092  
FXUS02 KWBC 200657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EST TUE FEB 20 2018  
 
VALID 12Z FRI FEB 23 2018 - 12Z TUE FEB 27 2018  
 
...CLASHING AIR MASSES SET STAGE FOR AN ARKLATEX TO TN/OH VALLEYS  
HEAVY RAINFALL THREAT...  
...ARCTIC AIR DROPS THROUGH THE WEST/CENTRAL U.S. WITH RECORD  
WARMTH ACROSS THE EAST...  
 
   
..PATTERN OVERVIEW
 
 
A BROAD UPPER RIDGE INITIALLY EXTENDING FROM THE GULF OF MX INTO  
THE SOUTHWESTERN ATLANTIC WILL CONTINUE TO ERODE AS A SERIES OF  
UPSTREAM SYSTEMS EFFECTIVELY LOWER HEIGHTS OVER THE EASTERN U.S. A  
BLOCKING PATTERN IS IN PLACE OVER THE EASTERN PACIFIC WITH A STOUT  
UPPER RIDGE REMAINING FIXED OVER THE REGION. THIS RESULTS IN A  
STEADY STREAM OF SOUTHWARD PROPAGATING SHORTWAVES WHICH RACE DOWN  
THE WESTERN COAST OF NORTH AMERICA AND EVENTUALLY INTO THE  
WESTERN/CENTRAL STATES. THE INITIAL SYSTEM MOVING FROM THE DESERT  
SOUTHWEST TOWARD THE GREAT PLAINS ON FRIDAY/SATURDAY SHOULD BE A  
PATTERN CHANGER AS IT WILL FINALLY SHIFT A QUASI-STATIONARY  
BOUNDARY ACROSS THE ARKLATEX/OZARKS REGION DOWNSTREAM. THIS COLD  
FRONT IS EXPECTED TO EXIT THE EASTERN SEABOARD BY LATE SUNDAY  
WHILE THE BOUNDARY STALLS ALONG THE IMMEDIATE GULF COAST INTO  
EARLY NEXT WEEK. ELSEWHERE...A PAIR OF ADDITIONAL SYSTEMS LOOM  
OVER THE WEST COAST...EACH KEEPING ACTIVE PRECIPITATION OVER THE  
REGION ACCOMPANIED BY RATHER LOW EXPECTED SNOW LEVELS.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
WHILE GLOBAL MODEL DIFFERENCES ARE EVIDENT DURING THE PERIOD...A  
PATTERN SHIFT WILL BE UNDERWAY AS THE PERSISTENT SUBTROPICAL RIDGE  
OVER THE SOUTHEASTERN U.S. BEGINS TO BUCKLE. CONSIDERING THE  
INITIAL BAND OF HEIGHT FALLS EJECTING OUT OF THE DESERT  
SOUTHWEST...MULTI-DAY ENSEMBLE SPAGHETTI PLOT COMPARISONS HAVE  
SHOWN A MARKED DECREASE IN SPREAD. HOWEVER...THERE IS PLENTY OF  
NOISE IN THE PLOTS INTO THE WEEKEND WITH EMERGING TIMING/AMPLITUDE  
DIFFERENCES. ABUNDANT MOVEMENT IS NOTED AMONG THE 00Z GUIDANCE  
WHICH HAS DECREASED THE SPREAD FROM THE FORMER CYCLE. BY  
24/1200Z...THE PREVIOUSLY OUTLYING 12Z CMC/UKMET HAVE MADE  
QUICKER/SLOWER ADJUSTMENTS...RESPECTIVELY...TO BRING BETTER MODEL  
CLUSTERING TO THE TABLE. HOWEVER...FURTHER SHORTWAVE DETAIL  
DIFFERENCES FURTHER COMPLICATE THE FORECAST OVER THE OH  
VALLEY/GREAT LAKES REGION. DEPENDING ON THE AMPLITUDE OF THIS  
FEATURE...A RATHER DEVELOPED CYCLONE MAY CROSS THE OH VALLEY/GREAT  
LAKES ON SUNDAY MORNING. ENSEMBLE LOW PLOTS INDICATE QUITE A BIT  
OF SCATTER WITH OPERATIONAL RUNS SHOWING VERY LITTLE CONSISTENCY  
IN PLACEMENT AND INTENSITY. MOST NOTABLY...THE 00Z GFS IS NOW THE  
DEEPEST SOLUTION FAVORING A 984-DM LOW OVER LAKE MI ON 25/1200Z.  
FURTHER...THE 12Z ECMWF APPEARED TO BE AN OUTLIER OF SORTS AS IT  
SITS WELL EAST OF ITS ENSEMBLE MEAN WITH ONLY ABOUT 5 OF ITS 50  
MEMBERS IN THE NEIGHBORHOOD. EVENTUALLY THIS ENTIRE TROUGH WILL  
SWEEP OFFSHORE WITH THE FRONTAL ZONE EXITING BY ROUGHLY SUNDAY  
EVENING.  
 
LOOKING FARTHER UPSTREAM...OPERATIONAL MODELS CONTINUE TO STRUGGLE  
WITH HEIGHT FALLS SWINGING THROUGH THE PACIFIC NORTHWEST ON SUNDAY  
MORNING. AS WAS THE CASE WITH THE PRECEDING SYSTEM...THE 00Z  
CMC/UKMET HAVE MADE POSITIVE ADJUSTMENTS WITH NOTABLE SLOWING  
TRENDS. THIS AT LEAST BRINGS THE CORE OF LOWER HEIGHTS BACK INTO  
THE PACIFIC NORTHWEST ON 25/1200Z. ENSEMBLE MEMBERS ARE RATHER  
SCATTERED ABOUT SO COULD SEE FUTURE ADJUSTMENTS IN LATER MODEL  
RUNS. THIS SHORTWAVE SHOULD EJECT EASTWARD INTO THE NORTH-CENTRAL  
U.S. BY EARLY MONDAY WHICH MAY SPIN UP ANOTHER AREA OF LOW  
PRESSURE. THE 00Z GFS/12Z ECMWF AGREE ON A WAVE OVER IA ON  
26/1200Z BUT PREVIOUS RUNS HAVE NOT BEEN CONSISTENT AS THE GFS HAS  
BEEN PLACING EMPHASIS ON A SYSTEM OVER SOUTH-CENTRAL CANADA  
RATHER. ELSEWHERE...AN EVEN MORE IMPRESSIVE UPPER LOW TAKES AIM ON  
THE NORTHERN CA/OR COAST ON TUESDAY WITH 500-MB HEIGHT DEPARTURES  
POSSIBLY EXCEEDING 2 TO 2.5 STANDARD DEVIATIONS. ACCOMPANYING THIS  
SYSTEM WOULD BE RATHER COLD 1000-500 MB THICKNESSES...POTENTIALLY  
BELOW 530-DM. WHILE OPERATIONAL MODELS AGREE ON THE FEATURE...THEY  
VARY WITH PLACEMENT AND WOULD EXPECT FUTURE CHANGES GIVEN IT IS A  
DAY 6/7 FORECAST.  
 
INITIALLY LEANED ON A MULTI-OPERATIONAL MODEL CONSENSUS OF THE 18Z  
GFS/12Z ECMWF WITH THE 12Z UKMET FOR DAY 3/FRIDAY BEFORE SLOWLY  
ADDING ENSEMBLE MEANS TO THE PICTURE. TRIED TO LOWER CONTRIBUTIONS  
FROM THE 12Z ECMWF ON DAY 5/SUNDAY GIVEN THE QUICKER OH  
VALLEY/LOWER GREAT LAKES WAVE. EVENTUALLY MOVED TOWARD A FULLY  
ENSEMBLE-BASED APPROACH INTO NEXT WEEK GIVEN GROWING DIFFERENCES  
ACROSS THE BOARD. AN EVEN SPLIT OF THE 18Z GEFS/12Z ECMWF ENSEMBLE  
MEANS WAS UTILIZED.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE BIG STORY WILL REMAIN THE HEAVY RAINFALL EXPECTED FROM THE  
ARKLATEX UP THROUGH THE OZARKS AND INTO THE OH VALLEY. WITH  
POSSIBLY 4 TO 6 INCHES OF RAIN POSSIBLE BEFORE THE START OF THIS  
PERIOD...ANY ADDITIONAL PRECIPITATION MAY EXACERBATE FLOODING  
ISSUES. WHILE AMOUNTS ARE TAME COMPARED TO THAT DURING THE SHORT  
RANGE...RIVERS WILL LIKELY BE AT VARIOUS FLOOD STAGES WHICH MAY  
POSE HAZARDOUS CONDITIONS TO SURROUNDING LOCATIONS. REGARDING  
EXPECTED RAINFALL...THE HEAVIER AMOUNTS SHOULD BE SHIFTING AWAY  
FROM THE ARKLATEX AND MOVING INTO THE OH/TN VALLEYS WHERE SOME OF  
THE BETTER DYNAMICS WILL RESIDE. EVENTUALLY THE SYSTEM WILL EXIT  
THE EAST COAST WITH SOME TRAILING BAND OF LIGHT/MODERATE RAIN  
EXTENDING OVER THE GULF COAST INTO EARLY NEXT WEEK. THERE ARE SOME  
HINTS OF ENHANCED ACTIVITY PUSHING TOWARD THE UPPER TX/LA COAST  
BUT DETAILS REMAIN NEBULOUS. REGARDING WINTRY PRECIPITATION...A  
RIBBON OF SNOW MAY FALL FROM THE UPPER MIDWEST INTO INTERIOR NEW  
ENGLAND. OUT WEST...A WET PERIOD IS LIKELY OVER THE PACIFIC  
NORTHWEST INTO THE UPPER INTERMOUNTAIN WEST. SNOW LEVELS SHOULD  
CONTINUE TO FALL WITH THE LOWERING THICKNESS VALUES. THE EARLIER  
MENTIONED UPPER LOW COULD BRING SOME HEAVIER PRECIPITATION TO  
AREAS OF NORTHERN CA BY MONDAY WITH SNOW POTENTIALLY HUGGING THE  
COAST GIVEN THE MAGNITUDE OF COLD AIR ALOFT.  
 
WITH SPRINGTIME MOISTURE RETURN LIFTING UP FROM THE LOWER MS  
VALLEY INTO THE EASTERN U.S...RATHER MILD OVERNIGHT TEMPERATURES  
WILL PREVAIL AS FORECAST ANOMALIES RANGE FROM 20 TO 30 DEGREES  
ABOVE CLIMATOLOGY. THIS WILL PARTICULARLY BE THE CASE OVER THE  
MID-SOUTH INTO THE TN/LOWER VALLEYS AS OVERNIGHT READINGS REMAIN  
IN THE UPPER 50S TO LOWER 60S. MANY DAILY TEMPERATURE RECORDS  
COULD BE BROKEN GIVEN SUCH WARM OVERNIGHT LOWS EXPECTED.  
CONDITIONS WILL REMAIN MILD DURING THE DAYS AS WELL ALTHOUGH  
RECORDS SHOULD BE BROKEN MORE ON AN ISOLATED BASIS. ON THE  
CONTRARY...COLD WEATHER WILL MAINTAIN ITS STRANGEHOLD OVER THE  
WESTERN/CENTRAL STATES. WHILE THE SHARPEST DEPARTURES FROM AVERAGE  
ARE FORECAST OVER THE NORTHERN ROCKIES...GENERALLY IN THE 20 TO 30  
DEGREE RANGE...THE ENTIRE REGION WILL BE THE CHILLY SIDE. NEAR  
FREEZING TEMPERATURES SHOULD CONTINUE TO BE A DAILY OCCURRENCE  
EACH MORNING OVER THE PACIFIC NORTHWEST AND EXTENDING INTO VAST  
SECTIONS OF CA. WHILE THE MAJOR CITY CENTERS SHOULD STAY ABOVE  
FREEZING...MANY OF THE COOLER VALLEY LOCATIONS CAN EXPECT A FREEZE.  
 
RUBIN-OSTER  
 

 
 
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