Prognostic Meteorological Discussion
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000 FXUS02 KWBC 161558 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1057 AM EST SAT DEC 16 2017 VALID 12Z TUE DEC 19 2017 - 12Z SAT DEC 23 2017 ...OVERVIEW... GUIDANCE CONTINUES TO SHOW A STRONG SIGNAL TOWARD AMPLIFICATION OF THE LARGE SCALE FLOW OVER NORTH AMERICA DURING THE NEXT WEEK. MEAN TROUGHING EXTENDING FROM THE BERING SEA TO HAWAI`I WILL HELP TO AMPLIFY A RIDGE JUST OFF THE WEST COAST AND EXTENDING WELL NORTHWARD THROUGH AND BEYOND ALASKA, WHILE POSITIVELY TILTED MEAN TROUGHING WILL BECOME ESTABLISHED FROM NEAR HUDSON BAY THROUGH THE CENTRAL U.S. AND LIKELY INTO NORTHERN MEXICO. THERE ARE SOME SIMILARITIES TO OTHER RECENT AMPLIFIED EPISODES OVER THE CONUS IN RECENT WEEKS, BUT A STARK DIFFERENCE THIS TIME SEEMS TO BE THE WESTERN ATLANTIC SUBTROPICAL RIDGE ATTEMPTING TO BUILD INTO THE SOUTHEASTERN STATES. THIS PATTERN WOULD EVENTUALLY FAVOR A PERSISTENT, SHARP, AND WAVY BAROCLINIC ZONE FROM NEAR THE SOUTHERN PLAINS NORTHEASTWARD AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO THE NORTH CENTRAL CONUS WHILE WARM AND MOIST AIR FROM THE GULF OF MEXICO SPREADS NORTH INTO THE LOWER MS VALLEY AND SOUTHEAST. ...GUIDANCE/PREDICTABILITY ASSESSMENT... CURRENT SATELLITE IMAGERY SEEMS TO BE SUPPORTING LATEST GUIDANCE TRENDS THROUGH THE 00Z/06Z CYCLES THAT UPPER TROUGH ENERGY NOW OVER THE WEST SHOULD FAIRLY SOON FORM A CLOSED LOW SUFFICIENTLY SEPARATED FROM THE WESTERLIES THAT IT WILL REACH NO FARTHER EAST THAN THE SOUTHERN ROCKIES/NORTHERN MEXICO BY THE START OF THE MEDIUM RANGE PERIOD. IMPROVED AGREEMENT IN THE SHORT RANGE EVOLUTION LEADS TO BETTER CLUSTERING THAN SEEN PREVIOUSLY AS THE REMAINING SHORTWAVE EJECTS ACROSS THE CENTRAL/EASTERN STATES. WIDE SPREAD/RUN TO RUN VARIABILITY IN PRIOR CYCLES STILL PROVIDE LINGERING UNCERTAINTY FOR THE FORECAST OF THIS FEATURE. THE CURRENT ARRAY OF GUIDANCE SUGGESTS THE 00Z-06Z GFS/GEFS MEAN RUNS MAY BE SOMEWHAT TOO SUPPRESSED WITH THE SURFACE REFLECTION CROSSING THE SOUTHERN TIER DURING THE FIRST HALF OF THE PERIOD. SIMILAR TO YESTERDAY, THROUGH ABOUT NEXT THU THE MAJORITY OF GUIDANCE APPEARS FAIRLY WELL CLUSTERED FOR THE UPPER TROUGH AMPLIFYING INTO THE WEST WITH AN EMBEDDED LOW CENTER DROPPING SOUTHEAST FROM NEAR HAIDA GWAII TO UTAH. THEN MODELS/ENSEMBLES DIVERGE AND VARY FROM RUN TO RUN REGARDING THE STRENGTH OF HEIGHT FALLS ACROSS THE NORTHERN TIER/GREAT LAKES AND HOW MUCH TROUGHING LINGERS OVER THE SOUTHWEST. DIFFERENCES IN TELECONNECTION FAVORED FLOW FOR THE SOUTHWEST U.S. RELATIVE TO NEGATIVE HEIGHT ANOMALIES NORTH OF HAWAI`I (HIGHER HEIGHTS) AND POSITIVE ANOMALIES NEAR THE ALASKA PANHANDLE (MORE TROUGHING) IN RECENT D+8 MEAN CHARTS SUPPORT THE IDEA THAT THE SOUTHWEST U.S. IS THE AREA OF GREATEST UNCERTAINTY. THIS FAVORS AN INTERMEDIATE SOLUTION FOR SOUTHWEST TROUGHING. LATE IN THE PERIOD THE 00Z/06Z GFS, 00Z CMC, AND 00Z ECMWF MEAN ARE CLOSEST TO A MIDDLE GROUND WHILE THE 00Z ECMWF STRAYS EASTWARD AND THE 06Z GEFS MEAN LAGS. FARTHER NORTHEASTWARD, ENSEMBLE CLUSTERING CONTINUES TO INDICATE THAT LOW PRESSURE EMERGING FROM THE WEST IS MOST LIKELY TO REACH THE GREAT LAKES BY EARLY DAY 6 FRI AND NORTHEAST OF MAINE BY SAT. OVERALL THE OPERATIONAL SOLUTIONS FROM THE PAST DAY HAVE TENDED TO GRAVITATE TOWARD THE WEAKER HALF OF THE SPREAD FOR LOW PRESSURE DEPTH. SIMILAR STRENGTH IN THE ENSEMBLE MEANS WOULD SUGGEST THAT THERE ARE STILL SOME MEMBERS THAT KEEP A DEEPER SCENARIO IN PLAY. FINALLY THERE HAS BEEN DECENT AGREEMENT/CONTINUITY FOR A WEAK CLIPPER SYSTEM THAT MAY REACH THE UPPER GREAT LAKES BY LATE IN THE PERIOD. SUCH FEATURES TEND TO BE IN THE LOWER HALF OF THE PREDICTABILITY SPECTRUM SO IT WILL NOT BE SURPRISING IF SOME ASPECTS OF THIS SYSTEM CHANGE IN SUBSEQUENT DAYS. FOR THE MOST PART THE UPDATED FORECAST REFLECTS ONLY MINOR REFINEMENTS TO CONTINUITY. DAYS 3-5 EMPHASIZED THE OPERATIONAL MODELS WITH GFS/ECMWF/UKMET/CMC WEIGHTS SET TO REFLECT CONSENSUS DETAILS. DAYS 6-7 STARTED WITH A MORE EVEN WEIGHT OF MODEL/ENSEMBLE GUIDANCE WITH THE ECMWF MEAN GIVEN HIGHER PRIORITY THAN THE 06Z GEFS MEAN GIVEN BETTER DEFINITION OF THE GREAT LAKES SYSTEM AND INTERMEDIATE UPPER TROUGH POSITION OVER THE SOUTHWEST. ...WEATHER HIGHLIGHTS/THREATS... THE PACIFIC NORTHWEST AND NORTHERN/CENTRAL ROCKIES WILL BE UNSETTLED EARLY IN THE PERIOD AS A STRONG LOW PRESSURE SYSTEM MOVES INLAND TUE-WED. HEAVY RAINS ALONG THE COAST AND MOUNTAIN SNOWS WILL BE POSSIBLE. FARTHER EAST, CONFIDENCE IS INCREASING WITH RESPECT TO HEAVY RAIN POTENTIAL FROM THE SOUTHERN PLAINS TO THE SOUTHEAST/SOUTHERN APPALACHIANS. EXPECT ONE ROUND OF ENHANCED ACTIVITY TUE-WED IN ASSOCIATION WITH THE EJECTING SOUTHERN ROCKIES SHORTWAVE AND ASSOCIATED FRONTAL WAVE, FOLLOWED BY AN EPISODE OF SHOWERS AND THUNDERSTORMS FRI-SAT AS AN UPSTREAM COLD FRONT CROSSES THE REGION. THE FRONT MAY STALL IN THE GULF OF MEXICO WITH OVERRUNNING MOISTURE MAINTAINING PERIODS OF RAINFALL OVER THE SOUTHERN STATES. A COUPLE ROUNDS OF SNOW ARE POSSIBLE ACROSS THE NORTHERN PLAINS/UPPER MIDWEST THU-SAT, THE FIRST WITH A SURFACE LOW MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT LAKES ON FRI WITH POTENTIAL FOR IT TO BE ACCOMPANIED BY FAIRLY BRISK WINDS DEPENDING ON SYSTEM STRENGTH. A CLIPPER SYSTEM QUICKLY CROSSING THE NORTHERN PLAINS/UPPER MIDWEST LATE FRI INTO SAT COULD BRING ANOTHER ROUND OF SNOW ALONG WITH A MUCH COLDER AIR MASS. TEMPERATURES WILL INITIALLY BE ABOVE AVERAGE ACROSS MUCH OF THE CENTRAL/EASTERN U.S. TUE-THU BEFORE AN AREA OF ARCTIC HIGH PRESSURE BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO FRI, BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S. THE AFOREMENTIONED SUBTROPICAL RIDGE WILL KEEP ABOVE AVERAGE TEMPS IN PLACE ACROSS THE SOUTHEAST INTO NEXT WEEKEND. RAUSCH/RYAN $$

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