Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 240658
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST WED DEC 24 2014

VALID 12Z SAT DEC 27 2014 - 12Z WED DEC 31 2014

...PATTERN/THREATS OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT...

THE UPCOMING NORTH AMERICAN MEDIUM-RANGE FLOW PATTERN REVEALS
AMPLIFIED NRN AND SRN STREAMS THAT INCREASINGLY PHASE WHILE
BECOMING MORE NRN STREAM DOMINANT AND COLD OVER TIME FOR MUCH OF
THE NATION.

A COMMON THREAD TO MOST GUIDANCE IS THE BUILDING OF A HIGHLY
AMPLIFIED MID-UPPER LEVEL RIDGE UP THROUGH THE ERN PACIFIC INTO
ALASKA AND NWRN CANADA. THIS SHOULD INCREASINGLY FAVOR OVER TIME
ANY MODEL AND ENSEMBLE SOLUTIONS THAT OFFER IMPULSES ALOFT AND
LOWER AMTOSPHERIC COLD AIR DIGGING DOWNSTREAM TO THE LEE OF THIS
RIDGE ON THE MORE AMPLIFIED SIDE OF THE FULL ENVELOPE OF ALL
SOLUTIONS. RUN TO RUN CONTINUITY FROM THE LATEST ROUND OF
DETERMINISTIC MODELS HAS AGAIN BECOME WELL LESS THAN
STELLAR...LENDING THE WPC ENSEMBLE MEAN BASED FORECAST APPROACH
THAT PRIMARILY FEATURES THE 12 UTC ECMWF ENSEMBLE MEAN BUT MIXES
IN SOME 12 UTC NAEFS AND WPC CONTINUITY TO HEDGE LINGERING ABOVE
NORMAL FORECAST SPREAD AND UNCERTAINTY.

IN THIS SCENARIO AN INITIALLY SEPARATED SRN STREAM AMPLIFIED
MID-UPPER LEVEL TROUGH ALOFT OVER THE SWRN US SAT SHEARS NEWD
ACROSS THE CENTRAL AND ERN US SUN/MON CHANNELING BETWEEN A SERN US
MEAN RIDGE AND EMERGING NRN STREAM TROUGHING SAGGING DOWN FROM
S-CENTRAL CANADA INTO THE N-CENTRAL US. THIS WOULD FAVOR AN
ASSOCIATED LEAD LOW/FRONTAL SYSTEM AND PCPN SWATH WITH INITIALLY
MODEST COOLING IN THE WAKE OF SYSTEM PASSAGE. THE TRACK OF THE LOW
IS QUITE VARIED IN GUIDANCE RANGING BY 12 UTC MON FROM THE GREAT
LAKES TO THE SERN US...BUT WPC PROGS THAT TRACK THE LOW FROM THE
LOWER MS VALLEY TO THE MID-ATLANTIC SUN/MON OFFER TEMPS COLD
ENOUGH FOR A POTENTIAL BAND OF SNOW ON THE NW PERIPHERY OF THE
OVERALL PCPN SHIELD FROM VICINITY OF THE MID-MS VALLEY NEWD TO THE
OH VALLEY STATES AND NRN MID-ATLANTIC/NORTHEAST. LOW/FRONTAL
POOLED AND WARM SECTOR MOISTURE SEEMS DEEP ENOUGH TO FUEL SOME
HEAVIER CONVECTIVE RAINS UNDERNEATH SHEARING NEWD FROM THE LOWER
MS/TN VALLEYS AND GULF COAST TO THE S-CENTRAL APPALACHIANS/SERN US
AND MID-ATLANTIC.

DEEPER COOLING AND DRYING SHOULD THEN OCCUR FROM THIS WEEKEND INTO
NEW YEARS AS A BLAST OF HIGH LATITUDE AIR IS SLATED TO DIG DOWN
OVER MUCH OF THE LOWER 48. HOWEVER...STILL EXPECT THAT DIGGING
EMBEDDED SHORTWAVES SHOULD FAVOR PERIODIC DYNAMICALLY ENHANCED
SNOWS PRIMARILY OVER FAVORED TERRAIN FROM THE NWRN US SEWD TO THE
ROCKIES...WITH LESS CLEAR FOCUS AS THESE IMPULSES EJECT OVERTOP
THE DOWNSTREAM CENTRAL AND ERN US COLD DOME OTHER THAN TO THE LEE
OF THE GREAT LAKES.

SCHICHTEL

$$




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