841  
FXUS21 KWNC 272143  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 27 2017  
 
SYNOPSIS: A MID-LEVEL LOW-PRESSURE SYSTEM IS FORECAST TO MOVE ACROSS THE  
NORTHWEST LATER THIS WEEK. DOWNSTREAM, A SURFACE LOW-PRESSURE SYSTEM IS  
PREDICTED TO MOVE ACROSS THE NORTHEAST DURING THE WEEKEND AND PUSH A COLD FRONT  
TO THE GULF OF MEXICO. HIGH PRESSURE IS FORECAST TO BUILD IN BEHIND THAT COLD  
FRONT, BEFORE THE FRONT SLOWLY LIFTS NORTHWARD DURING THE EARLY PORTIONS OF  
WEEK-2. ACROSS THE WESTERN PORTIONS OF THE CONTIGUOUS 48 STATES, MODEL  
FORECASTS DISAGREE DURING WEEK-2, SO UNCERTAINTY IS HIGH. A SURFACE  
LOW-PRESSURE SYSTEM IS FORECAST TO IMPACT THE ALEUTIANS EARLY IN THE PERIOD  
BEFORE MOVING EASTWARD AND IMPACTING SOUTH-CENTRAL MAINLAND ALASKA COASTAL  
REGIONS.  
 
HAZARDS  
 
HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS AND THE NORTHERN  
ROCKIES, SAT, DEC 30.  
 
HEAVY SNOW AROUND THE GREAT LAKES, SAT-WED, DEC 30-JAN 3.  
 
HIGH WINDS ACROSS PORTIONS OF THE MID-ATLANTIC AND THE NORTHEAST, SUN-MON, DEC  
31-JAN 1.  
 
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE  
NORTHERN PLAINS, THE TENNESSEE VALLEY, THE GREAT LAKES, THE NORTHERN ROCKIES,  
THE MIDDLE MISSISSIPPI VALLEY, THE NORTHEAST, THE SOUTHERN APPALACHIANS, THE  
UPPER MISSISSIPPI VALLEY, THE SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO  
VALLEY, SAT-TUE, DEC 30-JAN 2.  
 
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE  
TENNESSEE VALLEY, THE GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, THE  
NORTHEAST, THE SOUTHERN APPALACHIANS, THE UPPER MISSISSIPPI VALLEY, THE  
SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO VALLEY, WED, JAN 3.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF MAINLAND ALASKA, SUN, DEC 31.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE AND MAINLAND  
ALASKA, TUE, JAN 2.  
 
HIGH WINDS ACROSS PORTIONS OF MAINLAND ALASKA AND THE ALEUTIANS, SAT, DEC 30.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL  
PLAINS, THE MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY, THE CENTRAL  
APPALACHIANS, THE NORTHERN PLAINS, THE TENNESSEE VALLEY, THE GREAT LAKES, THE  
MIDDLE MISSISSIPPI VALLEY, THE NORTHEAST, THE SOUTHERN APPALACHIANS, THE UPPER  
MISSISSIPPI VALLEY, THE SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO VALLEY,  
THU-SUN, JAN 4-JAN 7.  
 
MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL  
PLAINS, THE MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY, THE CENTRAL  
APPALACHIANS, THE NORTHERN PLAINS, THE TENNESSEE VALLEY, THE MIDDLE MISSISSIPPI  
VALLEY, THE NORTHEAST, THE SOUTHERN APPALACHIANS, THE UPPER MISSISSIPPI VALLEY,  
THE SOUTHEAST, THE GREAT LAKES, AND THE OHIO VALLEY, THU-SAT, JAN 4-JAN 6.  
 
HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE SOUTHEAST, THE  
MID-ATLANTIC, THE SOUTHERN APPALACHIANS, AND THE TENNESSEE VALLEY, THU-SAT, JAN  
4-JAN 6.  
 
HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT LAKES,  
THE MID-ATLANTIC, THE NORTHEAST, THE CENTRAL APPALACHIANS, AND THE OHIO VALLEY,  
THU, JAN 4.  
 
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHERN ROCKIES, THE  
SOUTHERN PLAINS, CALIFORNIA, AND THE SOUTHWEST, THU-FRI, JAN 4-JAN 5.  
 
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHERN ROCKIES, THE  
CENTRAL PLAINS, THE SOUTHERN PLAINS, AND THE SOUTHWEST, SAT-SUN, JAN 6-JAN 7.  
 
SEVERE DROUGHT ACROSS THE CENTRAL PLAINS, THE LOWER MISSISSIPPI VALLEY, THE  
CENTRAL GREAT BASIN, THE NORTHERN PLAINS, HAWAII, THE NORTHERN ROCKIES, THE  
MIDDLE MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, AND THE SOUTHWEST.  
 
DETAILED SUMMARY  
 
FOR SATURDAY DECEMBER 30 - WEDNESDAY JANUARY 03: A MID-LEVEL TROUGH IS FORECAST  
TO MOVE ACROSS THE PACIFIC NORTHWEST ON SAT DEC 29, BRINGING MODERATE  
PRECIPITATION TO MUCH OF THE PACIFIC NORTHWEST. THE NORTHERN ROCKIES ARE LIKELY  
TO EXPERIENCE HEAVY SNOWS (GREATER THAN 8 INCHES), ESPECIALLY IN MONTANA AND  
WYOMING. AT THIS TIME, THE FORECAST CALLS FOR LESSER AMOUNTS OF SNOW OVER  
NEBRASKA AND SOUTH DAKOTA, THOUGH IMPACTS TO TRAVEL COULD BE REALIZED DURING  
SUNDAY IF HIGHER SNOWFALL TOTALS OCCUR.  
 
AREAS OF MODERATE TO HEAVY SNOW DURING THE WEEKEND ARE PREDICTED DOWNSTREAM OF  
THE GREAT LAKES. LAKE SURFACE TEMPERATURES ARE STILL ABOVE FREEZING IN MOST  
LOCATIONS THOUGH LIKELY DROPPING QUICKLY IN THE COMING DAYS. OPEN LAKES AND  
COLD AIR SET THE STAGE FOR LAKE ENHANCED SNOWS, AND MANY LOCATIONS JUST  
RECEIVED SIGNIFICANT SNOWS. MITIGATING FACTORS WILL BE THE WIND DIRECTION AND  
DROPPING LAKE TEMPERATURES.  
 
A LOW-PRESSURE SYSTEM IS FORECAST TO DEEPEN OFF THE NORTHEAST COAST. AS A  
RESULT, HIGH WINDS (GREATER THAN 35 MPH OVER LAND) ARE LIKELY FROM THE  
MID-ATLANTIC TO NEW ENGLAND. WIND SPEEDS IN THE 12Z MODEL RUNS ARE LOWER THAN  
THE 0Z MODEL SUITE, SO THERE IS SOME UNCERTAINTY WITH THE HIGH WIND HAZARD,  
WHICH CAN BE RE-EVALUATED TOMORROW.  
 
BEHIND THIS LOW-PRESSURE SYSTEM, STRONG AND COLD HIGH PRESSURE IS FORECAST TO  
BUILD SOUTHWARD FROM CANADA. MULTIPLE DAYS OF TEMPERATURES AT LEAST 12 DEGREES  
BELOW AVERAGE ARE FORECAST FOR MUCH OF THE EASTERN HALF OF THE CONTIGUOUS 48  
STATES. THE ECMWF ENSEMBLES MEAN IS SLIGHTLY WARMER THAN THE GEFS, WITH THE  
CANADIAN MORE ALIGNED WITH THE ECMWF, SO THERE IS SOME UNCERTAINTY ABOUT THE  
SOUTHWARD LIMIT OF THE COLD AIR. STILL, ODDS OF FREEZES ARE ANOMALOUSLY HIGH AS  
FAR SOUTH AS THE GULF COAST, ESPECIALLY ON NEW YEAR'S DAY. BY DAY 7, SOME  
MODERATION OF THE AIRMASS IS LIKELY OVER THE PLAINS, DO THE HAZARD ON DAY 7 IS  
RESTRICTED TO EAST OF THE MISSISSIPPI VALLEY.  
 
BEHIND THE COLD FRONT, WINDS OVER THE SOUTHERN PLAINS ARE LIKELY TO BE STRONG,  
BUT CERTAINTY ON REACHING HAZARDOUS CRITERIA IS LOW. HIGHER WINDS COULD LEAD TO  
RED FLAG CONDITIONS ON SATURDAY.  
 
LATER IN THE PERIOD, MODELS SHOW ACTIVITY ALONG THE COLD FRONT PREDICTED OVER  
THE GULF OF MEXICO. THE GFS, ECMWF AND CANADIAN DETERMINISTIC MODELS INDICATE  
THE POTENTIAL FOR FREEZING/FROZEN PRECIPITATION FROM THE LOWER MISSISSIPPI  
VALLEY TO THE SOUTHEAST EARLY NEXT WEEK. TIMING DIFFERENCES ARE QUITE LARGE, SO  
NO HAZARDS ARE POSTED. UPSTREAM, SOME MODELS INDICATE MOISTURE MOVING INTO  
NORTHERN CALIFORNIA AND OREGON ON DAYS 6 AND 7, WITH THE ECMWF DETERMINISTIC  
SOLUTIONS DEPICTING HIGHER PRECIPITATION AMOUNTS, WITH POTENTIALLY HAZARDOUS  
LEVELS IN THE CENTRAL SIERRA MADRES.  
 
ACROSS SOUTHERN ALASKA, MEAN LOW-PRESSURE IS FORECAST TO PERSIST NEAR THE  
ALEUTIANS. SHORTWAVES ARE FORECAST TO MOVE THROUGH THE PATTERN, ENHANCING  
PRECIPITATION OVER SOUTHWEST AND SOUTH-CENTRAL MAINLAND ALASKA ON DEC 31,  
BEFORE MOVING EAST. OFFSHORE FLOW INTO THIS LOW-PRESSURE SYSTEM IS LIKELY TO BE  
FUNNELED BY TERRAIN OVER THE PANHANDLE, LEADING TO LOCALLY HIGH WINDS CROSSING  
HAZARDS THRESHOLDS IN SOME VALLEYS. AS A SHORTWAVE MOVES EAST TOWARD SOUTHEAST  
ALASKA, THE THREAT HEAVY PRECIPITATION INCREASES BY JAN 2. MODELS ARE  
INDICATING 2-3 INCHES OF PRECIPITATION IN 24 HOURS, AND THE POTENTIAL FOR SOME  
HIGHER WINDS NEAR THE PANHANDLE.  
 
FOR THURSDAY JANUARY 04 - WEDNESDAY JANUARY 10: A DEEP TROUGH IS FORECAST TO  
DOMINATE MUCH OF THE CENTRAL AND EASTERN CONUS FOR MOST OF WEEK-2, ALTHOUGH IT  
IS EXPECTED TO WEAKEN TOWARDS THE MIDDLE OF THE PERIOD. THIS LEADS TO A SLIGHT  
RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MUCH OF THE CENTRAL AND EASTERN  
CONUS JAN 4-7, AND A MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FROM  
MIDWEST TO THE LOWER MISSISSIPPI VALLEY AND EASTWARD, DURING JAN 4-6. A HIGH  
RISK OF MUCH BELOW NORMAL TEMPERATURES IS INDICATED AROUND THE GREAT LAKES, JAN  
4, AND OVER THE CAROLINAS, GEORGIA AND NORTHERN ALABAMA JAN 4-6. TEMPERATURES  
ARE EXPECTED TO MODERATE OVER THE WESTERN AND CENTRAL CONUS DURING THE PERIOD,  
SO THE WESTERN EDGES OF THE SLIGHT AND MODERATE THREAT AREAS ARE LESS CERTAIN  
FOR LATER IN WEEK-2. ADDITIONAL UNCERTAINTY IS EVIDENT OVER FLORIDA WHEN  
COMPARING THE ECMWF, GEFS, AND CANADIAN MODELS, SO THE OFFICIAL OUTLOOK IS NOT  
AS INTENSE AS THE GEFS PROBABILITIES.  
 
SOME MODEL OUTPUTS SHOW TROUGHING MOVING INTO THE FOUR CORNERS REGION DURING  
THE EARLY PORTION OF WEEK-2. THIS WOULD SUPPORT MODERATE TO POTENTIALLY HEAVY  
SNOWS IN ARIZONA, NEW MEXICO, COLORADO, AND THE SOUTHERN HIGH PLAINS. ODDS ARE  
20-30% ACCORDING TO THE GEFS, WITH ECMWF AND CANADIAN ENSEMBLE MEANS ABOUT THE  
SAME.  
 
GFS AND ECMWF DETERMINISTIC SOLUTIONS BOTH DEPICT A STRONG STORM MOVING UP THE  
EAST COAST, BUT MOSTLY OFF-SHORE DURING NEXT THU-FRI, JAN 4-5. NO HAZARD IS YET  
POSTED AS IMPACTS OVER LAND ARE STILL TOO UNCERTAIN.  
 
THE U.S. DROUGHT MONITOR, UPDATED ON DEC 19, SHOWS AN INCREASE IN AREAS OF  
EXCEPTIONAL DROUGHT (D2-D4) TO 6.97% FROM 6.49%. MOST OF THE NEWLY INDICATED  
SEVERE DROUGHT WAS OVER THE SOUTHERN PLAINS.  
 
FORECASTER: MATTHEW ROSENCRANS  
 

 
 
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