Prognostic Meteorological Discussion
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FXUS21 KWNC 122024
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 12 2017

SYNOPSIS: THIS OUTLOOK PERIOD BEGINS WITH THE END OF THE AMPLIFIED UPPER-LEVEL
TROUGH OVER THE EASTERN U.S. THE WEEK-1 PERIOD HAZARDS ARE MOSTLY ATTRIBUTED TO
A SURFACE LOW-PRESSURE SYSTEM MOVING FROM THE PACIFIC NORTHWEST SOUTHEAST
ACROSS THE CONTINENTAL U.S. TO THE GULF COAST. AMPLIFICATION TO THE UPPER-LEVEL
PATTERN AND AN EMERGING UPPER-LEVEL TROUGH BUILDING INTO THE NORTHWEST AND
CENTRAL U.S. ARE LIKELY TO BE INFLUENTIAL FACTORS FOR WEEK-2. PERSISTENT
RIDGING IN ALASKA IS EXPECTED TO WEAKEN SLIGHTLY TOWARD THE END OF WEEK-1, ONLY
TO RE-INTENSIFY DURING WEEK-2.



HAZARDS

HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST,  SAT-SUN, DEC
16-DEC 17.

HEAVY RAIN ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND THE TENNESSEE
VALLEY, SUN-MON, DEC 17-DEC 18.

HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT
BASIN, SAT-SUN, DEC 16-DEC 17.

HIGH WINDS ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE SOUTHERN PLAINS,
FRI-SAT, DEC 15-DEC 16.

MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHEAST, FRI, DEC 15.

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

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

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

MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT LAKES
AND THE NORTHERN PLAINS, FRI-TUE, DEC 22-DEC 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
PLAINS, NORTHERN ROCKIES AND PACIFIC NORTHWEST, FRI-TUE, DEC 22-DEC 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHEAST,
WED-THU, DEC 20-DEC 21.

MODERATE RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND
ALASKA, FRI-TUE, DEC 22-DEC 26.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND ALASKA,
FRI-TUE, DEC 22-DEC 26.

SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY
AND THE TENNESSEE VALLEY, FRI-SUN, DEC 22-DEC 24.

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 FRIDAY DECEMBER 15 - TUESDAY DECEMBER 19: THE DAY 3-7 PERIOD IS LIKELY TO
BE A TRANSITION PERIOD IN THE UPPER-LEVEL PATTERN FOR MUCH OF THE CONTINENTAL
U.S. TO BEGIN THE PERIOD, MUCH BELOW NORMAL TEMPERATURES ARE LIKELY FOR DEC 15
IN THE NORTHEAST AS AN UPPER-LEVEL TROUGH MOVES OUT OF THE EASTERN U.S.
NORTHWARD TO EASTERN CANADA.



HIGH WINDS ARE LIKELY TO FORM OVER THE CENTRAL AND SOUTHERN PLAINS DEC 15 - 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. THERE IS A POSSIBILITY FOR A HEAVY RAIN EVENT ACROSS THE WESTERN AND
CENTRAL GULF COAST FOR DEC 15 WITH THE PROCESSION OF THE COLD FRONT, THOUGH
MODEL GUIDANCE SHOWS A LOT OF UNCERTAINTY.



HEAVY PRECIPITATION IS FORECAST IN THE PACIFIC NORTHWEST FOR DEC 16-17,
FOLLOWING A LOW-PRESSURE SYSTEM BRINGING A COLD FRONT THROUGH THE REGION. AMONG
THE MODEL GUIDANCE, THE STRENGTH OF THIS SYSTEM HAS WEAKENED COMPARED TO
PREVIOUS MODEL RUNS, SO THE THREAT FOR HEAVY PRECIPITATION (1 INCH IN 24 HOURS)
HAS BEEN TRIMMED TO AREAS OF HIGHER ELEVATION. THIS FRONTAL PASSAGE IS ALSO
LIKELY TO BRING HEAVY SNOW (EXCESS OF 6 INCHES) TO PARTS OF NORTHERN IDAHO AND
WESTERN MONTANA FOR DEC 16-17.



AS THE SURFACE LOW PROPAGATES EASTWARD, IT IS EXPECTED TO MOVE SOUTHEAST,
ACROSS THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST. ON DEC 17-18, HEAVY RAIN (>1
INCHES IN 24 HOURS) IS LIKELY ACROSS THE LOWER MISSISSIPPI VALLEY AND TENNESSEE
VALLEY, AS THE ASSOCIATED COLD FRONT MOVES THROUGH.



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 ALASKAN
PENINSULA FOR DEC 15-16. AS THE LOW-PRESSURE SYSTEM MOVES INTO GULF OF 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 15-16. 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.

FOR WEDNESDAY DECEMBER 20 - TUESDAY DECEMBER 26: WEEK-2 IS LIKELY TO FEATURE AN
AMPLIFICATION IN THE UPPER-LEVEL PATTERN. AN UPPER-LEVEL TROUGH IS EXPECTED TO
AMPLIFY OVER THE GREAT LAKES REGION AND THE NORTH-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.  A
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS POSTED ACROSS THE NORTHERN
TIER OF THE US TO THE PACIFIC NORTHWEST TOWARD THE MIDDLE AND END OF WEEK-2,
DEC 22-26. THE MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES IS FORECAST FOR
THE NORTHERN PLAINS AND WESTERN GREAT LAKES REGION WHERE THE GFS ENSEMBLE MEAN
INDICATES TEMPERATURES AVERAGING NEAR 10 DEGREES F BELOW NORMAL. THIS COLD
OUTBREAK MAY EXTEND THE MUCH BELOW NORMAL TEMPERATURES FURTHER SOUTH THROUGH
THE CENTRAL U.S. TOWARD THE END OF WEEK-2 AND BEYOND.



A SLIGHT RISK OF HEAVY PRECIPITATION IS FORECAST OVER THE LOWER MISSISSIPPI
RIVER VALLEY, RESULTING FROM A HIGH-PRESSURE SYSTEM OFF THE EAST COAST FOR DEC
22-24. SOUTHERLY RETURN FLOW IS LIKELY TO BRING ENHANCED MOISTURE INTO THE
REGION.



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 22-26, WITH A MODERATE RISK
OF MUCH ABOVE NORMAL TEMPERATURES FORECAST FOR MUCH OF THE INTERIOR OF ALASKA
OVER THE SAME TIME 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: CHRISTINA FINAN

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