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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