007
FXUS21 KWNC 101910
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 10 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. 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. PERIODS OF WET AND WINDY WEATHER ARE
ALSO POSSIBLE ALONG THE GULF OF ALASKA COASTLINE, BUT CONDITIONS ARE NOT
EXPECTED TO RISE TO HAZARD THRESHOLDS.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY
AND SOUTHERN PLAINS, THU-SUN, MAY 18-21.
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.
DETAILED SUMMARY
FOR SATURDAY MAY 13 - WEDNESDAY MAY 17:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY MAY 18 - WEDNESDAY MAY 24: 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
SOUTHWARD. THIS PATTERN FAVORS ANOMALOUS WARM TEMPERATURES ACROSS THE WEST,
WITH THE GEFS AND ECMWF REFORECAST TOOLS SHOWING THE HIGHEST PROBABILITIES
(>50% CHANCE) OF MAXIMUM TEMPERATURES EXCEEDING THE 85TH PERCENTILE ACROSS
PARTS OF THE NORTHWEST AT THE BEGINNING OF THE PERIOD. TEMPERATURES ARE NOT
PREDICTED TO REACH HAZARDOUS THRESHOLDS, WITH LIMITED AREAS ACROSS THE
NORTHWEST EXCEEDING THE 90S (DEG F) AND TEMPERATURES REMAINING BELOW 95 DEG F
FURTHER SOUTH IN THE CALIFORNIA VALLEY. WARM TEMPERATURES ARE ANTICIPATED TO
ENHANCE THE SNOWMELT RATE ACROSS THE REGION, 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 18-20). 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, WHILE LESS SUPPORTIVE, ALSO MAINTAINS A 20% CHANCE OF 3 DAY
PRECIPITATION TOTALS EXCEEDING 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 THU-SUN, MAY 18-21.
OVER THE PAST SEVERAL WEEKS, BELOW-NORMAL PRECIPITATION HAS FALLEN OVER MUCH OF
THE MIDWEST, INCLUDING PARTS OF THE CORN BELT. PRECIPITATION TOTALS OVER THE
LAST 60 DAYS WERE BELOW HALF OF NORMAL FROM THE CENTRAL GREAT PLAINS THROUGH
PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY, AND 30-DAY RAINFALL DEFICITS
EXCEEDED 3 INCHES OVER A SIGNIFICANT PORTION OF MISSOURI AND SOME ADJACENT
AREAS. HOWEVER, POSSIBLE MODERATE TO HEAVY PRECIPITATION DURING WEEK-1 AND NEAR
TO BELOW NORMAL TEMPERATURES PREDICTED DURING WEEK-2 REDUCES THE THREAT FOR
RAPID-ONSET DROUGHT OVER THIS REGION.
MID-LEVEL RIDGING IS ANTICIPATED ACROSS MOST OF ALASKA DURING WEEK-2.
REFORECAST TOOLS FROM THE GEFS AND CANADIAN ARE SHOWING SIGNALS FOR MUCH ABOVE
NORMAL TEMPERATURES INTO PORTIONS OF EASTERN INTERIOR ALASKA AT THE ONSET OF
THE PERIOD. THESE CHANCES ARE TEMPERED BY THE EUROPEAN REFORECAST WITH LIMITED
CHANCES FOR EXCEEDING THE 85TH PERCENTILE. DUE TO THE UNCERTAINTY NO
CORRESPONDING HAZARDS ARE CURRENTLY POSTED IN ALASKA. MID-MAY IS NEAR THE PEAK
TIME OF YEAR WHEN RAPID SNOW AND ICE MELT CAN LEAD TO FLOODING IF TEMPERATURES
ARE SIGNIFICANTLY ABOVE NORMAL. INCREASING CHANCES FOR ABOVE-NORMAL
TEMPERATURES MAY LEAD TO ACCELERATED SNOW AND RIVER ICE BREAKUP AND WILL
CONTINUE TO BE MONITORED FOR POSSIBLE FLOODING.
FORECASTER: RYAN BOLT
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