498
FXUS21 KWNC 111840
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 11 2023
SYNOPSIS: A BROAD AREA OF MID-LEVEL HIGH PRESSURE CONTINUES TO FAVOR
PREDOMINANTLY ABOVE-NORMAL TEMPERATURES ACROSS THE NORTH-CENTRAL STATES AND IN
MOST AREAS FROM THE ROCKIES WESTWARD TO THE PACIFIC COAST BRINGING CHANCES FOR
HAZARDOUS TEMPERATURES TO PARTS OF THE PACIFIC NORTHWEST. UNSEASONABLY WARM
SPRING TEMPERATURES COULD ENHANCE THE SNOWMELT RATE WHERE DEEP SNOWPACK IS
STILL IN PLACE ACROSS THE HIGHER ELEVATIONS, INCREASING THE POTENTIAL FOR
FLOODING ALONG SOME RIVERS, ESPECIALLY ACROSS THE NORTHWESTERN QUARTER OF THE
CONTIGUOUS U.S. (CONUS). IN CONTRAST, DECLINING SNOW COVER ACROSS THE
NORTH-CENTRAL CONUS MAY LEAD TO WANING OF THE FLOODING RISK IN THE REGION.
ELSEWHERE, SUBSTANTIAL PRECIPITATION COULD IMPACT AREAS OF THE LOWER
MISSISSIPPI VALLEY AND SOUTHERN PLAINS. ANTECEDENT DRYNESS ACROSS THE CENTRAL
PLAINS LEADS TO AN INCREASED RISK OF RAPID ONSET DROUGHT DURING THE PERIOD.
HAZARDS
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE PACIFIC NORTHWEST, FRI-MON,
MAY 19-22.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY
AND SOUTHERN PLAINS, FRI-SUN, MAY 19-21.
RAPID ONSET DROUGHT IN NORTHEAST KANSAS, NORTHWEST MISSOURI, AND SOUTHWEST IOWA.
POSSIBLE FLOODING FOR PORTIONS OF CALIFORNIA, NEVADA, UTAH, WYOMING, WESTERN
MONTANA, IDAHO, OREGON, AND WASHINGTON.
POSSIBLE FLOODING ALONG PORTIONS OF THE RED RIVER OF THE NORTH, THE JAMES
RIVER, AND THE MISSISSIPPI RIVER.
POSSIBLE FLOODING IN PARTS OF EASTERN INTERIOR ALASKA
DETAILED SUMMARY
FOR SUNDAY MAY 14 - THURSDAY MAY 18:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY MAY 19 - THURSDAY MAY 25: THERE CONTINUES TO BE GOOD AGREEMENT IN
THE MEAN WEEK-2 500-HPA HEIGHT ANOMALY FORECASTS BETWEEN THE DYNAMICAL MODELS.
AMPLIFIED MID-LEVEL RIDGING IS FORECAST OVER THE NORTHWESTERN CONUS AT THE
BEGINNING OF THE PERIOD, GRADUALLY WEAKENING THROUGHOUT WEEK-2 AND EXPANDING
SOUTH AND EAST. AT THE ONSET OF THE PERIOD, THE ECMWF AND GEFS HAVE HEIGHT
ANOMALIES GREATER THAN 180M OVER PORTIONS OF THE PACIFIC NORTHWEST. THIS
PATTERN FAVORS ANOMALOUS WARM TEMPERATURES ACROSS THE WEST, WITH THE GEFS AND
ECMWF REFORECAST TOOLS SHOWING THE HIGHEST PROBABILITIES (>60% CHANCE) OF
MAXIMUM TEMPERATURES EXCEEDING THE 85TH PERCENTILE ACROSS PARTS OF THE
NORTHWEST AT THE BEGINNING OF THE PERIOD. REFORECAST TOOLS AND UNCALIBRATED
MODELS SHOW INCREASING CHANCES FOR TEMPERATURES TO REACH 90 DEG F IN PARTS OF
EASTERN WASHINGTON EARLY IN THE PERIOD. UNLIKE THE CHANCES FOR HAZARDOUS HEAT
DURING WEEK-1, STRONG EASTERLY WINDS APPEAR TO BE LESS LIKELY, REDUCING CHANCES
FOR HAZARDOUS HEAT ALONG THE IMMEDIATE PACIFIC COAST. THEREFORE, A SLIGHT RISK
OF EXCESSIVE HEAT IS POSTED FOR PARTS OF THE WILLAMETTE VALLEY, EASTERN
WASHINGTON, AND THE SNAKE RIVER VALLEY FOR FRI-MON, MAY 19-22.
WARM TEMPERATURES ARE ANTICIPATED TO ENHANCE THE SNOWMELT RATE ACROSS THE
WESTERN CONUS, INDUCING RIVER FLOODING ESPECIALLY ACROSS HIGHER ELEVATIONS IN
THE WEST AND THE SOUTHERN CANADIAN ROCKIES, WHERE A FEW SITES STILL REPORT OVER
50 INCHES OF SNOW WATER CONTENT. THE ABOVE-NORMAL TEMPERATURES MAY CAUSE RAPID
SNOWMELT IN PARTS OF THE SIERRA NEVADA, GREAT BASIN, NORTHERN ROCKIES, AND
PACIFIC NORTHWEST, RESULTING IN AREAS AT HIGH RISK OF FLOODING AND MUDSLIDES
ALONG MANY RIVERS, AND IN AREAS ADJACENT TO THE HIGHER ELEVATIONS; HOWEVER,
EXPANSIVE, MULTI-STATE FLOODING IS NOT EXPECTED TO ENVELOP LARGE AREAS OF THE
INTERIOR WEST. FLOODING IS ONGOING OR ANTICIPATED ALONG PARTS OF THE PORTNEUF,
BEAR, HUMBOLDT, NACHES, AND OKANOGAN RIVERS, AMONG OTHERS. FLOODING CONTINUES
TO BE POSSIBLE ALONG SOME RIVERS IN THE NORTH-CENTRAL CONUS. HOWEVER, DUE TO
SNOW COVER BECOMING NEARLY DEPLETED IN THIS AREA AND ADJACENT CANADA, THE RIVER
FLOODING RISK IS ANTICIPATED TO WANE OVER THIS AREA.
AT THE BEGINNING OF THE PERIOD, MULTIPLE MODELS INDICATE THE POTENTIAL FOR
SURFACE HIGH PRESSURE FORMING OVER THE SOUTH-CENTRAL CONUS WHILE A SURFACE LOW
FORMS OVER THE WESTERN PORTIONS OF THE FOUR CORNERS, SOUTHERN NEVADA AND
SOUTHERN CALIFORNIA. THIS PATTERN MAY SUPPORT ENHANCED MOIST SOUTHERLY FLOW TO
PARTS OF THE FOUR CORNERS, RESULTING IN ENHANCED PRECIPITATION IN THE REGION.
BOTH REFORECAST TOOLS INDICATE A BROAD AREA WITH AT LEAST A 20% CHANCE OF 3-DAY
RAINFALL EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE, AND A HALF AN INCH OVER
THE SOUTHWESTERN CONUS, PRIMARILY DURING DAYS 8 TO 10 (MAY 19-21). THESE TOOLS
SHOW THE HIGHEST CHANCES (>60%) OF EXCEEDING THESE THRESHOLDS ACROSS ARIZONA
AND SOUTHERN UTAH. DUE TO THIS BEING THE DRIEST TIME OF THE YEAR IN A
RELATIVELY ARID REGION OF THE COUNTRY, ONLY A FEW TENTHS OF AN INCH FOR A
ONE-WEEK PERIOD IS CONSIDERABLY ABOVE NORMAL. IN ALMOST 130 YEARS OF RECORD,
MAY MONTHLY TOTALS AT PHOENIX, AZ HAVE ONLY EXCEEDED AN INCH FOUR TIMES, AND
ONLY THE EUROPEAN REFORECAST SHOWS A NON-TRIVIAL CHANCE FOR PRECIPITATION
TOTALS TO EXCEED A HALF AN INCH. THE UNCALIBRATED ENSEMBLE MEANS HAVE LESS
AGREEMENT COMPARED TO THE REFORECAST TOOLS, WITH THE GEFS HAVING A GREATER
SIGNAL FOR ENHANCED PRECIPITATION THAN THE ECMWF. BASED ON SIGNIFICANT
UNCERTAINTY REGARDING HEAVY PRECIPITATION POTENTIAL AND CONSIDERING CLIMATOLOGY
AT THIS TIME OF THE YEAR, NO HAZARD IS POSTED AT THIS TIME.
HOWEVER, IN PARTS OF THE LOWER-MISSISSIPPI VALLEY AND SOUTHERN PLAINS
REFORECAST TOOLS ARE IN FAIR AGREEMENT FOR A CHANCE OF HEAVY PRECIPITATION. THE
CANADIAN AND ECMWF REFORECAST TOOLS INDICATE AT LEAST A 20% CHANCE OF
PRECIPITATION EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND 3-DAY
PRECIPITATION TOTALS EXCEEDING 1 INCH DURING BOTH THE 8-10 DAY AND 10-12 DAY
PERIODS. THE GEFS IS MORE SUPPORTIVE OF A RISK FOR HEAVY PRECIPITATION TODAY
RELATIVE TO YESTERDAY AND ALSO HAS CHANCES FOR PRECIPITATION EXCEEDING THE 85TH
PERCENTILE AND 1 INCH IN THE LOWER-MISSISSIPPI VALLEY. UNCALIBRATED ENSEMBLES
ARE LESS SUPPORTIVE OF HIGHER PRECIPITATION TOTALS, HOWEVER, PORTIONS OF THIS
REGION HAVE SEEN HEAVY PRECIPITATION IN RECENT WEEKS AND MAY BE SUSCEPTIBLE TO
ADDITIONAL HEAVY PRECIPITATION. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION
IS POSTED FOR FRI-SUN, MAY 19-21.
IN PORTIONS OF NORTHEAST KANSAS, NORTHWEST MISSOURI, AND SOUTHWEST IOWA,
PRECIPITATION TOTALS OVER THE LAST 14, 30, AND 60 DAYS HAVE BEEN BELOW HALF OF
NORMAL FOR THOSE PERIODS. SOIL MOISTURE IN THESE REGIONS IS ALSO WELL
BELOW-NORMAL WITH NASA SPORT SOIL MOISTURE ANALYSIS SHOWING MUCH OF THE AREA
BELOW THE 20TH PERCENTILE. FORECASTS FROM THE WEATHER PREDICTION CENTER (WPC)
SHOW LITTLE PRECIPITATION IN THESE REGIONS DURING THE WEEK-1 PERIOD, AND THE
CPC WEEK-2 FORECAST FAVORS BELOW-NORMAL PRECIPITATION IN THESE REGIONS. AS WE
MOVE INTO THE SUMMER SEASON, THE RAPID ONSET DROUGHT RISK IS EXPECTED TO
INCREASE IN THESE AREAS AND SUBSEQUENT HAZARD IS POSTED FOR THE WEEK-2 PERIOD.
IN EASTERN INTERIOR ALASKA, WARMER TEMPERATURES DURING THE PAST WEEK HAVE LED
TO RISES IN MANY OF THE RIVERS IN THIS REGION. MUCH OF THE TANANA RIVER NOW HAS
OPEN WATER AS RIVER ICE CONTINUES TO MELT. MEANWHILE, UPRIVER ON THE YUKON NEAR
THE CANADIAN BORDER WATER LEVELS HAVE RISEN 7 FEET WITH WATER LEVELS CONTINUING
TO RISE. WITH WARMER TEMPERATURES DURING THE WEEK-1 PERIOD SIGNIFICANT RIVER
RISES DUE TO SNOW AND ICE MELT MAY CONTINUE. FURTHERMORE, RIVER ICE BREAKUP
COULD LEAD TO ICE-JAMS ON THE MAJOR RIVERS LEADING TO INCREASED CHANCES FOR
POSSIBLE FLOODING.
FORECASTER: RYAN BOLT
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