986  
FXUS02 KWBC 191534  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1033 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
 
 
VERY STRONG UPPER RIDGING OFF THE CAROLINAS WILL ONLY SLOWLY  
WEAKEN THROUGH THE PERIOD WITH TROUGHING REMAINING IN THE WESTERN  
CONUS. A LEAD SHORTWAVE WILL LIFT OUT OF THE FOUR CORNERS REGION  
THIS WEEKEND INTO THE GREAT LAKES AND NORTHEAST SUN/MON WITH  
ANOTHER PACIFIC SYSTEM DROPPING NEARING THE WEST COAST BY NEXT  
MONDAY. A WAVY FRONTAL BOUNDARY IN THE EAST WILL BE THE FOCUS FOR  
LOCALLY HEAVY RAINFALL THROUGH THE WEEK INTO THE START OF THE  
WEEKEND WITH WARM SPRING-LIKE TEMPERATURES TO ITS SOUTH AND WELL  
BELOW AVERAGE TEMPERATURES TO ITS NORTH.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
LATEST MODEL GUIDANCE SHOWED GOOD OVERALL AGREEMENT ON THE PATTERN  
TO START ON THURSDAY, WITH THE GUIDANCE AGAIN TRENDING A BIT  
FARTHER SOUTH WITH THE FRONT THROUGH VIRGINIA. IN THE WEST, UKMET  
BECAME THE FIRST TO DEPART FROM THE GOOD DETERMINISTIC CONSENSUS  
WITH A QUICKER EJECTION OF THE SHORTWAVE THAN THE GFS RUNS,  
FOLLOWED BY THE SLOWER ECMWF/CANADIAN. BLEND OF THOSE CAMPS WAS  
THE PREFERENCE AS THAT SHORTWAVE LIFTED NORTHEASTWARD (THOUGH LESS  
WEIGHT TO THE SLOWER CANADIAN GIVEN THE PROGRESSIVE SW FLOW). BY  
THE WEEKEND, GFS RUNS WERE DEEPER WITH HEIGHT FALLS OUT OF THE  
NORTHEAST PACIFIC (GULF OF ALASKA) AND WAS FARTHER SW THAN THE  
ECMWF ENSEMBLES. BASED MOST OF THE REST OF THE FORECAST ON THE 00Z  
ECMWF/ECMWF ENSEMBLE MEAN AND 06Z GEFS MEAN, THOUGH KEPT A SMALL  
PORTION OF THE GFS IN TO HELP RESOLVE SOME DETAIL IN THE EAST.  
THIS ALLOWED HEIGHTS TO BRIEFLY REBOUND THROUGH THE FOUR CORNERS  
REGION SUN/MON IN BETWEEN THE TWO SHORTWAVES.  
 
FRACASSO  
 
   
..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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