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Extended Forecast Discussion
 
(Latest Discussion - Issued 0650Z Dec 08, 2016)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
149 AM EST THU DEC 08 2016

VALID 12Z SUN DEC 11 2016 - 12Z THU DEC 15 2016

...ARCTIC AIR TO SURGE INTO THE LOWER 48 NEXT WEEK...


...OVERVIEW...

A MIDDLE/UPPER-TROPOSPHERIC CLOSED LOW IS FORECAST TO DROP
SOUTHWARD FROM NUNAVUT THIS WEEKEND AND THEN TURN EASTWARD THROUGH
SOUTHERN CANADA DURING THE MIDDLE OF THE WEEK. A SHORTWAVE
INITIALLY IN THE NORTHERN PACIFIC WILL PASS THROUGH THE INTERIOR
PACIFIC NORTHWEST AND ROCKIES SUNDAY BEFORE AMPLIFYING IN THE
PLAINS AHEAD OF THE CANADIAN UPPER LOW. A BROAD CYCLONIC
CIRCULATION WILL ENCOMPASS THE EASTERN HALF OF THE LOWER 48 BY
NEXT WED/THU WHILE ATTEMPTS TO PUSH INTO THE WESTERN STATES.

...GUIDANCE PREFERENCES...

THE LARGER/SYNOPTIC SCALE FLOW SHOWS GOOD AGREEMENT OUT TO ABOUT
NEXT WEDNESDAY AMONG THE ENSEMBLES. THE BIGGER QUESTION LIES IN
HOW THE PACIFIC SHORTWAVE ZIPS THROUGH THE FLOW ACROSS THE CONUS
NEXT SUN-TUE AHEAD OF THE COLD SURGE FROM THE NORTH. THE MODELS
HAVE NOT BEEN VERY CONSISTENT AND THE ENSEMBLE SIGNAL HAS TRENDED
TOWARD A SLOWER AND MORE AMPLIFIED SOLUTION IN THE ECMWF ENSEMBLES
BUT NOT SO IN THE GEFS ENSEMBLES. GIVEN THAT THE GFS WAS QUICKER
THAN THE ECMWF/UKMET/CANADIAN, OPTED TO SLOW DOWN THE PROGRESSION
OF THE FRONT THROUGH THE CENTRAL STATES SUN-MON AS THE SFC LOW
HEADS TOWARD THE GREAT LAKES. BY TUESDAY THE FRONT IS OFF THE
COAST AND THE ARCTIC SURGE WILL FOLLOW FROM NW TO SE THEREAFTER.
ANOTHER QUESTION LINGERS BY NEXT WEDNESDAY -- CAN THE ARCTIC SURGE
SPIN UP ANOTHER AREA OF LOW PRESSURE ALONG THE FRONT TO SLOW ITS
PROGRESSION EASTWARD NEXT TUE/WED? THOUGH POSSIBLE, DID NOT DEPICT
SUCH A FEATURE AT THIS POINT.

BACK TO THE WEST, A LARGE SPREAD IN ENSEMBLE TEMPERATURES EXPANDS
AROUND THE DIVIDE AS THE COLD AIR ATTEMPTS TO BLEED THROUGH TO THE
WEST AND SOUTH. SECOND TO THAT, BY NEXT THURSDAY THE MODELS
DIVERGE ON HOW QUICKLY TO LIFT THE ARCTIC DRAPE BACK TO THE NORTH
AS A PACIFIC SYSTEM APPROACHES THE AREA. LOTS OF ENSEMBLE SPREAD
LEADS TO LOWER CONFIDENCE, BUT USUALLY THE COLD AIR IS SLOWER TO
ERODE/RETREAT THAN FORECAST.


...WEATHER HIGHLIGHTS/THREATS...

PERSISTENT WEST TO EAST MID-LEVEL FLOW WILL KEEP THE WESTERN
REGION ACTIVE, ESPECIALLY OVER NORTHERN CALIFORNIA NORTHWARD ALONG
THE COASTAL RANGE AND CASCADES. ADDITIONAL UPLIFT VIA THE ARCTIC
BOUNDARY MOVING OUT OF MONTANA WILL PROVIDE ANOTHER MECHANISM FOR
SNOWFALL OVER THE BITTERROOTS AND SAWTOOTH RANGE. GENERALLY ABOVE
AVERAGE TEMPERATURES WILL CONTINUE FOR THE SOUTHWESTERN FIFTH OF
THE CONUS (CA/NV/UT/AZ AND WESTERN CO/NM).

WITH THE FORECAST ARCTIC SURGE COMING OUT OF SOUTHWEST CANADA INTO
THE NORTHERN PLAINS, AN IMPRESSIVE AREA OF WELL BELOW AVERAGE
TEMPERATURES WILL RUSH SOUTHWARD AND EASTWARD FROM THE NORTHERN
HIGH PLAINS THROUGH THE MIDWEST AND EVENTUALLY TO THE EAST COAST
BY WED/THU NEXT WEEK. TEMPERATURES MAY BE 20-30 DEGREES BELOW
AVERAGE AND MAY FLIRT WITH SOME RECORDS (PERHAPS RECORD LOW
MAXIMA) THOUGH THE LONGER PERIOD STATIONS GENERALLY HAVE SOME VERY
COLD RECORDS BY NOW (MID-DECEMBER).

NW FLOW AND COLD AIR OVER THE GREAT LAKES SHOULD SUPPORT A
MULTI-DAY LAKE EFFECT EVENT FROM MICHIGAN EASTWARD THROUGH NE
OHIO/NW PA INTO WESTERN/CENTRAL NY INCLUDING THE TUG HILL PLATEAU.
TRACK OF THE EXITING SFC LOW AND INCOMING SFC HIGH WILL DETERMINE
WHO SEES THE MOST SNOW, BUT SETUP LOOKS GOOD PROVIDED ENOUGH
MOISTURE CAN BE WRUNG OUT IN THE OVERALL DRY/COLD AIRMASS. GLOBAL
MODELS/ENSEMBLES GENERALLY HAVE A GREEN LIGHT FOR AT LEAST
LIGHT-MODERATE SNOW NEXT TUE-THU (AND PERHAPS BEYOND).


FRACASSO