Valid Saturday December 14, 2019 to Friday December 20, 2019
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST December 06 2019Synopsis
: Mid-level low pressure is
predicted over eastern North America and the Aleutians/Bering Sea region during
week-2, with mid-level high pressure forecast over portions of western North
America. Although these higher-level circulation features appear to be fairly
persistent, even slight variations in the strength or location of these
features will have a noticeable impact on storm tracks and temperature patterns
near the surface. Hazards
- Slight risk of much below
normal temperatures for the east-central CONUS, Sat-Wed, Dec 14-18.
- Slight risk of heavy precipitation for most of the East Coast states
extending westward over Alabama, and eastern portions of Tennessee and
Mississippi, Sat-Fri, Dec 14-20.
- Slight risk of heavy precipitation for western Washington and western
Oregon, Sat-Fri, Dec 14-20.
- Slight risk of heavy snow for the southern Alaska coast, from Kodiak Island
eastward to the northern half of the Alaska Panhandle, Sat-Fri, Dec 14-20.
- Slight risk of high winds over the northern and central Plains, Sat-Mon,
For Monday December 09 - Friday
December 13: WPC Days 3-7 U.S.
Hazards For Saturday December 14 -
Friday December 20:
Two low pressure systems are predicted to bring
significant precipitation to portions of the eastern CONUS during week-2. The
first storm system is forecast to bring heavy precipitation (1-inch or greater
per 24-hour period) to the Atlantic Coast states during the first few days of
week-2. Today's model runs generally depict precipitation mostly in the form of
rain, with mixed or frozen precipitation possible over the interior portions of
the Mid-Atlantic and Northeast. This potential event is still at least a week
away so the details regarding precipitation type, storm track, and timing may
all change substantially. The 12z GEFS predicts a second storm system will
develop somewhere near the Central Gulf Coast states before bringing another
round of precipitation to the Southeast, Mid-Atlantic, and Northeast later in
week-2. Therefore, a slight risk (20%) area of heavy precipitation is posted
from the Central Gulf Coast region eastward and northeastward to include most
of the East Coast during week-2.
Over the western CONUS, the expected deamplification of the mid-level ridge
should allow for increased onshore flow across western sections of both
Washington and Oregon. The 0z ECMWF ensemble solution is more robust regarding
precipitation amounts during the week-2 period than the GEFS, with the ECMWF
ensemble favoring liquid equivalent values of at least 2 inches, associated
with multiple storm systems. If correct, heavy snow can be expected over the
higher elevations of the Cascades and Coastal Ranges of the Pacific Northwest.
Due to significant differences between the various ensemble means and the
deterministic model runs, only a slight risk of heavy precipitation is posted
for the area. A slight risk of heavy snow is indicated for the southern coast
of Alaska, from about Kodiak Island to the northern half of the Alaska
Panhandle, where 1.5-2.0 inches of precipitation (liquid equivalent) is
generally expected in advance of the 500-hPa trough over the Bering Sea and
Aleutians. The 0z ECMWF ensemble and the 12 GEFS forecast temperatures below
freezing not only for mountainous regions, but the coastal lowlands as well. As
is the case with the other noted precipitation hazards, the combination of
multiple storm systems and differences in timing necessitate the hazard to be
broadly designated for the entire week-2 period.
Today's hazardous temperature map features a slight risk of much below
normal temperatures only over the east-central CONUS, from Dec 14-18. The GEFS
is generally colder than the ECMWF ensemble, and the exact placement of the
coldest temperature anomalies is uncertain; hence the designation of only a
slight risk area on the map. Analysis of the deterministic 12z ECMWF run
suggests that a mean trough develops over the northern and central Rockies
beginning before the start of week-2, and raises 500-hPa heights across the
Gulf Coast and Lower Mississippi Valley late in Week-2.
Across the northern and central Plains, conditions are likely to change
rapidly, as an early arctic air outbreak and related upslope flow transitions
to warmer, downslope flow several days later. A slight risk of high winds (at
or above the 85th percentile, and 25 mph or greater) is posted for this region
from Dec 14-16.
Forecaster: Anthony Artusa
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts