Prognostic Meteorological Discussion
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FXUS21 KWNC 171839
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 17 2017

SYNOPSIS: A SURFACE LOW IS EXPECTED TO TRACK FROM WESTERN TO NORTHEASTERN
CANADA OVER WEEK-1. ITS TRAILING COLD FRONT IS ANTICIPATED TO SWEEP ACROSS THE
LOWER 48 STATES THROUGHOUT THE DAY 3 TO 7 PERIOD. A SECONDARY COLD FRONT IS
ANTICIPATED TO ENTER THE PACIFIC NORTHWEST BY OCT 22 CAND MOVE ACROSS PARTS OF
THE U.S. BEHIND THE INITIAL COLD FRONT. SURFACE HIGH PRESSURE IS FORECAST TO
BUILD ACROSS MUCH OF THE CONTIGUOUS U.S. AHEAD OF, AND IN THE WAKE OF, THE COLD
FRONTS. A SERIES OF SURFACE LOW PRESSURE SYSTEMS ARE PREDICTED TO DEVELOP OVER
COASTAL PARTS OF MAINLAND ALASKA AS WELL AS THE GULF OF ALASKA DURING THE 3 TO
7 DAY PERIOD. SOME MODELS FAVOR AMPLIFIED MID-LEVEL TROUGHING THROUGHOUT WEEK-2
ACROSS THE EASTERN HALF OF THE U.S. WITH THE POTENTIAL FOR A SIGNIFICANT STORM
APPROACHING THE ALEUTIANS, SOUTH COAST OF ALASKA, AND THE ALASKA PANHANDLE BY
THE BEGINNING OF WEEK-2.

HAZARDS

HEAVY PRECIPITATION ACROSS PORTIONS OF CASCADES, SAT, OCT 21.

HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST, SAT, OCT 21.

HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT
BASIN, FRI-SAT, OCT 20-OCT 21.

HIGH WINDS ACROSS COASTAL PORTIONS OF NORTHERN CALIFORNIA AND THE PACIFIC
NORTHWEST, SAT, OCT 21.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE PACIFIC NORTHWEST,
FRI, OCT 20.

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHERN PLAINS, SAT, OCT 21.

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST, THE MIDDLE MISSISSIPPI VALLEY, THE
LOWER MISSISSIPPI VALLEY, THE OHIO VALLEY, AND THE TENNESSEE VALLEY, SUN, OCT
22.

HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, TUE, OCT 24.

HIGH WINDS FOR PARTS OF SOUTHWESTERN CALIFORNIA, SAT, OCT 21.

HIGH WINDS ACROSS PORTIONS OF THE CENTRAL ROCKIES, THE CENTRAL PLAINS, THE
NORTHERN PLAINS, AND THE NORTHERN ROCKIES, SUN-MON, OCT 22-OCT 23.

HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE, KENAI PENINSULA,
AND KODIAK ISLAND, WED-FRI, OCT 25-OCT 27.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PENINSULA AND THE ALEUTIANS, WED-THU,
OCT 25-OCT 26.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE ALASKA PENINSULA AND
THE ALEUTIANS, WED-THU, OCT 25-OCT 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE EASTERN HALF
OF THE CONUS, WED-THU, OCT 25-OCT 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF NORTHERN MAINLAND
ALASKA, WED-THU, OCT 25-OCT 26.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF CALIFORNIA AND
THE SOUTHWEST, WED-THU, OCT 25-OCT 26.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST, THE GREAT
LAKES, AND THE SOUTHERN PLAINS.

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

DETAILED SUMMARY

FOR FRIDAY OCTOBER 20 - TUESDAY OCTOBER 24: A SURFACE LOW IS PREDICTED TO FORM
OVER ALBERTA AT THE START OF THE PERIOD AND TRACK EASTWARD ACROSS CANADA
THROUGHOUT THE 3 TO 7 DAY PERIOD. ITS TRAILING COLD FRONT OVER THE CONUS IS
EXPECTED TO SWEEP ACROSS THE LOWER 48 STATES THROUGHOUT WEEK-1, BRINGING HEAVY
PRECIPITATION TO PARTS OF THE COUNTRY. AT THE BEGINNING OF THE PERIOD, AS THE
COLD FRONT STRETCHES FROM THE NORTHERN PLAINS SOUTHWESTWARD TO CALIFORNIA,
PARTS OF THE PACIFIC NORTHWEST MAY RECEIVE HEAVY RAIN ON OCT 21 AND HEAVY RAIN
AND HIGH ELEVATION SNOW ACROSS PARTS OF THE CASCADES OCT 21. SOME MODELS
INDICATE THE POSSIBILITY OF LOCALIZED AREAS RECEIVING 2 TO 3 INCHES OR GREATER
OF LIQUID EQUIVALENT IN A 24-HOUR PERIOD. ADDITIONALLY, PARTS OF THE NORTHERN
ROCKIES AND NORTHERN GREAT BASIN MAY RECEIVE HEAVY SNOW OCT 20 TO 21. SEVERAL
INCHES OF SNOW IS POSSIBLE ACROSS HIGH ELEVATION AREAS.



ANOTHER SURFACE LOW IS ANTICIPATED TO FORM IN THE EASTERN PACIFIC AND APPROACH
THE PACIFIC NORTHWEST COAST. THIS SURFACE LOW AS WELL AS ANOTHER LOW IN THE
GULF OF ALASKA, SUPPORTS STRONG WESTERLY ONSHORE FLOW TO THE PACIFIC NORTHWEST
COAST, BRINGING INCREASED CHANCES FOR HIGH SIGNIFICANT WAVE HEIGHTS (21 FEET OR
GREATER) TO COASTAL PARTS OF THE PACIFIC NORTHWEST. ON OCT 21, COASTAL PORTIONS
OF THE PACIFIC NORTHWEST AND NORTHWESTERN CALIFORNIA MAY EXPERIENCE HIGH WINDS
(40 KNOTS OR GREATER).



AS THE AFOREMENTIONED COLD FRONT TRAVERSES THE CONUS, PARTS OF THE SOUTHERN
PLAINS MAY RECEIVE HEAVY RAINFALL ON OCT 21, FOLLOWED BY THE SOUTHEAST, THE
MIDDLE MISSISSIPPI VALLEY, THE LOWER MISSISSIPPI VALLEY, THE OHIO VALLEY, AND
THE TENNESSEE VALLEY ON OCT 22. SOME MODELS INDICATE THE POTENTIAL FOR
ATMOSPHERIC CONDITIONS TO BE CONDUCIVE FOR SEVERE THUNDERSTORMS ALONG THE COLD
FRONT, MOST LIKELY ON OCT 21 ACROSS PARTS OF KANSAS, OKLAHOMA, AND NORTH TEXAS.
PARTS OF THE NORTHEAST MAY ALSO RECEIVE HEAVY RAIN OCT 24 AS THE COLD FRONT
APPROACHES THE EAST COAST. IT SHOULD BE NOTED THAT THERE ARE SIGNIFICANT
DIFFERENCES AMONGST MODELS REGARDING THE SPATIAL COVERAGE AND RAINFALL TOTALS.
THE 6Z DETERMINISTIC GFS INDICATES A SMALLER REGION, LIMITED TO MAINLY THE
NORTHEAST AS INDICATED, WHEREAS THE 0Z DETERMINISTIC ECMWF SHOWS A LARGER
REGION INCLUDING EASTERN NEW YORK, PENNSYLVANIA, AND MARYLAND. THE ENSEMBLE
MEANS OF EACH OF THESE MODELS FAVOR LOWER RAINFALL ACCUMULATIONS AND SMALLER
COVERAGE SIMILAR TO THE 6Z DETERMINISTIC GFS. LOCALIZED AREAS IN THE REGIONS
MENTIONED MAY RECEIVE UP TO 2 INCHES OR GREATER OF RAINFALL IN A 24-HOUR
PERIOD.



STRONG SURFACE HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE WESTERN CONUS
BEHIND THE COLD FRONT BY OCT 21. THIS PATTERN MAY SUPPORT MODERATE TO STRONG
OFFSHORE WINDS ACROSS PARTS OF SOUTHERN CALIFORNIA. THESE ELEVATED WIND SPEEDS
MAY LEAD TO INCREASED CHANCES FOR WILDFIRE RISK THROUGHOUT THE WEEKEND.
SUSTAINED WIND SPEEDS MAY REACH 30 KNOTS OR GREATER.



A SURFACE TROUGH IS EXPECTED TO FORM ACROSS THE NORTHERN PLAINS BY OCT 22. A
TIGHT PRESSURE GRADIENT ASSOCIATED WITH THIS TROUGH MAY BRING HIGH WINDS TO
PARTS OF THE CENTRAL ROCKIES, THE CENTRAL PLAINS, THE NORTHERN PLAINS, AND THE
NORTHERN ROCKIES OCT 22 TO 23. SOME AREAS MAY EXPERIENCE SUSTAINED WIND SPEEDS
REACHING 40 KNOTS OR GREATER.



A SURFACE LOW IS PREDICTED TO FORM OVER THE SOUTH COAST OF ALASKA BY OCT 21
SUPPORTING STRONG ONSHORE FLOW FROM THE GULF OF ALASKA TO PARTS OF THE SOUTH
COAST OF ALASKA AND THE ALASKA PANHANDLE. THIS STORM MAY BRING HEAVY SNOW TO
PARTS OF THE KENAI PENINSULA ON OCT 21. HOWEVER, DUE TO SIGNIFICANT MODEL
DIFFERENCES REGARDING ANTICIPATED PRECIPITATION TOTALS, A SPECIFIC RELATED
HAZARDS AREA IS NOT INDICATED AT THIS TIME.

FOR WEDNESDAY OCTOBER 25 - TUESDAY OCTOBER 31: TODAY THE 6Z GEFS AND 0Z ECMWF
ENSEMBLE HAVE COME INTO BETTER AGREEMENT REGARDING THE DEVELOPMENT OF A
MID-LEVEL TROUGH ACROSS THE EASTERN HALF OF THE CONUS THROUGHOUT WEEK-2. THIS
PATTERN MAY TRANSLATE TO MUCH BELOW NORMAL TEMPERATURES ACROSS THE EASTERN HALF
OF THE CONUS OCT 25 TO 26. THE 6Z GEFS SHOWS THIS TROUGH REMAINING IN PLACE
UNTIL OCT 28, WHEREAS THE 0Z ECMWF ENSEMBLE SHOWS A BRIEF RETURN TO MORE ZONAL
FLOW ON OCT 27 BEFORE REBUILDING A TROUGH THE FOLLOWING DAY. THERE IS A
POSSIBILITY OF MORE DAYS IN THE WEEK-2 PERIOD BEING INCLUDED FOR A MUCH BELOW
NORMAL TEMPERATURE HAZARD DEPENDING ON HOW THE MODELS EVOLVE IN THE NEXT FEW
DAYS. THE GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OF TEMPERATURES
FOR HIGHLIGHTED AREAS REACHING THE LOWEST 15TH PERCENTILE, WITH THE MINIMUM
TEMPERATURES OF SOME AREAS REACHING SUB-FREEZING TEMPERATURES. THIS COLD EVENT
COULD BE THE FIRST FREEZE FOR THE NORTHERN PART OF THE CORN BELT.



AN AMPLIFIED MID-LEVEL RIDGE IS PREDICTED TO BUILD ACROSS THE WESTERN HALF OF
THE CONUS AT THE BEGINNING OF WEEK-2. THIS PATTERN FAVORS A SLIGHT CHANCE OF
MUCH ABOVE NORMAL TEMPERATURES ACROSS PARTS OF CALIFORNIA AND THE SOUTHWEST OCT
25 TO 26. THE GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OR GREATER OF
MAXIMUM TEMPERATURES IN THE HIGHLIGHTED AREAS REACHING THE 85TH PERCENTILE AND
LOCALIZED AREAS REACHING UP TO 90 DEG F.



A STRONG SURFACE LOW PRESSURE SYSTEM IS FORECAST TO DEVELOP OVER THE NORTHERN
PACIFIC AND REACH THE WESTERN ALEUTIANS BY THE END OF WEEK-1. AS THIS STORM
TRACKS EASTWARD ALONG THE ALEUTIANS TO THE GULF OF ALASKA BY THE MIDDLE OF
WEEK-2, THERE IS AN INCREASED CHANCE OF HIGH WINDS (40 KNOTS OR GREATER) AND
SIGNIFICANT WAVE HEIGHTS (24 FEET OR GREATER), INITIALLY ACROSS THE WESTERN AND
CENTRAL ALEUTIANS OCT 25, SHIFTING EASTWARD TO THE ALASKA PENINSULA OCT 26.
STRONG ONSHORE FLOW AHEAD OF THIS STORM MAY BRING HEAVY RAIN AND SNOW TO PARTS
OF THE ALASKA PANHANDLE, KENAI PENINSULA, AND KODIAK ISLAND OCT 25 TO 27. THE
GEFS REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OR GREATER OF THESE REGIONS
RECEIVING 3-DAY PRECIPITATION ACCUMULATIONS REACHING OR EXCEEDING THE 85TH
PERCENTILE.



SURFACE HIGH PRESSURE IS ANTICIPATED TO DEVELOP OVER THE BEAUFORT SEA AT THE
BEGINNING OF WEEK-2. THIS PATTERN MAY SUPPORT A SLIGHT CHANCE OF MUCH BELOW
NORMAL TEMPERATURES ACROSS NORTHERN PARTS OF MAINLAND ALASKA OCT 25 TO 26. THE
GEFS REFORECAST TOOL INDICATES A SLIGHT CHANCE OF MINIMUM TEMPERATURES IN THIS
REGION REACHING THE LOWEST 15TH PERCENTILE, WITH TEMPERATURES REACHING 0 DEG F.

FORECASTER: MELISSA OU

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