388
FXUS02 KWBC 151900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST MON JAN 15 2024
VALID 12Z THU JAN 18 2024 - 12Z MON JAN 22 2024
...ANOTHER ARCTIC AIRMASS COMES INTO THE CENTRAL U.S. LATE WEEK,
BUT NOT AS EXTREME AS THE FIRST ONE...
19Z UPDATE: THE MODEL GUIDANCE IS IN ABOVE AVERAGE AGREEMENT ON
MOST ASPECTS OF THE FORECAST GOING FORWARD TO THE END OF THE WEEK
AND INTO THE WEEKEND, AND A MULTI-DETERMINISTIC MODEL BLEND
SUFFICED AS A GOOD OVERALL STARTING POINT IN THE FORECAST PROCESS,
WITH A LITTLE MORE WEIGHTING TO THE GFS/ECMWF GIVEN THEIR CLOSE
AGREEMENT TO THE RESPECTIVE ENSEMBLE MEANS. THE MODELS STILL
AGREE THAT THE DEVELOPING OFFSHORE LOW SOUTHEAST OF NEW ENGLAND
WILL BE FAR ENOUGH FROM THE COAST TO MITIGATE THE POTENTIAL
IMPACTS FROM THIS EVENT FRIDAY NIGHT, WITH THE CMC BEING THE
CLOSEST SOLUTION TO THE COAST. THERE IS EXCELLENT AGREEMENT ON
THE ARCTIC SURFACE HIGH OVER THE EASTERN HALF OF THE COUNTRY THIS
WEEKEND, BUT MORE MODEL DIFFERENCES EMERGE OVER THE EASTERN
PACIFIC THAT HAVE IMPACTS FOR THE WEST COAST FOR THE SECOND HALF
OF THE FORECAST PERIOD, MAINLY REGARDING TIMING AND NOT INTENSITY.
THE CMC BEGAN TO STRAY FROM THE CONSENSUS BY EARLY NEXT WEEKEND
WITH THE DEVELOPING UPPER RIDGE AXIS OVER THE CENTRAL U.S.
SITUATED WEST OF THE ENSEMBLE MEANS, SO THIS WAS WEIGHTED LESS IN
THE FORECAST FORECAST. IN TERMS OF QPF, THE NBM WAS TOO LIGHT
WITH EXPECTED LAKE EFFECT SNOW FOR THE THURSDAY-SATURDAY TIME
PERIOD, SO AN EQUAL BLEND OF GFS/ECMWF/NBM WAS USED AS A BASELINE,
AND THEN THIS RESULT WAS INCREASED ABOUT 75% DOWNWIND OF THE GREAT
LAKES TO BETTER ACCOUNT FOR LAKE EFFECT BANDING. THE PREVIOUS
FORECAST DISCUSSION, ALONG WITH UPDATED EXCESSIVE RAINFALL
OUTLOOK, ARE APPENDED BELOW FOR REFERENCE. /HAMRICK
--------------------------
..OVERVIEW
THE DEEP AND BROAD MEAN UPPER TROUGH THAT SHOULD DOMINATE MUCH OF
THE LOWER 48 THROUGH LATE WEEK WILL FINALLY GET DISPLACED INTO THE
EAST AND THEN THE ATLANTIC OVER THE WEEKEND, AS AN UPPER RIDGE
SHIFTS FROM THE WEST COAST THURSDAY ACROSS THE WESTERN AND THEN
CENTRAL U.S. BY THE WEEKEND. AHEAD OF THE RIDGE, MEAN
NORTHWESTERLY FLOW WILL BRING IN ANOTHER ARCTIC SURFACE HIGH,
PRODUCING MUCH BELOW NORMAL TEMPERATURES ACROSS THE CENTRAL U.S.
THURSDAY AND SPREADING INTO THE EAST LATE WEEK. WHILE TEMPERATURES
ARE FORECAST TO BE GENERALLY BE 15-30F BELOW AVERAGE, AT MOST
LOCATIONS THIS SECOND ROUND OF COLD TEMPERATURES SHOULD BE LESS
EXTREME THAN THE THE FIRST. MEANWHILE, ROUNDS OF PRECIPITATION ARE
EXPECTED FOR THE WEST COAST, AND IN THE EAST POSSIBLE LOW
DEVELOPMENT ALONG A SURFACE FRONTAL SYSTEM OR TWO COULD PRODUCE
SOME WINTER WEATHER DURING THURSDAY-SATURDAY, BUT WITH CONTINUED
SIGNIFICANT UNCERTAINTY FOR DETAILS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST MODELS/MEANS CONTINUE TO AGREE WELL FOR THE LARGE SCALE
EVOLUTION THROUGH THE PERIOD WITH MORE PROGRESSIVE FLOW THAN HAS
BEEN SEEN RECENTLY, AS THE UPPER RIDGE AXIS PUSHES THE MAIN TROUGH
THROUGH THE EAST AND AN UPSTREAM TROUGH COMES INTO THE WEST
THROUGH THE PERIOD. DETAILS OF THE FLOW SHOW SOME DIFFERENCES THAT
ARE CERTAINLY WITHIN REASON FOR THE MEDIUM RANGE TIMEFRAME, BUT
HAVE SENSIBLE WEATHER IMPACTS. ONE OF THESE MORE VARIABLE SYSTEMS
IS A SHORTWAVE CONSOLIDATING IN THE EAST-CENTRAL U.S. BY FRIDAY AS
ENERGIES FROM THE NORTHWEST AND NORTH COMBINE WITHIN THE BROAD
TROUGH. THIS STRONG SHORTWAVE SHOULD PIVOT THROUGH THE BASE OF THE
TROUGH AND HELP PRODUCE WESTERN ATLANTIC LOW PRESSURE ON FRIDAY,
WHILE ADDITIONAL LOW PRESSURE/FRONTAL SYSTEMS MAY BE FARTHER WEST,
MAKING FOR A COMPLEX SURFACE PATTERN BEFORE THESE FEATURES MAY
COMBINE. THE SURFACE LOW TRACK FRIDAY-SATURDAY HAS SOME EAST-WEST
SPREAD AND ALSO SOME NORTH-SOUTH SPREAD INDICATING TIMING
DIFFERENCES. MORE SPECIFICALLY, CMC RUNS HAVE BEEN ON THE EASTERN
SIDE OF THE SPREAD COMPARED TO CONSENSUS--BUT THE NEW 00Z ECMWF
HAS A SIMILAR EASTERN TRACK FARTHER FROM SHORE. THESE LOW TRACK
VARIATIONS LEAD TO MEANINGFUL DIFFERENCES AS FAR AS EAST COAST
EFFECTS.
FARTHER WEST, THE RIDGE AXIS TAKING A STEADILY EASTWARD TRACK
SEEMS WELL FORECAST BY THE MODELS. MEAN TROUGHING BEHIND THE RIDGE
BECOMES MORE QUESTIONABLE WITH THE DETAILS ESPECIALLY BY EARLY
NEXT WEEK, AS MULTIPLE SHORTWAVES SHOW VARIABILITY WITH TIMING AND
AMPLITUDE IN THE MODELS, AFFECTING SURFACE FRONTS AS WELL. THESE
WILL TAKE ADDITIONAL TIME TO RESOLVE ESPECIALLY SINCE THEY ARE
TRACKING FROM THE DATA-POOR PACIFIC THAT CAUSES MODELS TO
STRUGGLE. THE 12Z CMC SEEMED LIKE THE MAIN OUTLIER BY EARLY NEXT
WEEK AS IT MAINTAINS TROUGHING ACROSS THE EAST COAST, SERVING TO
AMPLIFY THE RIDGE MORE SO THAN OTHER GUIDANCE AND NOT ALLOWING FOR
SHORTWAVES TO PROPAGATE FROM THE WEST.
THE WPC FORECAST WAS BASED ON A MULTI-MODEL DETERMINISTIC BLEND
EARLY IN THE FORECAST PERIOD. GRADUALLY INCORPORATED THE GEFS AND
EC ENSEMBLE MEAN GUIDANCE WHILE PHASING OUT THE CMC (AND OF COURSE
THE TIME-LIMITED UKMET) AS THE PERIOD PROGRESSED, ENDING UP WITH
AN EVEN BLEND OF THE 18Z GFS AND GEFS MEAN AND 12Z ECMWF AND EC
MEAN BY DAY 7. THIS SMOOTHED OUT MODEL DIFFERENCES BUT MAINTAINED
SOME STRENGTH INDIVIDUAL SYSTEMS.
..WEATHER/HAZARDS HIGHLIGHTS
BY THE START OF THE PERIOD THURSDAY, MOST AREAS WILL HAVE WARMED
UP COMPARED TO THE FIRST SURGE OF ARCTIC AIR CURRENTLY AFFECTING
THE NATION, THOUGH WITH SOME LINGERING SLIGHTLY BELOW NORMAL
TEMPERATURES IN THE SOUTHEAST. HOWEVER, ANOTHER ARCTIC HIGH WILL
PROVIDE A REINFORCING SURGE OF COLD AIR THAT IS FORECAST TO PUSH
INTO THE NORTH-CENTRAL U.S. THURSDAY, WELL INTO THE
PLAINS/MISSISSIPPI VALLEY FRIDAY, AND INTO THE EAST SATURDAY.
BELOW NORMAL TEMPERATURES BY 15-30 DEGREES SHOULD BE WIDESPREAD,
AND THIS COLD EPISODE WILL STILL BE NOTABLE EVEN IF NOT QUITE AS
EXTREME AS THE CURRENT/SHORT RANGE COLD EVENT OVER MOST AREAS.
TEMPERATURES SHOULD MODERATE CLOSER TO NORMAL BY EARLY NEXT WEEK.
MEANWHILE WEST OF THE ROCKIES, RIDGING ALOFT WILL PROMOTE ABOVE
NORMAL TEMPERATURES INCREASING IN MAGNITUDE AND COVERAGE LATE WEEK
INTO THE WEEKEND, AND SHIFTING INTO THE NORTH-CENTRAL U.S. EARLY
NEXT WEEK.
THE WEST COAST STATES CAN EXPECT ROUNDS OF PRECIPITATION FOR THE
LATTER PART OF THIS WEEK INTO EARLY NEXT WEEK. ON THURSDAY
POSSIBLY HEAVY PRECIPITATION LOOKS TO FOCUS ACROSS WESTERN
WASHINGTON, WITH HEAVY SNOW FOR THE CASCADES. THE OLYMPIC
PENINSULA AND FAR NORTHWESTERN OREGON MAY HAVE JUST ENOUGH
MODERATE TO HEAVY RAINFALL TO WARRANT A PLANNED MARGINAL RISK AREA
FOR DAY 4. BY LATE FRIDAY, A ROUND OF PRECIPITATION LOOKS TO
AFFECT CALIFORNIA ALONG AND AHEAD OF A FRONTAL SYSTEM. MODELS
INDICATE THAT RAIN RATES COULD BE OVER HALF AN INCH PER HOUR AS
INSTABILITY MAY SNEAK ONSHORE, ALONG WITH GOOD DYNAMICAL SUPPORT
FOR HEAVY RAIN FROM A STRONG SHORTWAVE ALOFT. THEREFORE, THE
EXISTING MARGINAL RISK AREA REMAINS UNCHANGED IN THE EXCESSIVE
RAINFALL OUTLOOK FOR FRIDAY-FRIDAY NIGHT ACROSS PORTIONS OF
NORTHERN CALIFORNIA. PRECIPITATION CHANCES SHOULD REMAIN ACROSS
THE WEST COAST AND ALSO EXPAND FARTHER SOUTHEASTWARD OVER THE
WEEKEND. SOME LOCALLY MODERATE/HEAVY TOTALS ARE POSSIBLE OVER
FAVORED TERRAIN, DEPENDING ON UPPER LEVEL AND SURFACE DETAILS THAT
WILL BE BETTER RESOLVED IN COMING DAYS. EVENTUALLY BY EARLY NEXT
WEEK, THE PATTERN SHOULD ALLOW FOR GULF RETURN FLOW TO PRODUCE
RAIN CHANCES IN THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY.
FARTHER EAST, EXPECT CONTINUED PERIODS OF LAKE EFFECT SNOW WITH
PERSISTENCE OF COLD AIR AND BREEZY CONDITIONS INTO SATURDAY. LAKE
EFFECT SNOW WILL LIKELY TAPER OFF BY SUNDAY AS SURFACE HIGH
PRESSURE BUILDS INTO THE EAST. A FRONTAL SYSTEM WITH EMBEDDED LOW
PRESSURE REACHING THE EAST DURING THE LATTER PART OF THE WEEK MAY
PRODUCE SOME PRECIPITATION. THIS SYSTEM COULD INITIALLY SPREAD
SOME LIGHT SNOW ACROSS THE NORTHERN-CENTRAL PLAINS BY THURSDAY
WHILE SOME LIMITED GULF MOISTURE COMES INTO THE EAST-CENTRAL U.S.,
SHIFTING TO THE EAST COAST FRIDAY. THE PRECIPITATION TYPE IN THE
SOUTHEAST SHOULD STAY RAIN WHILE GENERALLY FROM THE OHIO VALLEY
INTO THE APPALACHIANS AND THE NORTHERN MID-ATLANTIC TO NORTHEAST
SHOULD HAVE SNOW. AREAS IN BETWEEN ARE MORE UNCERTAIN WITH
PRECIPITATION TYPE, WHILE UNCERTAINTY ALSO REMAINS WITH THE
COVERAGE AND AMOUNTS OF PRECIPITATION, WHICH WILL CONTINUE TO BE
REFINED IN FUTURE FORECASTS. THE MAJORITY OF GUIDANCE STILL
SUGGESTS THAT SNOWFALL WILL BE FAIRLY LIGHT, AND THE LATEST
PROBABILITIES FOR 0.25 INCH LIQUID IN THE FORM OF SNOW ARE NO MORE
THAN 10-30 PERCENT OR SO DURING THE THURSDAY-FRIDAY NIGHT TIME
FRAME, THOUGH SLIGHTLY HIGHER PERHAPS IN SOUTHERN NEW ENGLAND AND
BACK INTO THE CENTRAL APPALACHIANS.
TATE
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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