205
FXUS02 KWBC 240717
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
216 AM EST SAT DEC 24 2022
VALID 12Z TUE DEC 27 2022 - 12Z SAT DEC 31 2022
...A STRONG ATMOSPHERIC RIVER COULD CAUSE NOTABLE HEAVY
RAIN/FLOODING FOR THE WEST COAST ALONG WITH HEAVY MOUNTAIN SNOW IN
THE WEST NEXT WEEK...
..OVERVIEW
THE MODELS AND ENSEMBLE MEANS HAVE BEEN CONSISTENTLY SHOWING A
SIGNIFICANT UPPER LEVEL PATTERN CHANGE OVER THE COURSE OF NEXT
WEEK. THE EASTERN PACIFIC TROUGH/WESTERN U.S. RIDGE/EASTERN TROUGH
CONFIGURATION AT THE START OF THE WEEK LASTING INTO THE BEGINNING
OF THE PERIOD TUESDAY SHOULD MOVE EASTWARD WITH TIME, WITH A MEAN
TROUGH ULTIMATELY SETTLING OVER THE ROCKIES/HIGH PLAINS WHILE A
RIDGE BUILDS OVER THE EAST COAST. VIGOROUS SYSTEMS AHEAD OF THE
PACIFIC TROUGH, AND IN PARTICULAR A STORM REACHING THE WASHINGTON
COAST OR VANCOUVER ISLAND BY TUESDAY AND ITS TRAILING FRONT, WILL
BRING A STRONG ATMOSPHERIC RIVER WITH A LONG FETCH OF TROPICAL
MOISTURE INTO THE WEST DURING THE FIRST HALF OF NEXT WEEK. THIS
EVENT WILL LIKELY PRODUCE HEAVY RAIN, POTENTIALLY CAUSING FLOODING
ESPECIALLY FOR COASTAL REGIONS OF SOUTHERN OREGON AND CALIFORNIA,
ALONG WITH HEAVY SNOW POSSIBLE IN HIGHER ELEVATIONS OF THE WEST.
ADDITIONAL ROUNDS OF PRECIPITATION ARE LIKELY ACROSS MUCH OF THE
WEST THROUGH LATE NEXT WEEK. MEANWHILE FARTHER EAST, THE PATTERN
CHANGE WILL FLIP TEMPERATURES FROM THE PLAINS EASTWARD FROM BELOW
NORMAL TO ABOVE AS THE WEEK PROGRESSES. ADDITIONALLY, MOISTURE
SHOULD BEGIN TO FLOW NORTHWARD THROUGH THE MISSISSIPPI VALLEY AND
INCREASE PRECIPITATION CHANCES BY LATE IN THE WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE BEGINS THE MEDIUM RANGE PERIOD TUESDAY WITH
REASONABLY GOOD AGREEMENT WITH THE OVERALL PATTERN OF UPPER
TROUGHING JUST OFFSHORE THE WEST COAST, DOWNSTREAM RIDGING, AND
EASTERN U.S. TROUGHING. THERE WERE SOME SLIGHT DIFFERENCES WITH
THE EXACT TROUGH AXIS/SHORTWAVE DIFFERENCES IN THE EAST, WITH THE
12Z GFS ON THE FASTER SIDE AND THE 12Z CMC ON THE SLOWER SIDE, BUT
WITH BETTER AGREEMENT IN THE 18Z GFS/12Z ECMWF AND NOTHING A
MULTI-MODEL DETERMINISTIC BLEND COULD NOT HANDLE. THE INCOMING 00Z
GUIDANCE APPEARS TO BE BETTER ALIGNED WITH THESE DETAILS, AND THE
TROUGH QUICKLY EXITS THE REGION BY DAY 4/WEDNESDAY ANYWAY. IN THE
WEST, MODEL DIFFERENCES REMAINED MOST NOTABLE IN THE 12Z RUNS WITH
THE DEPTH OF THE SURFACE LOW PRESSURE SYSTEM MOVING TOWARD THE
PACIFIC NORTHWEST, WITH THE ECMWF IN THE 950S MB VERSUS THE UKMET
BEING IN THE 970S WHILE OTHER MODELS WERE GENERALLY IN THE 960S.
00Z RUNS ARE MAINLY IN THE 960S FOR A BIT BETTER AGREEMENT, THOUGH
THE UKMET IS IN THE LOW 970S. AGAIN A MULTI-MODEL DETERMINISTIC
BLEND SEEMED TO WORK WELL EARLY IN THE PERIOD TO PRODUCE A
REASONABLE CONSENSUS.
AS THE PERIOD PROGRESSES, THE WESTERN UPPER TROUGH IS THE MAIN
CONCERN AS IT MOVES EAST AND SLOWS. CMC AND ECMWF RUNS HAVE BEEN
SHOWING MULTIPLE SHORTWAVES MOVING THROUGH THE OVERALL TROUGH
WHILE UKMET AND GFS RUNS SHOW A PARTICULAR SHORTWAVE OR TWO
DIGGING AROUND DAY 5/THURSDAY TO PRODUCE A SLIGHTLY DEEPER AND
PERHAPS SLOWER TROUGH THAT IS ALSO INDICATED BY MANY GEFS MEMBERS.
ALL ARE WITHIN NORMAL SPREAD FOR THE FORECAST TIMEFRAME BUT THE
DIFFERENCES DO LEAD TO UNCERTAINTIES WITH FRONTAL POSITIONS
ESPECIALLY IN THE CENTRAL U.S. THE WPC FORECAST GRADUALLY
INCORPORATED AND INCREASED THE PROPORTIONS OF THE GEFS AND EC
ENSEMBLE MEANS THROUGH THE PERIOD TO ABOUT HALF GIVEN THE
INCREASING MODEL DIFFERENCES BUT THAT ARE STILL WITHIN NORMAL
MEDIUM RANGE SPREAD.
..WEATHER/HAZARD HIGHLIGHTS
A SERIES OF NORTHEASTERN PACIFIC SYSTEMS SHOULD BRING A MULTI-DAY
PERIOD OF PRECIPITATION TO MUCH OF THE WEST. A STRONG TO EXTREME
ATMOSPHERIC RIVER WITH A MOISTURE FETCH ALL THE WAY FROM THE
WESTERN PACIFIC IS LIKELY TO AFFECT THE WEST AND PEAK
MONDAY-TUESDAY TO PRODUCE CONSIDERABLY HIGH RAINFALL RATES AND
AMOUNTS PARTICULARLY ALONG THE SOUTHERN OREGON AND CALIFORNIA
COASTS. A SLIGHT RISK OF EXCESSIVE RAINFALL HAS BEEN MAINTAINED IN
WPC'S EXPERIMENTAL DAY 4 EXCESSIVE RAINFALL OUTLOOK DUE TO
FLOODING CONCERNS WITH THIS RAIN. HIGH WINDS ARE ALSO POSSIBLE
ACROSS PORTIONS OF THE WEST COAST AND GREAT BASIN--ESPECIALLY WITH
THE STRONG STORM TRACKING INTO OR NEAR THE PACIFIC NORTHWEST
TUESDAY. BY DAY 5/WEDNESDAY, LOCALLY HEAVY RAINFALL COULD BECOME
HAZARDOUS FOR PARTS OF SOUTHERN CALIFORNIA INTO ARIZONA AS THE
MOISTURE AXIS SHIFTS SOUTH. SOME MODEL GUIDANCE SHOWS ANOTHER
ROUND OF RAIN COULD MAKE IT INTO SOUTHWESTERN OREGON/NORTHWESTERN
CALIFORNIA AS WELL BY WEDNESDAY OVER AREAS THAT WOULD BE
HYDROLOGICALLY SENSITIVE. WHILE THERE IS GOOD AGREEMENT FOR ROUNDS
OF PRECIPITATION TO CONTINUE IN THE WEST THROUGH THE REST OF NEXT
WEEK WITH ONE OR MORE ADDITIONAL FRONTAL SYSTEMS, TIMING OF EACH
PRECIPITATION EVENT DOES VARY AFTER THE FIRST STRONG ATMOSPHERIC
RIVER. THE SUBSEQUENT PRECIPITATION EVENTS ARE NOT EXPECTED TO BE
AS INTENSE AS THE INITIAL ONE. REGARDLESS, FARTHER INLAND IN
HIGHER ELEVATIONS, HEAVY SNOW IS A THREAT IN THE CASCADES/SIERRA
NEVADA INTO THE ROCKIES IN THESE MULTIPLE ROUNDS OF PRECIPITATION
NEXT WEEK.
EAST OF THE ROCKIES, MOST AREAS SHOULD BE GENERALLY DRY
TUESDAY-WEDNESDAY OTHER THAN SOME LIGHT PRECIPITATION CHANCES
ACROSS THE NORTHERN TIER. BY THE LATTER HALF OF THE WEEK, MOISTURE
INFLOW FROM THE GULF OF MEXICO IS FORECAST TO RETURN TO THE
MISSISSIPPI VALLEY AHEAD OF A COLD FRONT AND UPPER TROUGH. WITHIN
THIS EXPANDING MOISTURE SHIELD, RAINFALL COULD BECOME LOCALLY
MODERATE TO HEAVY BY FRIDAY.
TEMPERATURES ACROSS THE EASTERN HALF OF THE LOWER 48 WILL STAY
10-15F BELOW AVERAGE ON TUESDAY (PERHAPS LOCALLY COLDER FOR LOWS
IN THE MIDWEST) BUT GRADUALLY MODERATE TOWARD NORMAL BY AROUND
MIDWEEK. ABOVE NORMAL TEMPERATURES BY 15-25F FOR LOWS AND 10-20F
IN TERMS OF HIGHS OVER THE WEST/HIGH PLAINS TUESDAY SHOULD MIGRATE
INTO THE CENTRAL AND EVENTUALLY THE EASTERN U.S. BY LATE WEEK AS
THE UPPER RIDGE REACHES THE EAST COAST.
TATE/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
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