Preliminary Forecasts
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000
FXUS02 KWNH 170647
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
146 AM EST SAT FEB 17 2018

VALID 12Z TUE FEB 20 2018 - 12Z SAT FEB 24 2018

...HEAVY RAINFALL THREAT FROM THE ARKLATEX TO THE TN/OH VALLEYS...
...FRIGID CONDITIONS FOR THE NORTHERN/CENTRAL ROCKIES AND GREAT
PLAINS...
...RECORD WARMTH FROM THE SOUTH-CENTRAL TO EASTERN U.S...


...PATTERN OVERVIEW...

LITTLE CHANGE IN THE SYNOPTIC-SCALE PATTERN IS EXPECTED DURING THE
PERIOD AS A SUBTROPICAL RIDGE HOLDS STRONG OVER THE SOUTHWESTERN
ATLANTIC. A MID-LEVEL RIDGE WITH AN INTENSITY POSSIBLY REACHING
594-DM MAY APPROACH OR EVEN EXCEED 2.5 STANDARD DEVIATIONS ABOVE
CLIMATOLOGY FOR MID-FEBRUARY. THE RESULTANT UPSTREAM PATTERN WILL
FAVOR BELOW AVERAGE HEIGHTS WITH AN UPPER TROUGH THAT CONTINUALLY
RE-LOADS ACROSS THE SOUTHWESTERN STATES. A STEADY PERIOD OF
SOUTHWARD PROGRESSING SYSTEMS ARE NOTED ALONG THE EASTERN PACIFIC
GIVEN THE STOUT UPPER RIDGE REMAINING IN PLACE FARTHER TO THE
WEST. THIS OVERALL SETUP OVER NORTH AMERICA WILL FAVOR A RATHER
SHARP AXIS OF MID-LEVEL CONFLUENCE EXTENDING FROM THE ARKLATEX UP
TOWARD THE TN/OH VALLEYS WHERE A HEAVY RAINFALL THREAT SHOULD
REMAIN IN PLAY.


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

THE MOST CONFIDENT PART OF THE UPCOMING FORECAST IS REGARDING THE
POSITIVE HEIGHT ANOMALY HOLDING STRONG OVER THE WESTERN ATLANTIC
DOWN INTO THE CENTRAL/EASTERN CARIBBEAN. TOWARD THE END OF THE
UPCOMING WORK WEEK...A NORTHERN EROSION OF THIS RIDGE IS EXPECTED
AS NORTHERN STREAM ENERGY HELPS SUFFICIENTLY LOWER HEIGHTS. THIS
WILL ALLOW A BACKDOOR FRONT TO SWEEP SOUTHWARD ACROSS THE
NORTHEASTERN U.S....THUS AFFORDING A COOL DOWN FROM THE RECORD
WARMTH. WHERE THE MODELS SEEM TO BE STRUGGLING THE MOST IS WITH
THE ACTIVE LONGWAVE TROUGH EXTENDING OVER THE WESTERN/CENTRAL
STATES.

CONSIDERING EACH EJECTING ELEMENT WITHIN THIS LONGWAVE PATTERN...A
SHARP BAND OF HEIGHT FALLS WILL BE IN THE PROCESS OF EJECTING OUT
OF THE SOUTHERN/CENTRAL ROCKIES WITH OPERATIONAL MODEL DIFFERENCES
EVIDENT BY AS EARLY AS DAY 3/TUESDAY. AS HAS BEEN COMMON IN RECENT
DAYS...THE 00Z GFS AND PRECEDING RUNS FAVOR A QUICKER DOWNSTREAM
PROGRESSION WHICH ALLOWS FOR SPATIAL DIFFERENCES IN FRONTAL
PLACEMENTS. WHILE THE GFS SOLUTIONS WOULD SUPPORT A COLD FRONT
PASSING THROUGH THE I-95 CORRIDOR ON WEDNESDAY...OTHER GLOBAL
MODELS KEEP THIS BOUNDARY BACK TOWARD THE APPALACHIANS AND MOVE
THE FRONT OFF THE EASTERN SEABOARD BY THE FOLLOWING DAY. THE
SOUTHWESTERN FLANK OF THIS BAROCLINIC ZONE SHOULD BE SLOWER MOVING
ACCOMPANIED BY A SERIES OF WEAK PERTURBATIONS ALONG THE FRONT.
THIS SUPPORTS AN EXTENDED PERIOD OF MODERATE/HEAVY RAINFALL FROM
THE ARKLATEX INTO THE OZARKS/LOWER MS VALLEY AS THE MID-LEVEL FLOW
GENERALLY ALIGNS WITH THIS MENTIONED BOUNDARY.

THE UPSTREAM PATTERN OVER THE EASTERN PACIFIC/WESTERN U.S. REMAINS
SOMEWHAT COMPLEX AS THERE IS A SEPARATION IN THE JETS. THE PRIOR
THREE RUNS OF THE GFS FAVOR A WESTWARD RETROGRESSION OF A
SHORTWAVE BACK TOWARD 130W WHICH IS NOT SUPPORTED BY OTHER GLOBAL
MODELS. OTHER SOLUTIONS FAVOR FORWARD PROGRESSION INTO THE DESERT
SOUTHWEST BY MID-WEEK ALTHOUGH ENSEMBLE SPAGHETTI PLOTS SHOW QUITE
A BIT OF SCATTER AS WAS EXPECTED. ADDITIONAL SHORTWAVE ENERGY
SWEEPS THROUGH THE PACIFIC NORTHWEST WITH EVENTUAL PLACEMENT
UNKNOWN GIVEN VAST MODEL SPREAD. THE 00Z/18Z GFS FAVOR ADJOINING
THIS SYSTEM WITH THE NORTHERN STREAM WHILE THE OTHER GUIDANCE TAKE
A MORE SOUTHWARD TRACK INTO THE CENTRAL GREAT BASIN/SOUTHWESTERN
U.S. ALL AND ALL...WHILE THE GUIDANCE AGREE ON A NEGATIVE HEIGHT
ANOMALY HOLDING ON OVER THE WESTERN THIRD OF THE NATION...THE
DETAILS VARY GREATLY.

WHILE THE GFS SOLUTIONS APPEARED ON THEIR OWN THROUGHOUT THE
PERIOD...DID NOT WANT TO DISCOUNT THEM JUST YET. THUS...FAVORED A
50/50 BLEND OF THE 12Z GFS/ECMWF THROUGH DAY 4/WEDNESDAY WHILE
QUICKLY ADDING ENSEMBLE MEANS TO THE PICTURE FOR THE REMAINING FEW
DAYS. GIVEN THE COMPLEXITY OF THE FLOW DEEPER INTO THE
PERIOD...LOWERED OPERATIONAL CONTRIBUTIONS TO AROUND 20 PERCENT BY
DAY 6/FRIDAY BEFORE GOING TO ALL ENSEMBLE MEANS THEREAFTER. THIS
WAS LED BY THE 12Z ECMWF ENSEMBLE MEAN WHICH RECEIVED HALF OF THE
WEIGHTING WHILE SPLITTING THE REMAINDER BETWEEN THE 12Z GEFS/NAEFS
ENSEMBLE MEANS.


...WEATHER HIGHLIGHTS/THREATS...

THE PERSISTENT HIGH-AMPLITUDE PATTERN WILL MAKE FOR AN ACTIVE
PERIOD IN TERMS OF PRECIPITATION ALONG WITH SOME VERY IMPRESSIVE
TEMPERATURE ANOMALIES. IN SOME CASES...DEPARTURES FROM CLIMATOLOGY
WILL BE ON THE ORDER OF 30 TO 40 DEGREES...PARTICULARLY WITHIN THE
DOME OF ARCTIC AIR. ON TUESDAY/WEDNESDAY...FRIGID TEMPERATURES
SHOULD FOCUS OVER THE NORTHERN/CENTRAL ROCKIES INTO THE GREAT
PLAINS. THE COLDEST AIR IS EXPECTED ACROSS THE NORTHERN ROCKIES AS
HIGHS BARELY MOVE ABOVE 0 DEGREES WHILE OVERNIGHT TEMPERATURES
DESCEND INTO THE NEGATIVE TEENS OR EVEN SLIGHTLY LOWER. SOME
MODERATION IS EXPECTED BEYOND WEDNESDAY ALTHOUGH COOLER THAN
AVERAGE TEMPERATURES WILL PREVAIL. EVEN THE PACIFIC NORTHWEST WILL
GET IN ON THE ACTION AS ARCTIC AIR CROSSES THE CONTINENTAL DIVIDE
AS OVERNIGHT LOWS PLUNGE INTO THE LOWER 20S ALL THE WAY TO SEATTLE
WA AND PORTLAND OR.

THE EXPANSIVE WARM SECTOR STRETCHING FROM THE SOUTH-CENTRAL U.S.
UP TOWARD NEW ENGLAND WILL FEATURE SOME EXTREME ANOMALIES AS
WELL...MAXIMIZING IN THE 30 TO 35 DEGREE RANGE FOR OVERNIGHT
MINIMA. WIDESPREAD RECORDS MAY BE BROKEN ON TUESDAY/WEDNESDAY
MORNINGS AS MORNING LOWS STAY IN THE 50S AS FAR NORTH AS THE OH
VALLEY/LOWER GREAT LAKES. CONSIDERING FORECAST HIGHS...70S ARE
EXPECTED UP INTO THE OH VALLEY...CENTRAL APPALACHIANS...AND
MID-ATLANTIC. A FEW 80 DEGREE READINGS COULD POP UP IN THIS REGION
THROUGH MID-WEEK. SOME COOLING IS EXPECTED LATER IN THE WEEK
ALTHOUGH THIS COULD BE BRIEF AS A WARM FRONT LIFTS NORTHWARD UP
TOWARD 40N LONGITUDE BY EARLY NEXT WEEKEND.

WITH UNIDIRECTIONAL FLOW LIKELY SETTING UP ACROSS THE ARKLATEX UP
THROUGH THE OZARKS AND INTO THE OH VALLEY...MULTIPLE ROUNDS OF
HEAVY RAINFALL IS LIKELY...PARTICULARLY IN THE TUESDAY THROUGH
THURSDAY TIME SPAN. THEREAFTER...UNCERTAINTIES IN THE PATTERN MAKE
HEAVY RAINFALL PLACEMENT DIFFICULT. HOWEVER...WITH THIS BOUNDARY
STILL LOOMING OVER THE CENTRAL/EASTERN U.S...ADDITIONAL
PRECIPITATION IS LIKELY GIVEN LITTLE CHANGE IN THE SYNOPTIC SETUP.
GENERALLY FLOODING CONCERNS MAY BE AN ISSUE WITH THIS MULTI-DAY
HEAVY RAINFALL THREAT IN PLACE. THE DURATION OF THIS EVENT COULD
LEAD TO GENERAL FLOODING CONCERNS BUT SPRINGTIME WARMTH/MOISTURE
RETURN COULD SUPPORT SOME FLASH FLOODING ISSUES GIVEN SUFFICIENT
INSTABILITY AND FAVORABLE BACKBUILDING ACTIVITY.
ELSEWHERE...UNSETTLED CONDITIONS ARE TO RETURN TO THE PACIFIC
NORTHWEST GIVEN THE SERIES OF SHORTWAVES MOVING THROUGH. MODIFIED
ARCTIC AIR WILL LOWER SNOW LEVELS APPRECIABLY TO BRING SOME
ACCUMULATING SNOWS TO VALLEY LOCALES IN ADDITION TO THE USUAL
HIGHER TERRAIN.


RUBIN-OSTER


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