532  
FXUS21 KWNC 141807  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EDT AUGUST 14 2018  
 
SYNOPSIS: A SURFACE LOW IS FORECAST TO EXIT THE NORTHEAST ON FRIDAY, WHILE A  
COLD FRONT SHIFTS SOUTH ACROSS THE CENTRAL U.S. BY EARLY NEXT WEEK. AN AREA OF  
UPPER-LEVEL LOW (HIGH) PRESSURE IS FORECAST TO DEVELOP OVER THE ALEUTIANS  
(NORTHEAST PACIFIC AND WESTERN NORTH AMERICA) DURING THE NEXT WEEK. THIS  
PATTERN IS LIKELY TO PERSIST THROUGH LATE AUGUST.  
 
HAZARDS  
 
HEAVY RAIN FOR PARTS OF THE NORTHEAST, FRI, AUG 17.  
 
PERIODS OF LOCALLY HEAVY RAIN FROM THE CENTRAL AND SOUTHERN GREAT PLAINS EAST  
TO THE LOWER AND MIDDLE MISSISSIPPI VALLEY, FRI-MON, AUG 17-20.  
 
EXCESSIVE HEAT FOR PARTS OF THE PACIFIC NORTHWEST, SUN-TUE, AUG 19-21.  
 
HEAVY RAIN FOR THE EASTERN ALASKA PENINSULA, KODIAK ISLAND, KENAI PENINSULA,  
AND SOUTHEAST MAINLAND ALASKA, SAT-SUN, AUG 18-19.  
 
A MODERATE RISK OF EXCESSIVE HEAT FOR PARTS OF THE PACIFIC NORTHWEST, WED, AUG  
22.  
 
A SLIGHT RISK OF EXCESSIVE HEAT FOR PARTS OF THE PACIFIC NORTHWEST, WED-TUE,  
AUG 22-28.  
 
A SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN GREAT PLAINS AND  
SOUTHWESTERN U.S., WED-FRI, AUG 22-24.  
 
A SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE ALASKA PENINSULA, KODIAK  
ISLAND, AND THE KENAI PENINSULA, WED-FRI, AUG 22-24.  
 
FLASH FLOODING POSSIBLE FOR PARTS OF ARIZONA, COLORADO, AND NEW MEXICO.  
 
FLOODING POSSIBLE ACROSS PARTS OF THE MID-ATLANTIC AND NORTHEAST, FRI-SAT, AUG  
17-18.  
 
FLOODING LIKELY ACROSS PARTS OF NORTHERN NEW JERSEY, FRI-SAT, AUG 17-18.  
 
SEVERE DROUGHT FOR PARTS OF THE MIDDLE TO LOWER MISSISSIPPI VALLEY, GREAT  
PLAINS, AND WESTERN U.S., AND THE BIG ISLAND OF HAWAII.  
 
DETAILED SUMMARY  
 
FOR FRIDAY AUGUST 17 - TUESDAY AUGUST 21: A SHORTWAVE TROUGH AND ASSOCIATED  
SURFACE LOW ARE FORECAST TO MOVE ACROSS THE NORTHEAST ON FRIDAY. THUNDERSTORMS  
WITH LOCALLY HEAVY RAIN (MORE THAN 1 INCH) ARE LIKELY ALONG THE TRACK OF THIS  
SURFACE LOW. THE DETERMINISTIC 0Z ECMWF MODEL INDICATES A LOW PRESSURE SYSTEM  
WITH HEAVY RAINFALL CROSSING THE MID-ATLANTIC THIS WEEKEND. HOWEVER, THE GFS  
DETERMINISTIC MODELS DEPICT LESS RAINFALL. DUE TO THESE MODEL DIFFERENCES, NO  
HEAVY RAIN HAZARD IS POSTED FOR THE MID-ATLANTIC AT THIS TIME. THIS POTENTIAL  
FOR HEAVY RAINFALL WILL BE CLOSELY MONITORED SINCE THE MID-ATLANTIC REMAINS  
EXCESSIVELY WET WITH ONGOING RIVER FLOODING.  
 
NORTHWEST FLOW ALOFT WITH EMBEDDED SHORTWAVE TROUGHS AND A COLD FRONT SHIFTING  
SOUTH ARE LIKELY TO RESULT IN A MULTI-DAY RISK OF HEAVY RAINFALL FROM THE  
CENTRAL GREAT PLAINS EAST TO THE MIDDLE AND LOWER MISSISSIPPI VALLEY. THE  
SPATIAL AREA FOR HEAVY RAINFALL IS BASED ON 24-HOUR PRECIPITATION AMOUNTS FROM  
THE GFS AND ECMWF ENSEMBLE MEANS. LOCAL AMOUNTS WITHIN THE DEPICTED HAZARD AREA  
MAY EXCEED 4 INCHES DURING THE 96-HOUR PERIOD. ISOLATED TO SCATTERED  
THUNDERSTORMS, ASSOCIATED WITH THE SUMMER MONSOON AND UPSLOPE FLOW, ARE  
EXPECTED TO BE CAPABLE OF TRIGGERING FLASH FLOODING ACROSS SOUTHEAST ARIZONA  
ALONG WITH THE CENTRAL TO SOUTHERN ROCKIES, THROUGH THE WEEKEND.  
 
NUMEROUS LARGE WILDFIRES CONTINUE TO BURN THROUGHOUT THE WESTERN U.S. THE  
LARGEST WILDFIRE EVER RECORDED IN CALIFORNIA, THE MENDOCINO COMPLEX FIRE, HAS  
BURNED NEARLY 350,000 ACRES AND IS CURRENTLY 68 PERCENT CONTAINED. ALTHOUGH AN  
ENHANCED WILDFIRE RISK AREA IS NOT POSTED ON THE MAP, THE LARGE NUMBER OF  
WILDFIRES IS LIKELY TO CONTRIBUTE TO POOR AIR QUALITY AT TIMES ACROSS THE  
WESTERN U.S. THROUGH LATE AUGUST. AN AMPLIFIED UPPER-LEVEL RIDGE (500-HPA  
HEIGHTS NEAR 588-DM) IS EXPECTED TO RESULT IN EXCESSIVE HEAT (MAXIMUM  
TEMPERATURES IN THE MID 90S TO LOWER 100S) ACROSS PARTS OF THE PACIFIC  
NORTHWEST FROM AUG 19 TO 21.  
 
AN AMPLIFYING UPPER-LEVEL TROUGH AND STRONG ONSHORE FLOW ARE EXPECTED TO BRING  
HEAVY RAINFALL TO PARTS OF ALASKA THIS WEEKEND. BASED ON A GFS AND ECMWF MODEL  
CONSENSUS, HEAVY RAIN (2 TO 4 INCHES, LOCALLY MORE) IS MOST LIKELY ACROSS THE  
EASTERN ALASKA PENINSULA, KODIAK ISLAND, THE KENAI PENINSULA, AND COASTAL  
SOUTHEAST MAINLAND ALASKA.  
 
FOR WEDNESDAY AUGUST 22 - TUESDAY AUGUST 28: THE ENSEMBLE MEANS ARE IN GOOD  
AGREEMENT AND CONSISTENT WITH AN AMPLIFIED UPPER-LEVEL TROUGH (RIDGE) OVER THE  
ALEUTIANS (NORTHEAST PACIFIC AND WESTERN NORTH AMERICA) DURING WEEK-2. THE  
SLIGHT RISK AREA FOR EXCESSIVE HEAT IS EXPANDED TO INCLUDE THE CENTRAL VALLEY  
OF CALIFORNIA BASED ON THE LATEST MODEL GUIDANCE. ON AUG 22, A MODERATE RISK OF  
EXCESSIVE HEAT IS POSTED FOR THE INTERIOR PACIFIC NORTHWEST WHERE THE GEFS  
REFORECAST TEMPERATURE TOOL INDICATES MAXIMUM TEMPERATURES HAVING GREATER THAN  
A 40 PERCENT CHANCE OF EXCEEDING THE 85TH PERCENTILE. ABOVE-NORMAL  
TEMPERATURES, DRY FUELS, AND POTENTIAL FOR LIGHTNING WITH DRY THUNDERSTORMS ARE  
LIKELY TO MAINTAIN AN ELEVATED WILDFIRE RISK ACROSS THE PACIFIC NORTHWEST  
CALIFORNIA, AND THE GREAT BASIN THROUGH LATE AUGUST.  
 
AN AMPLIFYING UPPER-LEVEL TROUGH IS EXPECTED TO RESULT IN A COLD FRONT SHIFTING  
WELL SOUTH INTO THE SOUTHERN GREAT PLAINS BY DAY 8. THIS EVOLVING PATTERN  
SUPPORTS A SLIGHT RISK OF HEAVY PRECIPITATION ACROSS PARTS OF THE SOUTHERN  
GREAT PLAINS, EARLY IN WEEK-2. A SLIGHT RISK OF HEAVY PRECIPITATION IS ALSO  
POSTED FOR PARTS OF THE SOUTHWESTERN U.S. WHERE CONVECTION ASSOCIATED WITH THE  
SUMMER MONSOON IS EXPECTED TO BE NEAR TO SLIGHTLY ABOVE NORMAL. AN AMPLIFIED  
UPPER-LEVEL TROUGH AND ANOMALOUS ONSHORE FLOW SUPPORT A SLIGHT RISK OF HEAVY  
PRECIPITATION FROM THE ALASKA PENINSULA EAST TO THE KENAI PENINSULA THROUGH AUG  
24.  
 
DESPITE AN ATMOSPHERIC KELVIN WAVE CROSSING THE WESTERN HEMISPHERE DURING  
MID-AUGUST, THERE IS NO STRONG SIGNAL AMONG THE MODELS FOR TROPICAL CYCLONE  
DEVELOPMENT ACROSS THE ATLANTIC BASIN DURING THE NEXT TWO WEEKS. THIS LACK OF A  
SIGNAL AMONG THE MODEL SOLUTIONS IS LIKELY RELATED TO BELOW NORMAL SEA SURFACE  
TEMPERATURES AND INCREASED WIND SHEAR WITH THE BACKGROUND STATE.  
 
THE U.S. DROUGHT MONITOR, VALID ON AUGUST 7, SHOWS A SLIGHT INCREASE IN SEVERE  
DROUGHT COVERAGE TO 21.06 PERCENT FROM 19.41 PERCENT LAST WEEK. DROUGHT BECAME  
MORE INTENSE OVER LARGE PORTIONS OF THE SOUTHERN PLAINS, MIDDLE MISSISSIPPI  
VALLEY, AND THE NORTHWEST.  
 
FORECASTER: BRAD PUGH  
 

 
 
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