657
FXUS21 KWNC 041907
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 04 2023
SYNOPSIS: A PERIOD OF COOLER THAN NORMAL TEMPERATURES OVER THE WESTERN
CONTIGUOUS U.S. (CONUS) SHOULD BE ENDING AS WEEK-2 BEGINS, THEN A BROAD AREA OF
MID-LEVEL HIGH PRESSURE SHOULD FAVOR PREDOMINANTLY ABOVE-NORMAL
TEMPERATURES.FOR MOST OF THE PERIOD. IN PARTS OF THE NORTH-CENTRAL AND WESTERN
CONUS, THIS IS EXPECTED TO INCREASE THE RATE OF SNOWMELT, ELEVATING THE
POTENTIAL FOR RIVER FLOODING. THIS IS ESPECIALLY TRUE IN THE NORTHWEST, WHERE
AREAS OF DEEP SNOWPACK ARE COMMON ACROSS THE HIGHER ELEVATIONS. MID-LEVEL LOW
PRESSURE ALONG WITH ENHANCED LOW-LEVEL MOISTURE FROM THE GULF OF MEXICO
SUPPORTS AN INCREASED RISK FOR HEAVY PRECIPITATION ACROSS THE SOUTH-CENTRAL
CONUS.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GREAT PLAINS, THE
SOUTHEAST, THE MISSISSIPPI VALLEY, AND ADJACENT AREAS, FRI-SUN, MAY 12-14.
POSSIBLE FLOODING FOR PORTIONS OF CALIFORNIA, NEVADA, UTAH, EASTERN 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 SUNDAY MAY 07 - THURSDAY MAY 11:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY MAY 12 - THURSDAY MAY 18: THERE IS GOOD AGREEMENT IN THE MEAN WEEK-2
500-HPA HEIGHT ANOMALY FORECASTS BETWEEN THE DYNAMICAL MODELS, THOUGH THE GEFS
FEATURES A LESS AMPLIFIED SOLUTION COMPARED TO THE ECMWF AND CANADIAN
ENSEMBLES. WEEK-2 MAY START OUT WITH BELOW-NORMAL TEMPERATURES IN THE WEST -
PARTICULARLY AROUND THE FOUR CORNERS REGION - BUT THIS SHOULD BE SCOURED OUT
QUICKLY AS ANOMALOUS MID-LEVEL RIDGING IS FORECAST TO ENVELOP MUCH OF THE CONUS
FOR MOST OF WEEK-2. THIS SHOULD LEAD TO GENERALLY ABOVE-NORMAL TEMPERATURES FOR
MUCH OF THE COUNTRY, BUT READINGS ARE EXPECTED TO REMAIN BELOW HAZARDS
THRESHOLDS, SO NO TEMPERATURE HAZARDS ARE POSTED; HOWEVER, THE RELATIVELY WARM
WEATHER COULD ACCELERATE SNOW MELT, WHICH WOULD INCREASE THE POTENTIAL FOR
FLOODING ALONG SOME RIVERS IN THE NORTH-CENTRAL AND THE WESTERN CONUS.
DECLINING SNOWPACK IS EXPECTED TO REDUCE THE CHANCES FOR SIGNIFICANT FLOODING
IN THE NORTH-CENTRAL STATES NEAR THE RED RIVER (OF THE NORTH), JAMES RIVER, AND
MISSISSIPPI RIVER LATER WEEK-2. IN CONTRAST, THE SNOWPACK REMAINS VERY DEEP IN
SOME HIGHER ELEVATIONS IN THE WEST AND THE SOUTHERN CANADIAN ROCKIES, WITH A
FEW SITES THERE STILL REPORTING OVER 100 INCHES OF SNOW WATER CONTENT. AS A
RESULT, THE ABOVE-NORMAL TEMPERATURES CREATE AN INCREASED RISK OF FLOODING IN
PARTS OF THE SIERRA NEVADA, GREAT BASIN, NORTHERN ROCKIES, AND PACIFIC
NORTHWEST.
WEEK-2 IS EXPECTED TO START WITH A WEAK MID-LEVEL TROUGH AXIS APPROXIMATELY
ALONG THE ROCKIES, AND A DECENT FETCH OF MOIST SUBTROPICAL AIR FROM THE GULF OF
MEXICO INTO THE PARTS OF THE CENTRAL AND WESTERN GULF COAST REGION, THE
SOUTHERN HALVES OF THE MISSISSIPPI VALLEY AND GREAT PLAINS, AND PARTS OF THE
SOUTHERN HIGH PLAINS. THE CYCLONICALLY-CURVED MID-LEVEL FLOW ALONG WITH THE
SURFACE INFLUX OF MILD AND MOIST AIR INCREASES THE CHANCES FOR HEAVY
PRECIPITATION IN THESE AREAS BEFORE CONDITIONS DISSIPATE AROUND THE MIDDLE OF
WEEK-2. BUT INCONSISTENCIES AMONG THE TOOLS REGARDING THE PLACEMENT AND AMOUNT
OF HEAVY PRECIPITATION - IF ANY - KEEPS THE RISK ONLY SLIGHTLY ELEVATED. IN
PARTICULAR, TOOLS DERIVED FROM THE LESS-AMPLIFIED GEFS KEEP PRECIPITATION
TOTALS BELOW HAZARDS THRESHOLDS.
OVER THE PAST SEVERAL WEEKS, BELOW-NORMAL PRECIPITATION HAS FALLEN OVER MUCH OF
THE MIDWEST, INCLUDING PART OF THE CORN BELT. 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 SHORTFALLS EXCEED 3 INCHES OVER A
SIGNIFICANT PORTION OF MISSOURI AND SOME ADJACENT AREAS. GIVEN THE ANTICIPATED
ABOVE-NORMAL TEMPERATURES, THIS RAISES THE POSSIBILITY OF RAPID-ONSET DROUGHT;
HOWEVER, AT LEAST SEVERAL TENTHS OF AN INCH OF PRECIPITATION IS ANTICIPATED
ACROSS THIS REGION DURING WEEK-1, AND WEEK-2 TEMPERATURES ARE NOT EXPECTED TO
BE HIGH ENOUGH TO SUBSTANTIALLY INCREASE SURFACE MOISTURE LOSS. IN ADDITION,
WINDS ARE NOT FORECAST TO BE PARTICULARLY HIGH, AND RELATIVE HUMIDITY IS LIKELY
TO BE BELOW-NORMAL, AND FOR THESE REASONS, THE RISK OF RAPID-ONSET DROUGHT DOES
NOT SEEM PARTICULARLY HIGH AT THIS TIME.
ACROSS ALASKA, THE DEEP MID-LEVEL TROUGH IN PLACE EARLY WEEK-2 IS EXPECTED TO
INDUCE CYCLOGENESIS OVER OR NEAR THE GULF OF ALASKA. THIS WILL LIKELY TRIGGER
ABOVE-NORMAL PRECIPITATION AND GUSTY WINDS ACROSS SOUTH-CENTRAL AND SOUTHEAST
ALASKA, BUT MOST TOOLS KEEP CONDITIONS BELOW HAZARDS THRESHOLDS AT THIS TIME.
THE MID-LEVEL TROUGH DISSIPATES QUICKLY BY THE MIDDLE OF THE PERIOD, WHICH
SHOULD END ANY STORMY WEATHER. WEEK-2 TEMPERATURE TOOLS ARE NOT CONSISTENT WITH
EACH OTHER; ENSEMBLE REFORECAST TOOLS ARE CONSIDERABLY WARMER THAN THE
CALIBRATED TOOLS. BUT IN ANY CASE, MOST INDICATORS KEEP TEMPERATURES BETWEEN
THE THRESHOLDS FOR COLD AND WARM HAZARDS, SO NO TEMPERATURE HAZARDS ARE POSTED.
MID-MAY IS NEAR THE PEAK TIME OF YEAR WHEN RAPID SNOW- AND ICE-MELT CAN
ENGENDER FLOODING IF TEMPERATURES ARE SIGNIFICANTLY ABOVE NORMAL. CHANCES FOR
SUCH FLOODING WILL NEED TO BE MONITORED CLOSELY, BUT AT THIS TIME THE POTENTIAL
APPEARS LOW DURING THE FORECAST PERIOD.
FORECASTER: RICH TINKER
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