EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 245 AM EDT MON MAR 26 2018 VALID 12Z THU MAR 29 2018 - 12Z MON APR 02 2018 ...HEAVY RAIN THREAT TO SPREAD FROM THE LOWER MS VALLEY NORTHEASTWARD TO THE APPALACHIANS... ...PATTERN OVERVIEW... A SOUTHERN STREAM SHORTWAVE WILL FINALLY BE ON MOVE AS IT ACCELERATES FROM THE ARKLATEX TOWARD THE CENTRAL APPALACHIANS THROUGH FRIDAY MORNING. THIS FEATURE IS FORECAST TO BECOME ABSORBED IN THE LARGE-SCALE LONGWAVE TROUGH TO THE NORTH WHILE ADDITIONAL HEIGHT FALLS ARE PRIMED TO SWING BACK THROUGH THE TN VALLEY IN ITS WAKE ON FRIDAY EVENING. A DEEPENING AREA OF LOW PRESSURE WOULD ULTIMATELY UNFOLD IN THIS SCENARIO WITH THIS WAVE LIKELY LIFTING INTO THE NORTHEASTERN U.S. DURING THE WEEKEND. THE HANDLING OF BOTH FEATURES REMAINS A CONTENTIOUS ASPECT OF THE UPCOMING FORECAST. WHILE THE RESULTANT PATTERN ACROSS THE COUNTRY FAVORS LONGWAVE TROUGHING FROM THE INTERMOUNTAIN WEST TO THE EASTERN SEABOARD...MODELS ARE AT ODDS WITH WHETHER TO EXTEND THIS NEGATIVE HEIGHT ANOMALY TOWARD THE WEST COAST OR BUILD HEIGHTS. ...MODEL EVALUATION/UNCERTAINTY ASSESSMENT/PREFERENCES... MANY UNCERTAINTIES LOOM EARLY IN THE FORECAST RESOLVING THE HANDLING OF THE INITIAL SHORTWAVE TRACKING TOWARD THE APPALACHIANS AND THE POTENTIAL FOR ADDITIONAL AMPLIFICATION IN ITS WAKE. THE 00Z GFS AND MANY PRECEDING RUNS PUT MORE EMPHASIS ON THE LEAD SYSTEM WITH A PRONOUNCED SURFACE CYCLONE LIFTING INTO NEW ENGLAND MIDDAY FRIDAY. WHILE SOME 12Z CMC/ECMWF ENSEMBLE MEMBERS DEPICT THIS SCENARIO...IT STILL SEEMS HEAVILY DRIVEN BY THE GEFS SOLUTIONS. IN ITS WAKE...THE ENSEMBLE SPAGHETTI PLOTS AND SOME OF THE 00Z MODELS OUTSIDE THE GFS SHOW AMPLIFIED FLOW SWEEPING THROUGH THE TN VALLEY ON FRIDAY EVENING. THIS WOULD INDUCE A POTENT SURFACE LOW WITH A TRACK NORTHEASTWARD TOWARD NEW ENGLAND EARLY IN THE WEEKEND. RUN-TO-RUN DIFFERENCES ARE NOTED WHICH DOES REDUCE THE CONFIDENCE IN TIMING/INTENSITY OF THE EVOLUTION OF RELEVANT FEATURES. HOWEVER...THERE IS A FAIRLY STRONG CONSENSUS FAVORING A BROAD LONGWAVE TROUGH EXTENDING FROM THE INTERMOUNTAIN WEST TO THE EASTERN U.S. WITH RATHER LOW HEIGHTS CENTERED UP OVER HUDSON BAY AS 500-MB HEIGHTS ARE BELOW 492-DM. ANOTHER ISSUE ARISES OVER THE EASTERN PACIFIC AND INTO THE WESTERN U.S. AS HIGH LATITUDE BLOCKING SETS UP OVER WESTERN AK THIS WEEKEND. THE HANDLING OF INDIVIDUAL FEATURES UNDERNEATH THIS BLOCK GETS A BIT MESSY AS SUGGESTED BY THE SCATTER IN THE ENSEMBLE SPAGHETTI PLOTS. THE 12Z CMC/ECMWF AND THEIR RESPECTIVE ENSEMBLES FAVOR THE BUILDING OF HEIGHTS ACROSS THE WEST WHILE THE LAST FEW GFS RUNS SHOW A TROUGH SWINGING THROUGH. CONSIDERING WHERE THESE DIFFERENCES EMERGE...IT APPEARS DRIVEN BY A LARGE UPPER LOW THE GFS DEPICTS OVER THE GULF OF AK WHICH PLUNGES SOUTHWARD...THUS LOWERING HEIGHTS. UNTIL THIS IS RESOLVED...MODELS WILL CONTINUE TO STRUGGLE WITH THE RESULTANT PATTERN DOWNSTREAM. THE MID-LATITUDE PATTERN DOES SHIFT EASTWARD BY DAY 7/APRIL 2...WHICH WILL HELP ADVANCE A FRONTAL ZONE TOWARD THE APPALACHIANS LATE IN THE PERIOD. GIVEN THE STRONGER CONSENSUS FORMED BY THE TRIO OF THE 12Z ECMWF/CMC/UKMET...DECIDED TO WEIGHT THE BLEND MORE HEAVILY IN THIS DIRECTION. THIS FAVORED THE SLOWER...MORE AMPLIFIED SCENARIO WITH LESS EMPHASIS ON THE LEAD WAVE AS SHOWN BY THE GFS RUNS. THROUGHOUT THE FORECAST...GENERALLY MINIMIZED USE OF THE GFS/GEFS MEAN WITH GREATER USE OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN WHILE BEING ACCOMPANIED BY A SMALLER FRACTION OF THE 12Z NAEFS ENSEMBLE MEAN. THE BLEND WAS MORE HEAVILY WEIGHTED TOWARD THE ENSEMBLES DURING THE DAY 6/7...APRIL 1/2 REALM. ...WEATHER HIGHLIGHTS/THREATS... HEAVY RAINFALL WILL REMAIN LIKELY FROM THE LOWER MS VALLEY NORTHEASTWARD ACROSS THE LOWER TN VALLEY AND INTO THE APPALACHIANS. ADDITIONALLY...IF THE THE SECOND WAVE WERE TO VERIFY...ANOTHER SWATH OF ORGANIZED PRECIPITATION WOULD UNFOLD OVER SECTIONS OF THE CENTRAL PLAINS EASTWARD INTO THE OH VALLEY/LOWER GREAT LAKES REGION AND NEW ENGLAND. WHILE THIS SYSTEM EXITS THE COUNTRY OVER THE WEEKEND...A BROAD SWATH OF PRECIPITATION MAY EXTEND ALONG AND NORTH OF A FRONTAL ZONE STRETCHING FROM THE UPPER INTERMOUNTAIN WEST INTO THE MIDDLE OF THE NATION. LARGE TIMING DIFFERENCES WITH THIS EVOLUTION PRECLUDES ANY ADDITIONAL DETAIL. REGARDING SNOWFALL THREATS...THE BEST CHANCES WOULD BE FOUND OVER THE HIGHER ELEVATIONS OF THE NORTHERN/CENTRAL ROCKIES AS WELL AS THE GREAT LAKES AND INTERIOR NEW ENGLAND LATE IN THE PERIOD. WITH ARCTIC AIR ABLE TO INVADE THE NORTHERN TIER...EXPECT BELOW NORMAL TEMPERATURES TO BE COMMONPLACE FROM THE NORTHERN ROCKIES EASTWARD INTO THE UPPER MIDWEST. FORECAST ANOMALIES WILL BE ON THE ORDER OF 20 TO 25 DEGREES REMOVED FROM CLIMATOLOGY WHICH SUPPORTS HIGHS ONLY IN THE 20S OVER THIS REGION. OVERNIGHT LOWS SHOULD PLUNGE INTO THE SINGLE DIGITS...PARTICULARLY OVER NORTHERN SECTIONS OF MT/ND/MN. THE EXPANSE OF COLD WILL EXTEND SOUTH AND EAST IN TIME BUT SHOULD MODIFY IN TIME. WHILE COLD GRASPS THE MIDDLE OF THE COUNTRY...IT SHOULD BE MILD ON BOTH THE WEST AND EAST COASTS. ACROSS THE LATTER...WARM ADVECTION IN ADVANCE OF THE SLOW MOVING TROUGH SHOULD BRING 60S UP THE I-95 CORRIDOR INTO THE DC METRO AREA BEFORE A COOLING TREND ENSUES BY THE WEEKEND. MEANWHILE...THE SOUTHWESTERN SECTOR OF THE U.S. SHOULD REMAIN UNDER GENERAL ZONAL FLOW TO FLAT RIDGING WHICH WILL KEEP TEMPERATURES ON THE WARMER SIDE. LOWS 90S WOULD BE POSSIBLE EACH DAY ACROSS THE DESERT SOUTHWEST. RUBIN-OSTER