Threats Assessment Discussion

408 
FXUS21 KWNC 161926
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 16 2018
SYNOPSIS: A STATIONARY FRONT IS FORECAST TO EXTEND FROM THE CENTRAL HIGH PLAINS 
EAST TO THE MID-ATLANTIC THIS WEEKEND. MEANWHILE, A COLD FRONT IS PREDICTED TO 
ENTER THE NORTHERN PLAINS EARLY IN THE PERIOD AND SINK SOUTHWARD BY THE 
BEGINNING OF NEXT WEEK. THESE FRONTS MAY BRING UNSETTLED WEATHER TO PARTS OF 
THE EASTERN HALF OF THE LOWER 48 STATES AS THEY PROGRESS SOUTHEASTWARD. AN AREA 
OF UPPER-LEVEL LOW (HIGH) PRESSURE IS FORECAST TO DEVELOP OVER THE ALEUTIANS 
(NORTHEAST PACIFIC AND WESTERN NORTH AMERICA) AND PERSIST THROUGH AT LEAST THE 
EARLY PART OF WEEK-2. TROPICAL STORM LANE IS FORECAST TO BECOME A MAJOR 
HURRICANE AS IT TRACKS WEST FROM THE EAST PACIFIC.
HAZARDS
HEAVY RAIN ACROSS PORTIONS OF THE EASTERN PORTION OF THE GREAT PLAINS, AND THE 
MISSISSIPPI VALLEY, SUN, AUG 19.
HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, THE CENTRAL APPALACHIANS, THE 
MIDDLE MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE UPPER MISSISSIPPI VALLEY, THE 
GREAT LAKES, AND THE OHIO VALLEY, MON-TUE, AUG 20-AUG 21.
HEAVY RAIN ACROSS COASTAL PORTIONS OF THE SOUTHEAST AND THE MID-ATLANTIC, 
SUN-MON, AUG 19-AUG 20, AND WED-THU, AUG 22-AUG 23.
FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE MID-ATLANTIC, THE LOWER 
MISSISSIPPI VALLEY, AND THE SOUTHERN PLAINS.
HEAVY RAIN ACROSS PORTIONS OF THE SOUTH COAT OF ALASKA AND KODIAK ISLAND, 
SUN-WED, AUG 19-AUG 22.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE CENTRAL PLAINS, THE 
NORTHERN PLAINS, THE GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, THE UPPER 
MISSISSIPPI VALLEY, AND THE OHIO VALLEY, FRI-TUE, AUG 24-AUG 28.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE DESERT SOUTHWEST, 
FRI-TUE, AUG 24-AUG 28.
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 SUNDAY AUGUST 19 - THURSDAY AUGUST 23: AN AMPLIFYING LONGWAVE PATTERN IS 
LIKELY TO OCCUR DURING THIS PERIOD AS AN UPPER-LEVEL TROUGH (RIDGE) BECOMES 
ESTABLISHED OVER THE ALEUTIANS (WESTERN NORTH AMERICA). DOWNSTREAM OF THE 
UPPER-LEVEL RIDGE, AN UPPER-LEVEL TROUGH OF VARYING AMPLITUDE FAVORS PERIODS OF 
HEAVY RAINFALL ACROSS THE CENTRAL AND EASTERN CONUS. A SURFACE LOW IS 
ANTICIPATED TO FORM OVER THE CENTRAL PLAINS ALONG THE STATIONARY FRONT AND 
TRACK NORTHEASTWARD, BRINGING HEAVY RAINFALL TO PARTS OF THE EASTERN GREAT 
PLAINS AND MISSISSIPPI VALLEY, AUG 19, SHIFTING TO PORTIONS OF THE NORTHEAST, 
THE CENTRAL APPALACHIANS, THE MIDDLE MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE 
UPPER MISSISSIPPI VALLEY, THE GREAT LAKES, AND THE OHIO VALLEY, AUG 20 TO 21. 
PARTS OF THE COASTAL CAROLINAS AND VIRGINIA MAY ALSO RECEIVE HEAVY RAINFALL, 
AUG 19 TO 20, AND AUG 22 TO 23. THESE HIGHLIGHTED AREAS MAY RECEIVE 1 TO 2 
INCHES OF RAINFALL IN 24-HOURS, LOCALLY MORE.
NUMEROUS LARGE WILDFIRES CONTINUE TO BURN THROUGHOUT THE WESTERN U.S. THE 
LARGEST WILDFIRE EVER RECORDED IN CALIFORNIA, THE MENDOCINO COMPLEX FIRE, HAS 
BURNED MORE THAN 360,000 ACRES AND IS CURRENTLY 76 PERCENT CONTAINED. THE CARR 
WILDFIRE IN SHASTA COUNTY OF NORTHERN CALIFORNIA CONTINUES TO INCREASE WITH 
MORE THAN 200,000 ACRES BURNED. THE LARGE NUMBER OF WILDFIRES IS LIKELY TO 
CONTRIBUTE TO POOR AIR QUALITY AT TIMES ACROSS THE WESTERN U.S. THROUGH LATE 
AUGUST. AN AMPLIFYING UPPER-LEVEL RIDGE (500-HPA HEIGHTS NEAR 588-DM) IS 
EXPECTED TO RESULT IN ANOMALOUSLY WARM TEMPERATURES (MAXIMUM TEMPERATURES IN 
THE MID 90S TO LOWER 100S, ANOMALIES REACHING 12 DEG F OR GREATER) ACROSS PARTS 
OF THE PACIFIC NORTHWEST ON AUG 20 AND 21. HOWEVER, SINCE THE TEMPERATURE 
ANOMALIES ARE EXPECTED TO BE LOCALIZED, AN EXCESSIVE HEAT AREA IS NOT SPECIFIED 
AT THIS TIME.
A DRYING TREND IS EXPECTED ACROSS MUCH OF THE DESERT SOUTHWEST THIS WEEKEND AS 
A THE 500-HPA RIDGE AXIS SHIFTS BACK TO THE SOUTH. THE MOST LIKELY AREA FOR 
FLASH FLOODING EXISTS ACROSS SOUTHEAST ARIZONA WHERE ISOLATED TO SCATTERED 
THUNDERSTORMS ARE FORECAST TO DEVELOP WITH SUFFICIENT LOW AND MID-LEVEL 
MOISTURE. 
AN AMPLIFYING UPPER-LEVEL TROUGH IS LIKELY TO RESULT IN AN INTENSIFYING LOW 
PRESSURE SYSTEM ACROSS THE NORTH PACIFIC BY THE WEEKEND. HEAVY RAIN (2 TO 3 
INCHES), DUE TO ENHANCED ONSHORE FLOW, IS FORECAST TO BRING HEAVY RAINFALL TO 
PARTS OF THE SOUTH COAT OF ALASKA AND KODIAK ISLAND AUG 19 TO 22.
FOR FRIDAY AUGUST 24 - THURSDAY AUGUST 30: THE ENSEMBLE MEANS REMAIN IN GOOD 
AGREEMENT ACROSS THE NORTH PACIFIC WITH THE AMPLIFIED UPPER-LEVEL TROUGH 
(RIDGE) OVER THE ALEUTIANS (NORTHEAST PACIFIC). THE SLIGHT CHANCE FOR EXCESSIVE 
HEAT ACROSS THE WEST HAS BEEN REMOVED DUE TO THE FACT THAT THE ECMWF ENSEMBLE 
WEEK-2 MEAN SHOWS MORE OF A ZONAL PATTERN COMPARED TO YESTERDAY'S RIDGE ACROSS 
THE NORTHWESTERN CONUS AND THE FORECAST PROBABILITIES FOR MUCH ABOVE NORMAL 
TEMPERATURES IN THE GEFS REFORECAST TEMPERATURE TOOL DOES NOT REACH 20 PERCENT. 
IN GENERAL THE WEEK-2 FORECAST INDICATES ABOVE-NORMAL TEMPERATURES. WARM 
TEMPERATURES, DRY FUELS, AND THE POTENTIAL FOR LIGHTNING WITH DRY THUNDERSTORMS 
ARE LIKELY TO MAINTAIN AN ELEVATED WILDFIRE RISK ACROSS THE PACIFIC NORTHWEST, 
NORTHERN CALIFORNIA, AND THE GREAT BASIN THROUGH LATE AUGUST.
SURFACE LOW PRESSURE IS PREDICTED TO FORM ACROSS THE CENTRAL CONUS AHEAD OF 
UPPER-LEVEL TROUGHING ACROSS THE NORTHWESTERN QUARTER OF THE CONUS AT THE 
BEGINNING OF WEEK-2. THIS FEATURE SUPPORTS A SLIGHT CHANCE OF HEAVY RAINFALL 
ACROSS PARTS OF THE CENTRAL PLAINS, THE NORTHERN PLAINS, THE GREAT LAKES, THE 
MIDDLE MISSISSIPPI VALLEY, THE UPPER MISSISSIPPI VALLEY, AND THE OHIO VALLEY, 
AUG 24 TO 28. THE GEFS AND ECMWF REFORECAST TOOLS INDICATE A 20 PERCENT CHANCE 
OF THESE AREAS RECEIVING 3-DAY PRECIPITATION ACCUMULATION AMOUNTS EXCEEDING THE 
85TH PERCENTILE COMPARED TO NORMAL.
DURING WEEK-2, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR THE DESERT 
SOUTHWEST WHERE CONVECTION ASSOCIATED WITH THE SUMMER MONSOON IS EXPECTED TO BE 
NEAR TO SLIGHTLY ABOVE NORMAL. THE REGION HIGHLIGHTED HAS BEEN DECREASED FROM 
YESTERDAY'S FORECAST, LIMITED MAINLY TO EASTERN ARIZONA AND SOUTHWESTERN NEW 
MEXICO AND WESTERN TEXAS, AUG 24 TO 28. THE SPATIAL AREA IS BASED ON 24-HOUR 
PRECIPITATION AMOUNTS FROM THE GFS ENSEMBLE MEANS ALONG WITH THE GEFS AND ECMWF 
REFORECAST TOOLS. A DEEP SURGE OF MOISTURE NORTH FROM THE GULF OF CALIFORNIA IS 
NOT ANTICIPATED AT THIS TIME.
AS OF 8AM PDT ON AUG 16, TROPICAL STORM LANE IS LOCATED AT 10.6N/127.6W IN THE 
EAST PACIFIC. LOW WIND SHEAR AND SUFFICIENTLY WARM WATERS ARE EXPECTED TO 
PROVIDE A FAVORABLE ENVIRONMENT FOR STRENGTHENING AND LANE IS FORECAST TO 
BECOME A MAJOR HURRICANE BY THE WEEKEND. THE DETERMINISTIC MODEL SOLUTIONS VARY 
ON THE SPEED AND TRACK OF THE TROPICAL CYCLONE, BUT THERE IS MODEL SUPPORT FOR 
THE STORM MOVING NEAR OR SOUTH OF THE BIG ISLAND OF HAWAII BY THE BEGINNING OF 
WEEK-2. REGARDLESS OF ITS TRACK, AN INCREASE IN SURF IS EXPECTED FOR THE 
HAWAIIAN ISLANDS NEXT WEEK. 
THE U.S. DROUGHT MONITOR, VALID ON AUGUST 14, SHOWS A SLIGHT DECREASE IN SEVERE 
DROUGHT COVERAGE TO 20.02 PERCENT FROM 21.06 PERCENT LAST WEEK. DROUGHT 
IMPROVEMENT IS NOTED ACROSS THE SOUTHERN PLAINS.
FORECASTER: MELISSA OU 
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