Prognostic Meteorological Discussion
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710
FXUS02 KWBC 190652
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
151 AM EST MON FEB 19 2018

VALID 12Z THU FEB 22 2018 - 12Z MON FEB 26 2018

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


...PATTERN OVERVIEW...

AN EXPANSIVE UPPER RIDGE WILL REMAIN FIXED OVER A BROAD REGION
STRETCHING FROM THE GULF OF MX INTO THE WESTERN ATLANTIC. UPSTREAM
HEIGHT FALLS SHOULD GRADUALLY ERODE THE NORTHERN EXTENSION OF THIS
POSITIVE HEIGHT ANOMALY. WHILE 500-MB HEIGHTS COULD EXCEED 594-DM
ON THURSDAY...THE VALUES SHOULD DROP CONSIDERABLY BY THE UPCOMING
WEEKEND. THIS WILL BE IN RESPONSE TO A SERIES OF EJECTING
SHORTWAVES WITHIN THE PERSISTENT WESTERN/CENTRAL U.S. MEAN UPPER
TROUGH. AS THESE FEATURES ADJOIN THE BROAD SOUTHWESTERLY FLOW
EXTENDING FROM THE ARKLATEX TO THE OH/TN VALLEYS...MULTIPLE ROUNDS
OF HEAVY RAINFALL ARE POSSIBLE ALONG A QUASI-STATIONARY BAROCLINIC
ZONE. WITH SEVERAL INCHES OF RAIN LIKELY BEFORE THURSDAY
MORNING...THE ADDITIONAL PRECIPITATION MAY LEAD TO WIDESPREAD
HYDROLOGIC ISSUES.


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

ON THURSDAY...GLOBAL MODELS HAVE ESTABLISHED SUFFICIENT AGREEMENT
WITH THE MEAN UPPER RIDGE ACROSS THE WESTERN ATLANTIC. BASED ON
THE 00Z GFS...INITIAL 500-MB HEIGHT DEPARTURES WILL BE AT LEAST
2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY WITH SUCH ANOMALIES
GRADUALLY DIMINISHING IN TIME. WITH HEIGHTS GRADUALLY LOWERING
OVER NEW ENGLAND...A BACK DOOR FRONT SHOULD DROP SOUTHWARD INTO
THE LOWER MID-ATLANTIC ON THURSDAY. THIS WILL SUPPLY COOLER
WEATHER TO THE NORTHEASTERN U.S. AS A 1040-MB ANTICYCLONE SETS UP
OVER SOUTHERN QUEBEC. THE BOUNDARY IN QUESTION ACTUALLY STRETCHES
BACK THROUGH THE TN VALLEY AND INTO THE OZARKS AND SOUTHERN PLAINS
WITH ITS POSITION GENERALLY MEANDERING TO THE NORTH AND SOUTH.
MULTIPLE UPSTREAM SHORTWAVES SHOULD ADJOIN THE AXIS OF MID-LEVEL
CONFLUENCE OVER THE CENTER OF THE COUNTRY LEADING TO MULTIPLE
ROUNDS OF HEAVY RAINFALL. IDENTIFYING THE TIMING AND PLACEMENT OF
EACH SUCH IMPULSE IS SOMEWHAT CHALLENGING IN SPITE OF THE
LARGE-SCALE AGREEMENT OVER THE WESTERN STATES.

CONSIDERING THE INITIAL POSITIVELY-TILTED TROUGH ACROSS THE
SOUTHWESTERN U.S...MULTI-CYCLE COMPARISONS HAVE SHOWN A SLOWING
TREND. THIS BRINGS MANY SOLUTIONS BACK TOWARD THE 12Z ECMWF
ENSEMBLES WHICH HAVE BEEN GENERALLY STABLE DATING BACK TO A FEW
RUNS AGO. BY 23/1200Z...THE 12Z ECMWF/UKMET ARE A BIT SLOWER THAN
THE WELL CLUSTERED ENSEMBLE MEANS BUT THERE ARE PLENTY OF ENSEMBLE
MEMBERS BACK ON THE WESTERN EDGE OF THE SPREAD. UNCERTAINTY STILL
REMAINS AS RUN-TO-RUN MODEL CONSISTENCY IS NOT GREAT WITH THE 00Z
UKMET ACCELERATING TOWARD THE EAST WHILE THE 00Z CMC SHIFTED BACK
TO THE 12Z ECMWF. THE ULTIMATE FATE OF THIS FEATURE IS UP FOR
DEBATE AS IT QUICKLY MOVES TOWARD THE MIDDLE MS VALLEY BY SOMETIME
THIS WEEKEND. THE 00Z/18Z/12Z GFS CYCLES HAVE BEEN QUICKER AND OF
HIGHER AMPLITUDE WHICH CARRIES A NOTABLE SURFACE CYCLONE INTO THE
OH VALLEY BY SUNDAY MORNING. RECENT RUNS OF THE CMC/UKMET/ECMWF
HAVE NOT SHOWN THIS AS THEY ARE MORE SHEARED ALOFT WITH ONLY THE
ECMWF FROM PREVIOUS DAYS DEPICTING A MORE INTENSE SURFACE
REFLECTION.
GIVEN THIS...WOULD SUGGEST THE GFS IS ON ITS OWN UNTIL FURTHER
NOTICE.

WITH A MEAN UPPER RIDGE CONTINUING TO DOMINATE THE EASTERN
PACIFIC...A COUPLE MORE NOTABLE SHORTWAVES ENTER THE PICTURE OVER
THE PACIFIC NORTHWEST ON DAY 5/FEBRUARY 24 AND DAY 7/FEBRUARY 26.
PLACEMENT DIFFERENCES ARE CONSIDERABLY ON 25/0000Z AMONG THE
OPERATIONAL MODELS WITH VERY LITTLE CONSISTENCY BETWEEN RUNS. THE
NET RESULT OF THESE HEIGHT FALLS IS TO SHIFT THE MEAN UPPER TROUGH
EASTWARD TOWARD THE MS RIVER VALLEY BY EARLY NEXT WEEK. EVENTUALLY
AN EVEN MORE INTENSE UPPER LOW LURKS OVER THE NORTHWESTERN U.S. BY
DAY 7/FEBRUARY 26 WITH ENSEMBLE MEANS EVEN SHOWING A NOTABLE DENT
IN THE MID-LEVEL HEIGHT FIELDS.

THROUGH FRIDAY...FELT THERE WAS ENOUGH CONFIDENCE IN THE PATTERN
TO SUPPORT A MULTI-OPERATIONAL MODEL CONSENSUS LED BY THE 12Z
ECMWF/GFS WITH SMALLER CONTRIBUTIONS FROM THE 12Z CMC/UKMET. FROM
DAY 5/SATURDAY ONWARD...SHIFTED TOWARD A LARGER COMBINATION OF THE
12Z ECMWF/ECMWF ENSEMBLE MEAN...70 PERCENT...WITH THE REMAINDER
GIVEN TO THE 12Z GEFS MEAN. THE FORMER EXHIBITED A GREATER DEAL OF
CONSISTENCY AND MANY SOLUTIONS HAVE MOVED IN ITS DIRECTION WITH
PRECEDING SYSTEMS.


...WEATHER HIGHLIGHTS/THREATS...

WITH SEVERAL INCHES OF RAINFALL OCCURRING BEFORE 22/1200Z...THERE
MAY ALREADY BE ONGOING HYDROLOGIC ISSUES ANYWHERE FROM THE
ARKLATEX INTO THE MO/OH RIVER VALLEYS. THE CURRENT DAY 1-3
PRECIPITATION FORECAST HIGHLIGHTS AN AXIS OF 4 TO 6 INCHES FROM
NORTHEAST TX INTO SOUTHERN AR. BASED ON THE LATEST OUTPUT FROM THE
GLOBAL GUIDANCE...ADDITIONAL RAINFALL IS LIKELY OVER THESE REGIONS
ON THURSDAY THROUGH PERHAPS SATURDAY. INTENSE 850-700 MB MOISTURE
FLUX ANOMALIES WILL OVERLAP THIS REGION WITH DEPARTURES FROM
AVERAGE LIKELY IN THE 3 TO 4 STANDARD DEVIATIONS RANGE AT TIMES.
WITH THIS EXPECTATION OF MORE INTENSE RAINFALL AND WET ANTECEDENT
CONDITIONS...ADDITIONAL FLOOD AND FLASH FLOOD CONCERNS WILL BE A
MAJOR ISSUE. WHILE DETAILS WILL CHANGE IN THE UPCOMING DAYS...THE
THREAT SHOULD REMAIN INTO THE WEEKEND AND EVEN BEYOND GIVEN ANY
APPRECIABLE RIVER LEVEL RISES.

OTHER AREAS OF INTEREST WILL BE ALONG THE NORTHERN EXTENT OF THIS
PRECIPITATION SHIELD AS SNOW COULD FALL ANYWHERE FROM THE UPPER
MIDWEST/GREAT LAKES AND INTO SECTIONS OF INTERIOR NORTHEAST. AND
ACROSS THE WESTERN U.S...WHILE THE SYSTEMS ARE NOT FORECAST TO TAP
INTO THE BEST PACIFIC MOISTURE FEEDS...IT SHOULD REMAIN UNSETTLED
OVER THE REGION. IT DOES APPEAR SNOW LEVELS WILL DROP CONSIDERABLY
AS 1000-500 THICKNESSES DECREASE MARKEDLY WITH A COUPLE OF THESE
SYSTEMS. THIS WOULD ULTIMATELY SPREAD SNOW INTO MANY OF THE LOWER
ELEVATIONS ACROSS THE NORTHWESTERN U.S.

WITH HIGH AMPLITUDE FLOW IN PLAY...A MARKED TEMPERATURE CONTRAST
WILL EXIST ALONG THIS MENTIONED BAROCLINIC ZONE. WITHIN THE
EXPANSIVE WARM SECTOR...THERE WILL BE MANY CHANCES TO BREAK DAILY
TEMPERATURE RECORDS FROM THE DEEP SOUTH ALL THE WAY INTO NEW
ENGLAND...PARTICULARLY ON THURSDAY. MORE SPECIFICALLY...IT WILL BE
THE WARM OVERNIGHT MINIMA THAT SHOULD BE THE STORY.  AS HIGH
THETA-E AIR SHOULD CONTINUE TO BE TRANSPORTED WITHIN AN ACTIVE
PLUME OF GULF MOISTURE...OVERNIGHT LOWS WILL BE AFFECTED GIVEN
OVERCAST/RAINY CONDITIONS. DEPARTURES FROM CLIMATOLOGY SHOULD
CONTINUE TO BE AROUND 15 TO 25 DEGREES WITH BELOW FREEZING
TEMPERATURES NOT BEING FOUND UNTIL INTERIOR NEW ENGLAND AND AT
TIMES THE LOWER GREAT LAKES.

ACROSS THE WESTERN/CENTRAL U.S...IT WILL REMAIN QUITE COLD AS
MODIFIED ARCTIC AIR INFILTRATES THE REGION. GENERALLY
SPEAKING...DEPARTURES FROM AVERAGE WILL BE AROUND 1O TO 20 DEGREES
ALTHOUGH SLIGHTLY HIGHER NUMBERS ARE EXPECTED ACROSS THE NORTHERN
ROCKIES AS HIGHS STRUGGLE TO ESCAPE THE SINGLE DIGITS AND TEENS ON
THURSDAY/FRIDAY. GIVEN EXPECTED LOWS BELOW FREEZING ALONG A
SIGNIFICANT CHUNK OF THE WEST COAST...MANY REGIONS CAN EXPECT A
FREEZE TO OCCUR. THIS COULD EVEN AFFECT SECTIONS OF THE CA COAST
ALTHOUGH THE MAJOR CITY CENTERS INCLUDING SAN FRANCISCO...LOS
ANGELES...AND SAN DIEGO WILL ESCAPE THIS.


RUBIN-OSTER


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