644
FXUS02 KWBC 152030
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
329 PM EST THU DEC 15 2022
VALID 12Z SUN DEC 18 2022 - 12Z THU DEC 22 2022
...BITTERLY COLD TEMPERATURES LIKELY TO SETTLE OVER THE NORTHERN
PLAINS FROM SUNDAY ONWARD...
..OVERVIEW
GUIDANCE SUGGESTS THE MEAN PATTERN OVER THE LOWER 48 SHOULD BECOME
LESS AMPLIFIED FOR ABOUT THE FIRST HALF OF NEXT WEEK, AS THE UPPER
LOW OVER THE GREAT LAKES/SOUTHEAST CANADA ON SUNDAY CONTINUES
EASTWARD AND ELONGATED MEAN TROUGHING (WITH ONE OR MORE EMBEDDED
LOWS) PREVAILS ACROSS SOUTHERN CANADA. MEANWHILE A COUPLE PACIFIC
SHORTWAVES MAY TRACK TO THE SOUTH OF THE STRONGEST WESTERLIES
EXPECTED OVER THE NORTHERN TIER. EXPECT PACIFIC FLOW TO SUPPORT
EPISODES OF RAIN AND HIGHER ELEVATION SNOW OVER THE NORTHWEST AND
ONE OR TWO PERIODS OF RAINFALL OVER THE GULF COAST REGION. THERE
IS GOOD AGREEMENT IN PRINCIPLE THAT AN UPPER RIDGE BUILDING INTO
THE EASTERN PACIFIC/WEST COAST BY WEDNESDAY-THURSDAY WILL
ENCOURAGE DOWNSTREAM TROUGH AMPLIFICATION INTO THE
PLAINS/MISSISSIPPI VALLEY. THIS TROUGH SHOULD INCREASE THE
POTENTIAL FOR ANOTHER PERIOD OF ACTIVE WEATHER OVER THE EAST AFTER
MIDWEEK AND PROVIDE A MORE SOUTHWARD AND EASTWARD PUSH FOR THE
COLD AIR INITIALLY PERSISTING OVER THE NORTHERN PLAINS. CLIMATE
PREDICTION CENTER OUTLOOKS INDICATE THE COLD PATTERN EAST OF THE
ROCKIES IS LIKELY TO PERSIST WELL BEYOND THE END OF THE MEDIUM
RANGE PERIOD.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MODELS ARE SLOWLY IMPROVING THEIR CLUSTERING FOR THE SYSTEM
WHOSE INFLUENCE ON NEW ENGLAND LINGERS THROUGH THE FIRST PART OF
THE PERIOD. LATEST RUNS APPEAR TO FOLLOW THROUGH ON YESTERDAY'S
EMERGING TRENDS FOR THE SURFACE LOW OFF NOVA SCOTIA (AS OF EARLY
SUNDAY) TO CURL BACK TO A POSITION JUST NORTHEAST OF MAINE BY
MONDAY AS THE UPPER LOW TRACKING OUT OF THE GREAT LAKES INTERACTS
WITH/REINFORCES LEADING DYNAMICS. THEN THE SYSTEM SHOULD FINALLY
DEPART BY TUESDAY.
AS IN PREVIOUS DAYS, GUIDANCE CONTINUES TO HAVE DIFFICULTY IN
RESOLVING THE PRECISE DETAILS OF FLOW DOWNSTREAM FROM A STRONG
UPPER HIGH NORTHWEST OF ALASKA. PRIMARY ISSUES HAVE INVOLVED THE
ULTIMATE PROGRESSION/SHAPE OF AN INITIAL TROUGH/CLOSED LOW OVER
SOUTHWESTERN CANADA, AND HOW MUCH TROUGHING PERSISTS OVER WESTERN
CANADA THROUGH MIDWEEK ALONG WITH AMPLITUDE OF FLOW EXTENDING INTO
THE WESTERN U.S. 00Z THROUGH 12Z GUIDANCE RUNS IN GENERAL HAVE
OSCILLATED BACK TO THE IDEA FROM A COUPLE DAYS AGO WITH RESPECT TO
INITIAL SOUTHWEST CANADA ENERGY PROGRESSING EASTWARD IN A MORE
DEFINED FASHION WHILE TRAILING FLOW IS A BIT LESS AMPLIFIED BUT
WITH LOWER NORTHWEST HEIGHTS THAN FORECAST BY THE 06Z/12Z GFS.
HOW MUCH TROUGHING LINGERS OVER WESTERN CANADA THROUGH TUESDAY
WILL DETERMINE SOME OF THE DETAILS OF DOWNSTREAM TROUGH
AMPLIFICATION AS UPPER RIDGING REACHES NEAR THE WEST COAST BY
THURSDAY. CURRENTLY THE 12Z UKMET HAS STRAYED FROM OTHER LATEST
SOLUTIONS, SHEARING THE LEADING ENERGY MORE WHILE AMPLIFYING THE
WESTERN TROUGH MORE BY THE END OF ITS RUN EARLY WEDNESDAY--BUT
SOME SPORADIC MODEL RUNS IN THE RECENT PAST DID SHOW SOME
VARIATION OF THAT EVOLUTION.
FORECAST PREFERENCE CONTINUED TO INCORPORATE AN OPERATIONAL MODEL
BLEND EARLY AND THEN TRANSITIONED TO A MODEL/ENSEMBLE MEAN MIX,
REFLECTING THE MOST COMMON ADJUSTMENTS IN LATEST GUIDANCE WHILE
TONING DOWN INCREASINGLY UNCERTAIN DETAILS LATER IN THE PERIOD.
THE EVENTUAL EVOLUTION TOWARD AN AMPLIFIED LARGE SCALE
CONFIGURATION SHOULD ULTIMATELY LEAD TO GREATER PREDICTABILITY BUT
THAT MAY NOT APPLY AS WELL TO IMPORTANT EMBEDDED DETAILS OVER THE
EASTERN HALF OF THE COUNTRY AROUND THE END OF THE FORECAST PERIOD
AND BEYOND.
..WEATHER/HAZARD HIGHLIGHTS
HEAVY SNOWS ACROSS NEW ENGLAND ASSOCIATED WITH A NOR'EASTER MOVING
NORTHWARD SHOULD BE COMING TO AN END BY SUNDAY. FAVORABLE WESTERLY
FLOW ON THE BACKSIDE OF THE MAIN UPPER LOW WILL CONTINUE TO
SUPPORT SOME LAKE EFFECT SNOWS OVER DOWNWIND AREAS FOR MULTIPLE
DAYS. A SHORTWAVE TRACKING ACROSS THE SOUTHERN TIER ALONG WITH AN
ASSOCIATED FRONTAL WAVE OVER THE GULF OF MEXICO COULD PRODUCE A
PERIOD OF ENHANCED RAINFALL NEAR THE GULF COAST AND INTO FLORIDA
DURING MONDAY-TUESDAY. THE NORTHWEST SHOULD SEE MULTIPLE EPISODES
OF RAIN AND HIGHER ELEVATION SNOW WITH HIGHEST TOTALS OVER FAVORED
TERRAIN. CONFIDENCE IS MODERATE AT BEST FOR DAY-TO-DAY DETAILS
REGARDING AMOUNTS AND COVERAGE BUT THERE IS A GENERAL SIGNAL THAT
TOTALS SHOULD INCREASE TOWARD WEDNESDAY-THURSDAY. ALSO AROUND
THAT TIME, UPPER TROUGH AMPLIFICATION OVER THE PLAINS-MISSISSIPPI
VALLEY AND ASSOCIATED LOW PRESSURE/FRONTAL SYSTEM COULD BRING
ANOTHER THREAT OF SNOW TO PARTS OF THE PLAINS AND UPPER MIDWEST
INTO GREAT LAKES, WITH RAIN FARTHER SOUTH. LOWER LATITUDE
WAVINESS LATE IN THE PERIOD, POSSIBLY EVOLVING NEAR THE SOUTHEAST
COAST BY THURSDAY, COULD ALSO BRING SOME MOISTURE TO PARTS OF THE
SOUTH AND EAST. CONFIDENCE IN ANY SPECIFICS IS LOW AT THIS TIME,
GIVEN THE VARIABILITY THAT GUIDANCE HAS HAD FOR PATTERN DETAILS
OVER RECENT DAYS.
THE MOST WIDESPREAD AND EXTREME COLD ANOMALIES WILL LIKELY BE
ACROSS THE NORTHERN PLAINS FROM SUNDAY ONWARD, WITH DAYTIME
TEMPERATURES AS COLD AS 30-40 DEGREES BELOW NORMAL (LOCALLY EVEN
COLDER ACROSS PARTS OF MONTANA), EQUATING TO WIDESPREAD AND
BITTERLY COLD SINGLE DIGIT TEMPERATURES WITH SOME BELOW ZERO
READINGS ACROSS EASTERN MONTANA AND NORTH DAKOTA. MORNING LOWS
COULD FALL TO AT LEAST 10-20 DEGREES BELOW ZERO (LOCALLY COLDER IN
SPOTS). SURROUNDING AREAS SHOULD SEE LESS EXTREME ANOMALIES OF
10-20F BELOW NORMAL. AFTER MIDWEEK EXPECT THE COLD AIR TO SURGE
FARTHER SOUTH AND EAST, BRINGING HIGHS DOWN TO 10-25F BELOW NORMAL
OVER THE SOUTHERN HALF OF THE PLAINS AND AN INCREASING PORTION OF
THE MISSISSIPPI VALLEY BY NEXT THURSDAY. THE EAST WILL BE CHILLY
TO FINISH THE WEEKEND AND MODERATE FOR A TIME, BEFORE STARTING TO
TREND COLDER AGAIN MID-LATE WEEK. THE WEST WILL TEND TO SEE
READINGS 5-15F BELOW NORMAL THROUGH SUNDAY AND THEN
CENTRAL/SOUTHERN AREAS SHOULD SEE A WARMING TREND. MODERATELY
ABOVE NORMAL READINGS WILL LIKELY BECOME MORE COMMON OVER THE
SOUTHERN TWO-THIRDS OF THE WEST BY WEDNESDAY-THURSDAY AS AN UPPER
RIDGE BUILDS TOWARD THE WEST COAST.
RAUSCH/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
HAZARDS:
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND
THE NORTHERN GREAT BASIN,
WED-THU, DEC 21-DEC 22.
- HEAVY RAIN ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND
THE SOUTHERN PLAINS, MON, DEC 19.
- HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST, TUE, DEC 20.
- HEAVY SNOW ACROSS PORTIONS OF THE NORTHEAST, SUN, DEC 18.
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE LOWER
MISSISSIPPI VALLEY.
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE
NORTHERN/CENTRAL PLAINS, THE
NORTHERN/CENTRAL ROCKIES, THE UPPER/MIDDLE MISSISSIPPI VALLEY, THE
NORTHERN GREAT BASIN, AND THE
GREAT LAKES, SUN-THU, DEC 18-DEC 22.
- MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE
CENTRAL/SOUTHERN PLAINS, THE MISSISSIPPI
VALLEY, THE TENNESSEE VALLEY, THE GREAT LAKES, AND THE OHIO
VALLEY, THU, DEC 22.
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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