Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Sun Dec 29 2019
Valid 12Z Wed Jan 01 2020 - 12Z Sun Jan 05 2020
...Heavy rainfall potential over the Lower Mississippi Valley to
the Southern Appalachians mid to late week...
...Overview...
Upper pattern will transition to broad troughing over the Lower 48
as a deep upper low initially over Baja California Wed lifts
toward the Southeast later this week. This will lead to
potentially heavy rainfall for the Lower Mississippi Valley
northeastward. An active storm track out of the northeast Pacific
will sustain several frontal passages into the Pacific Northwest,
causing rounds of rain.
...Guidance/Predictability Assessment...
Model guidance has come into general agreement regarding the
placement of the upper low over Baja California Wed. Northern
stream energy from the northern Pacific should drop southward
Wed/Thu, creating a trough with its axis located from the Northern
Plains to Four Corners region. The original southern stream energy
will wrap up into this trough around Thu night/Fri, but this
transition leads to some model spread in the position/eastward
extent of the trough, as well as issues with the surface low
placement and strength given the 500 mb vorticity differences. The
00Z UKMET was the farthest forward, with the GFS not far behind
and the 00Z ECMWF and CMC the farthest west. Leaned slightly more
toward the slower solutions by excluding the UKMET by that point
and weighting the 00Z EC/CMC a bit heavier than the GFS runs.
By the weekend, broad troughing seems to set in across the CONUS,
with embedded shortwaves in the flow coming across the Pacific and
into the U.S. Confidence is low with the details in this flow,
especially given the limited predictability coming from the
Pacific. GEFS ensemble members generally lie south of ECMWF and
CMC members in terms of 500 mb heights across the western half of
the U.S. Sat. A surface low will likely spin up ahead of one
shortwave somewhere in the north-central tier by Sat, but
placement and strength are still uncertain. Relied on the GEFS and
ECENS means to smooth out differences in details of the flow, but
expect the amplitude of shortwave features to end up greater.
...Weather Highlights/Threats...
As the upper low/trough lifts out of Mexico, moisture from the
Gulf will get drawn across the warm front along the coast which
will lead to an expanding area of rainfall and some embedded
convection. Heaviest rainfall may occur from Louisiana across
Mississippi into Alabama, especially Thu. The system is expected
to weaken Fri, but some rainfall is expected over areas farther
northeast over portions of the Tennessee Valley to Southern
Appalachians. Nearly daily bouts of precipitation are likely for
the Northwest into the Northern Rockies during the first several
days of January, focused over coastal areas and the Cascades.
Temperatures will generally be near to above average east of the
Rockies and near to below average in the West.
Tate/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