Prognostic Meteorological Discussion
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FXUS02 KWBC 230628
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
227 AM EDT SUN APR 23 2017

VALID 12Z WED APR 26 2017 - 12Z SUN APR 30 2017

...OVERVIEW AND GUIDANCE EVALUATION ...

THE UPPER PATTERN ACROSS THE LOWER 48 WILL BE DOMINATED BY ONE
POSITIVE HEIGHT ANOMALY SW OF CALIFORNIA IN THE PACIFIC OCEAN AND
ANOTHER OVER SE CANADA/WESTERN ATLANTIC. THAT WILL FORCE TROUGHING
IN BETWEEN, WITH WAVES DEPARTING THE ROCKIES AMPLIFYING IN THE
PLAINS AND THEN MOVING NORTHEASTWARD OUT OF THE PLAINS ACROSS THE
UPPER MS VALLEY INTO THE GREAT LAKES. IN ADDITION, A CLOSED LOW
OFF THE MID ATLANTIC COAST WILL SLOWLY LIFT NORTHEASTWARD TOWARD
CAPE COD BUT WEAKEN WITH TIME AS IT ENCOUNTERS INCREASED RIDGING
IN THE WESTERN ATLANTIC.

ACROSS THE PLAINS, THE MODELS WAVER ON TIMING/INTENSITY OF SFC LOW
PROGRESSION FROM THE PLAINS ACROSS THE UPPER MS VALLEY AND GREAT
LAKES...BUT EVERY FORECAST HAS LOW PRESSURE PROGRESSING ACROSS
THESE AREAS.  THE BEST AGREEMENT IS WITH THE FIRST OF THE WAVES
CROSSING THE GREAT LAKES ON THU 27 APR.

THE NEXT WAVE MOVING OUT OF THE SOUTHERN PLAINS INTO THE MS VALLEY
SAT/SAT NIGHT/EARLY SUN IS WHERE BIGGER DIFFERENCES DEVELOP.

DUE TO THE HIGH AMPLITUDE BLOCKING RIDGE JUST OFF THE EAST
COAST...FEW FORECASTS HAVE LOW PRESSURE GOING FURTHER EAST. THE
18-00Z GFS DO HAVE LOW PRESSURE GOING FURTHER WEST...THOUGH. THE
18Z GEFS/12Z ECMWF ENSEMBLE MEANS ALIGN WELL GIVEN THE RUN TO RUN
AND MODEL TO MODEL DIFFERENCES SO THEY WERE GIVEN GREATER WEIGHT
THAN THE OPERATIONAL RUNS.  CONFIDENCE REMAINS SOMEWHAT BELOW
NORMAL ON DAY 7.

TO THE EAST, BATTLE CONTINUES BETWEEN HIGH PRESSURE IN CANADA AND
THE WAVY FRONT CROSSING THE GREAT LAKES AND THEN THE NORTHEAST.
THE FRONT PROGRESSES THROUGH NEW ENGLAND AND SOUTH TOWARDS THE
MID-ATLANTIC NEXT WEEKEND. COLD ATLANTIC WATERS WOULD HELP BRING
SHALLOW COOL AIR SOUTHWARD ACROSS NEW ENGLAND...BUT STILL FURTHER
SOUTH IN THE MID ATLANTIC AS A STORM MOVES TOWARDS THE GREAT LAKES
AND MID ATLANTIC RIDGING HOLDS IN PLACE.

...WEATHER HIGHLIGHTS...

ACROSS THE PACIFIC NORTHWEST INTO THE NORTHERN AND CENTRAL
ROCKIES, MODERATE TO LOCALLY HEAVY PRECIPITATION IS EXPECTED
THROUGH THE PERIOD DUE TO PERSISTENT MOIST ONSHORE FLOW PLUS THE
PROXIMITY OF THE UPPER LEVEL JET AND DIVERGENCE MAXIMA ALOFT
CROSSING OR BORDER. SNOW IS EXPECTED ACROSS THE RANGES OF WA/OR
AND FARTHER INLAND ACROSS THE MOUNTAINS OF ID/UT/MT/WY/CO. THERE
IS POTENTIAL FOR LOCALLY HEAVY AMOUNTS IN THE MIDDLE TO LATTER
PART OF THE WEEK AS THE UPPER TROUGH PROGRESSES ACROSS WY AND CO.

RAINFALL BECOMES WIDESPREAD ACROSS THE PLAINS TO THE MS VALLEY IN
THE MIDDLE TO LATTER PORTION OF THE WEEK WITH BROAD SW FLOW ALOFT
AND SOUTHERLY FLOW AT LOWER LEVELS. SEE THE STORM PREDICTION
CENTER OUTLOOKS REGARDING THE POSSIBILITY OF SEVERE WEATHER.
LOCALLY HEAVY RAIN IS POSSIBLE IN THE MID MS VALLEY TO THE
SOUTHERN PLAINS LATE IN THE WEEK INTO NEXT WEEKEND AS GEFS
M-CLIMATE PRECIPITABLE WATER VALUES INCREASE TO THE 95-99
PERCENTILE FROM EASTERN OK AND NORTHEAST TEXAS NORTHEASTWARD
THROUGH MISSOURI TO IL. LIFT OF THIS ABUNDANT MOISTURE IS EXPECTED
WITH UPPER DIVERGENCE MAXIMA IN THE RIGHT ENTRANCE REGION OF THE
UPPER LEVEL JET PLUS SLOW MOVING FRONT TO SUPPORT WIDESPREAD
SHOWERS/STORMS.

MUCH OF THE EASTERN THIRD OF THE CONUS WILL SEE INCREASING
TEMPERATURES LATER THIS WEEK AFTER THE COASTAL LOW DEPARTS AND
BEFORE THE NEXT FRONT SINKS SOUTHWARD.  RECORD WARM TEMPERATURES
ARE A DISTINCT POSSIBILITY FROM THURSDAY ONWARD, STARTING OVER THE
LOWER RIO GRANDE VALLEY OF TEXAS AND THEN EXTENDING EASTWARD
ACROSS THE SOUTHEAST INTO THE MID-ATLANTIC.  BELOW AVERAGE
TEMPERATURES WILL REMAIN ANCHORED NEAR THE UPPER TROUGH AXIS
ACROSS THE CENTRAL ROCKIES AND NORTHERN PLAINS TO THE UPPER MS
VALLEY. THE PACIFIC NORTHWEST SHOULD REMAIN COOLER THAN AVERAGE
THROUGH THE PERIOD DUE TO PERSISTENT CLOUDS AND PERIODS OF RAIN
WHILE MOST OF CALIFORNIA...NEVADA AND ARIZONA ARE DRIER AND WARMER
WITH ABOVE AVERAGE TEMPERATURES DEVELOPING NEXT WEEKEND.

PETERSEN/FRACASSO


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