Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EST Tue Dec 17 2019
Valid 12Z Fri Dec 20 2019 - 12Z Tue Dec 24 2019
...Atmospheric River to bring heavy precipitation to the Northwest
later this week...
...Parts of the Southeast may see locally heavy rain this
weekend...
19Z Update: Models and ensembles are still not in good agreement
on the speed and expected evolution of the southern stream system
over the Gulf of Mexico this weekend and into next Monday. Both
the 00Z and 6Z runs of the GFS were notably faster with the low
across the Gulf Coast region compared to the remaining guidance
and the GEFS mean. The 12Z GFS has trended slower and a little
closer to the model consensus. The UKMET is on the slower side of
the model guidance and not favored by the ensemble means. The 12Z
ECMWF is now closer to the consensus compared to its 00Z run,
which was also farther west. The WPC forecast was based on a
blend of the 00Z ECMWF, 00Z CMC, GEFS/EC means, and some previous
WPC continuity. The previous forecast discussion is appended
below. /Hamrick
...Overview...
Upper troughing out of the Gulf of Alaska will slowly ease
southeastward into the West Coast by next week, with a several day
stretch of modest to locally heavy rain/snow from northern
California northward. At about the same time, a somewhat unusual
southern stream system is likely to dip into the Gulf of Mexico
and cross through Florida with its own locally heavy rain around
its circulation. Much of the rest of the CONUS will be dry from
the southern Rockies northeastward to the Great Lakes under upper
ridging.
...Guidance/Predictability Assessment...
Consensus of the recent models/ensembles off the West Coast
sufficed as a starting point with better than average
predictability despite some lingering timing differences. The 12Z
ECMWF was most preferred, followed by the GFS. The 12Z
UKMET/Canadian were displaced from the best clustering with other
timing issues downstream. As such, for the system into the Gulf,
the 12Z Canadian (and 18Z GFS) was likely too quick while the 12Z
UKMET was likely too slow with the overall progression. The 12Z
ECMWF/GFS were closer to the ensemble means which has been
somewhat stable in principle over the last several days (i.e.,
keeping the system separate from the northern stream). Still would
not trust either model verbatim given the unusual track/evolution,
but each run offers insight to what could transpire with respect
to heavy rain and possibly higher elevation snow in the southern
Appalachians.
...Weather Highlights/Threats...
Moisture plume off the Pacific should sink southward through
coastal OR/CA after Friday as the front moves onshore Saturday.
Multiple inches of precipitation are expected over a 2-3 day
period with a likely break in between for a given location.
Daytime temperatures will be near to above average overall with a
trend toward cooler temperatures as the front moves inland this
weekend. East of the Rockies, temperatures will be above average
with overall dry conditions and upper ridging overhead--some
locations 15 to 20 deg F above average. The Northeast will see a
moderation after a chilly Friday. Coastal/Gulf system will provide
at least modest rainfall for parts of the Southeast, depending on
its evolution, with possible locally heavy amounts. Some snow will
be possible on its northern fringe in the marginally colder air of
the southern Appalachians if the precipitation shield extends that
far north.
Fracasso
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of California, the Pacific
Northwest, and the Northern Great Basin, Fri-Sun, Dec 20-Dec 22.
- Heavy rain across portions of the Southeast, Sat-Mon, Dec 21-Dec
23.
- Heavy snow across portions of the Northern Rockies and the
Northern Great Basin, Fri, Dec 20.
- Heavy snow across portions of the Northern Rockies and the
Northern Great Basin, Fri-Sat, Dec 20-Dec 21.
- Flooding possible across portions of the Central Appalachians,
the Pacific Northwest, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Much below normal temperatures across portions of the Lower
Great Lakes, the Mid-Atlantic, the Northeast, and the Northern
Appalachians, Fri, Dec 20.
- High significant wave heights for coastal portions of California
and the Pacific Northwest, Fri-Sat, Dec 20-Dec 21.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml