336  
FXUS02 KWBC 290712  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
311 AM EDT THU SEP 29 2022  
 
VALID 12Z SUN OCT 02 2022 - 12Z THU OCT 06 2022  
 
...IAN FORECAST TO BRING HEAVY/EXCESSIVE RAINFALL AND BRISK TO  
STRONG WINDS TO PARTS OF THE APPALACHIANS AND MID-ATLANTIC THROUGH  
THIS WEEKEND AND POSSIBLY INTO EARLY NEXT WEEK...  
   
..OVERVIEW
 
 
THE PATTERN OVER THE LOWER 48 WILL BE FAIRLY BLOCKY TO START THE  
FORECAST ON SUNDAY, WITH A TROUGH/UPPER LOW OVER THE EAST  
(SUPPORTING THE EXTRATROPICAL REFLECTION OF IAN) AND A SLOW MOVING  
NORTHERN ROCKIES INTO PLAINS UPPER LOW BETWEEN RIDGES OVER THE  
WEST COAST AND CENTRAL-SOUTHERN PLAINS/MIDWEST. AFTER TUESDAY THE  
GUIDANCE SUGGESTS THAT FLOW WILL BEGIN TO TRANSITION TOWARD LARGER  
SCALE MEAN TROUGHING OVER EASTERN NORTH AMERICA IN RESPONSE TO  
MORE ROBUST UPPER RIDGING THAT EXTENDS INTO FAR WESTERN CANADA AND  
THE ALASKA PANHANDLE. MEANWHILE THE MODELS AND ENSEMBLE MEANS ARE  
STARTING TO SHOW A MORE COMMON EVOLUTION OVER THE EASTERN  
PACIFIC/WEST COAST BY WEDNESDAY-THURSDAY, WITH SOME WEAK SOUTHERN  
STREAM TROUGH ENERGY APPROACHING CALIFORNIA WHILE THE NORTHERN  
PART OF THE INITIAL TROUGH WEAKENS AS IT APPROACHES THE CORE OF  
POSITIVE HEIGHT ANOMALIES OVER THE ALASKA PANHANDLE/BRITISH  
COLUMBIA. THE TWO MAIN AREAS OF PRECIPITATION FOCUS WILL BE OVER  
THE MID-ATLANTIC WHERE MOISTURE FROM IAN MAY CONTINUE TO PRODUCE  
LOCALLY HEAVY RAINFALL INTO EARLY NEXT WEEK, AND PARTS OF THE  
ROCKIES/HIGH PLAINS WHERE THE UPPER LOW AND ASSOCIATED TROUGH WILL  
PRODUCE AREAS OF PRECIPITATION.  
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE STILL SHOWS CONSIDERABLE DISAGREEMENT FOR THE SPECIFICS  
OF THE EVOLUTION OVER THE EAST FROM SUNDAY ONWARD, INVOLVING THE  
EXTRATROPICAL REFLECTION OF IAN AND POTENTIAL INTERACTION OF ONE  
OR MORE NORTHERN STREAM IMPULSES. UKMET RUNS REMAIN ON THE OPEN  
AND PROGRESSIVE EXTREME BUT EVEN AMONG OTHER SOLUTIONS THERE IS A  
RANGE BETWEEN INCOMING ENERGY REINFORCING THE INITIAL UPPER LOW OR  
EJECTING IT (THEN POSSIBLY FORMING A NEW LOW). GIVEN THE TYPICAL  
SENSITIVITY OF SURFACE EVOLUTIONS TO SMALL SHORTWAVE DETAILS,  
CONFIDENCE IS EVEN LOWER FOR THE DETAILS OF LOW PRESSURE NEAR THE  
EAST COAST AFTER SUNDAY. MEANWHILE THE GFS CONTINUES TO GRAVITATE  
BACK TOWARD PRIOR REMAINING GUIDANCE FOR THE ROCKIES/PLAINS UPPER  
LOW THAT SHOULD OPEN UP BY MIDWEEK OR SO. A BLEND/ENSEMBLE  
APPROACH LOOKS BEST FOR RESOLVING THE VARYING SHORTWAVE DETAILS  
WITHIN SOUTHERN CANADA FLOW THAT BEGINS TO AMPLIFY INTO THE  
NORTHERN TIER STATES, GRADUALLY PUSHING A COLD FRONT INTO THE  
REGION TUESDAY-THURSDAY. FOR THE MOMENT GUIDANCE IS CONVERGING  
OVER THE EASTERN PACIFIC AFTER AT LEAST A COUPLE DAYS OF  
SIGNIFICANT SPREAD, WITH CONSENSUS NOW SHOWING A WEAK SOUTHERN  
STREAM TROUGH NEARING CALIFORNIA BY NEXT THURSDAY AFTER THE REST  
OF THE INITIAL TROUGH ERODES WHILE DOWNSTREAM MEAN RIDGE. THE  
UPDATED FORECAST STARTED WITH THE 18Z GFS/12Z ECMWF AND A LITTLE  
12Z CMC TO REPRESENT THE OVERALL PREFERRED SOLUTION FOR FEATURES  
OF INTEREST EARLY IN THE PERIOD, FOLLOWED BY SOME INCLUSION OF THE  
18Z GEFS/12Z ECMWF MEANS AS DETAIL SPREAD INCREASES OVER SOME  
AREAS.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE POTENTIALLY SLOW MOVING EASTERN U.S. UPPER LOW AND ASSOCIATED  
LOW PRESSURE (WHETHER INITIALLY FROM IAN OR EVENTUAL REDEVELOPMENT  
JUST OFF THE EAST COAST) MAY CONTINUE TO PRODUCE LOCALLY HEAVY  
RAINFALL OVER PARTS OF THE MID-ATLANTIC/APPALACHIANS THROUGH THE  
WEEKEND AND POSSIBLY INTO EARLY NEXT WEEK. BRISK TO STRONG WINDS,  
AIDED BY HIGH PRESSURE TO THE NORTH, SHOULD BE MOST NOTICEABLE  
OVER COASTAL AREAS. THE EXPERIMENTAL DAY 4 EXCESSIVE RAINFALL  
OUTLOOK VALID SUNDAY-SUNDAY NIGHT SHOWS A SLIGHT RISK OVER PARTS  
OF THE CENTRAL/SOUTH-CENTRAL APPALACHIANS EAST INTO THE  
MID-ATLANTIC. THE DAY 5 OUTLOOK FOCUSES A LINGERING SLIGHT RISK  
CLOSER TO THE COAST. GUIDANCE DIFFERENCES FOR SPECIFICS AT THE  
SURFACE AND ALOFT BY THIS TIME FRAME LEAD TO LOWER CONFIDENCE IN  
RAINFALL COVERAGE AND AMOUNTS, SO MONITOR UPCOMING FORECASTS FOR  
POSSIBLE ADJUSTMENTS.  
 
THE UPPER LOW EMERGING FROM THE NORTHERN ROCKIES AND THE TROUGH TO  
ITS SOUTH SHOULD PRODUCE AREAS OF PRECIPITATION FROM THE ROCKIES  
INTO THE HIGH PLAINS. SLOW MOVEMENT OF THE UPPER LOW AND  
SUFFICIENT MOISTURE COULD SUPPORT SOME AREAS OF LOCALLY MODERATE  
TO HEAVY RAINFALL MAINLY SUNDAY-MONDAY. HIGHEST ELEVATIONS UNDER  
THE UPPER LOW MAY SEE SOME SNOW. EXPECT PRECIPITATION TO END FROM  
THE NORTHWEST AND BECOME LIGHTER/MORE SCATTERED AS THE UPPER LOW  
OPENS UP AND THE SHORTWAVE CONTINUES ONWARD.  
 
MUCH OF THE EAST WILL SEE BELOW NORMAL HIGHS DURING THE FIRST HALF  
OF NEXT WEEK. PARTS OF THE MID-ATLANTIC INTO SOUTHEAST MAY BE  
10-15F BELOW NORMAL INTO EARLY NEXT WEEK WITH THE CLOUDS/RAIN  
ORIGINALLY ASSOCIATED WITH IAN, WHILE COOL HIGH PRESSURE PREVAILS  
OVER NEW ENGLAND. TEMPERATURES SHOULD FINALLY TREND CLOSER TO  
NORMAL BY WEDNESDAY-THURSDAY. ON THE OTHER HAND, PERSISTENT UPPER  
RIDGING WILL KEEP MUCH OF THE WEST WARMER THAN AVERAGE WITH SOME  
LOCATIONS IN THE NORTHWEST POTENTIALLY SEEING HIGHS 10-15F ABOVE  
NORMAL FOR MOST OF THE PERIOD. MODESTLY ABOVE NORMAL TEMPERATURES  
OVER THE PLAINS WILL EVENTUALLY SETTLE MORE OVER SOUTHERN AREAS  
WHILE LOCATIONS OVER THE FAR NORTHERN TIER MAY TREND COLDER THAN  
NORMAL BY NEXT THURSDAY.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, EXPERIMENTAL EXCESSIVE RAINFALL  
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES ARE  
AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page