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Valid Thursday October 19, 2017 to Monday October 30, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT October 16 2017Synopsis
: A surface low is expected to track
from western to eastern Canada over week-1. Its trailing cold front is
anticipated to sweep across the lower 48 states throughout the day 3 to 7
period into the beginning of week-2. Surface high pressure is forecast to build
across much of the Contiguous U.S. ahead of and in the wake of the cold front.
A series of surface low pressure systems are predicted to develop over 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 storm approaching the
Aleutians, South Coast of Alaska, and the Alaska Panhandle.
Detailed Summary For Thursday October 19 - Monday October 23:
- Heavy rain across portions of Northern
California and the Pacific Northwest, Thu, Oct 19.
- Heavy rain across portions of the Pacific Northwest, Sat-Sun, Oct 21-Oct 22.
- Episodes of heavy snow across high elevation areas of the Cascades,
Thu-Sun, Oct 19-Oct 22.
- Heavy snow across portions of the Northern Rockies, Fri-Sat, Oct 20-Oct 21.
- Heavy rain across portions of the Central Plains, the Southern Plains, and
the Mississippi Valley, Sat, Oct 21.
- Heavy rain across portions of the Southeast, the Lower Mississippi Valley,
the Southern Plains, and the Tennessee Valley, Sun, Oct 22.
- Flooding occurring or imminent across portions of the Southeast, the Great
Lakes, the Middle Mississippi Valley, and the Southern Plains.
- High winds across coastal portions of the Pacific Northwest, Thu, Oct 19.
- Heavy snow across portions of the Kenai Peninsula, Sun, Oct 22.
- High winds across parts of southern California, Sat-Sun, Oct 21-Oct 22.
- Severe Drought across the Middle Mississippi Valley, the Northern Plains,
the Northern Rockies, and Hawaii.
A surface low
is predicted to form over Alberta, Canada 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 across the Pacific Northwest, parts
of Northern California and the Pacific Northwest may receive heavy rain on Oct
19 and the Pacific Northwest Oct 21 to 22. Higher elevation areas in the
Cascades may receive episodes of heavy snow Oct 19 to 22. Some models indicate
the possibility of localized areas receiving 3 to 4 inches or greater of liquid
equivalent in a 24-hour period. Additionally, parts of the Northern Rockies in
Idaho may receive heavy snow Oct 20 to 21. Several inches of snow is possible
across high elevation areas. Another surface low pressure is forecast to
develop off the West coast of Canada. This feature supports high winds (40
knots or greater) across coastal parts of the Pacific Northwest Oct 19.
Strong surface high pressure is forecast to build across the western CONUS
behind the cold front by Oct 21. This pattern may support a prolonged period of
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.
As the aforementioned cold front traverses the CONUS, parts of the Central
Plains, the Southern Plains, and the Mississippi Valley may receive heavy
rainfall on Oct 21, followed by the Southeast, the Lower Mississippi Valley,
the Southern Plains, and the Tennessee Valley on Oct 22. Localized areas may
receive up to 2 inches or greater of rainfall in a 24-hour period.
A surface low is predicted to form over the South Coast of Alaska by Oct
22. This storm may bring heavy snow to parts of the Kenai Peninsula (liquid
equivalent totals of 2 inches or greater in a 24-hour period). For Tuesday
October 24 - Monday October 30:
In general, some models are indicating 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 southeastern quarter of the CONUS Oct 27 to 29. The 6Z GEFS shows a
more amplified trough than the 0Z ECMWF Ensemble, leading to significant
forecast uncertainty. The GEFS Reforecast Tool indicates a 20 percent chance of
temperatures in the southeastern quadrant of the CONUS being in the lowest 15th
percentile, with the minimum temperatures of some areas dipping into the 40s
(Deg F). The GEFS Reforecast Tool also indicates increased chances for heavy
rain across parts of Texas, Louisiana, the Lower Mississippi and Tennessee
Valley Oct 24 to 26. This is attributed to the placement of the subtropical
high pressure favoring strong southerly flow from the Gulf of Mexico into the
CONUS, in addition to surface low pressure north of the Yucatan Peninsula.
However, due to the 0Z ECMWF Ensemble having a less amplified trough, it does
not indicate precipitation amounts as high as the GEFS. Due to these
significant model differences, specific hazards areas are not highlighted at
this time for week-2.
A strong storm system may approach the Aleutians at the beginning of
week-2, tracking along the Gulf of Alaska throughout week-2. This storm may
bring heavy precipitation to southwestern mainland Alaska, the South Coast of
Alaska, and the Alaska Panhandle throughout week-2. The GEFS Reforecast Tool
shows broad areas of 20 percent, localized areas of 40 percent of heavy
precipitation. The 0Z ECMWF Ensemble favors a less intense storm, leading to
lower expected precipitation amounts. This significant model difference
precludes a related hazards area from being determined at this time but will be
monitored in the upcoming days.
Forecaster: Melissa Ou
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.