Prognostic Meteorological Discussion
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FXUS21 KWNC 132021
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 13 2017

SYNOPSIS: THE FORECAST UPPER-LEVEL PATTERN IS FORECAST TO BE ORIENTED MORE WEST
TO EAST OVER THE U.S. EARLY IN THE PERIOD, WITH A SERIES OF STORM SYSTEMS
FORECAST TO MOVE QUICKLY ACROSS THE NORTHERN TIER OF THE U.S. AND SOUTHERN
CANADA. STRONG SURFACE HIGH PRESSURE IS FORECAST TO PUSH SOUTHWARD OUT OF
CANADA INTO THE NORTHERN HIGH PLAINS DURING WEEK-2, BRINGING INCREASED ODDS OF
MUCH BELOW NORMAL TEMPERATURES TO THE NORTHERN AND CENTRAL U.S. STRONG SURFACE
LOW PRESSURE IS FORECAST TO AFFECT SOUTHERN ALASKA AT THE BEGINNING OF THE
PERIOD, GIVING WAY TO SURFACE HIGH PRESSURE IN THE GULF OF ALASKA LATER IN THE
PERIOD.

HAZARDS

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST AND THE LOWER MISSISSIPPI VALLEY,
SAT-SUN, DEC 16-DEC 17.

HIGH WINDS ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS, SAT, DEC 16.

HEAVY PRECIPITATION ACROSS PORTIONS OF SOUTHEAST ALASKA, SAT, DEC 16.

HIGH WINDS ACROSS PORTIONS OF MAINLAND ALASKA AND THE ALEUTIANS, SAT-MON, DEC
16-DEC 18.

MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF MAINLAND ALASKA, SAT, DEC 16.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE ALEUTIANS, SUN-MON,
DEC 17-DEC 18.

LOCALLY HEAVY SNOW OVER PORTIONS OF THE NORTHERN HIGH PLAINS, THU-SAT, DEC
21-DEC 23.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE MID-ATLANTIC,
THE NORTHEAST, AND THE GREAT LAKES, THU-FRI, DEC 21-DEC 22.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE PLAINS, THE
GREAT LAKES, THE MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE TENNESSEE
VALLEY, THE NORTHERN AND CENTRAL ROCKIES, THE PACIFIC NORTHWEST, THE NORTHERN
GREAT BASIN, THE MID-ATLANTIC, THE NORTHEAST, AND THE OHIO VALLEY, FRI-WED, DEC
22-DEC 27.

MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL AND
NORTHERN PLAINS, THE NORTHERN ROCKIES, THE MIDDLE AND UPPER MISSISSIPPI VALLEY,
AND THE GREAT LAKES, FRI-WED, DEC 22-DEC 27.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND ALASKA,
THU-WED, DEC 21-DEC 27.

MODERATE RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND
ALASKA, FRI-MON, DEC 22-DEC 25.

SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI
VALLEY, THE CENTRAL AND SOUTHERN APPALACHIANS, THE TENNESSEE VALLEY, THE
MID-ATLANTIC, THE SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO VALLEY, FRI-MON,
DEC 22-DEC 25.

SEVERE DROUGHT ACROSS THE LOWER MISSISSIPPI VALLEY, THE NORTHERN PLAINS,
HAWAII, THE NORTHERN ROCKIES, THE MIDDLE MISSISSIPPI VALLEY, THE SOUTHERN
PLAINS, AND THE SOUTHWEST.

DETAILED SUMMARY

FOR SATURDAY DECEMBER 16 - WEDNESDAY DECEMBER 20: DURING THE EARLY PERIOD, A
FAIRLY ZONAL PATTERN IS FORECAST OVER THE CONUS, AT LEAST COMPARED TO THE
PERIOD PRECEDING THE START OF THE OUTLOOK PERIOD. HIGH WINDS ARE LIKELY TO
LINGER OVER THE CENTRAL AND SOUTHERN PLAINS INTO DEC 16 AS A COLD FRONT MOVES
THROUGH THE REGION. THIS AREA COULD ALSO SEE AN ELEVATED THREAT FOR FIRE
WEATHER, DUE TO THE HIGH WINDS AND LOW RELATIVE HUMIDITY VALUES. AS THE
ASSOCIATED UPPER-LEVEL TROUGH LIFTS NORTHEASTWARD AND THE SURFACE COLD FRONT
SHIFTS EASTWARD, LOCALLY HEAVY RAIN IS EXPECTED OVER PARTS OF THE SOUTHEAST AND
LOWER MISSISSIPPI VALLEY THIS WEEKEND.



THE VARIOUS HEAVY PRECIPITATION SHAPES THAT HAD BEEN DEPICTED FOR PARTS OF THE
NORTHWEST HAVE BEEN DISCONTINUED BASED ON THE LATEST MODEL GUIDANCE AND
NATIONAL FORECAST PRODUCTS FROM THE WEATHER PREDICTION CENTER. THE FORECAST
SHORTWAVE TROUGH IS FORECAST TO BE SOMEWHAT WEAKER AND FARTHER NORTH COMPARED
TO PREVIOUS FORECAST CYCLES.



AN ANOMALOUSLY STRONG SURFACE LOW IS FORECAST TO MOVE OVER THE ALEUTIAN ISLANDS
AND ALONG THE ALASKAN SOUTH COAST AT THE BEGINNING OF THE PERIOD. THIS IS
LIKELY TO BRING HIGH WINDS (GUSTS GREATER THAN 30 KNOTS) TO THE ALASKA
PENINSULA FOR DEC 16. AS THE LOW-PRESSURE SYSTEM MOVES INTO GULF OF ALASKA AND
OVER SOUTHERN ALASKA, THERE IS A POSSIBILITY FOR HEAVY PRECIPITATION ALONG THE
SOUTHERN GULF COAST AND PARTS OF THE PANHANDLE.



ANOTHER LOW PRESSURE SYSTEM IS LIKELY TO MOVE EAST FROM THE BERING SEA FOR THE
LATTER HALF OF WEEK-1, BRINGING A SECOND THREAT OF HIGH WINDS (GUSTS GREATER
THAN 40 KNOTS) AND A THREAT FOR SIGNIFICANT WAVES FOR TO ALEUTIAN ISLANDS FOR
DEC 17-18. THE UPPER-LEVEL RIDGE THAT HAS BEEN PRESENT OVER CENTRAL ALASKA WILL
REMAIN IN THE BEGINNING OF THE PERIOD, CONTINUING THE LIKELIHOOD FOR MUCH ABOVE
NORMAL TEMPERATURES FOR DEC 16. AN UPPER-LEVEL TROUGH MAY MOVE THROUGH IN THE
MIDDLE OF THE PERIOD, BRINGING SOME MODERATION IN TEMPERATURES, BUT RIDGING IS
LIKELY TO RETURN TOWARD THE END OF WEEK-1 AND DURING WEEK-2 WITH SURFACE HIGH
PRESSURE FORECAST TO DOMINATE OVER THE GULF OF ALASKA.

FOR THURSDAY DECEMBER 21 - WEDNESDAY DECEMBER 27: WEEK-2 IS LIKELY TO FEATURE
AN AMPLIFICATION IN THE UPPER-LEVEL PATTERN, WITH ANOMALOUS RIDGING FORECAST
JUST OFF THE WEST COAST OF NORTH AMERICA.  THIS PATTERN AND TELECONNECTIONS
UPON A LARGE 500-HPA POSITIVE HEIGHT ANOMALY CENTER OVER THE GULF OF ALASKA ARE
SUPPORTIVE OF A COLD OUTBREAK OVER THE NORTHERN U.S.  A SLIGHT RISK OF MUCH
BELOW NORMAL TEMPERATURES IS POSTED ACROSS NORTHERN AND CENTRAL PARTS OF THE
CONUS FOR MUCH OF WEEK-2. THE MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES
IS FORECAST FROM THE NORTHERN PLAINS TO THE WESTERN GREAT LAKES AND SOUTHWARD
THROUGH THE PLAINS, WHERE THE 12Z GEFS MEAN INDICATES TEMPERATURES AVERAGING
NEAR 10 TO 15 DEGREES F BELOW NORMAL. ADDITIONALLY, THERE IS A GREATER THAN 20%
CHANCE THAT SUBZERO MINIMUM TEMPERATURE WILL REACH INTO PARTS OF THE PLAINS
WHERE EXPOSED WINTER WHEAT CROP COULD BE DAMAGED.



WITH ANOMALOUS HIGH PRESSURE FORECAST TO BUILD SOUTHWARD OUT OF CANADA, AN
UPSLOPE SNOW EVENT IS POSSIBLE OVER PARTS OF THE NORTHERN HIGH PLAINS. A HAZARD
IS DEPICTED DAYS 8-10 TO HIGHLIGHT THIS POTENTIAL HAZARD.



A SLIGHT RISK OF HEAVY PRECIPITATION IS FORECAST OVER THE LOWER MISSISSIPPI
RIVER VALLEY, THE SOUTHEAST, AND THE CENTRAL AND SOUTHERN APPALACHIANS FOR DEC
22-25, IN THE VICINITY OF THE MEAN FRONTAL ZONE. AREAS IN THE NORTHERN FRINGE
OF THE DEPICTED SHAPES AND POINTS NORTHWEST COULD SEE HAZARDOUS WINTER WEATHER
DURING THAT PERIOD.



PERSISTENT UPPER-LEVEL RIDGING OVER ALASKA IS LIKELY TO CONTINUE THE ANOMALOUS
TEMPERATURES THAT HAVE BEEN AFFECTING MUCH OF MAINLAND ALASKA FOR MOST OF THE
MONTH OF DECEMBER. A SLIGHT RISK FOR MUCH ABOVE NORMAL TEMPERATURES IS FORECAST
THROUGHOUT THE INTERIOR AND WESTERN ALASKA FOR DEC 21-27, WITH A MODERATE RISK
OF MUCH ABOVE NORMAL TEMPERATURES FORECAST FOR MUCH OF THE INTERIOR OF ALASKA
OVER A SLIGHTLY SHORTER PERIOD.



THE U.S. DROUGHT MONITOR, UPDATED ON DEC 5, SHOWS AN INCREASE IN AREAS OF
EXCEPTIONAL DROUGHT (D2-D4) FROM 4.88% TO 5.99%, A CHANGE OF 1.11% OVER THE
CONTINENTAL U.S. FROM THE PREVIOUS WEEK.

FORECASTER: STEPHEN BAXTER

$$




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