Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Mon Dec 30 2019
Valid 12Z Thu Jan 02 2020 - 12Z Mon Jan 06 2020
...Heavy rainfall potential over the Lower Mississippi Valley to
the Southern Appalachians Thu-Fri...
...Overview...
Upper troughing will traverse the central to eastern U.S. Thu-Sun,
ahead of which moderate to locally heavy rainfall is possible in
the south-central to southeastern U.S. Then flow from the North
Pacific creates a messy forecast for the beginning of next week,
but with only minor sensible weather concerns.
...Guidance/Predictability Assessment...
Model agreement for the beginning of the medium range period (Thu)
is fairly good, indicating a trough axis through the High Plains
and a surface low near Oklahoma. By Fri, the 00Z and 06Z GFS runs
were a bit farther east with the 500 mb trough than the 00Z ECMWF,
UKMET, and CMC, but the surface low positions were not too
different so did not completely discount the GFS. The trough
continues eastward Sat, but spread starts to increase in both the
East and the West. The 06Z GFS and 00Z ECMWF were deeper with the
eastern trough than the 00Z UKMET and CMC, but at least they all
have a similar pattern. Using a multi-model blend seemed to
suffice.
In the West, upper ridging in the Pacific Thu/Fri may shift
eastward into the Southwest by Sat/Sun. Recent model runs have
trended this direction, after previously indicating broad
troughing across the CONUS. But confidence is low in the strength
of the ridging, so used the 06Z GEFS mean and the operational
GFS/ECMWF for more amplified ridging blended with the flatter 00Z
EC mean to temper the ridging. In the northern stream, large
operational model differences arise over the northern Pacific/Gulf
of Alaska Sat which propagate into the northwestern U.S. Thus
there is low confidence in this aspect of the forecast for the
beginning of next week. The most substantial change from the
previous forecast was to back up the surface low moving across
southern Canada Sat-Mon compared to the previous forecast, given
the current trend for a slower shortwave. Used mostly the 06Z GEFS
mean and 00Z EC mean to create the forecast by next Mon at the
expense of much detail.
...Weather Highlights/Threats...
As the sharp trough lifts out of Mexico, moisture from the Gulf
will get drawn across the surface warm front along the coast,
which will lead to an expanding area of rainfall and some embedded
convection. This may be accentuated via additional weak surface
wave development along the front. Heaviest rainfall should occur
from Louisiana into Mississippi and Alabama on Thu, getting into
the Southern Appalachians by Fri. Nearly daily bouts of
precipitation are likely for the Northwest into the Northern
Rockies during the period, focused over coastal areas and the
Coastal Range/Cascades, with lesser amounts in the Idaho
Panhandle/northwestern Montana. Temperatures will generally be
near to above average east of the Rockies through the weekend,
with the Four Corners region seeing near to below average
temperatures.
Tate
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