Prognostic Meteorological Discussion
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FXUS21 KWNC 142028
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 14 2017

SYNOPSIS: THE 3-7 DAY PERIOD IS LIKELY TO BE DOMINATED BY TRANSIENT PATTERNS,
WHILE THE WEEK-2 PERIOD IS FORECAST FOR AN AMPLIFICATION OF THE UPPER-LEVEL
PATTERN. A SERIES OF FRONTS ARE FORECAST TO MOVE OVER THE SOUTHEAST AND CENTRAL
GULF COAST, WHILE A SHORTWAVE TROUGH WILL MOVE OVER THE NORTHWESTERN U.S. IN
THE MIDDLE OF WEEK-1. FOR WEEK-2, AN UPPER-LEVEL POSITIVELY-TILTED TROUGH IS
ANTICIPATED TO DEEPEN OVER THE CENTRAL U.S. AND THE SOUTHWEST, WITH RIDGING
MOVING OVER THE SOUTHEAST. STRONG LOW-PRESSURE SYSTEMS ARE EXPECTED TO MOVE
OVER SOUTHERN ALASKA IN WEEK-1, WHILE AN UPPER-LEVEL RIDGE IS LIKELY TO RETURN
AT THE END OF WEEK-1 AND PERSIST INTO WEEK-2.

HAZARDS

HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA, THE PACIFIC NORTHWEST, AND
THE NORTHERN GREAT BASIN, TUE, DEC 19.

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST, THE SOUTHERN APPALACHIANS, THE
LOWER MISSISSIPPI VALLEY, AND THE TENNESSEE VALLEY, SUN-TUE, DEC 17-DEC 19.

HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES, TUE, DEC 19.

HIGH WINDS ACROSS PORTIONS OF CALIFORNIA, SUN, DEC 17.

ENHANCED WILDFIRE RISK ACROSS PORT OF THE CALIFORNIA, SUN, DEC 17.

HIGH WINDS ACROSS PORTIONS OF MAINLAND ALASKA AND THE ALEUTIANS, SUN-TUE, DEC
17-DEC 19.

MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF MAINLAND ALASKA, THU, DEC 21.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE ALEUTIANS, MON-TUE,
DEC 18-DEC 19.

FROZEN PRECIPITATION ACROSS PORTIONS OF THE NORTHERN AND CENTRAL ROCKIES AND
NORTHERN AND CENTRAL PLAINS, FRI-SUN, DEC 22-DEC 24.

MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE PLAINS, THE
NORTHERN PLAINS, THE GREAT LAKES, AND THE ROCKIES, SUN-THU, DEC 24-DEC 28.

MODERATE RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND
ALASKA, SAT-TUE, DEC 23-DEC 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE PLAINS, THE
ROCKIES, THE GREAT LAKES, THE MISSISSIPPI VALLEY, THE PACIFIC NORTHWEST, AND
THE SOUTHWEST, SAT-THU, DEC 23-DEC 28.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND ALASKA,
FRI-THU, DEC 22-DEC 28.

MODERATE RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHEAST, THE LOWER
MISSISSIPPI VALLEY, AND THE TENNESSEE VALLEY, SUN-MON, DEC 24-DEC 25.

SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI
VALLEY, THE SOUTHEAST, AND THE SOUTHERN APPALACHIANS, SAT-TUE, DEC 23-DEC 26.

MODERATE RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHEAST, THE LOWER
MISSISSIPPI VALLEY, AND THE TENNESSEE VALLEY, SUN-MON, DEC 24-DEC 25.

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

DETAILED SUMMARY

FOR SUNDAY DECEMBER 17 - THURSDAY DECEMBER 21: FRONTAL ACTIVITY AND A FEW
SHORTWAVE TROUGHS ARE EXPECTED TO AFFECT THE DAY 3-7 PERIOD FOR THE CONUS. FIRE
WEATHER AND HIGH WIND HAZARDS ARE POSTED FOR SOUTHWEST CALIFORNIA FOR DEC 17,
AS THE SURFACE PRESSURE GRADIENT IN THE REGION INCREASES AND AN UPPER-LEVEL
SHORTWAVE TROUGH MOVES THROUGH THE AREA.



A SERIES OF FRONTS MOVING OVER THE SOUTHEAST AND THE CENTRAL GULF COAST IS
LIKELY TO BRING HEAVY RAIN (> 1.5 INCHES IN 24 HOURS) TO THE AREA OVER DEC
17-19. THE FIRST FRONT IS EXPECTED THROUGH ON DEC 17, WHICH IS MORE LIKELY TO
AFFECT THE LOWER MISSISSIPPI RIVER VALLEY. THE SECOND FRONT IS FORECAST MOVE
THROUGH ON DEC 19 AND IS MORE LIKELY TO AFFECT PARTS OF THE SOUTHEAST AND GULF
COAST.



AS AN UPPER-LEVEL TROUGH MOVES OVER THE PACIFIC NORTHWEST AND A SURFACE COLD
FRONT ADVANCES, HEAVY PRECIPITATION (>1.5 INCHES LIQUID EQUIVALENT IN 24 HOURS)
IS EXPECTED ALONG THE NORTHWESTERN COAST FOR DEC 19, WITH AREAS OF HEAVY SNOW
(> 6 INCHES IN 24 HOURS) THE NORTHERN CASCADES. PARTS OF THE NORTHERN ROCKIES
IN NORTHERN IDAHO AND NORTHWESTERN MONTANA ARE FORECAST TO RECEIVE HEAVY SNOW
(> 8 INCHES IN 24 HOURS) FOR DEC 19.



A SURFACE LOW IS FORECAST TO MOVE ALONG THE ALASKAN SOUTH COAST AT THE
BEGINNING OF THE PERIOD. THIS IS LIKELY TO BRING HIGH WINDS (GUSTS GREATER THAN
50 KNOTS) TO THE ALASKAN PENINSULA FOR DEC 17. HEAVY PRECIPITATION RESULTING
FROM THE SYSTEM IS EXPECTED TO END BEFORE THE START OF THIS FORECAST PERIOD.
ANOTHER STRONG LOW PRESSURE SYSTEM IS LIKELY TO MOVE EAST FROM THE BERING SEA
BRINGS A SECOND THREAT OF HIGH WINDS (GUSTS GREATER THAN 50 KNOTS) AND A THREAT
FOR SIGNIFICANT WAVES FOR TO ALEUTIAN ISLANDS FOR DEC 18-19. A STRONG SURFACE
GRADIENT IS EXPECTED AS HIGH PRESSURE MOVES INTO THE GULF OF ALASKA.



THE UPPER-LEVEL RIDGE THAT HAS BEEN PRESENT OVER CENTRAL ALASKA IS LIKELY TO BE
INTERRUPTED BY AN UPPER-LEVEL TROUGH IN THE BEGINNING OF WEEK-1, BRINGING SOME
MODERATION IN TEMPERATURES. RIDGING IS LIKELY TO RETURN TOWARD THE END OF
WEEK-1 AND A MUCH ABOVE NORMAL TEMPERATURES HAZARD IS POSTED FOR DEC 21.

FOR FRIDAY DECEMBER 22 - THURSDAY DECEMBER 28: 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 AND A POSITIVELY TILTED TROUGH OVER THE
CENTRAL U.S. 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. MODEL GUIDANCE HAS BEEN MORE CONFIDENT IN THE POSITIVELY
TILTED UPPER-LEVEL TROUGH DEVELOPING OVER THE CENTRAL U.S. AND DEEPENING DOWN
TOWARD THE SOUTHWEST. THE SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES POSTED
ACROSS NORTHERN AND CENTRAL PARTS OF THE CONUS FOR MUCH OF WEEK-2 HAS BEEN
EXPANDED TO INCLUDE PARTS OF THE SOUTHWEST. THE MODERATE RISK OF MUCH BELOW
NORMAL TEMPERATURES IS FORECAST FROM THE NORTHERN PLAINS TO THE WESTERN GREAT
LAKES AND SOUTHWARD THROUGH THE PLAINS FOR DEC 24-28, 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
CROPS 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 FROZEN
PRECIPITATION HAZARD IS POSTED FOR DEC 22-24 TO HIGHLIGHT THIS POTENTIAL HAZARD.

A SLIGHT RISK OF HEAVY PRECIPITATION IS FORECAST OVER THE LOWER MISSISSIPPI
RIVER VALLEY, THE SOUTHEAST, AND THE SOUTHERN APPALACHIANS FOR DEC 23-26 DUE TO
FRONTAL ACTIVITY. THIS AREA IS LIKELY TO BE A TRANSITION ZONE BETWEEN THE
UPPER-LEVEL TROUGH OVER THE CENTRAL U.S. AND THE RIDGING IN THE SOUTHEAST.



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 22-28, WITH A MODERATE RISK
OF MUCH ABOVE NORMAL TEMPERATURES FORECAST FOR MUCH OF THE INTERIOR OF ALASKA
OVER DEC 23-26.



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: CHRISTINA FINAN

$$



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