Threats Assessment Discussion

765 
FXUS21 KWNC 281943
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT APRIL 28 2017
SYNOPSIS: MID-LEVEL LOW PRESSURE IS ANTICIPATED ACROSS THE CENTRAL CONTIGUOUS 
U.S. DURING THE FIRST TWO DAYS OF THE OUTLOOK PERIOD, WITH A FORECAST TENDENCY 
TO LIFT NORTHWARD, TOWARD AND OVER THE UPPER GREAT LAKES REGION. MODERATELY 
STRONG SURFACE LOW PRESSURE INITIALLY NEAR IOWA IS FORECAST TO FOLLOW THE SAME 
GENERAL TRACK AS THE MID-LEVEL LOW PRESSURE AREA. SEVERAL TYPES OF HAZARDS ARE 
PREDICTED TO ACCOMPANY THIS STORM SYSTEM. UNCERTAINTY REMAINS HIGH REGARDING 
THE WEEK-2 CIRCULATION AND ITS IMPACTS TO THE CONTIGUOUS U.S.. ALASKA IS 
EXPECTED TO EXPERIENCE PERIODIC SURFACE LOW PRESSURE SYSTEMS OF SEASONABLE 
STRENGTH MOVING ACROSS THE BERING SEA AND GULF OF ALASKA THROUGHOUT THE OUTLOOK 
PERIOD.
HAZARDS
HEAVY RAIN ACROSS PORTIONS OF THE MID-ATLANTIC, THE LOWER AND MIDDLE 
MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE 
NORTHEAST, THE SOUTHERN APPALACHIANS, THE SOUTHEAST, THE SOUTHERN PLAINS, AND 
THE OHIO VALLEY, WED-THU, MAY 3-MAY 4.
HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST AND THE GREAT LAKES, MON-TUE, MAY 
1-MAY 2.
FLOODING POSSIBLE ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER MISSISSIPPI 
VALLEY, THE TENNESSEE VALLEY, THE GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, 
THE UPPER MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, AND THE OHIO VALLEY.
FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST, THE 
MID-ATLANTIC, THE SOUTHERN PLAINS, THE NORTHERN PLAINS, AND THE NORTHERN GREAT 
BASIN.
FLOODING LIKELY ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY, THE SOUTHERN 
PLAINS, THE MIDDLE MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE SOUTHEAST, THE 
GREAT LAKES, AND THE OHIO VALLEY.
HIGH WINDS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL PLAINS, THE GREAT LAKES, 
THE MIDDLE AND UPPER MISSISSIPPI VALLEY, AND THE OHIO VALLEY, MON-TUE, MAY 
1-MAY 2.
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHERN AND CENTRAL 
PLAINS, AND THE MIDDLE AND UPPER MISSISSIPPI VALLEY, MON, MAY 1.
HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE, WED, MAY 3.
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE SOUTHERN 
ROCKIES, THE CENTRAL GREAT BASIN, THE SOUTHWEST, CALIFORNIA, AND THE SOUTHERN 
PLAINS, SAT-SUN, MAY 6-MAY 7.
SEVERE DROUGHT ACROSS PARTS OF THE SOUTHEAST, CALIFORNIA, AND HAWAII.
DETAILED SUMMARY
FOR MONDAY MAY 01 - FRIDAY MAY 05: DURING THE FIRST FEW DAYS OF THE OUTLOOK 
PERIOD, A SURFACE LOW PRESSURE SYSTEM INITIALLY NEAR IOWA IS PREDICTED TO TRACK 
NORTHEASTWARD ACROSS THE UPPER GREAT LAKES REGION AND INTO CANADA. SEVERAL 
HAZARDS ARE EXPECTED TO ACCOMPANY THIS SYSTEM. FOR AREAS GENERALLY EAST AND 
SOUTH OF THE PREDICTED PATH OF THE LOW PRESSURE CENTER, THE CONCERN IS FOR 
HEAVY RAIN (1.5"-2.0") OVER PORTIONS OF UPSTATE NEW YORK AND NORTHERN NEW 
ENGLAND ON MAY 1-2, AND 1.5"-4.0" GENERALLY EAST OF A LINE EXTENDING FROM THE 
LOWER MISSISSIPPI VALLEY TO SOUTHERN NEW ENGLAND, ON MAY 3-4. ALTHOUGH NO 
SEVERE WEATHER SHAPES ARE DRAWN ON THE MAP FOR MAY 1, FAIRLY STRONG 
THUNDERSTORMS MAY STILL DEVELOP AHEAD OF THE LOW PRESSURE SYSTEM'S COLD FRONT 
IN THE REGION BETWEEN THE LOWER GREAT LAKES AND THE CENTRAL APPALACHIANS. THE 
STORM PREDICTION CENTER (SPC) NOTES A CORRIDOR OF STRONG SOUTH-SOUTHWEST 
500-HPA FLOW IS EXPECTED TO OVERSPREAD THIS AREA, BUT ANY SIGNIFICANT THREAT OF 
SEVERE WEATHER IS PREDICTED TO BE TEMPERED BY A LACK OF STEEPER MID-LEVEL LAPSE 
RATES, LIMITED BUOYANCY/INSTABILITY, AND CONCERNS REGARDING CLOUD COVER EARLIER 
IN THE DAY.
OTHER HAZARDS THAT ARE FORECAST TO ACCOMPANY THE AFOREMENTIONED LOW PRESSURE 
SYSTEM ARE HIGH WINDS AND MUCH BELOW-NORMAL TEMPERATURES. A BROAD AREA OF 
MARGINALLY HIGH WIND SPEEDS (APPROACHING 30 KTS) IS DEPICTED FROM EASTERN 
PORTIONS OF NEBRASKA AND KANSAS ACROSS THE MIDDLE AND UPPER MISSISSIPPI VALLEY, 
PORTIONS OF INDIANA AND OHIO, AND THE WESTERN AND CENTRAL GREAT LAKES REGION, 
MAY 1-2. AN AREA OF MUCH BELOW-NORMAL TEMPERATURES IS PREDICTED FOR PORTIONS OF 
THE UPPER MISSISSIPPI VALLEY (AND SMALL PARTS OF ADJACENT STATES), ON MAY 1. IN 
THIS AREA, DAYTIME MAXIMUM TEMPERATURES ARE ANTICIPATED TO RANGE FROM 20 TO 26 
DEGREES F BELOW-NORMAL. THIS WOULD INDICATE A HIGH OF 36 DEGREES F FOR DULUTH, 
MN, AND 50 DEGREES F FOR DES MOINES, IA. THESE PREDICTED DAYTIME HIGH 
TEMPERATURES ARE TYPICAL OF OVERNIGHT LOW TEMPERATURES FOR THESE LOCATIONS AT 
THIS TIME OF YEAR.
FLOODING IS POSSIBLE, LIKELY, IMMINENT, OR OCCURRING ACROSS THE CAROLINAS AND 
SOUTHERN VIRGINIA, CENTRAL AND SOUTH-CENTRAL PORTIONS OF THE MISSISSIPPI 
VALLEY, EASTERN OKLAHOMA, NORTHERN NORTH DAKOTA, AND PARTS OF IDAHO. AS THESE 
FLOOD SHAPES TEND TO CHANGE FAIRLY QUICKLY, IT IS RECOMMENDED THAT THE RIVER 
FORECAST CENTER HOME PAGE IS CONSULTED FOR THE VERY LATEST INFORMATION AT: 
HTTPS://WATER.WEATHER.GOV/AHPS/RFC/RFC.PHP. A COLD FRONT WHICH RECENTLY STALLED 
ACROSS NORTH CAROLINA AND TENNESSEE BROUGHT SEVERE STORMS TO THE REGION, 
RAINFALL AMOUNTS OF 2-10", AND FLASH FLOODING.
SEVERAL LOW PRESSURE SYSTEMS ARE FORECAST TO AFFECT SOUTHERN ALASKA DURING THIS 
PERIOD, AS THEY TRACK PRIMARILY SOUTH OF THE ALEUTIANS AND ACROSS THE NORTHERN 
GULF OF ALASKA. THE 0Z ECMWF AND 6Z GFS (ON APR 28) BOUNDARY LAYER WIND 
FORECASTS, ALONG WITH SEA-LEVEL PRESSURE GRADIENTS PREDICTED BY THE WEATHER 
PREDICTION CENTER (WPC), ANTICIPATE HIGH WINDS (NEAR 40 KTS) AHEAD OF ONE OF 
THESE CYCLONIC SYSTEMS AS IT MOVES ACROSS THE NORTHEASTERN GULF OF ALASKA ON 
MAY 3. THE MOST LIKELY AREA TO EXPERIENCE THESE STRONGER WIND SPEEDS SPANS THE 
PANHANDLE REGION FROM ABOUT YAKUTAT BAY TO SITKA.
FOR SATURDAY MAY 06 - FRIDAY MAY 12: SUBSTANTIAL UNCERTAINTY IS APPARENT IN THE 
WIDE RANGE OF SPAGHETTI MAP PERSPECTIVES OF THE MID-LEVEL CIRCULATION OVER 
NORTH AMERICA IN WEEK-2. SOME EASTWARD PROGRESSION OF THE CIRCULATION PATTERN 
IS ANTICIPATED, WITH A TROUGH PREDICTED TO BE JUST OFF THE EAST COAST, A RIDGE 
OVER THE NATION'S MIDSECTION, AND ANOTHER TROUGH TRAVERSING THE WESTERN CONUS. 
GIVEN THE EXPECTED PROGRESSIVE PATTERN, AND THE FORECAST OF MODEST 500-HPA 
HEIGHT ANOMALIES, LIMITED HAZARDOUS IMPACTS CAN BE DISCERNED AT THIS TIME.
A SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES IS FORECAST FOR SOUTHERN 
CALIFORNIA AND PARTS OF THE SOUTHWEST EARLY IN THE WEEK-2 PERIOD. THE AREA OF 
ANTICIPATED MUCH ABOVE-NORMAL TEMPERATURES INDICATED ON THE MAP IS WHERE THE 
GEFS HAS AT LEAST A 20% CHANCE OF EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE 
OF MAXIMUM TEMPERATURE, AND THE DAYTIME HIGH TEMPERATURE IS EXPECTED TO BE AT 
LEAST 85 DEGREES F.
ACCORDING TO THE U.S. DROUGHT MONITOR RELEASED ON APRIL 27TH (USING DATA 
THROUGH 8 AM EDT, APRIL 25TH), COVERAGE OF SEVERE, OR GREATER INTENSITY, 
DROUGHT THROUGHOUT THE CONUS DROPPED SLIGHTLY (FROM 1.44 PERCENT LAST WEEK TO 
1.07 PERCENT THIS WEEK). THIS IS THE LOWEST COVERAGE OF D2-D4 DROUGHT OVER THE 
CONUS SINCE THE INCEPTION OF THE U.S. DROUGHT MONITOR IN JANUARY 2000.
FORECASTER: ANTHONY ARTUSA 
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