098  
FXUS21 KWNC 262123  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 26 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 PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST, FRI, DEC 29.  
 
HEAVY SNOW ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE NORTHERN ROCKIES,  
FRI, DEC 29.  
 
HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES, SAT-MON, DEC 30-JAN 1.  
 
HIGH WINDS ACROSS PORTIONS OF THE MID-ATLANTIC, THE NORTHEAST, AND THE GREAT  
LAKES, SUN, DEC 31.  
 
MUCH BELOW NORMAL TEMPERATURES FROM THE NORTHERN ROCKIES TO THE GREAT PLAINS,  
MISSISSIPPI VALLEY, GREAT LAKES, OHIO VALLEY, TENNESSEE VALLEY, APPALACHIANS,  
NORTHEAST, MID-ATLANTIC AND PORTIONS OF THE SOUTHEAST, FRI-TUE, DEC 29-JAN 2.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF SOUTH COASTAL MAINLAND ALASKA, SUN, DEC  
31.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE, TUE, JAN 2.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT PLAINS,  
THE MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE  
GREAT LAKES, OHIO VALLEY, THE NORTHEAST, THE MID-ATLANTIC, THE SOUTHERN  
APPALACHIANS, AND THE SOUTHEAST, WED-SAT, JAN 3-JAN 6.  
 
MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FROM THE MISSISSIPPI VALLEY TO  
THE EAST COAST, WED-FRI, JAN 3-JAN 5.  
 
HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHEAST, THE  
CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE MID-ATLANTIC, THE SOUTHERN  
APPALACHIANS, THE SOUTHEAST, THE GREAT LAKES, AND THE OHIO VALLEY, WED-THU, JAN  
3-JAN 4.  
 
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE CENTRAL PLAINS, THE  
SOUTHERN ROCKIES, THE SOUTHERN PLAINS, AND THE SOUTHWEST, FRI-SAT, JAN 5-JAN 6.  
 
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 FRIDAY DECEMBER 29 - TUESDAY JANUARY 02: A MID-LEVEL TROUGH IS FORECAST TO  
MOVE ACROSS THE PACIFIC NORTHWEST ON SAT DEC 29, BRINGING HEAVY PRECIPITATION  
(UP TO 2.0 INCHES LOCALLY, MORE WIDESPREAD 1.0-1.5 INCHES) TO THE WESTERN  
PORTIONS OF WASHINGTON AND OREGON. PREDICTED PRECIPITATION TOTALS ARE LOWER  
TODAY THAN YESTERDAY, THOUGH SOME HEAVY SNOWS ARE STILL LIKELY IN THE CASCADES.  
FURTHER EAST, HEAVY SNOWS (LIKELY IN EXCESS OF 6 INCHES, THOUGH SOME MODELS  
SHOW SIGNIFICANTLY MORE) ARE LIKELY ACROSS THE BITTERROOTS AND SOUTHWARD INTO  
YELLOWSTONE NP AREA.  
 
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.  
 
THE LOW-PRESSURE SYSTEM RESPONSIBLE FOR THE LAKE ENHANCED SNOWS IS FORECAST TO  
DEEPEN OFF THE NORTHEAST COAST. AS A RESULT, HIGH WINDS (GREATER THAN 35 MPH  
OVER LAND, AND 40 KNOTS IN NEAR SHORE WATERS) ARE LIKELY FROM THE MID-ATLANTIC  
TO NEW ENGLAND. WIND SPEEDS ARE HIGHER IN THE ECMWF MODEL THAN THE GFS FOR THE  
MID-ATLANTIC AS THE GFS SPINS UP THE STORM FURTHER NORTH, THEN A SECOND, WEAKER  
LOW-PRESSURE SYSTEM IN JAN 1 NEAR THE CAROLINAS. SO THERE IS SOME UNCERTAINTY  
IN THE WIND SPEED HAZARD. ON THE NORTHERN SIDE OF THIS SYSTEM, MODERATE  
SNOWFALL IS LIKELY ACROSS MAINE AND POTENTIALLY EXTENDING TO CAPE COD, THOUGH  
ODDS ARE SLIGHTLY LOWER THAN YESTERDAY.  
 
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 MAY BE SOME WARMING OVER THE  
SOUTHERN AND CENTRAL PLAINS, ON DAY 4, AS WELL AS LESS SOUTHWARD PENETRATION OF  
THE COLDEST AIR OVER THE SOUTHEAST. STILL, ODDS OF FREEZES ARE ANOMALOUSLY HIGH  
AS FAR SOUTH AS THE GULF COAST, ESPECIALLY ON NEW YEAR'S DAY.  
 
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.  
 
INTO EARLY NEXT WEEK, MODELS SHOW ACTIVITY ALONG THE FRONT, WHICH BY THEN IS  
LIKELY TO BE STALLED OVER THE GULF OF MEXICO. ALL OF THE FORECASTS 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.  
 
ENSEMBLE MEANS BEGIN TO SHOW PRECIPITATION MOVING INTO NORTHERN CALIFORNIA  
DURING NEXT TUESDAY, BUT AMOUNTS ARE VARIED IN THE MEMBER SO THE MEANS ARE LOW.  
THE DETERMINISTIC EUROPEAN INDICATES LARGER TOTALS, WITH UP TO 2 INCHES IN THE  
CENTRAL AND SOUTHERN 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 OF SMALL AREAS. AS A SHORTWAVE MOVES EAST TOWARD THE  
PANHANDLE, THE THREAT OF WINDS SUBSIDES BUT THE THREAT OF HEAVY PRECIPITATION  
INCREASES BY JAN 2. MODELS ARE INDICATING 2-3 INCHES OF PRECIPITATION IN 24  
HOURS.  
 
FOR WEDNESDAY JANUARY 03 - TUESDAY JANUARY 09: 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 3-6, AND A MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FROM  
MIDWEST TO THE LOWER MISSISSIPPI VALLEY AND EASTWARD, DURING JAN 3-5. A HIGH  
RISK OF MUCH BELOW NORMAL TEMPERATURES IS INDICATED FROM THE GREAT LAKES TO NEW  
ENGLAND AND SOUTHWARD TO A LINE FROM CENTRAL ALABAMA TO ABOUT SAVANNAH, GA  
DURING JAN 3-4. 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 ECWMF, 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 CENTRAL HIGH PLAINS. ODDS ARE  
20-30% ACCORDING TO THE GEFS, WITH ECMWF AND CANADIAN ENSEMBLE MEANS ABOUT THE  
SAME.  
 
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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