305
FXUS21 KWNC 091840
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 09 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 WOULD 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 FROM THE LOWER
MISSISSIPPI VALLEY THROUGH THE SOUTH ATLANTIC STATES, AND PERIODS OF WET AND
WINDY WEATHER ARE POSSIBLE ALONG THE GULF OF ALASKA COASTLINE, BUT CONDITIONS
ARE NOT EXPECTED TO RISE TO HAZARD THRESHOLDS.
HAZARDS
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 FRIDAY MAY 12 - TUESDAY MAY 16:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY MAY 17 - TUESDAY MAY 23: 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
(>40% CHANCE) OF MAXIMUM TEMPERATURES EXCEEDING THE 85TH PERCENTILE ACROSS
PARTS OF THE NORTHWEST AT THE BEGINNING OF THE PERIOD. TEMPERATURES ARE NOT
DEEMED TO BE HAZARDOUS, 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 100 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 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 17-19). THESE TOOLS
SHOW THE HIGHEST CHANCES (>60%) CHANCE 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.
OVER THE PAST SEVERAL WEEKS, BELOW-NORMAL PRECIPITATION HAS FALLEN OVER MUCH OF
THE MIDWEST, INCLUDING PART 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, WITH
WEEK-2 MEAN TEMPERATURES TRENDING WARMER ACROSS THE EASTERN MAINLAND AND
SOUTHEAST COMPARED TO YESTERDAY. 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: MELISSA OU
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