Prognostic Meteorological Discussion
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FXUS02 KWBC 160627
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
127 AM EST TUE JAN 16 2018

VALID 12Z FRI JAN 19 2018 - 12Z TUE JAN 23 2018

...ACTIVE PATTERN FOR MUCH OF THE WEST WHILE HEAVY SNOW LOOMS FOR
THE UPPER MIDWEST/GREAT LAKES BY SUNDAY/MONDAY...


...PATTERN OVERVIEW...

THE MID/UPPER-LEVEL PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH
A SOUTHERN STREAM SHORTWAVE TRACKING FROM EXTREME SOUTH TX OUT
TOWARD THE NORTHERN GULF OF MX OVER THE WEEKEND. LARGE-SCALE
AMPLIFICATION BEGINS TO TAKE SHAPE WITHIN THE MID-LATITUDES AS
SHARPENING HEIGHT FALLS DIG INTO THE FOUR CORNERS REGION ON
SATURDAY. ADDITIONAL STRENGTHENING IS EXPECTED AS THE SYSTEM
ATTAINS A NEGATIVE TILT OVER THE MO RIVER VALLEY ON SUNDAY
EVENING. CONSEQUENTLY...A SIGNIFICANT WAVE OF LOW PRESSURE SWINGS
UP TOWARD THE UPPER GREAT LAKES ON MONDAY WITH A TRACK INTO
SOUTHERN QUEBEC BY THE FOLLOWING DAY. MEANWHILE...AS THE INITIAL
SYSTEM MOVES FROM THE WEST COAST TOWARD THE SOUTHERN HIGH PLAINS
THIS WEEKEND...ADDITIONAL UPPER TROUGHING IS LIKELY TO MOVE INTO
THE PACIFIC NORTHWEST/NORTHERN CA BY SUNDAY WITH EASTWARD
PROGRESSION THEREAFTER. BY DAY 7/JANUARY 23...A PAIR OF TROUGHS
SHOULD INHABIT THE COUNTRY...ONE ACROSS THE WESTERN/CENTRAL STATES
WHILE THE OTHER EXITS NEW ENGLAND.


...GUIDANCE ASSESSMENT/UNCERTAINTIES/PREFERENCES...

TO BEGIN THE PERIOD ON FRIDAY...LOW-AMPLITUDE SHORTWAVES WILL BE
IN THE PROCESS OF EXITING LOWER NEW ENGLAND WHICH GIVES WAY TO
MARKED HEIGHT RISES ACROSS THE EASTERN U.S. THIS WEEKEND. WITHIN
THE SOUTHERN STREAM...THERE HAVE BEEN TIMING DIFFERENCES EJECTING
A COMPACT CLOSED LOW FROM SOUTH TX EASTWARD INTO THE ADJACENT GULF
WATERS. THE GFS/ECMWF HAVE GENERALLY REMAINED ON THE SLOWER SIDE
OF THE ENVELOPE WITH THE EXCEPTION BEING THE 12Z GFS WHICH APPEARS
TO BE AN OUTLIER. CONSIDERING THE 570-DM ENSEMBLE SPAGHETTI
PLOTS...A MAJORITY OF THE 12Z ECMWF ENSEMBLE MEMBERS SUPPORT THIS
SLOWER PROGRESSION. EVENTUALLY THIS SYSTEM SHOULD FIZZLE OUT WHILE
MOVING INTO THE NORTHEASTERN GULF OF MX LATER IN THE WEEKEND.

ON THE HEELS OF THIS FEATURE...LARGE-SCALE HEIGHT FALLS TAKE SHAPE
ACROSS THE SOUTHWESTERN U.S. WITH EVENTUALLY EJECTION INTO THE
SOUTHERN HIGH PLAINS BY SUNDAY MORNING. MULTI-DAY ENSEMBLE
SPAGHETTI TRENDS SHOW ONLY MINIMAL CHANGES WITH AGREEMENT IN
DEVELOPING A STRONG AREA OF LOW PRESSURE INTO THE MIDDLE MS VALLEY
BY 22/0000Z. A SUB-990 MB CYCLONE SHOULD BE CROSSING INTO THE
UPPER GREAT LAKES ON MONDAY MORNING WITH THE 12Z ECMWF ENSEMBLE
MEAN BEING THE STRONGEST AMONG THE ENSEMBLE SOLUTIONS. EVENTUALLY
A COLD FRONTAL PASSAGE WILL OCCUR ALONG THE I-95 CORRIDOR BY LATE
MONDAY WITH LOWER HEIGHTS LINGERING ACROSS THE NEW ENGLAND INTO
THE END OF THE FORECAST PERIOD.

WHILE THIS LATTER SYSTEM IS OF HIGHER CONFIDENCE...THE TRAILING
EVOLUTION OF THE PATTERN IS LESS CERTAIN. THE GFS/CMC SOLUTIONS
APPEAR TO BE ON ONE SIDE WHICH FAVORS A FLATTER/QUICKER
PROGRESSION FROM THE PACIFIC NORTHWEST TOWARD THE NORTH-CENTRAL
U.S. ON THE OTHER END...THE 12Z ECMWF AND A MAJORITY OF ITS
ENSEMBLE MEMBERS FAVOR A SOLUTION THAT DIGS A TROUGH BACK INTO THE
SOUTHWESTERN U.S. WHICH APPEARS TO BE ITS THEME THE PAST COUPLE OF
RUNS.

GIVEN SOLID MODEL AGREEMENT BETWEEN THE 18Z GFS/12Z
ECMWF...FAVORED AN EQUAL BLEND OF THESE SOLUTIONS THROUGH DAY
4/SATURDAY BEFORE INCORPORATING SOME ENSEMBLE INFLUENCES FROM DAY
5/SUNDAY ONWARD. DECIDED TO FAVOR THE 12Z ECMWF ENSEMBLE MEAN MORE
STRONGLY DEEPER INTO THE PERIOD AS IT WAS THE ONLY ENSEMBLE
SOLUTION WITH A DEEP GREAT LAKES LOW. THE 18Z/12Z GEFS MEAN WERE
SEVERAL MB WEAKER WHICH IS NOT REFLECTIVE OF THE STRONG NATURE THE
OPERATIONAL GUIDANCE HAS BEEN SHOWING.


...WEATHER HIGHLIGHTS/THREATS...

FOR THE MOST PART...ABOVE AVERAGE TEMPERATURES SHOULD DOMINATE A
MAJORITY OF THE FORECAST AS STRONG WARM ADVECTION ENSUES IN
ADVANCE OF THE DEEP UPPER TROUGH. ON FRIDAY...READINGS
APPROXIMATELY 15 TO 20 DEGREES ABOVE CLIMATOLOGY SHOULD BE
COMMONPLACE FROM THE SOUTHERN/CENTRAL HIGH PLAINS UP INTO THE
UPPER MIDWEST. SUCH DEPARTURES FROM AVERAGE WILL GRADUALLY SLIDE
EASTWARD REACHING THE EASTERN U.S. ON SUNDAY/MONDAY. A FAIRLY
SHARP TEMPERATURE GRADIENT WILL SET UP ALONG THE BOUNDARY WITH
HIGHS ON SUNDAY IN THE 50S ACROSS CENTRAL IL/IN WHILE TEENS LOOM
BACK ACROSS THE UPPER MIDWEST. ACROSS THE WESTERN U.S...OVERALL
TEMPERATURES SHOULD BE SLIGHTLY BELOW NORMAL FOR MUCH OF THE
PERIOD GIVEN ACTIVE PRECIPITATION.

REGARDING PRECIPITATION...SOME CONVECTION COULD HUG THE TX COAST
ON FRIDAY IN ADVANCE OF THE SOUTHERN STREAM CLOSED LOW. ACROSS THE
WESTERN U.S...WIDESPREAD LOWER ELEVATION RAIN/MOUNTAIN SNOW IS
LIKELY THROUGH SATURDAY WITH A MUCH HEAVIER BATCH OF RAIN/SNOW
HITTING NORTHERN CA UP INTO THE PACIFIC NORTHWEST ON
SUNDAY/MONDAY. SOME OF THE GUIDANCE HAS 24-HOUR AMOUNTS
APPROACHING THE 2 TO 3 INCH RANGE AS AN ATMOSPHERIC RIVER EVENT
SETS UP. CONSIDERING THE IMPACTS FROM THE DEEP UPPER TROUGH
LIFTING TOWARD THE UPPER MIDWEST...HEAVY SNOW WILL INITIALLY BE
POSSIBLE OVER THE CENTRAL ROCKIES. AS THE SURFACE LOW BEGINS TO
WIND UP ACROSS THE MIDDLE MS VALLEY/UPPER GREAT LAKES...THE
POTENTIAL FOR MODERATE/HEAVY SNOW IS LIKELY ANYWHERE FROM THE
CENTRAL PLAINS INTO THE UPPER GREAT LAKES. EVEN SOME BLIZZARD
CONDITIONS MAY BE A POSSIBILITY IF THE SURFACE LOW DEEPENS AS MUCH
AS SUGGESTED. FARTHER SOUTH ALONG THE FRONT...LIGHT/MODERATE
RAINFALL SHOULD SPREAD ALONG AND AHEAD OF THE BOUNDARY.



RUBIN-OSTER


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