248
FXUS02 KWBC 200616
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
215 AM EDT FRI OCT 20 2023
VALID 12Z MON OCT 23 2023 - 12Z FRI OCT 27 2023
...MODERATE TO HEAVY RAINFALL INCREASINGLY LIKELY FOR PARTS OF THE
SOUTHERN AND CENTRAL U.S. EARLY TO MID NEXT WEEK...
..OVERVIEW
THE MEDIUM RANGE PERIOD BEGINS MONDAY WITH TROUGHING EXITING THE
EAST COAST TO BE REPLACED BY A DEVELOPING UPPER RIDGE LASTING MUCH
OF THE WEEK. OUT WEST, AN UPPER LOW IS LIKELY TO DIVE INTO THE
SOUTHWEST/NORTHERN MEXICO BEFORE WEAKENING AND LIFTING INTO THE
SOUTHERN U.S. LATER NEXT WEEK. ADDITIONAL ENERGY LOOKS TO MOVE
INTO THE NORTHWEST AROUND MID-WEEK, BUT MODEL GUIDANCE CONTINUES
TO SHOW CONSIDERABLE UNCERTAINTY IN THE TRACK AND STRENGTH OF THIS
ENERGY AND ANY POTENTIAL INTERACTIONS WITH THE UPPER LOW OVER THE
SOUTHWEST. REGARDLESS, IT SEEMS LIKELY REMNANT ENERGY/MOISTURE
FROM WHAT IS CURRENTLY HURRICANE NORMA COULD STREAM NORTHWARD TO
SPREAD POTENTIALLY HEAVY RAINFALL INTO PARTS OF THE SOUTHERN AND
CENTRAL U.S. AHEAD OF A STRONG COLD FRONT, WHILE ON THE BACKSIDE,
SNOW CHANCES INCREASE IN THE NORTHERN ROCKIES AND PERHAPS EVEN
INTO PARTS OF THE NORTHERN PLAINS LATE NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS IN REASONABLE AGREEMENT FOR AT LEAST THE FIRST
HALF OF THE PERIOD, ASIDE FROM THE GFS CONTINUING TO BE NOTABLY
FASTER WITH THE REMNANT ENERGY OF NORMA THROUGH NORTHERN MEXICO.
AFTER WEDNESDAY, THE 18Z/OCT 19 GFS RUN WAS SIGNIFICANTLY FASTER
TO EJECT THE SOUTHWEST UPPER LOW INTO THE SOUTHERN/CENTRAL U.S.
LATE WEEK ALLOWING IT TO BECOME ABSORBED INTO NORTHERN STREAM
TROUGHING. THE REMAINING DETERMINISTIC MODELS AS WELL AS TO SOME
DEGREE THE ENSEMBLE MEANS, SEEM TO BE TRENDING TOWARDS MAINTAINING
THE SOUTHERN LOW AS A SEPARATE FEATURE, AND SLOWER TO TRACK
NORTHEASTWARD. THE EVOLUTION OF THIS IS DIRECTLY RELATED TO
INCOMING ENERGY INTO THE NORTHWEST AROUND THE SAME TIME, WHICH
STILL HAS A TON OF SPREAD AND MODEL RUN TO RUN VARIABILITY. AT
LEAST THROUGH THE LATEST 12Z/18Z OCT 19 MODEL RUNS, THE GFS IS
MUCH STRONGER SHOWING AN UPPER LOW OVER THE NORTHWEST AND AN
EVENTUAL RATHER WOUND UP AND DEEP SURFACE CYCLONE ACROSS THE
NORTHERN PLAINS DAY 7. MEANWHILE, THE CMC KEEPS THE ENERGY NORTH
IN CANADA WITH UPPER RIDGING OVER THE NORTHWEST AND HIGH PRESSURE
AT THE SURFACE. THE ECMWF AND THE MEANS SUPPORT SOME SORT OF
ENERGY TRACKING ACROSS THE NORTHERN TIER, JUST MUCH WEAKER THAN
THE GFS. NEW 00Z RUNS FOR TONIGHT GENERALLY CONTINUE TO FOLLOW
SUIT WITH THEIR OWN PREVIOUS RUNS, SUGGESTING A VERY UNCERTAIN AND
LOW CONFIDENCE FORECAST AT BEST RIGHT NOW.
THE WPC FORECAST FOR TONIGHT USED A BLEND OF THE ECMWF, CMC, AND
UKMET FOR ABOUT THE FIRST HALF OF THE PERIOD. AFTER THAT, QUICKLY
TRANSITIONED TO A MOSTLY ENSEMBLE MEAN BLEND TO HELP MITIGATE
SIGNIFICANT LATE PERIOD DIFFERENCES. WAS ABLE TO CONTINUE MINOR
CONTRIBUTIONS OF THE ECMWF (WHICH WAS THE DETERMINISTIC MODEL
CLOSEST TO THE ENSEMBLE MEANS) THROUGH DAY 7 JUST FOR A LITTLE
EXTRA SYSTEM DEFINITION. THIS APPROACH MAINTAINS GOOD CONTINUITY
WITH THE PREVIOUS WPC FORECAST AS WELL.
..WEATHER/HAZARDS HIGHLIGHTS
THE BETTER CONSENSUS SUGGESTS MONDAY SHOULD BE RELATIVELY QUIET
ACROSS THE COUNTRY, THOUGH RECENT RUNS OF THE GFS HAVE BEEN RATHER
PERSISTENT IN DEPICTING THE ONSET OF LOCALLY HEAVY RAINFALL OVER
THE SOUTHERN PLAINS AS EARLY AS MONDAY, SO THIS IS SOMETHING TO
WATCH. STILL, GIVEN THE UNCERTAINTY, OPTED TO KEEP THE DAY 4
EXCESSIVE RAINFALL OUTLOOK BLANK, BUT A RISK AREA MAY NEED TO BE
INTRODUCED IN FUTURE UPDATES IF OTHER PIECES OF GUIDANCE TREND
FASTER LIKE THE GFS. BY TUESDAY, FAVORABLE MOISTURE AND
INSTABILITY FROM NORMA REMNANTS SHOULD BEGIN SPREADING IN EARNEST
INTO THE SOUTHERN PLAINS, WITH AT LEAST LOCALLY HEAVY RAINFALL
LIKELY, POSSIBLY SPREADING INTO PARTS OF THE MIDWEST ALONG THE
STRONG FRONTAL BOUNDARY. A BROAD MARGINAL RISK FOR EXCESSIVE
RAINFALL WAS INTRODUCED ON THE DAY 5/TUESDAY ERO, STRETCHING FROM
EASTERN NEW MEXICO NORTHWARD INTO THE UPPER MIDWEST. IT IS
POSSIBLE AN EMBEDDED SLIGHT RISK MAY BE NEEDED SOMEWHERE, BUT AT
LEAST THE SOUTHERN HALF OF THIS REGION HAS BEEN VERY DRY LATELY
AND THERE IS STILL A CONSIDERABLE AMOUNT OF UNCERTAINTY ON TIMING
AND RAINFALL AMOUNTS. RAINFALL MAY LINGER OR SPREAD VERY SLOWLY
EASTWARD LATER NEXT WEEK, BUT SHOULD TREND LIGHTER AS THE BETTER
MOISTURE WEAKENS AND THE UPPER LOW OPENS UP AND MOVES EAST. OUT
WEST, PROBABILITIES FOR MEANINGFUL SNOW AT HIGHER ELEVATIONS OF
THE NORTHERN ROCKIES SHOULD START INCREASING BY MONDAY/TUESDAY,
WITH SOME POTENTIAL FOR GENERALLY LIGHTER SNOWFALL AMOUNTS EVEN
EXTENDING INTO PARTS OF THE NORTHERN PLAINS LATER IN THE WEEK, BUT
HIGHLY DEPENDENT ON THE CURRENTLY VERY UNCERTAIN UPPER LEVEL
PATTERN EVOLUTION.
WELL ABOVE NORMAL TEMPERATURES WILL SPREAD FROM THE PLAINS INTO
THE MIDWEST/NORTHEAST MONDAY-WEDNESDAY WITH THE GREATEST ANOMALIES
(15-20 DEGREES ABOVE NORMAL) LIKELY MONDAY ACROSS THE CENTRAL
PLAINS. TO THE EAST, MORNING LOW TEMPERATURES ON MONDAY MAY BE
COLD ENOUGH FROM THE GREAT LAKES INTO THE APPALACHIANS TO SUPPORT
A GENERALLY FROST THREAT FOR MANY, BUT SHOULD MODERATE THEREAFTER
AS UPPER RIDGING BUILDS INTO THE REGION. TEMPERATURES WILL TREND
COOLER WEDNESDAY AND BEYOND FOR MUCH OF THE WEST, BUT ESPECIALLY
FOR THE NORTHERN ROCKIES/PLAINS WHERE DAYTIME HIGHS COULD BE 10-20
DEGREES BELOW NORMAL BEHIND THE COLD FRONT.
SANTORELLI
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 FORECAST, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES 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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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