Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4
467
FXUS02 KWBC 111538
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1038 AM EST MON DEC 11 2017

VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017

...OVERVIEW...

UPPER RIDGING IN THE WEST WILL FINALLY BE KNOCKED DOWN LATE THIS
WEEK BUT BROAD CYCLONIC FLOW WILL STILL DOMINATE THE EAST. THIS
WILL GUIDE A SERIES OF MAINLY PACIFIC SYSTEMS ACROSS THE LOWER 48
AS THE ARCTIC BOUNDARY WOBBLES NEAR THE US/CANADIAN BORDER. A SLOW
MODERATING TREND WILL UNFOLD ACROSS THE EAST THROUGH THE WEEKEND
WHILE THE WEST WILL SEE NEAR TO ABOVE AVERAGE TEMPERATURES.
PRECIPITATION WILL FINALLY COME BACK TO THE PACIFIC NORTHWEST BY
FRIDAY BUT LIMITED TO WA/OR.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

LATEST 00Z GUIDANCE SUITE OFFERED A REASONABLE STARTING POINT TO
THE FORECAST FOR THU-FRI (BLEND OF THE GFS/ECMWF/UKMET/CANADIAN).
00Z ECMWF/CANADIAN WERE IN THE MIDDLE OF THE SPREAD ACROSS THE
LOWER MS VALLEY AS A POSITIVELY TILTED TROUGH SWINGS THROUGH THE
REGION (GFS QUICKER, UKMET SLOWER) WHICH FIT BEST WITH THE
ENSEMBLE MEANS. TO THE NORTHWEST THERE WAS MUCH MORE DISAGREEMENT
ON TIMING OF THE SHORTWAVE INTO WESTERN WASHINGTON FRIDAY THOUGH
THE ECMWF AND ITS ENSEMBLES HAVE BEEN MORE CONSISTENT THAN THE
GFS/GEFS. FOR NEXT SUN/MON, UPPER TROUGH WILL PROGRESS EASTWARD
OUT OF THE CENTRAL PLAINS AND CARRY THAT FRONT TO THE EAST COAST
WHILE ANOTHER FLOWS THROUGH THE NORTHWEST INTO THE HIGH PLAINS.
AGAIN, TIMING DIFFERENCES EXPAND WITH TIME PER THE DETERMINISTIC
MODELS AS THEY SPLIT THE TROUGH AND EITHER FOCUS ON THE NORTHERN
OR SOUTHERN PORTION. A CONSENSUS BLEND SERVED TO MINIMIZE CHANGES
FROM OVERNIGHT. THIS SUGGESTED LEAVING ANOTHER SOUTHERN POTENTIAL
VORTICITY TAIL JUST WEST OF BAJA CALIFORNIA AS HEIGHTS BUILD
THROUGH THE WESTERN CARIBBEAN.

FRACASSO


...WEATHER HIGHLIGHTS/THREATS...

THERE WILL ONLY BE A BRIEF PERIOD OF BELOW AVERAGE TEMPERATURES
AND THIS APPEARS TO BE CONFINED TO LOCATIONS EAST OF THE MS RIVER
ON THURSDAY/FRIDAY. THE FINAL ARCTIC SURGE SHOULD BRING A
REINFORCEMENT OF COLD AIR TO THE REGION WITH THE MOST ANOMALOUS
TEMPERATURES BEING OVER NEW ENGLAND WITH ANOMALIES IN THE 10 TO 20
DEGREE BELOW CLIMATOLOGY RANGE. THIS SUGGESTS HIGHS ON THURSDAY
SHOULD STRUGGLE TO GET OUT OF THE UPPER SINGLE DIGITS AND TEENS
OVER INTERIOR NEW ENGLAND. OTHERWISE...MUCH OF THE COUNTRY SHOULD
SEE ABOVE AVERAGE TEMPERATURES AS THE FLOW FLATTENS AND BECOMES
MORE PACIFIC DOMINANT. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES
WILL RAISE TEMPERATURES INTO THE 50S AS FAR NORTH AS EASTERN MT ON
FRIDAY. THIS IS EASILY 15 TO 20 DEGREES ABOVE AVERAGE.

LOOKING AT PRECIPITATION DURING THE PERIOD...SOME LAKE EFFECT
ACTIVITY IS POSSIBLE ALTHOUGH IT SHOULD BE MORE TAME GIVEN A
CHANGING SYNOPTIC-SCALE PATTERN. WHILE PRECIPITATION CHANCES WILL
LOOM THROUGHOUT THE PERIOD...IT SHOULD BE WARM ADVECTION DRIVEN
VERSUS MESOSCALE BANDING RELATED. OUT WEST...CONDITIONS WILL BE
MUCH MORE ACTIVE THAN THEY HAVE BEEN THIS MONTH GIVEN THE
FLATTENING OF THE RIDGE. WIDESPREAD RAIN/SNOW SHOULD BLANKET THE
PACIFIC NORTHWEST AND LOCATIONS DOWNSTREAM INTO THE CENTRAL GREAT
BASIN/UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. GIVEN A LACK OF
COLD AIR...SNOWFALL SHOULD BE RELEGATED TO THE HIGHER ELEVATIONS.
THIS THREAT COULD CONTINUE THROUGH DAY 7/DECEMBER 18 GIVEN THE
SEMI-ZONAL FLOW IN PLACE. ELSEWHERE...WARM ADVECTION SHOWERS ARE
POSSIBLE FROM THE WESTERN GULF COAST TOWARD THE TN
VALLEY/SOUTHEASTERN U.S. THIS WEEKEND ALTHOUGH MODEL/ENSEMBLE
SPREAD OF QPF IS HIGH.


RUBIN-OSTER

$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.