Prognostic Meteorological Discussion
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000
FXUS02 KWBC 221558
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1158 AM EDT SAT APR 22 2017

VALID 12Z TUE APR 25 2017 - 12Z SAT APR 29 2017

...OVERVIEW...

UPPER PATTERN ACROSS THE LOWER 48 WILL BE DOMINATED BY ON POSITIVE
HEIGHT ANOMALY SW OF CALIFORNIA IN THE PACIFIC OCEAN AND ANOTHER
OVER SE CANADA/WESTERN ATLANTIC. THAT WILL FORCE TROUGHING IN
BETWEEN, GENERALLY ALONG THE SPINE OF THE ROCKIES INTO THE PLAINS
LATER NEXT WEEK. THIS SHOULD KEEP A STORM TRACK OUT OF THE
NORTHEAST PACIFIC, THROUGH THE INTERIOR WEST, AND THEN
NORTHEASTWARD OUT OF THE PLAINS INTO THE GREAT LAKES. IN ADDITION,
A CLOSED LOW OFF THE MID ATLANTIC COAST WILL SLOWLY LIFT
NORTHEASTWARD TOWARD CAPE COD BUT GENERALLY WEAKEN WITH TIME AS IT
ENCOUNTERS INCREASED RIDGING IN THE WESTERN ATLANTIC.


...GUIDANCE EVALUATION......

MODELS CONTINUE TO STRUGGLE WITH THE FLOW ACROSS THE
NORTHERN/NORTHEASTERN PACIFIC, WITH THE RECENT 00Z-06Z GFS
DISPLACED FROM THE LOOSE CLUSTERING OF THE ECMWF/UKMET/CANADIAN
MIDWEEK. ACROSS THE PLAINS, THE MODELS WAVER ON TIMING OF SFC LOW
PROGRESSION FROM THE PLAINS INTO THE GREAT LAKES THROUGH AN
OTHERWISE PREDICTABLE (SEMI-STATIONARY) UPPER FLOW PATTERN. THE
ENSEMBLES MEANS HAVE ALSO SHIFTED WITH THEIR RESPECTIVE PARENT
CORES (GEFS WITH THE GFS AND ECMWF ENSEMBLE MEAN WITH THE HRES
ECMWF) THOUGH THE RECENT 00Z RUNS WERE A BIT MORE IN LINE WITH
EACH OTHER (WHICH DOES NOT NECESSARILY INCREASE FORECASTER
CONFIDENCE ON SPECIFICS). OPTED TO USE A BLEND OF THE LATEST
GUIDANCE WITH OVERNIGHT CONTINUITY (GENERALLY THE GFS/ECMWF AND
THEIR MEANS) UNTIL THURSDAY WHEN THE GFS TAKES A LOW PRESSURE
FARTHER NORTH INTO THE GULF OF ALASKA COMPARED TO THE OTHER MODELS
AND MANY ENSEMBLE MEMBERS (THOUGH NOT MANY GEFS MEMBERS). TO THE
EAST, BATTLE CONTINUES BETWEEN HIGH PRESSURE IN CANADA AND THE
WAVY FRONT ACROSS THE GREAT LAKES. GIVEN THE TIME OF YEAR,
BACKDOOR FRONT INTO/THROUGH NEW ENGLAND AND MAYBE THE MID-ATLANTIC
LOOKS MORE LIKELY WITH THE RECENT ENSEMBLES, DESPITE THE BUILDING
UPPER RIDGING. COLD ATLANTIC WATERS WOULD HELP BRING SHALLOW COOL
AIR SOUTHWARD TO ABOUT 40N BUT THIS REMAINS A BIG VARIABLE IN
PLACEMENT AND STRENGTH (E.G. ENSEMBLE RANGE IN MAX TEMPS NEXT SAT
IN BOSTON IS ABOUT 40 DEG F -- LOW 40S TO LOW 80S). COURSE OF
LEAST REGRET SAYS STAY IN THE MIDDLE UNTIL A CLEARER SIGNAL
EMERGES.


...WEATHER HIGHLIGHTS...

ACROSS THE PACIFIC NORTHWEST INTO THE INTERIOR WEST, MODERATE TO
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 NEAR THE CA/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 NEXT WEEK AS THE
UPPER TROUGH DIGS IN.

PRECIPITATION CHANCES WILL ABATE AS THE COASTAL LOW WEAKENS AND
MOVES AWAY FROM THE MID-ATLANTIC AND NEW ENGLAND COAST. ANOTHER
ROUND OF SHOWERS IS POSSIBLE AS THE FRONT FROM THE WEST MOVES INTO
THE NORTHEAST LATE IN THE WEEK.

RAINFALL BECOMES WIDESPREAD ACROSS THE PLAINS TO THE MS VALLEY IN
THE MIDDLE TO LATTER PORTION OF NEXT WEEK WITH BROAD SW FLOW ALOFT
AND SOUTHERLY FLOW AT LOWER LEVELS. THE STORM PREDICTION CENTER
HAS HIGHLIGHTED THE SOUTHERN PLAINS FOR THE POSSIBILITY OF SEVERE
WEATHER NEXT THU/FRI. LOCALLY HEAVY RAIN IS POSSIBLE IN THE MID MS
VALLEY TO THE SOUTHERN PLAINS LATE NEXT WEEK -- GEFS M-CLIMATE
PERCENTILE VALUES INCREASE TO OVER 99 PERCENT FROM EASTERN OK
NORTHEASTWARD THROUGH MISSOURI TO EASTERN IA AND WESTERN IL.

TEMPERATURES OF 10 TO 15 DEGREES ABOVE AVERAGE ARE EXPECTED TO
SPREAD ACROSS THE CENTRAL AND SOUTHERN PLAINS AHEAD OF THE LEAD
FRONT. MUCH OF THE EASTERN THIRD OF THE CONUS WILL SEE INCREASING
TEMPERATURES LATER NEXT 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 SOUTHERN CALIFORNIA INTO ARIZONA SHOULD ESCAPE MOST OF THE
TROUGHING AND MAINTAIN NEAR TO ABOVE AVERAGE TEMPERATURES.


FRACASSO/PETERSEN


$$





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