Threats Assessment Discussion

818 
FXUS21 KWNC 241929
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT APRIL 24 2018
SYNOPSIS: DURING THE 3-7 DAY FORECAST PERIOD, A SURFACE LOW PRESSURE SYSTEM IS 
PREDICTED TO MOVE ACROSS THE WESTERN CONUS, WHILE SEVERAL COLD FRONTS ARE 
EXPECTED TO TRANSIT THE CENTRAL AND EASTERN CONUS. OVER THE ALASKAN DOMAIN, A 
STORM SYSTEM IS FORECAST TO TRACK EASTWARD ACROSS THE BERING SEA AND ALEUTIANS 
AND WEAKEN SUBSTANTIALLY BEFORE REACHING THE WEST COAST OF ALASKA ABOUT ONE 
WEEK FROM NOW. IN GENERAL, A MUCH QUIETER WEATHER PATTERN IS EXPECTED DURING 
THE 3-7 DAY PERIOD ACROSS THE CONUS AND ALASKA COMPARED TO RECENT WEEKS. DURING 
WEEK-2, AN INCREASE IN STORM ACTIVITY IS ANTICIPATED OVER THE CENTRAL AND 
EASTERN LOWER 48 STATES.
HAZARDS
HIGH WINDS OVER THE CENTRAL AND SOUTHERN HIGH PLAINS REGION, TUE-WED, MAY 1-2.
FLOODING IS POSSIBLE, LIKELY, IMMINENT OR OCCURRING ACROSS PARTS OF THE 
NORTH-CENTRAL CONUS, AND CENTRAL GULF COAST REGION.
SLIGHT RISK OF HEAVY PRECIPITATION OVER THE SOUTHERN PLAINS AND LOWER 
MISSISSIPPI VALLEY, WED-FRI, MAY 2-4.
MODERATE RISK OF HEAVY PRECIPITATION FOR MUCH OF THE SLIGHT RISK AREA, CENTERED 
NEAR THE ARKLATEX, WED-FRI, MAY 2-4. 
SEVERE DROUGHT ACROSS PORTIONS OF THE SOUTHEAST, THE GREAT PLAINS, THE CENTRAL 
AND SOUTHERN ROCKIES, THE CENTRAL AND SOUTHERN INTERMOUNTAIN REGION, AND 
SOUTHERN CALIFORNIA.
DETAILED SUMMARY
FOR FRIDAY APRIL 27 - TUESDAY MAY 01: A PROGRESSIVE MID-LEVEL CIRCULATION 
PATTERN IS PREDICTED DURING THE 3-7 DAY FORECAST PERIOD. INITIALLY, A CUTOFF 
LOW AT 500-HPA IS FORECAST NEAR THE COAST OF NORTHERN CALIFORNIA, WITH A RIDGE 
CENTERED NEAR THE ROCKIES, AND A TROUGH CENTERED OVER THE EAST-CENTRAL STATES. 
BY THE END OF THE 3-7 DAY PERIOD, THE INITIALLY CUT-OFF 500-HPA LOW IS 
PREDICTED TO BECOME OPEN-WAVE, AND REACH AS FAR EAST AS THE NORTHERN HIGH 
PLAINS. THE AFOREMENTIONED RIDGE IS EXPECTED TO REACH THE ATLANTIC SEABOARD BY 
DAY 7, AND THE TROUGH INITIALLY CENTERED OVER THE EAST-CENTRAL CONUS IS 
EXPECTED TO BE LOCATED FROM THE ATLANTIC PROVINCES OF EASTERN CANADA SOUTHWARD 
ACROSS THE WESTERN ATLANTIC OCEAN.
EARLY IN THIS PERIOD, THE CUTOFF LOW PREDICTED NEAR THE NORTHERN CALIFORNIA 
COAST IS EXPECTED TO SLOWLY PUSH INLAND ACROSS THE WESTERN CONUS. PRECIPITATION 
AMOUNTS OVER PARTS OF THE PACIFIC NORTHWEST AND MUCH OF MONTANA ARE EXPECTED TO 
BE SUB-HAZARDOUS (LESS THAN 1-INCH PER 24-HOUR PERIOD). AS THIS CUTOFF LOW 
EVOLVES INTO AN OPEN-WAVE 500-HPA TROUGH, IT IS EXPECTED TO AMPLIFY ACROSS THE 
WESTERN CONUS. NORMALLY, HIGHER WINDS ASSOCIATED WITH SHORTWAVES ROTATING 
AROUND THE BASE OF THE LONGWAVE TROUGH HELP TO ELEVATE THE CHANCES OF CRITICAL 
WILDFIRE CONDITIONS OVER THE SOUTHWEST. HOWEVER, WITH PERIODS OF PRECIPITATION 
EXPECTED DURING THE 3-7 DAY PERIOD, AND THE ONGOING GREEN-UP OF VEGETATIVE 
FUELS, THE RISK OF WILDFIRES IS EXPECTED TO DIMINISH. APART FROM THE POTENTIAL 
WILDFIRE ISSUE, THERE IS A HIGH WIND HAZARD POSTED (AT LEAST 30 KNOTS) FOR THE 
CENTRAL AND SOUTHERN HIGH PLAINS REGION, MAY 1. EARLY TO MID-SPRING IS A 
FAVORABLE TIME FOR HIGH WIND EVENTS ACROSS THIS REGION DUE TO A COMBINATION OF 
FACTORS; THE LOWER ATMOSPHERE IS GETTING WARMER, SETTING UP DEEP CONVECTION 
CURRENTS, AND INDIVIDUAL SHORTWAVES ROUNDING THE BASES OF MID-LEVEL TROUGHS CAN 
TRANSFER SOME OF THEIR MOMENTUM (IN THE FORM OF STRONG WINDS) DOWN TO THE 
SURFACE THROUGH VERTICAL MIXING.
ACROSS THE GULF COAST REGION, NORTHERLY LOW-LEVEL FLOW ASSOCIATED WITH A 
CONTINENTAL POLAR AIR MASS IMPLIES OFFSHORE FLOW AND DRY TRAJECTORIES, APR 
28-29. BY APR 30, AS A 1030-HPA SURFACE HIGH MOVES OFF THE SOUTHEAST COAST, THE 
LOW-LEVEL FLOW IS EXPECTED TO REVERSE, BRINGING INCREASINGLY RICH GULF MOISTURE 
NORTHWARD ACROSS THE REGION. THIS IS PREDICTED TO SET THE STAGE FOR THE 
DEVELOPMENT OF WIDESPREAD RAINFALL AND STRONG TO POTENTIALLY SEVERE 
THUNDERSTORMS ACROSS THE CENTRAL AND EAST-CENTRAL STATES OVER THE ENSUING FEW 
DAYS.
AS THE MID-LEVEL RIDGE (INITIALLY NEAR THE ROCKIES) PROGRESSES EASTWARD, IT IS 
EXPECTED TO CONTRIBUTE TOWARD DAYTIME TEMPERATURES ABOUT 12-16 DEGREES ABOVE 
NORMAL FROM THE NORTHERN AND CENTRAL ROCKIES EASTWARD TO THE GREAT LAKES AND 
OHIO VALLEY, APR 28-30. THESE TEMPERATURE DEPARTURES EQUATE TO DAILY HIGHS 
RANGING FROM ABOUT 75-80 DEGREES F.
BOTH ANTECEDENT AND PREDICTED SHORT-TERM PRECIPITATION MAY RESULT IN STREAM AND 
RIVER FLOODING OVER PORTIONS OF THE NORTHERN GREAT PLAINS AND UPPER MISSISSIPPI 
VALLEY (THOUGH MOST OF THE SNOW THAT FELL A WEEK AGO HAS SINCE MELTED DUE TO 
THE HIGHER SUN ANGLE IN APRIL), AND THE CENTRAL GULF COAST REGION.
ACROSS THE ALASKAN DOMAIN, SEVERAL LOW PRESSURE SYSTEMS ARE PREDICTED TO MOVE 
ACROSS THE BERING SEA AND ALASKA, THOUGH NONE WARRANT A HAZARDOUS DEPICTION ON 
THE MAP AT THIS TIME.
FOR WEDNESDAY MAY 02 - TUESDAY MAY 08: THE HIGH WIND HAZARD PREDICTED FOR THE 
CENTRAL AND SOUTHERN HIGH PLAINS REGION ON DAY 7 (MAY 1) CONTINUES INTO DAY 8 
(MAY 2).
AS NOTED IN THE 3-7 DAY SECTION, INCREASINGLY MOIST RETURN FLOW FROM SURFACE 
HIGH PRESSURE MOVING OFF THE SOUTHEAST COAST IS EXPECTED TO PRIME THE 
ATMOSPHERE ACROSS THE CENTRAL AND EAST-CENTRAL CONUS FOR WIDESPREAD RAIN AND 
THUNDERSTORM ACTIVITY FROM MAY 2-4. A SLIGHT RISK OF HEAVY RAIN IS PREDICTED 
OVER THE LOWER SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY, MAY 2-4. A 
SIGNIFICANT SUBSET OF THIS REGION IS COVERED BY A MODERATE RISK OF HEAVY RAIN 
FOR THE SAME PERIOD. THE BROAD 3-DAY EVENT IS DUE PRIMARILY TO TIMING 
DIFFERENCES AND INHERENT UNCERTAINTIES BETWEEN THE ECMWF AND GFS ENSEMBLE 
MEANS. NON-HAZARDOUS RAINFALL IS EXPECTED TO EXTEND FARTHER NORTH INTO THE 
MIDWEST, OHIO VALLEY, AND GREAT LAKES REGION. THE RISK OF STRONG TO POTENTIALLY 
SEVERE THUNDERSTORM ACTIVITY IS LIKELY TO BE ENHANCED, ESPECIALLY OVER THE 
LOWER SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY, AS A RESULT OF THE EXPECTED 
CONFLUENCE OF VARIOUS MID-LEVEL AND SURFACE WEATHER FEATURES (SUCH AS AN 
APPROACHING 500-HPA TROUGH, A DRYLINE AND ASSOCIATED ELEVATED MIXED LAYER, 
SEVERAL BAROCLINIC ZONES, INCREASED WIND SHEAR, AND INCREASED INSTABILITY).
TEMPERATURES OVER THE GREAT LAKES REGION AND NORTHEAST ARE ANTICIPATED TO BE 
WARMER THAN NORMAL DURING THE FIRST FEW DAYS OF THE WEEK-2 PERIOD, BUT NOT 
REACH HAZARDOUS THRESHOLDS.
FOR MAINLAND ALASKA, THERE IS AN INCREASED CONCERN FOR RIVER ICE BREAKUP AND 
RESULTANT FLOODING, AS WE MOVE INTO EARLY MAY.
THE US DROUGHT MONITOR, VALID ON APRIL 17, SHOWS A 1 CLASS IMPROVEMENT IN 
DROUGHT COVERAGE OVER THE NORTHERN PLAINS. THERE WAS DEGRADATION IN DROUGHT 
CONDITIONS OVER PARTS OF THE FOUR CORNERS REGION. OVERALL, THE SEVERE DROUGHT 
COVERAGE DURING THE PAST WEEK HAS SLIGHTLY DECREASED TO 16.33% FROM 16.69% LAST 
WEEK.
FORECASTER: ANTHONY ARTUSA 
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