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Prognostic Meteorological Discussion
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FXUS02 KWBC 191533
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1132 AM EDT SUN MAY 19 2013

VALID 12Z WED MAY 22 2013 - 12Z SUN MAY 26 2013

...OVERVIEW AND MODEL PREFERENCES...

AN AMPLIFIED FLOW PATTERN IS FCST TO PERSIST THIS WEEK INTO NEXT
WEEKEND WITH TROUGHS ALONG BOTH WEST/EAST COASTS AND A RIDGE OVER
THE CENTRAL STATES AND A CLOSED ANTICYCLONE OVER NORTHERN MEXICO.
DEEPER LOW IN THE PAC NW IS FORECAST TO YIELD 500MB HEIGHTS ABOUT
1 TO 2 STD DEVIATIONS BELOW NORMAL BUT ABOUT 1/2 STD DEVIATION
BELOW NORMAL IN THE EAST. RIDGING ALONG 100W BY LATE IN THE WEEK
SHOULD AVERAGE AROUND 2 STD DEVIATIONS ABOVE NORMAL ALL THE WAY
FROM THE RIO GRANDE TO THE CANADIAN ARCHIPELAGO AROUND 75N.

WITH ROBUST MERIDIONAL FLOW WEST OF THE PLAINS AND THE STRONG
RIDGE TO THE EAST... THE CLOSED LOW OVER THE PAC NW HAS LITTLE
CHANCE OF PUSHING EAST PAST THE DIVIDE UNTIL LATE IN THE PERIOD.
BETTER THAN AVERAGE AGREEMENT IS SEEN AMONG THE GUIDANCE THROUGH
FRI/D5 BEFORE SMALLER DETAILS AMONG THE MODELS AMPLIFY. THE
QUESTIONS BECOME... 1... HOW DEEP WILL THE ENERGY BY THAT ROTATES
THROUGH THE BASE OF THE TROUGH IN NORCAL AND 2... HOW QUICKLY DOES
THE UPPER LOW OR A HEALTHY CHUNK OF ITS ENERGY TRY TO PUSH
EASTWARD BY SUN/D7. OPTED TO SPLIT THE DIFFERENCE ON BOTH COUNTS
AMONG THE SOLUTIONS GIVEN THE UNCERTAINTY IN THE STRENGTH OF
UPSTREAM ENERGY AND THE START OF BREAKING DOWN THE RIDGING ALONG
THE US/CANADIAN BORDER. DISCOUNTED THE QUICKER GFS RUNS ON ACCOUNT
OF THE STRONG RIDGE TO THE EAST IN FAVOR OF THE 00Z ECMWF AND BOTH
GEFS/ECENS MEANS.

IN THE EAST... THE MODELS/ENSEMBLES HAVE TRENDED A BIT QUICKER
WITH THE WEAKENING MIDWEST CLOSED LOW SHEARING OUT AS IT MOVES
FROM THE UPPER MS VALLEY INTO THE GREAT LAKES AND THEN NORTHEAST.
ECMWF HAS BEEN QUITE CONSISTENT IN ROTATING ENERGY THROUGH QUEBEC
NEAR NORTHERN NYS/NEW ENGLAND WHILE THE GFS AND GEFS MEMBERS HAVE
BEEN SLOWLY TRENDING THAT WAY. ECENS MEAN HAS LOWERED HEIGHTS OVER
ITS PAST FOUR RUNS SO PUT MORE WEIGHT IN ITS FORECAST WHICH
RESULTED IN A BIT COOLER FORECAST THAN THE GFS AND MOS IN THE
NORTHEAST/MID-ATLANTIC SAT/D6 AS THE TROUGH SWINGS THROUGH.
DOWNPLAYED THE GFS AND OTHER MODELS THAT DEVELOP A SFC LOW OFF THE
FL/SOUTHEAST COAST AS PERHAPS ONLY 20 PERCENT OF THE ENSEMBLES
SHOW A LOW AT ALL.

BY NEXT WEEKEND... USED A INCREASING BLEND OF THE 06Z GEFS MEAN
AND 00Z ECENS MEAN WITH LESS AND LESS WEIGHT FROM THE 00Z ECMWF
WHICH APPEARED TOO QUICK TO PUSH THE FRONT OFF THE EAST COAST.


...SENSIBLE WEATHER HIGHLIGHTS...

BELOW NORMAL TEMPERATURES ARE EXPECTED INVOF THE SLOW-MOVING
UPPER-LEVEL TROUGH AND CUTOFF LOW IN THE PACIFIC NORTHWEST TO
NORTHERN ROCKIES. UPPER JET MAXIMA AND ASSOCIATED UPPER DIVERGENCE
MAXIMA SUPPORT WIDESPREAD SHOWERS ACROSS NORTHERN ID/NORTHWEST MT
AND POTENTIAL FOR LOCALLY HEAVY RAIN. LIKEWISE WIDESPREAD MODERATE
TO LOCALLY HEAVY RAINS ARE FCST IN ADVANCE OF THE UPPER TROUGH
ACROSS THE GREAT LAKES TO NEW ENGLAND AS ENHANCED 850-700 MB
MOISTURE FLUXES OCCUR AND OVERLAP WITH UPPER DIVERGENCE MAXIMA IN
THE RIGHT ENTRANCE REGION TO THE 300 MB JET. SFC FRONT MAY ATTEMPT
TO BRING COOLER/DRIER AIR INTO THE DEEP SOUTH BUT SHOULD LOSE A
BIT OF ITS PUNCH AS ITS UPPER SUPPORT LIFTS THROUGH CANADA.

THE SOUTHERN ROCKIES SHOULD BE FAR ENOUGH AWAY FROM THE UPPER LOW
TO ALLOW NEAR TO ABOVE NORMAL TEMPS AND LITTLE IF ANY PRECIP. THE
ABOVE NORMAL TEMPS SHOULD EXPAND INTO THE WESTERN AND SOUTHERN
PLAINS AS AREAS OF 700 MB TEMPS 1-2 STD DEVIATIONS ABV NORMAL
APPEAR NEXT WEEKEND PER THE ENSEMBLES. SOUTHERLY FLOW AHEAD OF THE
TROUGH SHOULD FOCUS PRECIP OVER THE CENTRAL PLAINS WHERE
CONVECTION IS POSSIBLE PER SOME MODELS... BUT THIS IS CONTINGENT
ON HOW MUCH RIDGING REMAINS IN PLACE.


FRACASSO/PETERSEN

$$
National Weather Service Weather Forecast Office
