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