Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 AM EST Wed Dec 18 2019
Valid 12Z Sat Dec 21 2019 - 12Z Wed Dec 25 2019
...Heavy rain possible over parts of the Southeast this weekend...
...Overview...
Upper troughing off the West Coast will be reinforced from the
northwest and slowly move onshore next week, maintaining an
unsettled pattern. Concurrently, a somewhat unusual southern
stream system will track through the Gulf of Mexico and across
Florida late this weekend. Locally heavy rain will be possible
around its circulation. Much of the rest of the CONUS will be dry
from the southern Rockies/Plains northeastward to the Great
Lakes/Northeast under upper ridging.
...Guidance/Predictability Assessment...
In the West, consensus of the GFS/ECMWF/Canadian served as a good
starting point as the models will continue to shift in the
interaction/timing of the lead and secondary shortwaves in the
overall longwave trough. These fit within the current and
multi-cycle ensemble envelope with expected differences that grew
larger with time. In the Southeast, 12Z ECMWF/Canadian lied in
between the quicker GFS runs and the slower UKMET. The 18Z GEFS
mean and 12Z ECMWF ensemble mean clustered well with the
ECMWF/Canadian, so a consensus near there was preferred. The GFS,
though quicker, was still included as it was well within the
middle 50% of ensembles with respect to forward speed.
...Weather Highlights/Threats...
Moisture plume associated with the short term atmospheric river
into the Pac NW should continue to sink southward through coastal
OR/CA as the cold front moves onshore Saturday. An additional
several inches of precipitation are expected over before the
focused plume diminishes and precipitation becomes lighter through
central and then southern coastal California. As the upper trough
and surface front move inland next week, precipitation will spread
through the southern Great Basin and Four Corners region.
Temperatures will trend cooler behind the front.
The Gulf system will provide at least modest rainfall for much of
the Southeast with possible locally heavy amounts. Models have
been showing over five inches over several days with sufficient
moisture in the lower atmosphere. Some snow will be possible on
its northern fringe in the marginally colder air of the southern
Appalachians. Temperatures will be above average east of the
Rockies (and outside the Southeast) with overall dry conditions
from eastern Montana/Colorado to the Plains where some locations
will see temperatures 15 to 20 deg F above average. The Northeast
will see a moderation to above average temperatures by Sunday and
especially Monday.
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
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