433
FXUS21 KWNC 021806
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 02 2023
SYNOPSIS: EARLY IN WEEK-2, FORECAST MID-LEVEL LOW PRESSURE OVER THE WESTERN
CONTIGUOUS U.S. (CONUS) IS EXPECTED TO BRING ANOMALOUSLY COLD TEMPERATURES TO
PRIMARILY THE WEST COAST STATES, WHICH MAY ADVERSELY AFFECT DEVELOPING
AGRICULTURE ACROSS PARTS OF NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST.
SURFACE LOW FORMATION ALONG WITH INCREASED RETURN FLOW FROM THE GULF OF MEXICO
SUPPORTS INCREASED CHANCES FOR HEAVY PRECIPITATION ACROSS THE SOUTH-CENTRAL
CONUS. SPRINGTIME FLOODING, ASSOCIATED WITH SNOWMELT, IS LIKELY TO PERSIST
ACROSS PARTS OF THE WEST, ALONG WITH THE RED RIVER OF THE NORTH, JAMES RIVER,
AND UPPER TO MIDDLE MISSISSIPPI RIVER.
HAZARDS
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF NORTHERN CALIFORNIA,
AND THE PACIFIC NORTHWEST, WED, MAY 10.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GREAT PLAINS,
MISSISSIPPI, TENNESSEE AND OHIO VALLEYS, AND SOUTHEAST, WED-FRI, MAY 10-12.
POSSIBLE FLOODING FOR PORTIONS OF CALIFORNIA, IDAHO, NEVADA, OREGON, AND UTAH.
POSSIBLE FLOODING ALONG PORTIONS OF THE RED RIVER OF THE NORTH, JAMES RIVER,
AND MISSISSIPPI RIVER.
DETAILED SUMMARY
FOR FRIDAY MAY 05 - TUESDAY MAY 09:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY MAY 10 - TUESDAY MAY 16: TOWARDS THE MIDDLE OF NEXT WEEK,
DYNAMICAL MODEL 500-HPA HEIGHT FORECASTS CONTINUE TO FEATURE A BROADLY
AMPLIFIED RIDGE OVER MUCH OF NORTH AMERICA, WITH WEAKLY POSITIVE HEIGHT
DEPARTURES EXTENDING FROM THE NORTH-CENTRAL CONUS TO THE MID-ATLANTIC. FARTHER
WEST, ANOMALOUS MID-LEVEL TROUGHING IS PREDOMINATELY FAVORED, HOWEVER THERE ARE
SIGNIFICANT DISCREPANCIES BETWEEN THE GEFS AND ECMWF ENSEMBLES IN REGARDS TO
STRENGTH AND EVOLUTION OF THIS FEATURE. THE 0Z AND 6Z GEFS KEEP MUCH OF THE
NEGATIVE HEIGHT ANOMALIES FOCUSED OVER THE EASTERN PACIFIC, WHEREAS THE 0Z
ECMWF FAVORS A MORE PROGRESSIVE AND AMPLIFIED SOLUTION, SHIFTING THE TROUGH
AXIS EASTWARD OVER THE INTERIOR WEST EARLY IN THE PERIOD. AS A RESULT, THERE
ARE NON-TRIVIAL DIFFERENCES IN THE TEMPERATURE AND PRECIPITATION GUIDANCE OVER
THE WEST, AS WELL AS IN THE STRENGTH OF SURFACE LOW FORMATION EJECTING OUT OF
THE ROCKIES, WHICH ALSO YIELDS INCREASED UNCERTAINTY IN THE OUTLOOK DUE TO THE
VARIED HAZARD POTENTIAL DOWNSTREAM.
WHILE THE EASTERN TWO-THIRDS OF THE CONUS ARE FAVORED TO EXPERIENCE
UNSEASONABLY WARM TEMPERATURES TIED TO ANOMALOUS RIDGING ALOFT, THE WEST IS
FORECAST TO REMAIN ON THE COLD SIDE BEFORE TEMPERATURES ARE EXPECTED TO
MODERATE LATER IN THE PERIOD. THE MORE EXPANSIVE TROUGHING FAVORED IN THE ECMWF
IS REFLECTED IN THE REFORECAST TOOL, WHICH DEPICTS A FAIRLY BROAD COVERAGE OF
MINIMUM TEMPERATURES FALLING BELOW THE 15TH PERCENTILE FROM THE WEST COAST TO
THE ROCKIES. WHILE THE GEFS IS WEAKER WITH THESE COLD SIGNALS, BOTH TOOLS
MAINTAIN AT LEAST A 20% CHANCE OF NIGHTTIME TEMPERATURES FALLING BELOW 40
DEGREES F WHICH MAY ADVERSELY AFFECT DEVELOPING AGRICULTURE IN PORTIONS OF
NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST. THEREFORE, A SLIGHT RISK OF MUCH
BELOW NORMAL TEMPERATURES REMAINS ISSUED FOR DAY 8 (MAY 10). INCREASED WIND
SPEEDS ARE ALSO POSSIBLE ASSOCIATED WITH STRENGTHENING SURFACE PRESSURE
GRADIENTS EVIDENT IN THE ENSEMBLES, HOWEVER NO CORRESPONDING HAZARDS ARE POSTED
GIVEN THAT TROUGHING IS FAVORED TO DEAMPLIFY AS IT MOVES INLAND AND THERE IS
NO SUPPORT FROM THE REFORECAST TOOLS.
FOR PRECIPITATION, ENSEMBLES CONTINUE TO FAVOR SURFACE LOW FORMATION IN THE LEE
OF THE ROCKIES AND INCREASED RETURN FLOW FROM THE GULF OF MEXICO EARLY IN THE
PERIOD. COMPARED TO THE GEFS, THE RAW ECMWF FEATURES A STRONGER MEAN SURFACE
LOW AND HIGHER MEAN DAILY PRECIPITATION AMOUNTS, CONSISTING OF AT LEAST A 40%
(20%) CHANCE OF EXCEEDING AN INCH (2 INCHES) EXTENDING FROM SOUTHEASTERN TEXAS
TO THE TENNESSEE VALLEY DURING DAYS 8-10. GIVEN THESE HEIGHTENED SIGNALS, A
MODERATE RISK FOR HEAVY PRECIPITATION WAS STRONGLY CONSIDERED FOR THE
SOUTH-CENTRAL CONUS, HOWEVER THE WEAKER PRECIPITATION SIGNALS EVIDENT IN BOTH
THE RAW AND REFORECAST GEFS GUIDANCE PRECLUDES THIS HAZARD DESIGNATION AT THIS
TIME. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION REMAINS ISSUED FOR MAY
10-12, AND IS SLIGHTLY EXPANDED OVER THE SOUTH-CENTRAL CONUS TO ACCOUNT FOR
INCREASED SIGNALS IN THE TOOLS. WITHIN THE HIGHLIGHTED REGION, THUNDERSTORM
ACTIVITY IS POSSIBLE GIVEN THE ABOVE-NORMAL TEMPERATURES AND ELEVATED DEWPOINT
TEMPERATURES (>60 DEGREES F) FAVORED IN THE WARM SECTOR OF THE PREDICTED MEAN
SURFACE LOW. REGARDLESS OF WHETHER PRECIPITATION EXCEEDS HAZARD THRESHOLDS, THE
ABOVE-NORMAL PRECIPITATION FAVORED IS LIKELY TO PROVIDE SOME RELIEF TO AREAS IN
THE CORN BELT AFFECTED BY ANOMALOUS DRYNESS THIS SPRING. PRECIPITATION DEFICITS
DURING THE LAST 30 DAYS CONTINUE TO RUN 2 INCHES BELOW NORMAL ACROSS PORTIONS
OF MISSOURI, SOUTHEASTERN IOWA, AND CENTRAL ILLINOIS.
DESPITE SOME OF THE ANOMALOUS COLD FAVORED ACROSS PARTS OF THE WEST EARLY IN
THE PERIOD, WARMER SPRINGTIME TEMPERATURES HAVE LED TO CONTINUED SNOWMELT,
ELEVATING FLOWS ALONG STREAMS AND RIVERS. AS THIS RISK IS ANTICIPATED TO
PERSIST WELL INTO MAY, A BROAD POSSIBLE FLOODING AREA REMAINS POSTED FOR MANY
PARTS OF CALIFORNIA, NEVADA, OREGON, IDAHO, UTAH AND WYOMING. FARTHER EAST
WHERE TEMPERATURES ARE FAVORED TO BE ABOVE-NORMAL, ACCELERATED SPRING SNOWMELT
DURING WEEK-2 IS POSSIBLE, THUS THE CORRESPONDING FLOODING AREAS ALSO REMAIN
POSTED ALONG THE RED RIVER OF THE NORTH, AND THE JAMES RIVER OVER THE DAKOTAS,
AS WELL AS ALONG THE UPPER AND MIDDLE MISSISSIPPI RIVER. MANY RIVER GAUGES IN
THE HIGHLIGHTED AREAS ARE FORECAST TO REMAIN ABOVE FLOOD STAGE.
OVER ALASKA, DISCREPANCIES EXIST IN THE REFORECAST TOOLS LEADING TO CONTINUED
UNCERTAINTY IN HAZARD POTENTIAL OVER THE STATE. THE GEFS REFORECAST TOOL
CONTINUES TO LEAN WARM, WITH INCREASED CHANCES FOR MAXIMUM TEMPERATURES
EXCEEDING THE 85TH PERCENTILE OVER PORTIONS OF THE MAINLAND, WHEREAS THE ECMWF
REFORECAST TOOL MAINTAINS A COLDER SOLUTION WHICH IS CONSISTENT WITH BOTH RAW
AND BIAS-CORRECTED GUIDANCE AMONG THE DYNAMICAL MODELS. SHOULD THE MAINLAND
EXPERIENCE A TRANSITION TO A WARMER PATTERN, THIS MAY ACCELERATE SNOW AND ICE
MELT DURING THE SPRING BREAKUP SEASON.
FORECASTER: NICK NOVELLA
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