984
FXUS21 KWNC 041838
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 04 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FAVORED TO WEAKEN OVER THE NORTH-CENTRAL
AND NORTHEASTERN CONTIGUOUS U.S. (CONUS) EARLY IN THE PERIOD, WHILE MID-LEVEL
HIGH PRESSURE DOMINATES THE SOUTHERN AND WESTERN CONUS, ENHANCING THE RISK OF
EXCESSIVE HEAT THERE. HEAT MAY BE PARTICULARLY PROBLEMATIC OVER THE SOUTHERN
PLAINS EARLY IN THE PERIOD, AND ACROSS MUCH OF THE NORTHWESTERN CONUS DURING
THE MIDDLE OF WEEK-2. THE UNUSUALLY HIGH 500-HPA HEIGHTS SHOULD BEGIN TO ERODE
SLOWLY LATER IN WEEK-2, BRINGING AN END TO THE EXCESSIVE HEAT RISK ACROSS THE
CONUS. ACROSS THE SOUTH, ANTECEDENT HEAT AND DRYNESS COUPLED WITH LITTLE TO NO
RAINFALL AND CONTINUED ABOVE-NORMAL TEMPERATURES SUPPORT A RAPID ONSET DROUGHT
RISK ACROSS PARTS OF THE LOWER MISSISSIPPI VALLEY, SOUTHERN PLAINS, RIO GRANDE
VALLEY, AND SOUTHERN ROCKIES.
HAZARDS
HIGH RISK OF EXCESSIVE HEAT OVER MOST OF TEXAS AND THE SOUTHERN HIGH PLAINS,
SAT, AUG 12.
MODERATE RISK OF EXCESSIVE HEAT FROM THE SOUTHERN ROCKIES TO THE LOWER
MISSISSIPPI VALLEY, SAT-MON, AUG 12-14.
MODERATE RISK OF EXCESSIVE HEAT IN MUCH OF THE PACIFIC NORTHWEST AND THE
NORTHERN SACRAMENTO VALLEY, SUN-WED, AUG 13-16.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE SOUTHERN AND WESTERN CONUS,
SAT-THU, AUG 12-17.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF SOUTHERN ALASKA, SAT-SUN, AUG 12-13.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE FOUR CORNERS AND SOUTHERN CALIFORNIA,
SAT-FRI, AUG 12-18.
RAPID ONSET DROUGHT RISK ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY,
SOUTHERN PLAINS, AND SOUTHWEST.
DETAILED SUMMARY
FOR MONDAY AUGUST 07 - FRIDAY AUGUST 11:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY AUGUST 12 - FRIDAY AUGUST 18: HEADING INTO EARLY WEEK-2, DYNAMICAL
MODELS HAVE BEEN CONSISTENT IN FAVORING A COOLER PATTERN OVER THE CENTRAL AND
NORTHERN CONUS WITH ANOMALOUS 500-HPA TROUGHING ENVELOPING THE NORTH-CENTRAL
AND NORTHEASTERN CONUS. CONCURRENTLY, ABOVE-NORMAL MID-LEVEL HEIGHTS ARE
FAVORED ACROSS THE SOUTHERN TIER OF THE U.S., EXPANDING INTO THE NORTHWEST FOR
SEVERAL DAYS AROUND THE MIDDLE OF THE PERIOD, BRINGING VARYING RISKS OF
EXCESSIVE HEAT TO MUCH OF THE SOUTHERN AND WESTERN STATES BEFORE HEIGHTS BEGIN
TRENDING DOWNWARD ACROSS MOST OF THE CONUS AS WEEK-2 ENDS. THE PROBABILISTIC
EXTREME TOOLS (PETS) REFLECT THESE ELEVATED RISKS FOR EXCESSIVE HEAT ACROSS THE
SOUTHERN AND WESTERN CONUS THROUGH MOST OF WEEK-2, BEFORE DECLINING MID-LEVEL
RIDGIING ALLOWS TEMPERATURES TO DROP CLOSER TO NORMAL THROUGHOUT IN THIS BROAD
AREA.
MOST TOOLS SHOW A PERIOD OF MUCH ABOVE NORMAL 500-HPA HEIGHTS OVER AND WEST OF
THE PACIFIC NORTHWEST FOR SEVERAL DAYS AROUND THE MIDDLE OF THE PERIOD,
INCREASING THE CHANCES FOR EXCESSIVE HEAT IN THIS REGION. CONDITIONS COULD
BECOME PARTICULARLY EXTREME IN THE LOWER COLUMBIA RIVER VALLEY (INCLUDING THE
PORTLAND, OR AREA), OVER THE VALLEYS EAST OF THE CASCADES, AND IN THE NORTHERN
SACRAMENTO VALLEY. HOWEVER, DESPITE SOMEWHAT CLOSER TO NORMAL TEMPERATURES
EARLY AND LATER WEEK-2, TEMPERATURES ARE EXPECTED TO AVERAGE 5 TO 15 DEG. F
ABOVE NORMAL FOR THE PERIOD AS A WHOLE, WITH HIGHS APPROACHING 105 DEG. F AS
FAR NORTH AS THE PORTLAND, OR AREA FOR ONE OR TWO DAYS AROUND THE MIDDLE OF
WEEK-2.
FARTHER SOUTH, OPPRESSIVE HEAT DURING WEEK-1 IS EXPECTED TO CONTINUE INTO EARLY
WEEK-2 ACROSS THE SOUTH-CENTRAL STATES BEFORE A SLOW COOLING TREND BEGINS. A
HIGH RISK OF EXCESSIVE HEAT IS IDENTIFIED FOR DAY 8 (AUG. 12) ACROSS MUCH OF
THE SOUTHERN PLAINS AND ADJACENT SOUTHERN ROCKIES, WITH A MODERATE RISK FOR
EXCESSIVE HEAT EXTENDING FOR TWO MORE DAYS IN A BROAD SWATH FROM THE SOUTHERN
ROCKIES THROUGH THE LOWER MISSISSIPPI VALLEY. A SLIGHT RISK FOR EXCESSIVE HEAT
COVERS A LARGE PROPORTION OF THE SOUTHERN AND WESTERN CONUS WHERE BROADLY
ABOVE-NORMAL 500-HPA HEIGHTS ARE EXPECTED THROUGH MOST OF WEEK-2. ENSEMBLE
MODEL MEANS SHOW A SLOW DECLINE IN 500-HPA HEIGHTS LATER WEEK-2, FINALLY
BRINGING AN END TO EXCESSIVE HEAT RISKS THROUGHOUT THE CONUS ON DAY 14 (AUG.
17).
BY THE OUTSET OF WEEK-2, ENSEMBLES CONTINUE TO SHOW SURFACE LOW PRESSURE OVER
THE PLAINS AND MISSISSIPPI VALLEY LEADING TO INCREASED PRECIPITATION AMOUNTS
OVER PORTIONS OF THE CENTRAL AND EASTERN CONUS. THIS IS REFLECTED IN THE GEFS
AND ECMWF PETS, DEPICTING INCREASED CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE
85TH PERCENTILE AND AN ONE INCH TOTAL. HOWEVER, THE TOOLS ARE INCONSISTENT IN
IDENTIFYING SPECIFIC TIMES AND LOCATIONS WITH INCREASED HEAVY PRECIPITATION
RISK, VARIOUSLY HIGHLIGHTING PARTS OF THE CENTRAL GREAT PLAINS, MIDDLE
MISSISSIPPI VALLEY, GULF COAST STATES, AND EASTERN SEABOARD AT DIFFERENT TIMES.
NO CORRESPONDING PRECIPITATION HAZARD IS ISSUED AT THIS TIME DUE TO THIS LARGE
UNCERTAINTY. IN ADDITION, THE ATLANTIC OCEAN TYPICALLY EXPERIENCES AN UPTICK IN
TROPICAL CYCLONE ACTIVITY LATER IN AUGUST, BUT WITHOUT SPECIFIC SYSTEMS BEING
IDENTIFIED FOR WEEK-2, THIS ONLY SERVES TO FURTHER INCREASE UNCERTAINTY.
A SLIGHT RISK OF EPISODES OF HIGH WINDS REMAINS ISSUED OVER MUCH OF THE
SOUTHWESTERN CONUS AND IS VALID FOR THE ENTIRETY OF WEEK-2 DUE TO THE
ANOMALOUSLY HIGH TEMPERATURES AND BELOW NORMAL PRECIPITATION FAVORED. PET
GUIDANCE CONTINUES TO SHOW AT LEAST A 20% CHANCE OF WIND SPEEDS EXCEEDING THE
85TH PERCENTILE FOR PARTS OF THE PERIOD. ANY ANOMALOUSLY STRONG WINDS IN THE
SLIGHT RISK REGION WILL EXACERBATE ANTECEDENT DRY AND WARM CONDITIONS,
INCREASING CHANCES FOR THE RAPID DEVELOPMENT AND SPREAD OF WILDFIRES.
A RAPID ONSET DROUGHT (ROD) RISK REMAINS POSTED ACROSS PARTS OF THE LOWER
MISSISSIPPI VALLEY, TEXAS, SOUTHERN NEW MEXICO, AND SOUTHEASTERN ARIZONA DUE TO
THE COMBINATION OF INCREASING 30-DAY PRECIPITATION DEFICITS, LITTLE TO NO
RAINFALL DURING THE NEXT TWO WEEKS, AND LIKELIHOOD FOR A PROLONGED PERIOD OF
HIGH EVAPOTRANSPIRATION RATES ASSOCIATED WITH MUCH ABOVE-NORMAL TEMPERATURES.
GUIDANCE CONTINUES TO INDICATE AMPLIFIED MID-LEVEL TROUGHING NEAR THE
ALEUTIANS, CONCURRENT WITH THE AFOREMENTIONED BUILDING OF MID-LEVEL HEIGHTS
OVER WESTERN NORTH AMERICA. THIS PATTERN WOULD GENERATE A STRONG SURFACE
LOW-PRESSURE SYSTEM OVER OR WEST OF THE GULF OF ALASKA EARLY IN WEEK-2,
ENHANCING THE SOUTHERLY FLOW OF MARITIME AIR FROM THE NORTH PACIFIC OCEAN INTO
LOWER MAINLAND AND SOUTHEASTERN ALASKA. THE GEFS AND ECMWF PETS ARE AGAIN
FAIRLY SUPPORTIVE OF HEAVY PRECIPITATION AND HIGH WINDS ACROSS THE SOUTHERN
TIER OF ALASKA, AND GIVEN THE POTENTIALLY STRONG SURFACE PRESSURE GRADIENT, A
SLIGHT RISK OF HIGH WINDS IS POSTED FOR DAYS 8-9 (AUG. 12-13). HOWEVER, THERE
IS LESS CERTAINTY ABOUT THE SURFACE AND MID-LEVEL MOISTURE INFLUX AND THE
MID-LEVEL HEIGHT PATTERN IN THE GUIDANCE, AND EXTREME PRECIPITATION AMOUNTS DO
NOT SEEM AS LIKELY AS A STRONG WIND HAZARD, SO NO RELATED PRECIPITATION HAZARD
IS POSTED. THE UNCERTAINTY IS EXACERBATED BY THE POTENTIAL INCURSION OF
TROPICAL SYSTEMS OR THEIR REMNANTS INTO THE MID-LEVEL FLOW PATTERN AT HIGHER
LATITUDES. THIS SITUATION WILL CONTINUE TO BE MONITORED IN UPCOMING OUTLOOKS.
FORECASTER: RICH TINKER
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