592
FXUS02 KWBC 121855
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
155 PM EST THU DEC 12 2024
VALID 12Z SUN DEC 15 2024 - 12Z THU DEC 19 2024
..OVERVIEW
THE FORECAST CALLS FOR A CHANGE IN THE WEATHER PATTERN NEXT WEEK,
SHIFTING FROM A SERIES OF PROGRESSIVE SHORTWAVES TO MEAN RIDGING
OVER THE WEST COAST AND MEAN TROUGHING DOWNSTREAM OVER THE EASTERN
TWO-THIRDS OF THE NATION. A COUPLE SHORTWAVES AND ASSOCIATED
FRONTAL SYSTEMS WILL MOVE INTO THE WEST AND PRODUCE ROUNDS OF
PRECIPITATION, LOW ELEVATION RAIN AND HIGH ELEVATION SNOW, THIS
WEEKEND THROUGH TUESDAY. THESE SHORTWAVES WILL PROGRESS ACROSS THE
CENTRAL AND EASTERN U.S., TRIGGERING PRECIPITATION OVER THE PLAINS
THAT WILL SPREAD EAST WITH TIME. WHEN THE PATTERN CHANGES MID-NEXT
WEEK, THE WEST WILL TREND DRIER UNDER THE UPPER RIDGE WHILE THE
EASTERN U.S. TRENDS WETTER UNDER UPPER TROUGHING.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS GENERALLY IN GOOD AGREEMENT EARLY IN THE PERIOD
AS A SERIES OF SHORTWAVES AND FRONTAL SYSTEMS MOVE ACROSS THE
MIDWEST AND GREAT LAKES REGION, WITH SOME RELATIVELY MINOR
DIFFERENCES IN TIMING/DETAILS OF SURFACE FEATURES. THE MOST
IMPACTFUL DIFFERENCES CONCERN A WAVE OF LOW PRESSURE MOVING NORTH
ALONG A FRONTAL BOUNDARY FROM THE ARK-LA-TEX REGION TO THE OHIO
VALLEY, AND TIMING AND STRENGTH OF THIS FEATURE WILL AFFECT WHERE
THE FOCUS FOR HEAVY PRECIPITATION WILL SET UP SUNDAY INTO MONDAY.
MODEL AGREEMENT DECREASES SIGNIFICANTLY AS THE PATTERN CHANGES,
WITH DIVERSE SOLUTIONS ON HOW TO HANDLE ENERGY MOVING OVER THE WEST
COAST RIDGE AND THROUGH THE MEAN TROUGH IN THE EAST. THE GFS HAS
SHOWN THE BEST RUN TO RUN CONSISTENCY FOR THIS TIME PERIOD WHILE
THE ECMWF HAS THE MOST RUN TO RUN VARIABILITY. YESTERDAY'S 12Z
ECMWF STRAYED FROM THE GENERAL MODEL CONSENSUS CONCERNING A
SHORTWAVE MOVING OVER THE CENTRAL/SOUTHERN PLAINS MID-NEXT WEEK,
BRINGING THIS FEATURE FURTHER SOUTH AND STRENGTHENING IT MORE THAN
OTHER GUIDANCE SUGGESTS (INCLUDING AI MODELS). THE 00Z ECMWF FELL
MORE IN LINE WITH CONSENSUS, WITH A WEAKER WAVE IN THIS REGION, BUT
TODAY'S 12Z RUN FLIP FLOPPED BACK TO A SOUTHERN/STRONGER SOLUTION.
THE ECMWF-AIFS CONSISTENTLY SHOWS A NORTHERN/WEAKER SOLUTION,
WHICH LENDS MORE CONFIDENCE TO THIS SOLUTION. THE CMC SOLUTION LIES
IN THE MIDDLE GROUND BETWEEN THE GFS AND ECMWF.
BASED ON GUIDANCE COMPARISONS, THE FIRST HALF OF THE FORECAST
CONSISTED OF A BLEND OF THE 00Z/06Z GFS, 00Z ECMWF, 00Z CMC, AND
00Z UKMET, WITH THE MOST WEIGHT GIVEN TO THE GFS. FOR THE SECOND
HALF, THE WEIGHT PUT ON THE ECMWF AND UKMET WAS DECREASED AND
ENSEMBLE MEANS FROM THE GEFS AND ECENS WERE ADDED.
..WEATHER/HAZARDS HIGHLIGHTS
A SHORTWAVE WITH AN EMBEDDED COMPACT UPPER LOW WILL LIKELY TRACK
FROM THE MIDWEST/WESTERN GREAT LAKES THROUGH OR NEAR THE EASTERN
GREAT LAKES DURING SUNDAY-MONDAY, PRODUCING AN AREA OF
PRECIPITATION FROM THE GREAT LAKES SOUTHWARD AND THEN CONTINUING
EASTWARD. PARTS OF THE APPALACHIANS/NORTHEAST MAY SEE SOME WINTRY
PRECIPITATION BUT IT MOST SHOULD BE LIGHT, AS PROBABILITIES FOR
0.25 INCH LIQUID IN THE FORM OF SNOW REMAIN BELOW 30 PERCENT.
THE NEXT SYSTEM WILL CROSS THE INTERIOR WEST AND ROCKIES ON
SUNDAY, PRODUCING SNOW OVER THE NORTHERN-CENTRAL ROCKIES BUT WITH
TOTALS LOWER THAN FORECAST OVER THE NORTHERN ROCKIES/NORTHWEST ON
SATURDAY. EXPECT CONTINUED PROGRESSION OF THE UPPER TROUGH WITH
LEADING COLD FRONT AND INCORPORATION OF SOME GULF MOISTURE TO
DEVELOP AN AREA OF PRECIPITATION FROM THE ARK-LA-TEX REGION
NORTHEAST TO THE GREAT LAKES BY MONDAY-MONDAY NIGHT. MOST OF THIS
ACTIVITY WILL BE IN THE FORM OF RAIN EXCEPT FOR SOME SCATTERED
POCKETS OF WINTRY WEATHER IN THE FAR NORTHERN TIER. GUIDANCE
CONTINUES TO SUGGEST GREATER INSTABILITY WITH THIS SYSTEM RELATIVE
TO ITS PREDECESSOR, LEADING TO HIGHER RAINFALL RATES WITHIN
PORTIONS OF AN AREA EXTENDING FROM THE SOUTHERN HALF OF THE EASTERN
PLAINS INTO THE LOWER OHIO VALLEY. FOR THE TIME BEING THE DAY 5
EXCESSIVE RAINFALL OUTLOOK DEPICTS NO RISK AREAS, GIVEN A
COMBINATION OF CURRENT GUIDANCE SPREAD FOR HEAVIEST RAINFALL
MAGNITUDE/LOCATION AS WELL AS GENERALLY DRY ANTECEDENT CONDITIONS.
HOWEVER SOME RAINFALL EXPECTED IN THE SHORT TERM MAY TEMPER THE
INITIAL DRYNESS AND IMPROVED CLUSTERING TOWARD HEAVIER RAINFALL
WOULD INCREASE THE POTENTIAL FOR RUNOFF ISSUES.
THE NORTHERN HALF OF THE WEST COAST SHOULD SEE ANOTHER EPISODE OF
RAIN AND HIGHER ELEVATION SNOW ON MONDAY, WITH MOISTURE EXTENDING
EAST THROUGH THE ROCKIES BY TUESDAY. DETAILS OF THE SURFACE PATTERN
AND CORRESPONDING PRECIPITATION COVERAGE AND AMOUNTS OVER THE
CENTRAL/EASTERN U.S. BECOME INCREASINGLY UNCERTAIN BY NEXT
WEDNESDAY-THURSDAY.
WITHIN A GENERALLY MILD PATTERN ACROSS THE LOWER 48, WARMEST
ANOMALIES SHOULD EXTEND FROM THE EASTERN SLOPES OF THE ROCKIES TO
JUST WEST OF THE APPALACHIANS SUNDAY-MONDAY AND REACH THE EAST ON
TUESDAY AHEAD OF A PROGRESSIVE COLD FRONT. EXPECT A DECENT AREA OF
HIGHS 10-15 DEGREES ABOVE NORMAL AND ANOMALIES UP TO PLUS 15-25
DEGREES IN PLACES FOR MORNING LOWS. LOCALIZED AREAS WITH SLIGHTLY
BELOW NORMAL HIGHS SHOULD INCLUDE THE CENTRAL WEST COAST SUNDAY-
MONDAY, THE FAR NORTHERN TIER TUESDAY-WEDNESDAY, AND POSSIBLY PARTS
OF THE SOUTH BY NEXT THURSDAY.
DOLAN/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 FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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