Preliminary Forecasts
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FXUS02 KWNH 180656
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
156 AM EST THU JAN 18 2018

VALID 12Z SUN JAN 21 2018 - 12Z THU JAN 25 2018

...POTENTIAL BLIZZARD CONDITIONS ACROSS THE UPPER MIDWEST/GREAT
LAKES ON SUNDAY/MONDAY...


...PATTERN OVERVIEW...

WHILE A BROAD AND STRETCHED OUT BAND OF VORTICITY SLIDES ACROSS
THE FL PENINSULA ON SUNDAY MORNING...A WELL DEVELOPED SHORTWAVE
WILL BE IN THE PROCESS OF DIGGING THROUGH THE SOUTHERN HIGH
PLAINS. RAPID STRENGTHENING OF THE ATTENDANT SURFACE CYCLONE WILL
ENSUE ON SUNDAY NIGHT/MONDAY MORNING ACROSS THE MIDDLE MS
VALLEY/UPPER GREAT LAKES REGION. AFTER LIKELY DUMPING HEAVY SNOW
OVER SECTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER MIDWEST AND
INTO THE GREAT LAKES...THE DEEP SURFACE LOW WILL PUSH INTO
SOUTHERN QUEBEC BY TUESDAY WITH A COLD FRONTAL PASSAGE EXPECTED
ALONG THE I-95 CORRIDOR BY 23/1200Z. ACROSS THE WESTERN U.S...A
WEAKENING SYSTEM IS FORECAST TO PUSH INTO THE PACIFIC NORTHWEST ON
SUNDAY AFTERNOON WITH ADDITIONAL LONGWAVE ENERGY CARVING OUT A
TROUGH ACROSS THE FOUR CORNERS REGION BY TUESDAY. MID-LEVEL
HEIGHTS SHOULD BRIEFLY BUILD OVER THE WEST COAST BEFORE RENEWED
TROUGHING RETURNS TO THE REGION BY MID-WEEK WITH GRADUAL
ACCELERATION INLAND BY DAY 7/JANUARY 25.


...GUIDANCE ASSESSMENT...MODEL UNCERTAINTIES...AND PREFERENCES...

CONSIDERING THE INITIAL SHORTWAVE CROSSING INTO THE FL PENINSULA
ON SUNDAY...THE PREVIOUSLY QUICKER 12Z CMC/UKMET HAVE MADE SLOWER
ADJUSTMENTS IN THEIR LATEST RUNS. THIS BRINGS THE 00Z UKMET
SOLUTION IN LINE WITH THE 00Z/18Z GFS RUNS BUT STILL REMAIN
QUICKER THAN THE MORE WESTERN 12Z ECMWF. FARTHER UPSTREAM...MODEL
DIFFERENCES REMAIN AN ISSUE WITH THE DEVELOPING CYCLONE ACROSS THE
CENTRAL PLAINS ON SUNDAY AFTERNOON WITH THE 00Z/18Z GFS OVER
SOUTHERN IA WHILE THE REMAINING CONSENSUS IS BACK NEAR THE
VICINITY OF SOUTHERN KS. SUCH UNCERTAINTIES PROJECT IN THE FUTURE
AS THE LOW CONTINUES DEEPENING INTO THE UPPER MIDWEST/GREAT LAKES
EARLY NEXT WEEK. ENSEMBLE LOW PLOTS THE PAST FEW MODEL CYCLES HAVE
SHOWN THE SPREAD HAS DECREASED A BIT BUT A DIVIDE REMAINS BETWEEN
THE GEFS/ECMWF ENSEMBLE CAMPS. IN TERMS OF SUPPORT FROM OTHER
MODELS...THE 00Z/12Z CMC REMAIN CLOSER TO THE ECMWF WITH THE
SOUTHWESTWARD TRENDING 00Z UKMET JOINING THIS SLOWER GROUP.
THUS...THE OVERNIGHT WPC MEDIUM RANGE PACKAGE COULD BE A TAD FAST.
MULTI-DAY ENSEMBLE SPAGHETTI TRENDS HAVE SHOWN A SLOWING WITH THE
ECMWF RUNS BEING MOST EMPHATIC ABOUT THIS SCENARIO. EVENTUALLY THE
MID-LEVEL PATTERN RELAXES AS LOWER HEIGHTS LIFT UP INTO EASTERN
CANADA BY THE MIDDLE OF NEXT WEEK.

ACROSS THE EASTERN PACIFIC AND ADJACENT WEST COAST STATES...AN
UPPER LOW WILL BE SPINNING OFF THE COAST OF BRITISH COLUMBIA ON
21/1200Z. AS THIS SYSTEM MOVES INLAND AND WEAKENS...LONGWAVE
ENERGY SHOULD PUSH INTO THE PACIFIC NORTHWEST WHILE BEING
STRETCHED AND SHEARED UPON MIGRATION DOWNSTREAM. GENERAL TROUGHING
TAKES SHAPE FARTHER INLAND ACROSS MUCH OF THE INTERIOR WEST WITH A
MEAN TROUGH POSITION ALONG THE HIGH PLAINS BY 24/0000Z AS SHOWN BY
THE 00Z GFS/12Z ECMWF AND 12Z ECMWF ENSEMBLE MEAN. THE CMC
SOLUTIONS HAVE BEEN FLIP-FLOPPING A BIT WHILE NOT BEING ALIGNED
WITH THE ENSEMBLE MEANS. MULTI-DAY COMPARISONS OF RELEVANT
ENSEMBLE SPAGHETTI PLOTS SHOW A LOT OF NOISE WITH LITTLE REDUCTION
IN FORECAST SPREAD. MOST ENSEMBLE MEMBERS HAVE TRENDED QUICKER
ALTHOUGH A VAST MAJORITY OF THE ECMWF MEMBERS HAVE STUCK TO A
SLOWER PROGRESSION.

WHILE MID-LEVEL RIDGING WILL REBUILD OVER THE WEST COAST BY
MID-WEEK...A DEEP UPPER TROUGH LOOMS OFFSHORE WITH A PROGRESSION
INLAND BY PERHAPS WEDNESDAY AFTERNOON. BASED ON THE ENSEMBLE
MEANS...THIS LONGWAVE PATTERN SHOULD ACCELERATE INLAND WITH THE
00Z GFS/12Z ECMWF BOTH SHOWING A POTENT SYSTEM DIGGING INTO THE
SOUTHWESTERN U.S. BY 26/0000Z.

REGARDING PREFERENCES...TOOK A THREE-WAY CONSENSUS OF THE 18Z GFS
AND 12Z ECMWF/CMC FOR THE FIRST COUPLE OF DAYS OF THE PERIOD. AS
MENTIONED EARLIER...THE SLOWING TREND IN THE 00Z GUIDANCE SUGGESTS
THE FORECAST MAY BE TOO QUICK GIVEN SUFFICIENT USE OF THE 18Z
GFS/GEFS MEAN SOLUTIONS. BY DAY 5/TUESDAY...STARTED TO INCORPORATE
MORE OF THE 18Z GEFS MEAN AND 12Z ECMWF/NAEFS ENSEMBLE MEANS
ALTHOUGH DID MAINTAIN APPROXIMATELY 20 PERCENT OF THE OPERATIONAL
COMPONENT TO THE BLEND OUT TO DAY 7/JANUARY 25.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

IMPRESSIVE WARMTH IS EXPECTED AHEAD OF THE DEEPENING CYCLONE OVER
THE CENTRAL U.S. AFTERNOON TEMPERATURES SHOULD BE AROUND 1O TO
POSSIBLY 20 DEGREES ABOVE CLIMATOLOGY FROM THE MO RIVER VALLEY OUT
TOWARD THE LOWER GREAT LAKES ON SUNDAY/MONDAY. THIS WILL RAISE
HIGHS INTO THE 50S ACROSS THIS STRETCH OF THE COUNTRY WITH SUCH
READINGS STRETCHING EASTWARD TOWARD MUCH OF THE I-95 CORRIDOR BY
TUESDAY. OVERNIGHT LOWS SHOULD BE EVEN MORE ANOMALOUS WITH
EXPECTED VALUES AROUND 20 TO 25 DEGREES ABOVE AVERAGE WITHIN THE
EXPANSIVE WARM SECTOR. MEANWHILE...A RATHER ACTIVE PATTERN ACROSS
THE WESTERN U.S. WILL LIKELY KEEP TEMPERATURES AROUND TO SLIGHTLY
BELOW CLIMATOLOGY THROUGHOUT THE PERIOD.

ON SUNDAY...MODERATE TO LOCALLY HEAVY RAINFALL IS POSSIBLE UP AND
DOWN THE COLD FRONT STRETCHING FROM THE LOWER MS VALLEY UP INTO
THE MIDDLE MS VALLEY. WITHIN THE COMMA-HEAD OF PRECIPITATION
AROUND THE DEEPENING LOW CENTER...EXPECT HEAVY SNOW TO FALL
ANYWHERE FROM NEB UP INTO THE UPPER MIDWEST/GREAT LAKES WITH
BLIZZARD CONDITIONS POSSIBLE AT TIMES DUE TO THE TIGHT PRESSURE
GRADIENT. SOME RESIDUAL SNOW SHOWERS ARE LIKELY OVER THE GREAT
LAKES GIVEN CYCLONIC FLOW ALOFT REMAINING WHILE FRONTAL RAINFALL
REMAINS IN THE FORECAST OUT TOWARD THE EASTERN SEABOARD ON
MONDAY/TUESDAY. BASED ON THE 00Z GFS/12Z ECMWF...AMPLE RETURN FLOW
FROM THE GULF OF MX BY WEDNESDAY/THURSDAY MAY BRING HEAVY RAINFALL
BACK IN THE PICTURE ACROSS AREAS OF THE CENTRAL/EASTERN GULF
COAST. THE SIGNAL IS STRONG ENOUGH TO SUPPORT ADDING SOME HEAVIER
AMOUNTS INTO THE MANUAL DAY 6/7 QPF.

ACROSS THE WESTERN U.S...MULTIPLE ROUNDS OF PRECIPITATION ARE
LIKELY WITH LOWER ELEVATION/VALLEY FLOOR RAIN AND MOUNTAIN SNOW.
THE FIRST WAVE PRIMARILY AFFECTS NORTHERN CA UP TOWARD THE
OLYMPICS/CASCADES WITH PRECIPITATION SPREADING INTO THE UPPER
INTERMOUNTAIN WEST/CENTRAL GREAT BASIN THEREAFTER. MODELS SHOW THE
NEXT SURGE MOVING INTO SIMILAR REGIONS ON TUESDAY/WEDNESDAY WHICH
WILL KEEP CONDITIONS ON THE WET SIDE FOR THE PERIOD. MULTI-DAY
PRECIPITATION TOTALS MAY BE QUITE IMPRESSIVE...PARTICULARLY OVER
AREAS OF CONTINUED UPSLOPE EFFECTS.


RUBIN-OSTER

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