340  
FXUS21 KWNC 101828  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT OCTOBER 10 2017  
 
SYNOPSIS: A SURFACE LOW IS FORECAST TO DEVELOP ACROSS THE MIDDLE MISSISSIPPI  
VALLEY THIS WEEKEND WITH A SUBSEQUENT TRACK NORTHEAST TO THE GREAT LAKES. A  
TROPICAL WAVE OF SURFACE LOW PRESSURE IS EXPECTED TO MOVE WEST FROM THE BAHAMAS  
AND CROSS SOUTH FLORIDA ON OCT 13 AND 14. SURFACE HIGH PRESSURE IS FORECAST TO  
BUILD ACROSS THE GREAT BASIN BY OCT 14 WITH OFFSHORE WINDS RETURNING TO  
SOUTHERN CALIFORNIA. A STRONG AREA OF UPPER-LEVEL LOW PRESSURE IS LIKELY TO  
DEVELOP OVER WESTERN NORTH AMERICA BY WEEK-2.  
 
HAZARDS  
 
SEVERE WEATHER FOR PARTS OF THE CENTRAL GREAT PLAINS AND LOWER MISSOURI VALLEY,  
SAT, OCT 14.  
 
HEAVY RAIN FOR PARTS OF THE CENTRAL GREAT PLAINS, MIDDLE AND UPPER MISSISSIPPI  
VALLEY, AND GREAT LAKES, SAT-SUN, OCT 14-15.  
 
MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF THE WESTERN U.S., FRI-SUN, OCT  
13-15.  
 
HIGH WINDS FOR SOUTHERN CALIFORNIA, SAT-SUN, OCT 14-15.  
 
HEAVY RAIN FOR THE COASTAL PACIFIC NORTHWEST, TUE, OCT 17.  
 
PERIODS OF HEAVY RAIN FOR THE COASTAL PACIFIC NORTHWEST, WED-FRI, OCT 18-20.  
 
PERIODS OF HEAVY SNOW FOR THE CASCADES AND PARTS OF THE NORTHERN ROCKIES,  
WED-FRI, OCT 18-20.  
 
A SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR SOUTHEAST MAINLAND ALASKA,  
WED-TUE, OCT 18-24.  
 
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 FRIDAY OCTOBER 13 - TUESDAY OCTOBER 17: A VIGOROUS SHORTWAVE TROUGH IS  
FORECAST TO EJECT OUT OF THE WESTERN U.S., PROMOTING SURFACE LOW DEVELOPMENT  
ACROSS THE MIDDLE MISSISSIPPI VALLEY THIS WEEKEND. THE DETERMINISTIC ECMWF  
MODEL REMAINS MUCH STRONGER WITH THE INTENSITY OF THE SHORTWAVE TROUGH AND  
ASSOCIATED SURFACE LOW. ALTHOUGH DIFFERENCES EXIST ON THE STRENGTH OF THE  
SURFACE LOW, MODEL SOLUTIONS ARE VERY CONSISTENT WITH A SWATH OF HEAVY RAINFALL  
(1 TO 3 INCHES) FROM THE LOWER MISSOURI VALLEY NORTHEAST TO THE GREAT LAKES ON  
OCT 14 AND 15. ON THE NORTHWEST SIDE OF THE SURFACE LOW TRACK, ACCUMULATING  
SNOW MAY OCCUR ACROSS THE UPPER GREAT LAKES. DUE TO MARGINALLY COLD THERMAL  
PROFILES, HEAVY SNOW IS NOT EXPECTED AT THIS TIME. INSTABILITY AND LOW-LEVEL  
MOISTURE ARE FORECAST TO BE SUFFICIENT TO SUPPORT SEVERE THUNDERSTORMS ALONG A  
FRONT ADVANCING SOUTH ACROSS THE CENTRAL GREAT PLAINS AND LOWER MISSOURI VALLEY  
ON OCT 14.  
 
SURFACE HIGH PRESSURE (1032-HPA) IS EXPECTED TO BUILD OVER THE GREAT BASIN FROM  
OCT 13 TO 15, RESULTING IN MUCH BELOW-NORMAL TEMPERATURES (NEGATIVE ANOMALIES  
OF 10 TO 15 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.  
 
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 ON OCT 17.  
 
A TROPICAL WAVE IS FORECAST TO APPROACH SOUTH FLORIDA BY THE WEEKEND. ALTHOUGH  
THIS WAVE OF LOW PRESSURE COULD BRING LOCALLY HEAVY RAIN (MORE THAN 1 INCH) TO  
THE SOUTHERN FLORIDA PENINSULA, THE DAILY RAINFALL AMOUNTS ARE NOT EXPECTED TO  
BE HAZARDOUS.  
 
A LOW PRESSURE SYSTEM IS FORECAST TO TRACK ACROSS THE ALEUTIANS AND ALASKA  
PENINSULA ON OCT 13 AND 14. SINCE THE SURFACE LOW IS EXPECTED TO BE MODERATE  
(972-HPA OR HIGHER) IN STRENGTH, WIND SPEEDS AND WAVE HEIGHTS ARE MOST LIKELY  
TO REMAIN BELOW HAZARDS CRITERIA.  
 
HEAVY RAIN, ASSOCIATED WITH HURRICANE IRMA, 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 WEDNESDAY OCTOBER 18 - TUESDAY OCTOBER 24: 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 0Z ECMWF ENSEMBLE MEAN  
INDICATES A MAXIMUM OF 6 INCHES ACROSS THE CASCADES DURING THE WEEK-2 PERIOD.  
BASED ON THE GEFS PRECIPITATION TOOL AND PRECIPITATION FORECASTS FROM THE 6Z  
GFS ENSEMBLE MEAN, THE MOST LIKELY TIME FOR HEAVY RAIN AND HIGH-ELEVATION SNOW  
ACROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES IS FROM OCT 18 TO 20. 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 LIKELY ACROSS ALASKA DURING WEEK-2 WHEN AN AMPLIFIED  
UPPER-LEVEL TROUGH BECOMES ESTABLISHED. THIS AMPLIFIED TROUGH ALONG WITH  
GUIDANCE FROM THE GEFS TEMPERATURE TOOLS SUPPORT A SLIGHT RISK OF MUCH  
BELOW-NORMAL TEMPERATURES ACROSS SOUTHEAST MAINLAND ALASKA DURING WEEK-2.  
 
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: BRAD PUGH  
 

 
 
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