TO  
BEGIN AFFECTING THE PACIFIC NORTHWEST EARLY NEXT WEEK. THEREFORE, A HEAVY RAIN  
HAZARD (AMOUNTS GREATER THAN 1.5 INCHES) IS POSTED FOR COASTAL OREGON AND  
WASHINGTON OCT 16-18.  
 
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 THURSDAY OCTOBER 19 - WEDNESDAY OCTOBER 25: 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 WEEK-2. THE ENSEMBLE MEAN SOLUTIONS  
INDICATE 4-6 INCHES PF PRECIPITATION (LIQUID EQUIVALENT) ACROSS PARTS OF THE  
REGION DURING THE WEEK-2 PERIOD. BASED ON THE GEFS PRECIPITATION TOOL AND  
PRECIPITATION FORECASTS FROM THE ENSEMBLE MEANS, THE MOST LIKELY TIME FOR HEAVY  
RAIN ACROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES IS FROM OCT 19 TO 20.  
THE HIGHEST ELEVATIONS OF THE CASCADES, BITTERROOTS, AND NORTHERN ROCKIES COULD  
SEE HEAVY SNOW (AMOUNTS IN EXCESS OF 8 INCHES). DUE TO UNCERTAINTY ON THE  
SOUTHERN EXTENT OF THE STORM TRACK, NORTHERN CALIFORNIA IS NOT INCLUDED IN ANY  
HEAVY PRECIPITATION HAZARDS AT THIS TIME. THE EVOLVING LONGWAVE PATTERN ALSO  
SUPPORTS AN INCREASING RISK OF SEVERE WEATHER DOWNSTREAM ACROSS THE GREAT  
PLAINS, LATER IN WEEK-2.  
 
A MAJOR PATTERN CHANGE IS EXPECTED ACROSS ALASKA DURING WEEK-2 WHEN AN  
AMPLIFIED UPPER-LEVEL TROUGH LIKELY BECOMES ESTABLISHED. 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 3, INDICATES THAT SEVERE TO  
EXCEPTIONAL (D2-D4) DROUGHT COVERS 4.73 PERCENT OF THE CONTINENTAL U.S. WITH  
THE MOST INTENSE DROUGHT CONDITIONS ACROSS EASTERN MONTANA.  
 
FORECASTER: RANDY SCHECHTER  
 

 
 
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