Prognostic Meteorological Discussion
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000
FXUS02 KWBC 030642
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 AM EST SAT DEC 03 2016

VALID 12Z TUE DEC 06 2016 - 12Z SAT DEC 10 2016

...OVERVIEW...

DEEP/COLD UPR TROUGHING WILL PROGRESS FROM WEST TO EAST ACROSS THE
LOWER 48 DURING THE PERIOD.  GUIDANCE AGREES ON THE LARGE SCALE
EVOLUTION BUT WITH SIGNIFICANT DISPARITY IN EMBEDDED
DETAILS/TIMING OF THE OVERALL TROUGH AFTER WED.  WHILE THIS TROUGH
REACHES THE EAST LATE IN THE WEEK... UPSTREAM FLOW OVER THE
MID-LATITUDE PACIFIC AND EXTENDING INTO THE U.S. WILL TREND
FLATTER.  THIS WILL LIKELY LEAD TO LOWER PREDICTABILITY FOR SHRTWV
DETAILS BY LATE IN THE PERIOD.  WELL AHEAD OF THE UPR TROUGH THE
GUIDANCE IS CONTINUING TO MAKE SOME ADJUSTMENTS FOR THE SHRTWV
ENERGY ORIGINALLY OVER NRN MEXICO AND CROSSING THE EAST ON TUE.


...MODEL EVALUATION/PREFERENCES...

DAYS 3-4 TUE-WED CONSIST PRIMARILY OF THE 00Z ECMWF.. 12Z-18Z
GFS.. AND 18Z GEFS MEAN.  COMPARED TO OPERATIONAL GUIDANCE THE
ECMWF MEAN CONTINUES TO APPEAR BEHIND THE CURVE IN PROPERLY
DEPICTING THE DETAIL AND TIMING OF EJECTING NRN MEXICO ENERGY.
THERE HAS BEEN SOMEWHAT OF A SLOWER TREND OVER THE PAST DAY IN THE
OPERATIONAL MODELS AND THUS FAR THIS TREND SEEMS TO BE EXTENDING
INTO THE NEW 00Z RUNS.  ACROSS THE NRN TIER THE GENERAL TREND HAS
BEEN TO EXTEND HGT FALLS SOMEWHAT FARTHER EWD.  COMBINATION OF
THIS TREND ALONG WITH SLOWER EJECTION OF THE ENERGY TO THE SE
RESULTS IN A LOWER PROPORTION OF EARLY TUE TN VLY SFC LOW PRES
MAKING IT INTO THE GRTLKS.  INSTEAD THERE IS MORE EMPHASIS ON A
SLOWER WRN ATLC WAVE LATE TUE ONWARD.  TIMING SPREAD BEGINS TO
DEVELOP WITH ENERGY CROSSING THE WEST WITH THE RESULTING BLEND
STAYING WITHIN THE MIDDLE PORTION OF THE ENVELOPE.

FOR DAYS 5-7 THU-SAT THE FCST USES A BLEND OF THE 18Z GEFS/12Z
ECMWF MEANS AS RECENT ECMWF/UKMET RUNS LEAN TO THE SLOWER SIDE OF
THE GUIDANCE SPREAD FOR THE CNTRL-ERN TROUGH AND GFS RUNS STRAY
TOWARD THE FASTER SIDE.  SO FAR THERE DOES NOT APPEAR TO BE A
PRONOUNCED TREND FAVORING ONE SIDE OF THE SPREAD OR THE OTHER...
THUS FAVORING THE ENSEMBLE MEANS WHICH KEEP THE OVERALL TIMING
FAIRLY CLOSE TO CONTINUITY.  BY THIS PART OF THE FCST THE
ADJUSTMENTS MADE FOR COMPONENTS OF THE UPR TROUGH LEAD TO A
GENERAL TREND TO DELAY DEVELOPMENT OF DEEPEST SFC LOW PRES UNTIL
ERN CANADA/NRN NEW ENGLAND IN CONTRAST TO PRIOR DAYS WHEN SUCH
POTENTIAL EXTENDED AS FAR WWD AS THE GRTLKS.

WITH FLOW COMING INTO THE NORTHWEST ON THE SRN SIDE OF A STRONG
NERN PAC SYSTEM... THE ECMWF/UKMET RUNS ARE ALSO ON THE SLOWER
SIDE OF THE SPREAD WHILE THE ENSEMBLE MEANS PROVIDE A GOOD MIDDLE
GROUND AND MINIMAL CHANGE IN CONTINUITY.  AS ERN PAC INTO
WRN-CNTRL U.S. FLOW FLATTENS DAY 7 SAT ENSEMBLE MEMBERS BECOME
MORE DIVERSE IN REGARD TO LOCATING INDIVIDUAL SHRTWVS.
TELECONNECTIONS ON RIDGING/POSITIVE HGT ANOMALIES NEAR/JUST NW OF
ALASKA SUGGEST FLOW SHOULD RETURN TO SOME TYPE OF POSITIVELY
TILTED CONUS TROUGH... PERHAPS SHORTLY BEYOND DAY 7... BUT THERE
ARE VARIOUS WAYS THIS CAN BE ACHIEVED FROM A MULTI-DAY MEAN
PERSPECTIVE.


...WEATHER HIGHLIGHTS/THREATS...

THE UPR TROUGH PROGRESSING FROM WEST TO EAST ACROSS THE LOWER 48
WILL BE ACCOMPANIED BY MUCH BELOW NORMAL TEMPS... INITIALLY OVER
THE WEST/PLAINS AND THEN REACHING THE EAST.  EXPECT THE MOST
EXTREME ANOMALIES TO BE OVER THE ERN SLOPES OF THE NRN-CNTRL
ROCKIES INTO THE PLAINS TUE-THU WITH HIGHS 20-30F OR SO BELOW
NORMAL.  ANOMALIES FOR MINS SHOULD BE SOMEWHAT LESS EXTREME EXCEPT
OVER MONTANA.  AS THE COLD AIR PUSHES EWD AND SFC HIGH PRESSURE
EVENTUALLY BUILDS INTO THE SOUTHEAST THE SRN HALF OF THE EAST WILL
LIKELY BE COLDEST RELATIVE TO NORMAL WITH MINUS 10-20F ANOMALIES.
WHILE THERE IS STILL SOME UNCERTAINTY IN SFC DETAILS... OVERALL
PATTERN WILL FAVOR A PERIOD OF COLD AND BRISK CYCLONIC FLOW ACROSS
THE GRTLKS/NORTHEAST AND ENHANCED LAKE EFFECT SNOW LATE IN THE
WEEK.  AHEAD OF THE COLD AIR THERE WILL BE AN AREA OF MOSTLY RAIN
ACROSS PORTIONS OF THE EAST ON TUE.  THEN UNCERTAINTY IN FRONTAL
TIMING OVER THE EAST LEADS TO LOWER THAN DESIRED CONFIDENCE IN
DETERMINING COVERAGE/INTENSITY OF RNFL BEFORE THE COLD AIR ARRIVES.

ACROSS THE WEST... EARLY PERIOD TROUGH CROSSING THE REGION WILL
BRING SOME AREAS OF SNOW TO FAVORED TERRAIN.  THEN THE PRIMARY
EMPHASIS WILL BE ON MSTR PUSHING INTO THE NORTHWEST BY THE LATTER
HALF OF THE WEEK AND THEN EXTENDING INTO THE NRN ROCKIES.  FAVORED
LOCATIONS ALONG THE PAC NW/EXTREME NRN CA COAST AND THE CASCADES
WILL LIKELY SEE THE HIGHEST PCPN TOTALS.  WRN U.S. TEMPS WILL
MODERATE COMPARED TO TUE-WED BUT THERE SHOULD BE ENOUGH LINGERING
COLD AIR TO PRODUCE SNOW AT FAIRLY LOW ELEVS.

RAUSCH

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