Preliminary Forecasts
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FXUS02 KWNH 220555
PREEPD

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

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

...OVERVIEW AND MODEL EVALUATION......

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. THAT WILL FORCE TROUGHING IN BETWEEN, GENERALLY
ALONG THE SPINE OF THE ROCKIES. 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.

THE MODELS/ENSEMBLES CLUSTER WELL TOGETHER FOR TUE-THU OF NEXT
WEEK.
THE CHANGE FROM 24 HOURS AGO WAS FOR THE MODEL/ENSEMBLE MAJORITY
TO ALLOW THE NORTHERN STREAM TROUGH ACROSS THE GREAT LAKES TO BE
MORE PROGRESSIVE...SLOWING COLDER AIR TO SPILL FORM CANADA THROUGH
THE THE NORTHERN PLAINS/UPPER MS VALLEY AND THEN GREAT LAKES.

DIFFERENCES DEVELOP STARTING THU WITH THE TIMING OF LOWS ALONG THE
NORTHERN STREAM WAVES OVER THE NORTHEAST PACIFIC APPROACHING
BRITISH COLUMBIA AND ALSO TIMING AND INTENSITY OF THE WAVES
TRAVERSING FROM THE SOUTHERN PLAINS TOWARDS THE GREAT LAKES.

THE 12Z CANADIAN FIT A BIT BETTER WITH THE ECMWF AND GEFS
MEAN/ECMWF ENSEMBLE MEAN THU 27 AND FRI 28 APR. A BLEND OF THE 18Z
GFS/GEFS AND 12Z GFS/ECMWF/ECENS MEAN WAS A GOOD STARTING POINT
THROUGH THURSDAY 27 APR. THEREAFTER, THE GFS RUNS GO ASTRAY FROM
THE OTHER SOLUTIONS SO ARE CONSIDERED LOW PROBABILITY.  THE 12Z
CANADIAN WAS BLENDED WITH THE 12Z ECMWF/ECMWF ENSEMBLE MEAN AND
18Z GEFS MEAN...AS THE CANADIAN GLOBAL CLUSTERED WELL WITH THE
ECMWF MEAN LOW POSITION NEXT FRI-SAT IN THE UPPER MS VALLEY AND
GREAT LAKES.  ON SAT 29 APR..T.HE ECMWF AND CANADIAN AND GFS ALL
HAVE ANOTHER SOUTHERN PLAINS WAVE MOVE NORTHEAST...BUT EACH HAS A
DIFFERENT SOLUTION.  WITH THE MEANS AGREEING BETTER...MORE
WEIGHTING WAS GIVEN TOWARDS THE ECMWF AND GEFS MEANS.  THE MEANS
MAY NOT BE THE PERFECT SOLUTION EITHER IF THE OPERATIONAL RUNS ARE
CORRECT IN DEPICTING A STRONGER CYCLONE TO DEVELOP WITHIN THE
FRONTAL ZONE. CONSEQUENTLY CONFIDENCE FOR THE CENTRAL US CYCLONE
POSITION AND INTENSITY NEXT SAT 29 APR IS BELOW AVERAGE.

EAST OF THE CYCLONE TRACK AND ASSOCIATED FRONT...HIGH PRESSURE UP
AND DOWN THE EASTERN SEABOARD NEXT FRI AND SAT LEADS TO TRANQUIL
WEATHER.

...WEATHER HIGHLIGHTS...

ACROSS THE PACIFIC NORTHWEST AND NORTHERN CA, 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. PRECIPITATION
CHANCES WILL BE HIGH NEAR THE COASTAL LOW OVER OR NEAR THE
CAROLINAS AND ADJACENT COASTAL WATERS MONDAY THEN EXTENDING ACROSS
THE MID-ATLANTIC STATES INTO SOUTHERN NEW ENGLAND MIDWEEK.

WITH THE DEEP LAYER TROUGH ACROSS THE WEST, SNOW IS EXPECTED
ACROSS THE RANGES OF WA/OR AND FURTHER INLAND ACROSS THE MOUNTAINS
OF ID/WY/CO. THERE IS POTENTIAL FOR LOCALLY HEAVY AMOUNTS IN THE
MIDDLE TO LATTER PART OF NEXT WEEK AS THE UPPER TROUGH DIGS IN.
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. SEE THE THE STORM PREDICTION
CENTER SEVERE WEATHER OUTLOOKS.  IN ADDITION TO SEVERE
WEATHER...LOCALLY HEAVY RAIN IS POSSIBLE IN THE MID MS VALLEY TO
THE SOUTHERN PLAINS.

TEMPERATURES OF 10 TO 15 DEGREES ABOVE AVERAGE ARE EXPECTED TO
SPREAD ACROSS  INTO 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. RECORD WARM TEMPERATURES ARE A DISTINCT POSSIBILITY NEXT
WEEKEND...STARTING OVER THE LOWER RIO GRANDE VALLEY OF
TEXAS...WHERE THE FIRST 100 DEGREE HIGHS OF THE SEASON ARE
FORECAST.  THE POTENTIAL FOR RECORD HIGHS THEN EXTENDS ACROSS THE
SOUTHEAST....TN VALLEY...AND UPPER OH VALLEY AND CENTRAL
APPALACHIANS.   BELOW AVERAGE TEMPERATURES WILL DEVELOP 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.

PETERSEN


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