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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0648Z Feb 18, 2018)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
148 AM EST SUN FEB 18 2018

VALID 12Z WED FEB 21 2018 - 12Z SUN FEB 25 2018

...ARCTIC AIR DROPS THROUGH THE WEST-CENTRAL U.S. WITH RECORD
WARMTH ACROSS THE EAST...
...CLASHING AIR MASSES SET STAGE FOR ARKLATEX TO TN/OH VALLEYS
HEAVY RAINFALL THREAT...


...PATTERN OVERVIEW...

A SUBTROPICAL RIDGE ACROSS THE SOUTHWESTERN ATLANTIC WILL CONTINUE
TO EXERT ITS DOMINANCE ON THE MID-LATITUDE PATTERN. CONSIDERING
TELECONNECTIONS WITH THIS PERSISTENT POSITIVE HEIGHT ANOMALY...THE
UPSTREAM PATTERN WILL FEATURE BROAD UPPER TROUGHING OVER THE
WESTERN/CENTRAL U.S. THIS SUPPORTS THE MOST ANOMALOUS COLD WEATHER
OVER THE NORTHERN/CENTRAL ROCKIES AND ADJACENT GREAT PLAINS. THE
PARTICULAR SET UP IN PLACE ALSO SUPPORTS A HEAVY RAINFALL SIGNAL
AS UNIDIRECTIONAL SOUTH-SOUTHWESTERLY FLOW PREVAILS FROM THE
ARKLATEX UP TOWARD THE OH/TN VALLEYS. THIS SHOULD BE AN ISSUE FOR
MULTIPLE DAYS DURING THE PERIOD AS THE WESTERN U.S. TROUGH
CONTINUALLY RE-LOADS. NOT A WHOLE LOT OF CHANGE IS EXPECTED IN THE
PATTERN GIVEN PERSISTENT RIDGING OVER THE SOUTHWESTERN ATLANTIC
AND OUT OVER THE EASTERN PACIFIC.


...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES...

STARTING ON WEDNESDAY MORNING...A 594-DM MID-LEVEL RIDGE WILL BE
CENTERED JUST SOUTHWEST OF BERMUDA WHICH IS ANYWHERE FROM 2.5 TO
POSSIBLY 3 STANDARD DEVIATIONS ABOVE CLIMATOLOGY. GRADUAL NORTHERN
EROSION OF THIS ANTICYCLONE CENTER WILL TAKE PLACE AS UPSTREAM
SHORTWAVES FLATTEN THE FLOW OFFSHORE OF THE NORTHEASTERN U.S.
ULTIMATELY THIS SHOULD AFFORD A FRONTAL PASSAGE ACROSS THE I-95
CORRIDOR BY LATE WEDNESDAY WITH THE GFS DEPICTING A SLOWING TREND
OVER THE PAST SEVERAL RUNS. THIS BRINGS ITS SOLUTION MORE IN
ALIGNMENT WITH THE 12Z ECMWF/CMC WHICH HAVE BEEN A TAD MORE STABLE
IN RECENT CYCLES. BY THURSDAY...HEIGHTS BEGIN TO RISE EAST OF THE
MS RIVER AGAIN WHICH WILL CAUSE THE FRONTAL ZONE TO STALL AND
MEANDER FROM THE MID-ATLANTIC BACK THROUGH THE TN VALLEY AND INTO
THE OZARKS. THIS PARTICULAR BOUNDARY SHOULD REMAIN A FOCUS FOR
MULTIPLE ROUNDS OF HEAVY RAINFALL WITH A PLACEMENT LIKELY BEING
DRIVEN BY MESOSCALE PROCESSES AND WHERE DIFFERENTIAL HEATING ZONES
SET UP.

WHILE GENERAL MID-LEVEL SOUTH-SOUTHWESTERLY FLOW SHOULD BE
ESTABLISHED ACROSS THE CENTRAL/EASTERN U.S...HOW THIS OCCURS IS
UNCERTAIN GIVEN MANY UPSTREAM DIFFERENCES. INITIALLY...A LONGWAVE
TROUGH WILL BE POSITIONED OVER THE SOUTHWESTERN U.S. WHILE THE
00Z/18Z GFS CONTINUE TO FAVOR A SOUTHWARD PROPAGATING IMPULSE
PARALLELING THE CA COAST. ENSEMBLE SPAGHETTI PLOTS SHOW THE 12Z
GEFS MEMBERS TAKE THIS MORE ELONGATED PATH AROUND THE MEAN TROUGH
AS OTHER SOLUTIONS LOOM FARTHER DOWNSTREAM. AS THIS SYSTEM PUSHES
TOWARD THE PERSISTENT REGION OF MID-LEVEL CONFLUENCE OVER THE
CENTRAL U.S...ADDITIONAL SHORTWAVE ENERGY DIGS THROUGH THE WESTERN
U.S. ON FRIDAY. THERE ARE SOME TIMING DIFFERENCES EVIDENT AMONG
THE OPERATIONAL/ENSEMBLE MODELS WITH A VAST MAJORITY OF THE 12Z
ECMWF ENSEMBLES FAVORING A SLOWER/MORE WESTWARD DISPLACED
SOLUTION. LIKE PRECEDING SYSTEMS...THIS FEATURE TOO WILL MARCH
TOWARD THE CENTER OF THE COUNTRY WITH ADDITIONAL ENERGY DROPPING
DOWN INTO THE PACIFIC NORTHWEST BY NEXT WEEKEND. GIVEN THE
MULTITUDE OF FEATURES TO ACCOUNT FOR DURING THE
PERIOD...UNCERTAINTY BUILDS LATE IN THE PERIOD WITH DETAILS WITHIN
THIS ACTIVE TROUGH.

GIVEN THE CONSISTENCY OF THE 12Z ECMWF...KEPT A SIGNIFICANT CHUNK
OF ITS SOLUTION IN THE BLEND THROUGH DAY 5/FRIDAY WITH SMALLER
CONTRIBUTIONS FROM THE 12Z GFS. MAINTAINED ROUGHLY 75 PERCENT OF
THE 12Z ECMWF/ECMWF ENSEMBLE MEAN THROUGHOUT THE FORECAST PERIOD
WITH NONE OF THE OPERATION RUN UTILIZED BEYOND DAY 5/FRIDAY. THE
CONFIDENCE IN THE FORECAST DOES DETERIORATE INTO NEXT WEEKEND
GIVEN SO MANY UNKNOWNS WITHIN THE ACTIVE PATTERN ACROSS THE
WESTERN/CENTRAL STATES.


...WEATHER HIGHLIGHTS/THREATS...

WITH HIGH-AMPLITUDE FLOW AND ANOMALOUS MOISTURE IN THE
PICTURE...IT SHOULD BE A RATHER ACTIVE PERIOD WITH BOTH DAILY
RECORD TEMPERATURES AND POSSIBLE FLOODING ISSUES. BEGINNING WITH
THE FORMER...WELL ABOVE AVERAGE READINGS ARE TO PREVAIL FROM THE
LOWER MS VALLEY UP INTO THE NORTHEASTERN U.S. DEPARTURES FROM
CLIMATOLOGY SHOULD BE ON THE ORDER OF 20 TO 30 DEGREES ON
WEDNESDAY AS HIGHS SURGE INTO THE 70S AS FAR NORTH AS THE
MID-ATLANTIC. WHILE SOME REDUCTION IN THESE ANOMALIES IS EXPECTED
AS A BACK DOOR FRONT STALLS OVER THE MID-ATLANTIC/TN VALLEY
REGION...OVERNIGHT WARMTH SHOULD CONTINUE THROUGHOUT NEXT WEEK.
WELL ABOVE AVERAGE TEMPERATURES ARE LIKELY WITH READINGS ABOVE
FREEZING UP INTO THE LOWER GREAT LAKES AND NEW ENGLAND AT TIMES.
OVERALL...DAILY TEMPERATURE RECORDS SHOULD BE BROKEN DURING THE
PERIOD...PARTICULARLY ON WEDNESDAY AND INTO THE FOLLOWING MORNING.

ON THE OPPOSITE SIDE OF THE SPECTRUM...FRIGID TEMPERATURES SHOULD
CONTINUE ACROSS MUCH OF THE WESTERN/CENTRAL U.S. EXPECTED LOWS AND
HIGHS MAY DEPART ANYWHERE FROM 20 TO 30 DEGREES FROM MID/LATE
FEBRUARY CLIMATOLOGY ACROSS THE CENTRAL/NORTHERN ROCKIES AND
ADJACENT GREAT PLAINS. EVEN THE WEST COAST CAN EXPECT COLDER
TEMPERATURES AS BELOW FREEZING LOWS MOVE ALONG INTERSTATE 5 FROM
THE PACIFIC NORTHWEST DOWN INTO NORTHERN/CENTRAL CA. WHILE WELL
BELOW AVERAGE TEMPERATURES SHOULD CONTINUE OVER THE NORTH-CENTRAL
U.S. INTO NEXT WEEKEND...THE ANOMALIES DO WEAKEN OVER A MAJORITY
OF THE WESTERN/CENTRAL STATES.

HEAVY RAINFALL SHOULD BE AN ONGOING ISSUE ACROSS THE ARKLATEX WITH
THE DAY 3 QPF SUGGESTING AREAL AVERAGES OF 2 TO 4 INCHES OVER THIS
REGION THROUGH 21/1200Z WITH SLIGHTLY LIGHTER AMOUNTS EXTENDING
INTO THE OZARKS AND MO VALLEY. WHILE THE FIRST BATCH OF HEAVY
RAINS WILL FALL FROM THE ARKLATEX INTO THE OZARKS AND TN/OH
VALLEYS...THERE ARE SUGGESTIONS ANOTHER ROUND SHOULD OCCUR ON
FRIDAY/SATURDAY. PLACEMENT IS UNCERTAIN AT THIS POINT WITH THE 12Z
ECMWF FAVORING SIMILAR LOCATIONS WHILE THE 00Z GFS PUSHES THE NEW
AXIS WELL TO THE NORTH. IT WILL PROVE TO BE A PERIOD FILLED WITH A
DISTINCT POSSIBILITY FOR FLOODING ISSUES GIVEN THE MULTI-DAY
NATURE OF THE EVENT. ADDITIONALLY...SPRINGTIME MOISTURE RETURN
COUPLED WITH PERSISTENT TRAINING WILL LIKELY CAUSE FLASH FLOODING
CONCERNS TO INCREASE AS WELL. MEANWHILE...TO THE NORTH OF THIS
SHIELD OF RAINFALL...A SLIVER OF WINTRY PRECIPITATION COULD FALL
OVER THE GREAT LAKES. OUT WEST...LIGHT TO MODERATE RAINFALL IS
EXPECTED ALONG THE WEST COAST WITH HIGHER ELEVATIONS SNOWS.
AMOUNTS DO NOT APPEAR SUBSTANTIAL GIVEN THE MORE NORTH/SOUTH TRACK
OF THESE DISTURBANCES.


RUBIN-OSTER