Prognostic Meteorological Discussion
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FXUS21 KWNC 181945
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 18 2017

SYNOPSIS: A COLD FRONT IS ANTICIPATED TO SWEEP ACROSS THE EASTERN HALF OF THE
CONTIGUOUS U.S. THROUGHOUT THE DAY 3 TO 7 PERIOD. SURFACE HIGH PRESSURE IS
FORECAST TO BUILD ACROSS MUCH OF THE CONTIGUOUS U.S. AHEAD OF, AND IN THE WAKE
THE COLD FRONT. MEANWHILE, A SURFACE LOW PRESSURE SYSTEM IS PREDICTED TO
DEVELOP OVER THE GULF OF ALASKA, BRINGING STRONG ONSHORE FLOW TO THE PACIFIC
NORTHWEST AT THE BEGINNING OF THE PERIOD. A SERIES OF STORMS ARE EXPECTED TO
FORM OVER THE ALEUTIANS AND THE GULF OF ALASKA THROUGHOUT WEEK-1 AND WEEK-2.
SOME MODELS FAVOR AMPLIFIED MID-LEVEL TROUGHING THROUGHOUT WEEK-2 ACROSS THE
EASTERN HALF OF THE U.S. FROM THE BEGINNING TO MIDDLE OF WEEK-2, TRANSITIONING
TO MORE ZONAL FLOW THROUGHOUT THE CONUS.

HAZARDS

HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST, SAT-SUN, OCT 21-OCT 22.

HEAVY SNOW ACROSS PORTIONS OF THE CASCADES, SAT-SUN, OCT 21-OCT 22.

HIGH WINDS ACROSS COASTAL PORTIONS OF NORTHERN CALIFORNIA AND THE PACIFIC
NORTHWEST, SAT-SUN, OCT 21-OCT 22.

HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT
BASIN, 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 RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE MIDDLE MISSISSIPPI
VALLEY, THE LOWER MISSISSIPPI VALLEY, AND THE SOUTHERN PLAINS, SAT, OCT 21.

SEVERE WEATHER ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE SOUTHERN PLAINS,
SAT, OCT 21.

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

HEAVY RAIN ACROSS PORTIONS OF THE EASTERN FOURTH OF THE CONUS, MON-TUE, OCT
23-OCT 24.

HIGH WINDS ACROSS PORTIONS OF SOUTHERN CALIFORNIA, SAT-MON, OCT 21-OCT 23.

ENHANCED WILDFIRE RISK ACROSS PORTIONS OF SOUTHERN CALIFORNIA, SUN-MON, OCT
22-OCT 23.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE, SAT, OCT 21.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE AND SOUTH COAST OF ALASKA,
TUE, OCT 24.

HIGH WINDS ACROSS PORTIONS OF THE ALEUTIANS AND SOUTH COAST OF ALASKA, WED, OCT
25.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF THE ALEUTIANS AND SOUTH
COAST OF ALASKA, WED, OCT 25.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE EASTERN HALF
OF THE CONUS, THU-SAT, OCT 26-OCT 28.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF SOUTHERN
CALIFORNIA AND THE SOUTHWEST, THU-SAT, OCT 26-OCT 28.

HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE, THU-SAT, OCT
26-OCT 28.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE AND SOUTH COAST OF ALASKA,
THU-FRI, OCT 26-OCT 27.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PARTS OF THE SOUTH COAST OF ALASKA
AND KODIAK ISLAND, THU, OCT 26.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF NORTHERN MAINLAND
ALASKA, THU-FRI, OCT 26-OCT 27.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST.

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

DETAILED SUMMARY

FOR SATURDAY OCTOBER 21 - WEDNESDAY OCTOBER 25: SURFACE LOW PRESSURE IS
ANTICIPATED TO FORM IN THE GULF OF ALASKA AT THE BEGINNING OF THE PERIOD,
BRINGING STRONG ONSHORE FLOW TO THE PACIFIC NORTHWEST. THIS PATTERN FAVORS
INCREASED CHANCES FOR HEAVY RAIN ACROSS PARTS OF THE PACIFIC NORTHWEST, HIGH
ELEVATION SNOW ACROSS THE CASCADES, AND HIGH WINDS ACROSS COASTAL PORTIONS OF
NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST, OCT 21 TO 22. PARTS OF THE
PACIFIC NORTHWEST AND CASCADES MAY RECEIVE LIQUID EQUIVALENT TOTALS OF 3 INCHES
OR GREATER IN A 24-HOUR PERIOD. SUSTAINED WIND SPEEDS IN THE HIGHLIGHTED REGION
MAY REACH 30 KNOTS OR GREATER. THIS ONSHORE FLOW OF MOISTURE MAY ALSO BRING
HEAVY SNOW TO PARTS OF THE NORTHERN ROCKIES ON OCT 21. LOCALIZED AREAS MAY
RECEIVE SEVERAL INCHES OF SNOW.



A SURFACE LOW IS FORECAST TO FORM OVER ALBERTA BY OCT 22. ITS TRAILING COLD
FRONT IS ANTICIPATED TO BRING HIGH WINDS (40 KNOTS OR GREATER) TO PARTS OF THE
CENTRAL ROCKIES, THE CENTRAL PLAINS, THE NORTHERN PLAINS, AND THE NORTHERN
ROCKIES OCT 22 TO 23 AS IT MOVES EASTWARD ACROSS THE NORTHERN ROCKIES AND
NORTHERN PLAINS.



A COLD FRONT IS EXPECTED TO SWEEP ACROSS THE EASTERN HALF OF THE U.S.
THROUGHOUT WEEK-1, BRINGING HEAVY RAIN TO PARTS OF THE CENTRAL PLAINS, THE
MIDDLE MISSISSIPPI VALLEY, THE LOWER MISSISSIPPI VALLEY, AND THE SOUTHERN
PLAINS OCT 21, FOLLOWED BY THE SOUTHEAST AND THE LOWER MISSISSIPPI VALLEY OCT
22. LOCALIZED AREAS MAY RECEIVE UP TO 2 INCHES OR GREATER OF RAINFALL IN A
24-HOUR PERIOD. ATMOSPHERIC CONDITIONS MAY BE CONDUCIVE TO SEVERE THUNDERSTORMS
ACROSS PARTS OF THE CENTRAL PLAINS AND THE SOUTHERN PLAINS OCT 21. AS THE COLD
FRONT PROGRESSES EASTWARD, PARTS OF THE EASTERN FOURTH OF THE CONUS MAY RECEIVE
HEAVY RAIN OCT 23 TO 24. A BROAD AREA IS HIGHLIGHTED ACROSS BOTH DAYS FOR
REGIONS THAT MAY RECEIVE HEAVY RAINFALL. LOCALIZED AREAS IN THIS REGION MAY
RECEIVE UP TO 4 INCHES OR GREATER OF RAINFALL IN A 24-HOUR PERIOD. THERE IS A
POSSIBILITY FOR SOME COASTAL PARTS OF THE EAST COAST TO EXPERIENCE HIGH WINDS
OCT 25 AS THE COLD FRONT EXITS THE CONUS. HOWEVER, AN ADDITIONAL RELATED HIGH
WIND HAZARD CANNOT BE INCLUDED DUE TO SIGNIFICANT MODEL DIFFERENCES.



STRONG SURFACE HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE WESTERN CONUS
THROUGHOUT THE 3 TO 7 DAY PERIOD. THIS PATTERN MAY SUPPORT MODERATE TO STRONG
OFFSHORE WINDS ACROSS PARTS OF SOUTHERN CALIFORNIA OCT 21 TO 23. THESE ELEVATED
WIND SPEEDS MAY LEAD TO INCREASED CHANCES FOR WILDFIRE RISK OCT 22 TO 23.
SUSTAINED WIND SPEEDS MAY REACH 30 KNOTS OR GREATER.



A SERIES OF SURFACE LOWS ARE PREDICTED TO FORM OVER THE GULF OF ALASKA BY OCT
21 SUPPORTING HIGH WINDS ACROSS PARTS OF THE ALASKA PANHANDLE OCT 21 AND THE
ALASKA PANHANDLE AND SOUTH COAST OF ALASKA OCT 24.  A STRONG STORM SYSTEM IS
EXPECTED TO FORM SOUTH OF THE ALEUTIANS BY THE END OF WEEK-1, INCREASING THE
LIKELIHOOD FOR HIGH WINDS AND SIGNIFICANT WAVE HEIGHTS ACROSS COASTAL PORTIONS
OF THE ALEUTIANS AND SOUTH COAST OF ALASKA OCT 25. LOCALIZED AREAS MAY
EXPERIENCE SUSTAINED WIND SPEEDS OF 40 KNOTS OR GREATER AND SIGNIFICANT WAVE
HEIGHTS OF 24 FEET OR GREATER.

FOR THURSDAY OCTOBER 26 - WEDNESDAY NOVEMBER 01: THERE IS GOOD AGREEMENT
BETWEEN THE 6Z GEFS AND 0Z ECMWF ENSEMBLE REGARDING THE DEVELOPMENT OF A
MID-LEVEL TROUGH ACROSS THE EASTERN HALF OF THE CONUS THROUGHOUT THE BEGINNING
TO MIDDLE OF WEEK-2. THIS PATTERN MAY TRANSLATE TO MUCH BELOW NORMAL
TEMPERATURES ACROSS THE EASTERN HALF OF THE CONUS OCT 26 TO 28. MODELS INDICATE
A TRANSITION TO MORE ZONAL FLOW FOR THE REST OF WEEK-2. 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 SOUTHERN CALIFORNIA AND THE
SOUTHWEST, OCT 26 TO 28. 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 SOUTH OF THE
ALEUTIANS BY THE END OF WEEK-1. AS THIS STORM TRACKS EASTWARD ALONG THE
ALEUTIANS TO THE GULF OF ALASKA BY OCT 27, THERE IS AN INCREASED CHANCE OF HIGH
WINDS (40 KNOTS OR GREATER) OCT 26 TO 27, AND SIGNIFICANT WAVE HEIGHTS (21 FEET
OR GREATER) OCT 26. STRONG ONSHORE FLOW ASSOCIATED WITH THIS STORM MAY BRING
HEAVY RAIN AND SNOW TO PARTS OF THE ALASKA PANHANDLE OCT 26 TO 28. THE GEFS
REFORECAST TOOL INDICATES A 20 PERCENT CHANCE OR GREATER OF 3-DAY PRECIPITATION
AMOUNTS 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 26 TO 27. 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.



THE U.S. DROUGHT MONITOR, VALID ON OCTOBER 10, INDICATES THAT SEVERE TO
EXCEPTIONAL (D2-D4) DROUGHT COVERS 4.03 PERCENT OF THE CONTINENTAL U.S. WITH
THE MOST INTENSE DROUGHT CONDITIONS ACROSS NORTHEASTERN MONTANA.

FORECASTER: MELISSA OU

$$




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