Valid Sunday October 21, 2018 to Thursday November 01, 2018
US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM
EDT October 18 2018
Synopsis: A front is forecast to remain
stationary over the Gulf of Mexico during the 3-7 day period with surface low
pressure development along this front on Oct 23 or 24. Concurrently, another
cold front is expected to move across the central and eastern CONUS, while a
North Pacific storm system is predicted to impact parts of the South Coast of
Alaska and the northern Alaska Panhandle. Mid-level low pressure is anticipated
to dominate the eastern half of the CONUS, and southwestern Alaska during
Week-2, with mid-level high pressure elsewhere.
Heavy rain shifting east from the western Gulf Coast and lower
Mississippi Valley to the Southeast, Tue-Thu, Oct 23-25.
Much below normal temperatures across parts of the eastern U.S., Sun-Mon,
Heavy precipitation across portions of the Alaska Panhandle and mainland
Alaska, Sun, Oct 21.
High winds across portions of the Alaska Panhandle and mainland Alaska,
Sun, Oct 21.
High significant wave heights for coastal portions of mainland Alaska, Sun,
Flooding possible across portions of the Southern Plains.
Flooding occurring or imminent across portions of the Southern Plains, the
Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the
Southeast, the Great Lakes, and the Ohio Valley.
Severe drought for parts of the Northeast, the Great Plains, the western
U.S., and the Alaska Panhandle.
Moderate risk of heavy precipitation for parts of the eastern U.S.,
Fri-Sun, Oct 26-28.
Slight risk of heavy precipitation for parts of the eastern and
south-central CONUS, Fri-Sun, Oct 26-28.
Slight risk of much below normal temperatures for parts of the
south-central and eastern CONUS, Mon-Thu, Oct 29-Nov 1.
For Sunday October 21 - Thursday October
25: Several mid-tropospheric shortwave troughs are forecast to move
through a broader longwave trough located over the northeastern quarter of the
Lower 48 states. Colder air masses are also expected to move across this
portion of the country, with much below normal minimum temperatures (8 degrees
F or more below normal) depicted on the map on Oct 21-22. Although these
predicted temperature anomalies are not that anomalous, the greater concern is
for portions of this region to experience the end of their growing season with
a killing frost. Frost or a light freeze could extend as far south as the
southern Appalachians and adjacent Piedmont areas the Carolinas on Oct 22.
Across the Great Lakes region, these colder air masses result in an increased
risk of lake-effect snow bands. Most of the expected lake-effect snow will
likely be limited to localized parts of western New York by this time period.
This is still relatively early in the season for lake-effect snow, and amounts
are forecast to be relatively light, but could make for slippery driving
As a shortwave trough approaches from the west, surface low development is
forecast along a stationary front across the Gulf of Mexico. Heavy rainfall (1
to 3 inches) is expected to accompany this low pressure system when it tracks
east across the northern Gulf of Mexico from October 23 to 25. Although
forecast confidence remains high that this surface low cyclogenesis occurs,
there is model spread on its track and how far north the heavier rainfall
A North Pacific storm system is forecast to impact the South Coast of
Alaska and the Panhandle region during the 3-7 day period. Dynamical models
have had some difficulty in resolving whether this might be a single storm
system, or as was predicted yesterday, back-to-back systems. In either case, an
extended period of hazardous conditions appears likely for south-central and
southeastern Alaska, and the Panhandle region. Periods of heavy precipitation
(low elevation rain and mountain snow, 3 inches or more liquid equivalent),
strong winds (40 knots or greater) and high significant wave heights (20-30
feet) are favored, as depicted on the map.
The U.S. Drought Monitor, valid October 16, indicates a decrease in severe
to exceptional drought (D2 to D4) coverage from 14.4 to 13.51 percent during
the past week. Continued improvement occurred across northeast Kansas,
Missouri, Texas, and Arizona.
For Friday October 26 - Thursday
November 01: The Arctic Oscillation (AO) index is forecast to become
negative during late October as positive 500-hpa height anomalies develop at
the higher latitudes of the Northern Hemisphere. This negative AO index would
be a major change in the pattern since this index has continued to be positive
throughout most of the summer and early fall. The evolving longwave pattern
with a strong blocking ridge over the north Atlantic increases chances for a
stormy end to October along the East Coast. The low pressure system that is
expected to emerge from the Gulf Coast later next week is most likely to move
north near the East Coast early in Week-2. The moderate risk of heavy
precipitation (Oct 26 to 28) from the Gulf Coast north to New England is based
on where the 6Z/12Z GFS ensemble mean indicates more than 0.5 inch per 24
hours. If this low pressure system strengthens rapidly, then the risk of other
hazards such as high winds and coastal flooding would increase for the
mid-Atlantic and Northeast. A broader slight risk area for heavy precipitation
captures the uncertainty of its spatial extent.
The GFS and ECMWF ensemble means are in excellent agreement with an
amplified 500-hpa trough and corresponding negative height anomalies over the
central and eastern CONUS during Week-2. Periods of cold air advection are
likely to maintain below normal temperatures across much of the central and
eastern U.S. through at least the beginning of November. A slight risk of much
below normal temperatures (Oct 29 to Nov 1) is forecast from the southern Great
Plains east to the Ohio Valley and parts of the Southeast based on where the
GEFS temperature tool indicates minimum temperatures having a 20 percent chance
of falling below the 15th percentile and where a frost or freeze could occur.
Forecaster: Melissa Ou
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.