Preliminary Forecasts
Issued by NWS

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349
FXUS02 KWNH 070645
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 AM EST WED DEC 07 2016

VALID 12Z SAT DEC 10 2016 - 12Z WED DEC 14 2016

...OVERVIEW AND PREFERENCES...

FOR DAYS 3/4 SAT 10 DEC-SUN 11 DEC THE MANUAL PRODUCT SUITE USED A
BLEND OF THE 18 UTC GFS/GEFS ENSEMBLE MEAN AND 12 UTC ECMWF/ECMWF
ENSEMBLE MEAN/12Z CANADIAN ENSEMBLE MEAN. MORE WEIGHTING IS GIVEN
TO THE ENSEMBLE MEANS ON DAYS 6 AND 7 WHERE THE 12Z ECMWF/18Z GFS
MODELS HAVE DIFFERENT SOLUTIONS...AND RUN TO RUN CHANGES THAT
INCREASE THE UNCERTAINTY.

ON DAYS 3-4 AGREEMENT IS GOOD THAT ONE AREA OF LOW PRESSURE
REMAINS JUST OFF THE PACIFIC NORTHWEST COAST WITH A STATIONARY
FRONT EXTENDING INLAND ACROSS THE NORTHERN ROCKIES TO A FRONTAL
SYSTEM DEVELOPING ON THE IN CENTRAL PLAINS THAT MOVES NORTHEAST
TOWARDS THE UPPER MS VALLEY AND UPPER GREAT LAKES SUN 11 DEC.

THE DIFFERENCES START TO GROW SUN INTO MON AS THE LOW MOVES INTO
SOUTHEAST CANADA WITH A TRAILING COLD FRONT MOVING ACROSS THE
NORTHEAST TO THE OH VALLEY AND APPALACHIANS.
THE 12Z ECMWF HAS A MORE AMPLIFIED 500 MB WAVE THAN MOST ECMWF
ENSEMBLE MEMBERS WITH THE MEAN INDICATING A LOWER AMPLITUDE/LONGER
WAVELENGTH SYSTEM AND THUS A LOW PROBABILITY OF A SECOND CYCLONE
DEVELOPING ON THE FRONT.

FOLLOWING THIS SYSTEM WILL BE A SURGE OF COLDER AIR ACROSS THE
LAKES TO THE NORTHEAST...FOLLOWED BY ANOTHER COLDER SURGE SOUTH OF
SOUTHWEST CANADA INTO THE NORTHERN PLAINS.  THE COLD FRONT AT THE
LEADING EDGE OF THE PLAINS ARCTIC SURGE HAS A FEW WAVES OF LOW
PRESSURE POSSIBLE ALONG THE FRONT WITH THE 18Z GFS DEVELOPING A
WAVE THAT ON 12Z TUE 13 DEC HAS SURFACE LOW PRESSURE NEAR THE
MI/IN BORDER.  THE 00Z GFS NO LONGER HAS THE WAVE
DEVELOPING...WHILE THE ECMWF/CANADIAN GLOBAL LOW PRESSURE IS JUST
NORTH OF LK SUPERIOR. THE ENSEMBLE MEANS LIKE THE LOCATION OF THE
FURTHER NORTH LOCATION OF LOW PRESSURE ON 12Z TUE SO THE MAJORITY
CLUSTER WAS USED.

THIS SYSTEM IS FORECAST TO DEVELOP AS IT MOVES ACROSS SOUTHEAST
CANADA...FOLLOWED BY THE SURGE OF ARCTIC AIR MOVING EAST FROM THE
NORTHERN PLAINS ACROSS THE UPPER MS VALLEY AND GREAT LAKES.  THE
12Z ECMWF DEVELOPED A WAVE ON THE ARCTIC FRONT SUN NIGHT INTO
EARLY MON IN THE OH VALLEY...TRACKING LOW ACROSS THE NORTHEAST.
THIS A DISTINCT MINORITY AMONG THE ECMWF ENSEMBLES...SO MORE
WEIGHTING WAS GIVEN TO THE ENSEMBLE MEAN.

ADDITIONAL SPREAD DEVELOPS OVER THE EASTERN PACIFIC AND ADJACENT
POSITIONS OF THE PACIFIC NORTHWEST AND NORTHERN CA AS THE
ECMWF/GFS/CANADIAN GLOBAL ALL SHOW LOW PRESSURE DEVELOPING ON A
WAVY SURFACE FRONT WITH EACH MODEL INDICATING A DIFFERING CYCLONE
TRACK/SPEED/INTENSITY.  THE STRONGER 12Z ECMWF SOLUTION WITH A
CYCLONE OFF THE OR/WA BORDER 00Z TUE 13 DEC AND THEN OFF THE WA
COAST 12Z TUE 13 DEC LEADS TO STRONG ONSHORE FLOW AND A POTENTIAL
HEAVY PRECIP EVENT IN SOUTHWEST OR/NORTHWEST CA.   THE ECMWF
ENSEMBLE MEMBERS ARE SPREAD WIDE WITH THE LACK OF A DEFINED
CLUSTER OF SOLUTIONS. THE WIDE SPREAD LEADS TO LOW CONFIDENCE IN
ANY PARTICULAR SOLN...AND GREATER WEIGHTING ON THE ENSEMBLE MEAN
FORECASTS.

...WEATHER HIGHLIGHTS/THREATS...

OVER THE WEST THE LARGE SCALE PATTERN WILL LIKELY PROMOTE
PERSISTENT VALLEY RAIN AND INLAND/HIGHER ELEV SNOW ACROSS THE
PACIFIC NORTHWEST/NRN CA/NORTHERN GREAT BASIN/NORTHERN ROCKIES
WITH A PRONOUNCED COASTAL/TERRAIN FOCUS FOR HIGHEST AMTS. THE
POTENTIAL EXISTS FOR ONE OR MORE EPISODES OF LOCALLY HVY
PCPN...MOST LIKELY OVER NORTHWEST CALIFORNIA AND/OR WRN OREGON.
ABOVE NORMAL TEMPS PERSISTS IN THE DESERT SOUTHWEST THROUGH THE
WEEKEND AND EARLY NEXT WEEK BUT BECOME BELOW NORMAL IN THE CENTRAL
TO NORTHERN GREAT BASIN/CENTRAL TO NORTHERN ROCKIES/PACIFIC
NORTHWEST.

WITH THE FORECAST ARCTIC SURGE COMING OUT OF SOUTHWEST CANADA INTO
THE NORTHERN PLAINS...AN IMPRESSIVE AREA OF TEMPERATURES 20-30
DEGREES BELOW NORMAL DEVELOPS IN THE NORTHERN HIGH PLAINS AND
SPREADS EAST...WITH HIGH TEMPERATURES POSSIBLY STAYING A FEW
DEGREES BELOW ZERO F IN NORTH DAKOTA DAYS 5 THROUGH 7. EXPECT
TEMPS TO BE 10-20F BELOW NORMAL OVER MUCH OF THE UPPER MS VALLEY
DAY 5 SPREADING INTO THE GREAT LAKES AND OH VALLEY/MID MS VALLEY
AND CENTRAL PLAINS DAYS 6-7.

THE SYSTEM SAT TRAVERSING FROM THE CENTRAL PLAINS TO UPPER MS
VALLEY AND UPPER LAKES HAS POTENTIAL TO PRODUCE SEVERAL INCHES OF
SNOW ALONG THE STORM TRACK...WITH LIGHTER AMOUNTS IN THE
DOWNSTREAM WARM ADVECTION ACROSS THE LOWER LAKES AND NORTHEAST.
HEAVY SNOWS ARE EXPECTED AGAIN IN THE WA CASCADES OVER THE WEEKEND
WITH LIGHT TO MODERATE SNOW IN RANGES FURTHER INLAND IN
ID/WY/CO/NORTHERN UT THAT CONTINUE INTO THE EARLY AND MIDDLE PARTS
OF NEXT WEEK.

PETERSEN

$$





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