216
FXUS21 KWNC 081913
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 08 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE IS ANTICIPATED TO PERSIST OVER THE SOUTHERN
CONTIGUOUS U.S. (CONUS) DURING WEEK-2, LEADING TO THE INCREASED POTENTIAL FOR
EXCESSIVE HEAT FOR THE SOUTHERN AND WESTERN CONUS. SURFACE LOW PRESSURE OVER
SOUTHERN ARIZONA AND HIGH PRESSURE OVER COLORADO RESULTS ENHANCED RISK FOR HIGH
WINDS OVER THE DESERT SOUTHWEST, WHILE LOW PRESSURE BOTH AT THE SURFACE AND
ALOFT OVER THE ALEUTIAN ISLANDS LEADS TO A CHANCE FOR HIGH WINDS ALONG THE
SOUTHERN ALASKAN COAST. A POTENTIAL MOISTURE SURGE FROM TROPICAL ACTIVITY IN
THE EAST PACIFIC INCREASES THE RISK FOR FLASH FLOODING ACROSS SOUTHERN
ARIZONA. ABNORMALLY DRY CONDITIONS FROM ARIZONA EAST ACROSS THE SOUTHERN CONUS
TO LOUISIANA SUPPORTS ELEVATED POTENTIAL FOR RAPID ONSET DROUGHT (ROD) ACROSS
THE REGION.
HAZARDS
MODERATE RISK FOR EXCESSIVE HEAT FOR PORTIONS OF THE DESERT SOUTHWEST, SOUTHERN
PLAINS, AND LOWER MISSISSIPPI VALLEY, WED-FRI, AUG 16-18.
MODERATE RISK FOR EXCESSIVE HEAT FOR PORTIONS OF THE NORTHWEST CONUS, WED, AUG
16.
SLIGHT RISK FOR EXCESSIVE HEAT FOR MUCH OF THE WESTERN CONUS, WED-FRI, AUG
16-18.
SLIGHT RISK FOR EXCESSIVE HEAT FOR MUCH OF THE SOUTHERN CONUS, WED-TUE, AUG
16-22.
SLIGHT RISK FOR HIGH WINDS FOR MUCH OF THE DESERT SOUTHWEST AND PORTIONS OF THE
SOUTHERN ROCKIES AND THE SOUTHERN GREAT BASIN, WED-TUE, AUG 16-22.
SLIGHT RISK FOR HIGH WINDS FOR PORTIONS OF THE SOUTHERN COAST OF ALASKA,
WED-FRI, AUG 16-18.
FLASH FLOODING POSSIBLE FOR PORTIONS OF SOUTHERN ARIZONA.
RAPID ONSET DROUGHT RISK ACROSS PORTIONS OF THE DESERT SOUTHWEST, SOUTHERN
PLAINS, AND LOWER MISSISSIPPI VALLEY.
DETAILED SUMMARY
FOR FRIDAY AUGUST 11 - TUESDAY AUGUST 15:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY AUGUST 16 - TUESDAY AUGUST 22: ENSEMBLE MODEL SOLUTIONS FROM THE
GEFS, ECMWF, AND CMCE ALL DEPICT POSITIVE 500-HPA HEIGHT ANOMALIES CENTERED
OVER THE DESERT SOUTHWEST IN EXCESS OF 595 DM AT THE ONSET OF THE WEEK-2
PERIOD. TOWARDS THE SECOND HALF OF THE WEEK THE RIDGE WEAKENS SOMEWHAT BUT
HEIGHT MAXIMA REMAINS ABOVE 590 DM. THIS PATTERN FAVORS CONTINUED ANOMALOUSLY
WARM TEMPERATURES ACROSS THE SOUTHERN CONUS, LEADING TO THE INCREASED POTENTIAL
FOR EXCESSIVE HEAT. THE PROBABILISTIC EXTREMES TOOL (PET) OF ALL THREE
ENSEMBLES AFOREMENTIONED GENERALLY AGREE ON AT LEAST A 40% PROBABILITY OF
MAXIMUM TEMPERATURES EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE FOR A BROAD
SWATH EXTENDING FROM INTERIOR SOUTHERN CALIFORNIA TO THE LOWER MISSISSIPPI
VALLEY FOR AUG 16-18. THEREFORE A MODERATE RISK FOR EXCESSIVE HEAT IS POSTED
FOR THESE REGIONS.
A MODERATE RISK FOR EXCESSIVE HEAT IS ALSO POSTED FOR PORTIONS OF THE
NORTHWESTERN CONUS FOR AUG 16. AT THE BEGINNING OF THE WEEK-2 PERIOD THE
ANOMALOUS SUBTROPICAL RIDGE FEATURES A NORTHWARD EXTENSION ALONG THE WEST
COAST, TRANSLATING TO THE PETS INDICATING AT LEAST A 40% CHANCE OF HIGH
TEMPERATURES EXCEEDING THE 85TH PERCENTILE. THIS FEATURE IS SHORT-LIVED
HOWEVER, SO THIS MODERATE RISK AREA IS ONLY POSTED FOR AUG 16. ADDITIONALLY,
THE PETS INDICATE AT LEAST A 20% PROBABILITY OF MAXIMUM TEMPERATURES EXCEEDING
THE 85TH PERCENTILE FOR MUCH OF THE WESTERN AND SOUTHERN CONUS DURING THE
WEEK-2 PERIOD, THUS A SLIGHT RISK FOR EXCESSIVE HEAT IS POSTED FOR THESE
REGIONS. THIS IS SPLIT INTO TWO AREAS, ONE COVERING THE WESTERN CONUS FOR AUG
16-18, AND THE OTHER COVERING THE SOUTHERN CONUS, AUG 16-22. A FINAL NOTE ABOUT
THE EXCESSIVE HEAT FOR THE SOUTHERN CONUS: THE NATIONAL BLEND OF MODELS
INDICATES THAT ALONG THE GULF COAST NIGHTTIME LOW TEMPERATURES HAVE THE
POTENTIAL TO BREAK RECORDS FOR WARMEST LOW TEMPS ON RECORD, FURTHER
EXACERBATING THE POTENTIAL IMPACTS OF THIS HEAT WAVE.
ANOTHER PROMINENT FEATURE IN THE WEEK-2 SYNOPTIC PICTURE IS A THERMAL LOW OVER
THE MOUTH OF THE COLORADO RIVER AND SURFACE HIGH PRESSURE OVER COLORADO,
RESULTING IN AN ENHANCED PRESSURE GRADIENT OVER THE DESERT SOUTHWEST. THIS
RESULTS IN THE INCREASED POTENTIAL FOR HIGH WINDS THROUGHOUT THE FORECAST
PERIOD. THE ECMWF AND CMCE PETS SHOW THIS WELL, ALTHOUGH THE GEFS DOES NOT
SHARE THIS FEATURE. HOWEVER, THE GEFS PET TENDS TO UNDERESTIMATE EXTREME WIND
POTENTIAL, SO A SLIGHT RISK FOR HIGH WINDS IS POSTED FOR MUCH OF THE SOUTHWEST
CONUS FOR THE WHOLE WEEK-2 PERIOD.
AT THE ONSET OF WEEK-2 ENSEMBLE MODEL SOLUTIONS SHOW TROUGHING OVER THE
ALEUTIANS AND ANOMALOUS RIDGING OVER THE NORTH PACIFIC, RESULTING IN AN
ENHANCED PRESSURE GRADIENT AND RESULTANT STRONG ONSHORE FLOW FOR THE SOUTHERN
ALASKAN COAST. THIS GRADIENT WEAKENS AND SHIFTS THROUGHOUT THE WEEK-2 PERIOD,
SO A SLIGHT RISK FOR HIGH WINDS IS POSTED ALONG THE SOUTHERN ALASKAN COAST FOR
AUG 16-18 ONLY.
TROPICAL CYCLONE ACTIVITY IN THE EASTERN PACIFIC IS FORECAST TO INCREASE IN THE
COMING WEEKS, BRINGING INCREASING THE LIKELIHOOD FOR A MOISTURE SURGE FROM THE
GULF OF CALIFORNIA INTO THE FOUR CORNERS REGION LATER IN THE FORECAST PERIOD.
MODEL DEPICTIONS OF THIS SURGE CURRENTLY LIMIT ITS NORTHERN EXTENT AND FORECAST
RAINFALL TOTALS WHICH DOES NOT MEET PREDETERMINED HAZARDS CRITERIA PRECLUDING A
DESIGNATED RISK FOR HEAVY PRECIPITATION. HOWEVER, GIVEN THE DRY ANTECEDENT
CONDITIONS AND THE POTENTIAL FOR LOCALIZED HEAVY RAINFALL, A RISK FOR FLASH
FLOODING IS POSTED FOR SOUTHERN ARIZONA.
DROUGHT CONCERNS ARE FORECAST TO INCREASE ACROSS PORTIONS OF THE SOUTHERN CONUS
FROM ARIZONA EAST TO LOUISIANA AS THE HEAT OF SUMMER CONTINUES AND A
PERSISTENCE OF BELOW-NORMAL PRECIPITATION IS FORECAST FOR THE REGION. WHILE
SOME PRECIPITATION IS PREDICTED FOR PORTIONS OF LOUISIANA AND THE MONSOON
REGION OF THE DESERT SOUTHWEST IN THE COMING WEEK, ACCUMULATIONS ARE NOT
EXPECTED TO SIGNIFICANTLY IMPACT SOIL MOISTURE SO NO CHANGES ARE MADE TO THE
ROD OUTLOOK.
FORECASTER: DANNY BARANDIARAN
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