Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1256 AM 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 and Guidance/Predictability Assessment...
Broad troughing will settle over the central and eastern states by
next week after a reversal of the pattern at the beginning of the
period this Thursday. The models and ensembles were in good
agreement to start the forecast but drifted apart by the weekend
and especially by next Monday. The 12Z ECMWF and Canadian offered
reasonable clustering with the 18Z GEFS mean and 12Z ECMWF
ensemble mean to use as a starting point, with added details from
the 12Z UKMET and 12Z/18Z GFS to start.
Areas of contention were first with the weakening upper low
(likely just a sharp trough by 12Z Thu) over Mexico, lifting to
the northeast. With three separate streams crossing 100W, the
models have wavered on how each interacts (or doesn't) with each
other aloft, and consequently at the surface. The 12Z/18Z GFSs
were likely too quick with the Mexican trough but perhaps too slow
with the trailing shortwave out of the Rockies, and it was not
preferred past Thursday. Lead and trailing surface systems will
attempt to merge together by the time they reach New England
around late next Fri into Sat.
Next area of contention lied in the west, with a large amount of
uncertainty in the upstream central North Pacific flow. This has
led to large timing differences in how quickly or slowly to lower
heights in the West starting Sat and then progress eastward and
southward next Sun/Mon. Again there, the ECMWF/Canadian paired
closer to the ensemble means as the GFSs were nearly completely
out of phase. By next Mon, no deterministic model really fit the
lead-in pattern suggested by the means, though the degree of
spread couldn't rule out any model either. Favored the ensemble
means in whole at the expense of any detail.
...Weather Highlights/Threats...
As the weakening upper low or 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 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 period, focused over coastal areas and the
Coastal Range/Cascades in addition to the Idaho
Panhandle/northwestern Montana. Temperatures will generally be
near to above average east of the Rockies through the weekend and
near to below average in the West. By next week, more of the
country may see below average temperatures than above as troughing
becomes established.
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