Threats Assessment Discussion

046 
FXUS21 KWNC 142033
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 14 2018
SYNOPSIS: UPPER LEVEL LOW PRESSURE FORECASTED OFF THE WEST COAST OF THE US WILL 
CONTINUE TO FAVOR AN ONSHORE FLOW AND CONTINUED CHANCES FOR HEAVY PRECIPITATION 
FOR COASTAL WASHINGTON AND OREGON THROUGHOUT MOST OF THE DAY 3-7 PERIOD. BY 
DAY-7, UPPER LEVEL HIGH PRESSURE IS FORECAST TO BUILD IN ALLOWING THESE REGIONS 
TO DRY OUT. THE EASTERN US IS PROJECTED TO BE MAINLY UNDER THE INFLUENCE OF 
UPPER LEVEL LOW PRESSURE, WITH SOME UNCERTAINTY TOWARD THE END OF THE 3-7 DAY 
PERIOD REGARDING THE PATTERN EVOLUTION IN REGARDS TO SURFACE LOW DEVELOPMENT. 
THIS UNCERTAINTY WILL CONTINUE TO DOMINATE INTO WEEK-2 AS THE TROUGHING IN THE 
EAST MAY FAVOR CYCLONE DEVELOPMENT, BUT MODEL SIGNALS REMAIN INCONSISTENT WITH 
RESPECT TO BOTH TIMING AND MAGNITUDE, PRECLUDING ANY SPECIFIC THREATS FROM 
BEING ISSUED. 
HAZARDS
HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE NORTHERN 
GREAT BASIN, MON-THU, DEC 17-DEC 20.
FLOODING POSSIBLE ACROSS PORTIONS OF THE SOUTHEAST, THE MIDDLE MISSISSIPPI 
VALLEY, THE MID-ATLANTIC, THE SOUTHERN PLAINS, AND THE OHIO VALLEY.
FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST, THE 
MID-ATLANTIC, AND THE SOUTHERN PLAINS.
FLOODING LIKELY ACROSS PORTIONS OF THE SOUTHEAST.
HIGH WINDS ACROSS PORTIONS OF THE PACIFIC NORTHWEST, MON-TUE, DEC 17-DEC 18.
HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE PACIFIC NORTHWEST, 
MON-TUE, DEC 17-DEC 18.
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF SOUTHWEST MAINLAND ALASKA, 
WED-THU, DEC 19-DEC 20.
SEVERE DROUGHT ACROSS THE CENTRAL PLAINS, THE CENTRAL ROCKIES, THE CENTRAL 
GREAT BASIN, THE SOUTHERN PLAINS, THE NORTHERN GREAT BASIN, THE SOUTHERN 
ROCKIES, CALIFORNIA, THE ALASKA PANHANDLE, THE PACIFIC NORTHWEST, AND THE 
SOUTHWEST.
DETAILED SUMMARY
FOR MONDAY DECEMBER 17 - FRIDAY DECEMBER 21: ON 12/17 SURFACE LOW PRESSURE IS 
FORECAST TO BE MOVING OFF THE NEW ENGLAND COAST. IN CONTRAST TO YESTERDAY, THE 
DETERMINISTIC RUN OF TODAY'S 00Z ECMWF NOW KEEPS THE SYSTEM CLOSER TO THE COAST 
COMPARED TO THE 06Z GFS. THE ECMWF SOLUTION WOULD RESULT IN A PERIOD OF 
MODERATE TO HEAVY SNOW FOR EASTERN MAINE ON 12/17. HOWEVER, THE ENSEMBLE 
SOLUTIONS FROM TODAY'S ECMWF ARE MORE IN LINE WITH THE GFS, KEEPING THE 
HEAVIEST PRECIPITATION OFFSHORE. NO WINTRY HAZARDS ARE BEING POSTED ON TODAY'S 
MAP DUE TO LACK OF MODEL AGREEMENT, WITH THE MOST FAVORED SCENARIO BEING 
LIGHTER SNOW SHOWERS AND GUSTY WINDS ON THE BACK SIDE OF THE DEPARTING SURFACE 
LOW. LAKE EFFECT SNOW SHOWERS DUE TO A NORTHERLY FETCH OVER LAKES ONTARIO AND 
ERIE WILL ALSO BE POSSIBLE ON 12/17. WINDS ARE FORECAST TO BE GUSTY IN THE 
NORTHEAST IN GENERAL EARLY NEXT WEEK DUE TO A TIGHT PRESSURE GRADIENT 
DEVELOPING AS SURFACE HIGH PRESSURE BUILDS IN FROM THE MIDWEST.
THE WEST COAST, PARTICULARLY OREGON AND WASHINGTON, IS FORECAST TO SEE AN 
ONSLAUGHT OF PACIFIC MOISTURE THROUGHOUT MOST OF THE 3-7 DAY PERIOD AS 
TROUGHING IN THE EAST PACIFIC RESULTS IN AN ATMOSPHERIC RIVER EVENT WITH HEAVY 
PRECIPITATION (COASTAL RAIN AND MOUNTAIN SNOW). SURFACE LOW PRESSURE AND AN 
ASSOCIATED SURGE OF MOISTURE LOOKS TO ARRIVE LATE ON 12/17 INTO 12/18, AND 
LINGER INTO 12/20. FOR 12/17 AND 12/18, MORE IMPACTS ARE FORECASTED OVER 
COASTAL OREGON, ALONG WITH WASHINGTON, BEFORE SHIFTING NORTH TO MAINLY JUST 
WASHINGTON BY 12/20. HIGH SIGNIFICANT WAVE HEIGHTS OF ABOUT 20-30 FEET ARE 
FORECAST ALONG WITH HIGH WINDS FROM THE WEST-SOUTHWEST (SUSTAINED 40 KNOTS WITH 
HIGHER GUSTS) AS THE SURFACE LOW MOVES ASHORE. WINDS ARE FAVORED TO SHIFT MORE 
SOUTH-SOUTHWESTERLY BY 12/20. THE HAZARD FOR HIGH WINDS AND SIGNIFICANT WAVE 
HEIGHTS WILL BE DISCONTINUED AFTER 12/18 AS THE THREAT BECOMES MORE MARGINAL, 
BUT SOME LINGERING IMPACTS MAY REMAIN THROUGH 12/20 ESPECIALLY FARTHER NORTH. 
MODEL GUIDANCE CONTINUES TO SUPPORT 2"-5" OF LIQUID WITH ISOLATED HIGHER TOTALS 
FOR THE ENTIRE PERIOD. THIS IS ESPECIALLY TRUE FOR COASTAL WASHINGTON WHICH IS 
FORECAST TO HAVE IMPACTS THROUGH 12/20. BY 12/21 RIDGING IS FORECAST TO BUILD 
OVER THE WEST COAST, ENDING THE THREAT OF HEAVY PRECIPITATION.
MUCH BELOW NORMAL TEMPERATURES ARE FORECAST FOR SOUTHWEST MAINLAND ALASKA FOR 
12/19 AND 12/20 AS TROUGHING OVER THE STATE IS FORECAST TO LEAD TO VERY COLD 
OVERNIGHT LOW TEMPERATURES (-20 DEG. F OR LOWER POSSIBLE) AND SUBZERO HIGH 
TEMPERATURES IN SOME AREAS. THERE IS LESS CONFIDENCE ON 12/21 AS TEMPERATURES 
MAY BEGIN TO SLOWLY MODERATE AS FORECASTED 500-HPA HEIGHTS BEGIN TO INCREASE.
ON 12/20 AND 12/21 ATTENTION WILL TURN TO THE EASTERN US AS A DIGGING TROUGH 
MAY RESULT IN SURFACE LOW DEVELOPMENT BUT CONFIDENCE IS VERY LOW. TODAY'S 00Z 
AND 12Z DETERMINISTIC ECMWF MODEL RUNS DEVELOP A SURFACE CYCLONE ALONG THE EAST 
COAST. THE 00Z CANADIAN ALSO SUPPORTS THIS SYSTEM. IN CONTRAST, THE 06Z AND 12Z 
DETERMINISTIC GFS RUNS ARE NOT AS AMPLIFIED WITH THE TROUGH OVER THE EASTERN 
US, AND SHOW CYCLONE DEVELOPMENT TOO FAR OFF THE COAST TO HAVE ANY MEANINGFUL 
IMPACTS, ALTHOUGH SOME INDIVIDUAL ENSEMBLE MEMBERS DO SUPPORT THE MORE 
AMPLIFIED SOLUTION. THE KEY COMPONENT WHICH NEEDS TO BE RESOLVED IS HOW QUICKLY 
DOES THE PATTERN AMPLIFY, AS A MORE AMPLIFIED PATTERN WILL TEND TO FAVOR A 
STRONGER CYCLONE. FOR NOW, NO HAZARDS ARE POSTED IN ASSOCIATION WITH THIS 
THREAT BUT WILL BE MONITORED FOR INCLUSION IN NEXT WEEK'S OUTLOOKS IF THE 
POTENTIAL FOR THIS SYSTEM INCREASES.
FOR SATURDAY DECEMBER 22 - FRIDAY DECEMBER 28: DURING THE WEEK-2 PERIOD, AN 
AMPLIFYING 500-HPA CIRCULATION PATTERN IS PREDICTED, WITH A TROUGH OVER EASTERN 
NORTH AMERICA, AND A RIDGE OVER WESTERN NORTH AMERICA. THIS FORECASTED PATTERN 
FAVORS MUCH DRIER CONDITIONS FOR THE WEST COAST, BRINGING AN END TO THE 
ATMOSPHERIC RIVER EVENTS DOMINATING THE 3-7 DAY PERIOD.
MODEL GUIDANCE CONTINUES TO WAVER IN REGARDS TO THREATS DURING WEEK-2, BUT 
OVERALL TROUGHING PATTERN IN THE EAST MAY RESULT IN SEVERAL OPPORTUNITIES FOR A 
SURFACE LOW PRESSURE SYSTEM TO DEVELOP BRINGING INCREASED PRECIPITATION CHANCES 
TO THE EASTERN US IN WEEK-2. HOWEVER, DUE TO NO CONSISTENCY AMONG THE MODELS, A 
THREAT IS NOT BEING POSTED AT THIS TIME.
THE U.S. DROUGHT MONITOR, VALID DECEMBER 11TH, INDICATES A SLIGHT DECREASE IN 
SEVERE TO EXCEPTIONAL DROUGHT (D2 TO D4) COVERAGE FROM 11.34 PERCENT LAST WEEK 
TO 11.20 PERCENT  THIS WEEK. IMPROVEMENT IS NOTED ACROSS SOUTHERN CALIFORNIA 
WHERE EXTREME DROUGHT (D3) HAS BEEN REMOVED. HOWEVER COVERAGE OF SEVERE DROUGHT 
(D3) HAS INCREASED OVER OREGON AND MODERATE DROUGHT (D1) HAS EXPANDED OVER 
SOUTHERN FLORIDA.
FORECASTER: THOMAS COLLOW 
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