Climate Prediction Center

US HAZARDS ASSESSMENT
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
300 PM EDT AUGUST 13 2009

SYNOPSIS: A SLOW-MOVING COLD FRONT IS FORECAST TO CROSS THE UPPER MIDWEST AND GREAT LAKES REGION EARLY IN THE PERIOD. THE FRONT IS EXPECTED TO TRIGGER HEAVY SHOWERS AS IT ADVANCES EASTWARD. BEHIND THE FRONT, COOLER AIR WILL SPREAD ACROSS THE NORTHWEST AND INTO THE NORTHERN PLAINS. THE TROPICS ARE FORECAST TO BE ACTIVE IN BOTH THE ATLANTIC AND EASTERN PACIFIC. THE VIGOROUS TROPICAL WAVE LOCATED IN THE EASTERN ATLANTIC NEAR THE CAPE VERDE ISLANDS BEARS WATCHING, AS FORECAST MODELS SHOW DEVELOPMENT FOR THIS SYSTEM AND A TRACK THAT TAKES THE STORM IN THE GENERAL DIRECTION OF THE PUERTO RICO AREA. THE IMPACT, IF ANY, ON THE U.S. EAST COAST IS VERY UNCERTAIN AT THIS TIME. A STRONG FLOW OF MOISTURE OUT OF THE GULF OF ALASKA IS FORECAST TO PRODUCE PRODIGIOUS RAINFALL AMOUNTS OVER ALASKA'S EASTERN SOUTH COAST AND NORTHERN PANHANDLE.

HAZARDS

HEAVY RAIN OVER ALASKA'S EASTERN SOUTH COAST AND NORTHERN PANHANDLE DURING AUGUST 16-20.
HEAVY RAIN FROM KANSAS AND OKLAHOMA INTO IOWA AND MISSOURI ON AUGUST 16-18.
SEVERE DROUGHT ACROSS PARTS OF CALIFORNIA, NEVADA, WASHINGTON, MONTANA, TEXAS, OKLAHOMA, AND THE UPPER MIDWEST. SOME RELIEF IS LIKELY FOR MONTANA, THE UPPER MIDWEST, AND OKLAHOMA.
CONTINUED FLOODING ALONG THE JAMES RIVER IN SOUTH DAKOTA.

DETAILED SUMMARY

FOR SUNDAY AUGUST 16 - TUESDAY AUGUST 18: AN UPPER-LEVEL LOW IN CONJUNCTION WITH A STRONG FLOW OF MOIST SOUTHWESTERLY AIR FROM THE GULF OF ALASKA, A SURFACE LOW PRESSURE AREA NEAR THE SOUTH COAST OF ALASKA, AND REMNANT MOISTURE FROM A PACIFIC TYPHOON ARE FORECAST TO RESULT IN VERY HEAVY RAINFALL ALONG THE EASTERN SOUTH COAST OF ALASKA AND THE NORTHERN PANHANDLE. FORECAST MODELS INDICATE THAT POTENTIAL RAINFALL TOTALS FOR THE EVENT COULD EXCEED 12 INCHES IN SOME LOCATIONS.

ELSEWHERE, A COLD FRONT IS FORECAST TO TRAVEL FROM THE NORTHERN PLAINS ON SUNDAY TO THE EASTERN GREAT LAKES REGION ON TUESDAY. THE SLOW-MOVING COLD FRONT IS EXPECTED TO TRIGGER HEAVY SHOWERS OVER THE NORTHERN AND CENTRAL PLAINS, WITH THE HEAVIEST RAINFALL TOTALS FORECAST FROM KANSAS AND WESTERN OKLAHOMA INTO IOWA AND MISSOURI. THERE IS A POSSIBLITY FOR SEVERE THUNDERSTORMS ACCOMPANYING THE FRONT AS IT ADVANCES THROUGH THE PLAINS AND INTO THE MIDWEST. AHEAD OF THE FRONT, ABNORMALLY HIGH TEMPERATURES WILL COVER THE UPPER MIDWEST ON SUNDAY, WITH THE COOLER AIR REPLACING THE HEAT ON MONDAY AND TUESDAY AS THE FRONT ADVANCES EASTWARD.

HIGH PRESSURE OVER THE EASTERN SEABOARD THIS PERIOD IS FORECAST TO TRANSPORT MOIST AIR NORTHWARD FROM THE GULF OF MEXICO INTO THE SOUTHEAST. RAINFALL TOTALS ARE EXPECTED TO CONTINUE TO APPROACH HAZARDOUS LEVELS FROM GEORGIA AND FLORIDA INTO THE CAROLINAS AS A TROPICAL WAVE NOW NEAR THE LESSER ANTILLES ENTERS THE GULF OF MEXICO AND CONTRIBUTES TO THE MOISTURE SUPPLY.

IN THE EASTERN PACIFIC, A NEW TROPICAL STORM, GUILLERMO, IS FORECAST TO TRACK WESTWARD. THE STORM COULD START TO AFFECT HAWAII BY TUESDAY, BUT THE IMPACTS, IF ANY, ARE UNCERTAIN AT THIS TIME.

IN THE EASTERN TROPICAL ATLANTIC, THE LATEST OFFICIAL ADVISORIES FROM THE TROPICAL PREDICTION CENTER NO LONGER INDICATE TROPICAL DEPRESSION 2 DEVELOPING INTO A TROPICAL STORM. IN CONTRAST, THE VIGOROUS TROPICAL WAVE LOCATED NEAR THE CAPE VERDE ISLANDS HAS A BETTER CHANCE OF INTENSIFICATION. THIS SYSTEM IS EXPECTED TO MOVE WESTWARD, BUT IS NOT FORECAST TO AFFECT THE GREATER OR LESSER ANTILLES DURING THIS PERIOD.

FOR WEDNESDAY AUGUST 19 - SUNDAY AUGUST 23: THE STRONG FLOW OF MOISTURE FROM THE SOUTHWEST IS FORECAST TO CONTINUE TO RESULT IN HEAVY RAINS FOR SOUTHERN ALASKA INTO THURSDAY. TO THE SOUTH, THE COLD FRONT THAT HAD EARLIER CROSSED THE PLAINS INTO THE MIDWEST IS FORECAST TO CONTINUE INTO THE NORTHEAST BY WEDNESDAY, TO BE FOLLOWED BY ANOTHER FRONT ON THURSDAY-FRIDAY. MODELS DIFFER ON THE LOCATION AND SPEED OF THE UPPER-LEVEL TROUGH EXPECTED TO TRACK EASTWARD ACROSS THE PLAINS AND MIDWEST. THE ECMWF AND CANADIAN MODELS ARE SLOWER THAN THE GFS IN BRINGING THE TROUGH EASTWARD. AS A RESULT, THE GFS MAY BE SOMEWHAT TOO FAST IN BRINGING A COLD HIGH PRESSURE AREA SOUTHWARD FROM CANADA INTO THE UPPER MIDWEST ON FRIDAY.

IN THE ATLANTIC TROPICS, MODELS SHOW CONSIDERABLE UNCERTAINTY ABOUT THE EVENTUAL TRACK OF THE TROPICAL WAVE NOW NEAR THE CAPE VERDE ISLANDS. MODELS ARE BECOMING QUITE CONSISTENT IN TAKING THE WAVE IN A WEST-NORTHWEST DIRECTION THAT WOULD PUT A TROPICAL SYSTEM IN THE VICINITY OF PUERTO RICO AROUND AUGUST 20. MODEL STORM TRACKS BEYOND THIS TIME SHOW CONSIDERABLE SPREAD, BOTH AMONG THE DIFFERENT MODELS, AND IN TERMS OF TEMPORAL CONSISTENCY FOR THE SAME MODEL. TODAY'S 6Z OPERATIONAL GFS RUN, FOR EXAMPLE, TOOK THE STORM OUT OF HARM'S WAY WELL EAST OF THE U.S. COASTLINE, WHILE THE 12Z RUN TAKES THE STORM ON A TRACK THAT IS FARTHER TO THE WEST AND WOULD THREATEN THE EAST COAST.

THE OVERALL UPPER AIR PATTERN SUGGESTED BY MODEL CONSENSUS INDICATES RIDGING OVER THE WEST AND TROUGHING OVER THE EAST. THIS IS EXPECTED TO LEAD TO BELOW-NORMAL TEMPERATURES FOR THE EAST AND ABOVE-NORMAL FOR THE PACIFIC STATES. THE FATE OF ANY TROPICAL STORM THAT APPROACHES THE EAST COAST WILL HINGE ON THE EXACT CONFIGURATION OF THE UPPER LEVEL FLOW PATTERN, AND THIS IS NOT KNOWN WITH ANY CONFIDENCE AT THIS TIME.

FOR MONDAY AUGUST 24 - THURSDAY AUGUST 27: THE MEAN UPPER AIR PATTERN IS FORECAST TO CONTINUE TO FAVOR TROUGHING AND COOL AIR OVER THE EAST AND RIDGING AND WARMTH OVER THE WEST. MODELS ARE INCONSISTENT REGARDING THE EVENTUAL TRACK IN THE WESTERN ATLANTIC OF THE TROPICAL SYSTEM DISCUSSED ABOVE, BUT THE TROPICS ARE EXPECTED TO REMAIN ACTIVE, AND THIS SYSTEM AND OTHERS SHOULD BE MONITORED CLOSELY.

FORECASTER: DOUG LECOMTE

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