Prognostic Meteorological Discussion
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FXUS21 KWNC 201958
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 20 2017

SYNOPSIS: A COLD FRONT IS ANTICIPATED TO SWEEP ACROSS THE EASTERN HALF OF THE
CONTIGUOUS U.S. EARLY IN THE DAY 3 TO 7 PERIOD, WITH A REINFORCING SHOT OF
COLDER AIR LATER NEXT WEEK. HIGH PRESSURE IS FORECAST TO BUILD IN ACROSS THE
WEST EARLY NEXT WEEK, ONLY TO BE DISLODGED BY THE SECOND COLD FRONT, WITH HIGH
PRESSURE BEHIND THAT FRONT BUILDING SOUTHWARD FROM CANADA. A STRONG
LOW-PRESSURE SYSTEM IS FORECAST TO IMPACT WESTERN AND SOUTHERN ALASKA DURING
NEXT WEEK, WITH MORE EXTENDED IMPACTS TO WESTERN ALASKA. SOME MODELS FAVOR
LOW-PRESSURE ACROSS THE EASTERN CONTIGUOUS U.S. DURING WEEK-2, BUT MODEL
SOLUTIONS EXHIBIT INCREASED UNCERTAINTY.

HAZARDS

HIGH WINDS ACROSS SOUTHERN CALIFORNIA, MON-TUE, OCT 23-OCT 24.

MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF CALIFORNIA, MON-TUE, OCT
23-OCT 24.

ENHANCED WILDFIRE RISK ACROSS PORTIONS OF THE CALIFORNIA, MON-TUE, OCT 23-OCT
24.

HEAVY RAIN ACROSS FROM THE SOUTHEAST TO THE GREAT LAKES AND NORTHEAST, MON-TUE,
OCT 23-OCT 24.

HEAVY RAIN ACROSS PORTIONS OF THE NORTHEAST, WED, OCT 25.

HIGH WINDS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL PLAINS, THE NORTHERN AND
CENTRAL ROCKIES, THE MIDDLE MISSISSIPPI VALLEY, CALIFORNIA, AND THE UPPER
MISSISSIPPI VALLEY, MON-TUE, OCT 23-OCT 24.

HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE AND MAINLAND
ALASKA, MON-TUE, OCT 23-OCT 24.

HEAVY PRECIPITATION ACROSS PORTIONS OF THE ALASKA PANHANDLE AND MAINLAND
ALASKA, WED-FRI, OCT 25-OCT 27.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE AND SOUTHERN MAINLAND
ALASKA, MON-TUE, OCT 23-OCT 24.

HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE, SOUTHERN MAINLAND ALASKA,
AND THE ALEUTIANS, TUE-WED, OCT 24-OCT 25.

HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF SOUTHERN MAINLAND ALASKA
AND THE ALEUTIANS, TUE-THU, OCT 24-OCT 26.

FLOODING POSSIBLE ACROSS PORTIONS OF THE PACIFIC NORTHWEST.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE SOUTHEAST.

FLOODING LIKELY ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE NORTHERN GREAT
BASIN.

SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL
PLAINS, THE LOWER MISSISSIPPI VALLEY, THE CENTRAL APPALACHIANS, THE TENNESSEE
VALLEY, THE GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE
SOUTHERN APPALACHIANS, THE UPPER MISSISSIPPI VALLEY, THE SOUTHEAST, THE
SOUTHERN PLAINS, AND THE OHIO VALLEY, SAT-SUN, OCT 28-OCT 29.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL GREAT
BASIN, CALIFORNIA, AND THE SOUTHWEST, SAT, OCT 28.

MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE MIDDLE
MISSISSIPPI VALLEY, THE GREAT LAKES, AND THE OHIO VALLEY, SAT, OCT 28.

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

DETAILED SUMMARY

FOR MONDAY OCTOBER 23 - FRIDAY OCTOBER 27: A COLD FRONT IS FORECAST TO PUSH
ACROSS THE EASTERN CONUS DURING NEXT WEEK, WITH A WAVE OF LOW-PRESSURE MOVING
FROM SOUTH TO NORTH ALONG THE FRONT.  THIS WAVE OF LOW-PRESSURE IS EXPECTED TO
SUPPORT HEAVY RAINS (UP TO 4.0 INCHES IN 48 HOURS) ACROSS PORTIONS OF THE
TENNESSEE VALLEY. SOME ISOLATED SEVERE WEATHER IS POSSIBLE FROM THE SOUTHERN
GREAT PLAINS TO THE LOWER MISSISSIPPI VALLEY, LIKELY IN THE FORM OF A SQUALL
LINE, ON SUNDAY.  AS THE LOW-PRESSURE SYSTEM MOVES NORTHWARD, HEAVY RAINS ARE
FORECAST TO SPREAD TO THE MID-ATLANTIC AND NORTHEAST. SOME MODELS INDICATE WIND
SPEEDS APPROACHING 30 MPH NEAR THE NORTHEAST COAST, LATER ON TUESDAY AND INTO
WEDNESDAY, OCT 25, THOUGH MODEL UNCERTAINTY IN THE WIND SPEEDS IS TOO HIGH TO
DEPICT A HAZARD.



HIGH PRESSURE IS LIKELY TO BUILD OVER THE ROCKIES, IN A POSITION WHERE A HIGH
PRESSURE SYSTEM CAN SUPPORT HIGH WINDS OVER CALIFORNIA. THE STRONG WINDS ARE
LIKELY TO CONTRIBUTE TO CRITICAL FIRE WEATHER CONDITIONS OVER SOUTHERN
CALIFORNIA ON OCT 23-24, WITH MARGINAL FIRE WEATHER CONDITIONS EXTENDING TO THE
24TH. DURING MON-TUE, OCT 23-24, STRONG WINDS (GREATER THAN 35 MPH) ARE ALSO
FORECAST OVER THE NORTHERN AND CENTRAL PLAINS ON THE NORTHERN PERIPHERY OF THE
HIGH PRESSURE SYSTEM.



A SECOND COLD FRONT IS FORECAST TO MOVE SOUTHWARD OVER THE GREAT PLAINS LATER
NEXT WEEK. SOME UPSLOPE FLOW IS POSSIBLE IN MONTANA AND WYOMING, LEADING TO
LOCALLY MODERATE SNOWS, THOUGH THE MOISTURE SUPPLY WILL BE LIMITED, RESTRICTING
POTENTIAL SNOWFALL TOTALS. WPC AND YOUR LOCAL WFO WILL HAVE THE LATEST
INFORMATION ON THAT SCENARIO.



MULTIPLE LOW-PRESSURE SYSTEMS ARE FORECAST TO IMPACT ALASKA.  HIGH WINDS
(EXCEEDING 40 MPH), HIGH SIGNIFICANT WAVE HEIGHTS, AND HEAVY PRECIPITATION ARE
LIKELY FOR SOUTHEAST ALASKA AND THE ALASKA PANHANDLE EARLY NEXT WEEK, MON-TUE,
WITH THE FIRST SYSTEM. A SUBSEQUENT LOW-PRESSURE SYSTEM IS FORECAST TO DEVELOP
NEAR THE ALEUTIANS, AND TRACK EASTWARD TOWARD SOUTHERN COASTAL ALASKA,
SPREADING HEAVY PRECIPITATION (EXCEEDING 3 INCHES IN 24 HOURS), HIGH WINDS
(LIKELY IN EXCESS OF 50 MPH IN SOME LOCATIONS), AND HIGH SIGNIFICANT WAVE
HEIGHTS. THE SECOND ROUND OF IMPACTS ARE LIKELY TO BEGIN ON OCT 24, AND SPREAD
EASTWARD, WHILE LASTING THROUGH OCT 27.

FOR SATURDAY OCTOBER 28 - FRIDAY NOVEMBER 03: 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 FOR THE EARLY PORTION OF WEEK-2,
THEN DISAGREEMENT AND UNCERTAINTY INCREASE QUICKLY. THIS PATTERN MAY TRANSLATE
TO MUCH BELOW NORMAL TEMPERATURES ACROSS THE EASTERN HALF OF THE CONUS OCT 28
TO 29. MODELS INDICATE A TRANSITION TO A MORE ZONAL FLOW FOR THE REST OF
WEEK-2. THE GEFS REFORECAST TOOL INDICATES A 40 PERCENT CHANCE OF TEMPERATURES
REACHING THE LOWEST 15TH PERCENTILE IN THE AREA HIGHLIGHTED AS A MODERATE RISK,
WITH THE MINIMUM TEMPERATURES OF SOME AREAS REACHING SUB-FREEZING TEMPERATURES.
THIS COLD EVENT COULD BE THE FIRST FREEZE FOR PORTIONS OF THE OHIO VALLEY AND
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 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.



THE U.S. DROUGHT MONITOR, VALID ON OCTOBER 10, INDICATES THAT SEVERE TO
EXCEPTIONAL (D2-D4) DROUGHT COVERS 2.98 PERCENT OF THE CONTINENTAL U.S., DOWN
FROM 4.03 PERCENT OF THE CONUS.  THE LARGEST CHANGES WERE OVER THE NORTHERN
PLAINS AND MIDWEST.

FORECASTER: MATTHEW ROSENCRANS

$$



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