Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
155 AM 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 Wednesday 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.
...Guidance/Predictability Assessment...
Despite some lingering timing differences, the 12Z/18Z models
generally agreed on the upper low progression Wed/Thu over Mexico
and a deterministic model consensus was used as a starting point.
By Thursday, the various streams within the continental-wide
trough axis (between 100-110W) differed quickly, and the
UKMET/Canadian became quicker overall with the pattern. Preferred
to rely on the steadier consensus of the 18Z GFS and 12Z ECMWF
with their ensemble means which steadily took the southern system
northeastward out through the Southeast (with trailing height
falls on its heels) and the northern stream through the Great
Lakes (uncertainty in speed there as well). Ensembles were still
split between very slow southern stream solutions and very quick
weaker solutions. Middle ground position seemed prudent.
By next weekend, ensembles still show a large amount of spread in
how quickly height falls will progress through the West and
interior West. Degree of spread has led to a canceling of
quicker/slower waves and a rather unlikely flat flow out of the
Pacific into broad CONUS troughing. With some agreement among the
GFS/ECMWF/Canadian in wave tracking, opted to use about 1/3 to 1/2
weighting of the GFS/ECMWF with their means to give some amplitude
to the forecast. Confidence is low in timing given the quick
progression out of the central Pacific by then.
...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 on Thursday. With the system
weakening on Friday, generally light to modest rainfall is
expected over areas farther east into the Carolinas. 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.
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