Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1256 PM EST Tue Jan 07 2020
Valid 12Z Fri Jan 10 2020 - 12Z Tue Jan 14 2020
...Late week/weekend storm to bring a heavy rain threat to the
Ark-La-Tex region/Lower-Mid Mississippi Valley and northeastward,
possible severe weather to parts of the South, and wintry
precipitation potential to the central Plains/Midwest through
Great Lakes/Northeast...
...Active pattern over the Northwest...
18 UTC Update...
No major changes to the ongoing forecast were made during this
update. A blend relying heavily on the 00Z ECMWF/UKMET/06Z GFS was
used during days 3-5 (Fri-Sun). Quite a bit more emphasis was
placed on the ECMWF/UKMET relative to the GFS. The GFS was weaker
and faster than the overall consensus with the surface low
expected to develop across the lower Mississippi Valley Fri
night/Sat, which is then expected to track northeastward across
New England on Sun. The ECMWF/UKMET solutions were much better
clustered with the ensemble means in showing a slower and somewhat
deeper low pressure system, and these solutions were preferred.
The trend toward greater confidence in these solutions resulted in
a relatively small westward shift in the heaviest QPF amounts
during the day 4-5 time frame.
Later in the forecast period the GFS continued to differ from the
ECMWF and from the overall ensemble consensus. The GFS was weaker
with the subtropical ridge across the Southeast by early next
week, which results in a more suppressed and faster-moving low
pressure system across the southern tier Mon-Tue. Given the
evolution of the large-scale/hemispheric pattern, with a building
anomalous upper ridge between Alaska and Hawaii, this favors a
persistent mean trough along the U.S. West Coast and a stronger
subtropical ridge in the Southeast. The ECMWF was much closer to
this idea, and has, for a couple runs, shown an area of low
pressure developing across the Southern Plains by Mon and moving
northeast toward the Ohio Valley or Great Lakes by Tue. Ensemble
means also show a fair degree of support for this idea. Given that
this solution is also supported by hemispheric teleconnections,
opted to continue using about 30 percent of the 00Z ECMWF through
day 7 along with the remainder comprised of ECENS/GEFS ensemble
means.
Ryan
Previous Discussion (issued at 0706 UTC)...
...Overview/Guidance Assessment...
A mean pattern aloft featuring an amplified western U.S. trough
held in place by strong ridges over the east-central Pacific and
off the Southeast U.S. will produce significant weather over
portions of the lower 48 during the period. Individual shortwaves
within the mean flow will support individual events. This pattern
will promote a contrast between very warm temperatures over the
eastern half of the country (many areas averaging 10-25F above
normal for the five-day period) and below normal readings over the
northern Plains (up to 20-35F below average one or more days) as
well as to a lesser extreme over much of the West.
Guidance is still in the process of trying to converge on the
ultimate solution for how a shortwave over the Rockies as of early
Fri will eject northeastward, with associated low pressure
tracking from the southern Plains through the Northeast in the
late week-weekend time frame. CMC runs had been the one holdout
for a slower/closed feature aloft but the new 00Z run has finally
joined the 12Z ECMWF. GFS runs have tended to be on the fast side
of the spectrum for this system and that continues to be the case
with the 00Z run actually straying ahead of the 18Z run.
Meanwhile the 00Z UKMET is a bit slower than the previous run.
Slowest solutions may not be ideal given the progressive nature of
the southwesterly flow aloft, favoring at least intermediate
timing. Latest trends toward a flatter shortwave reaching the
East Coast look good given the strong positive height anomalies
which multi-day means have persistently forecast just offshore.
Upstream the guidance agreement is steadily improving for the
robust though weakening system currently expected to tracking into
Washington state on Sat. 00Z CMC details aloft leave some
question marks but other wise a blend depicts consensus well.
Through the weekend the supporting dynamics will drop into the
long-term western mean trough position. By days 6-7 Mon-Tue
forecast details become more uncertain. In general expect this
western energy to eject beyond the Rockies while additional energy
aloft and surface system drop southeastward into and beyond the
Northwest. Notable timing differences exist for the system
reaching the Northwest while models/ensembles become quite varied
in how the weekend western energy ejects to the east/northeast.
Over the past couple days a number of operational model runs have
displayed a general signal for low pressure over east-central
North America by early day 7 Tue while the ensemble means (18Z/00Z
GEFS being best defined) have focused low pressure over the Great
Lakes region--a reasonable starting point given the full guidance
spread. Resolving specifics next Mon-Tue will be important for
determining the character of another potentially significant
rainfall event over some areas over/east of the Mississippi Valley.
The updated forecast incorporated a mostly model-based blend among
the 12Z and 00Z/06 ECMWF runs (to account for shortwave
differences through the period) as well as 18Z GFS/12Z UKMET for
days 3-5 Fri-Sun followed by a transition toward a more even
model/ensemble mean blend by day 7 Tue. The result provided good
continuity with most aspects of the forecast.
...Weather Highlights/Threats...
The late week through weekend storm system will produce a wide
variety of significant weather across the eastern half of the
country. Persistent Gulf inflow ahead of the developing low
pressure/frontal system tracking out of the southern Plains will
provide the moisture to produce heavy rainfall from the Ark-La-Tex
region and Mid-Lower Mississippi Valley northeastward into the
Tennessee/Ohio Valleys and possibly beyond. Currently expect the
axis of highest rainfall to extend from Arkansas into Ohio, but
this is still subject to adjustment given uncertainty in exact
details of system evolution/timing. At the same time the Storm
Prediction Center is monitoring the potential for severe weather
over portions of the South. Check latest SPC products for updated
information regarding the severe threat. Meanwhile over the past
couple days guidance has been steadily increasing the potential
for meaningful cold sector winter weather within an axis from the
central Plains/Midwest through the Great Lakes/New England.
Continue to expect the front trailing from the surface low to drop
into the Gulf of Mexico briefly and then return north as a warm
front early next week.
Additional ejecting western shortwave energy may support another
central-eastern U.S. system which could produce another heavy
rainfall event, but with lesser confidence in specifics due to
current guidance spread at that time frame. Thus far the best
heavy rain signal for this second event continues to be from the
Lower Mississippi Valley into Tennessee Valley/southern
Appalachians--somewhat southeast of the expected axis of the first
event. Some locations that may be affected by either or both
events should monitor flash flood/river flood potential due to
recent significant rainfall.
Over the West, guidance is consistent in highlighting favored
terrain from the Pacific Northwest into northwestern California
for highest rain/mountain snow totals during the period as
multiple systems affect the region. The trajectory of moisture
still does not appear to support anomalously high precipitable
water values/extreme intensity of precipitation in most cases, but
persistence of the rain/snow may lead to several inches liquid
over the five-day period. Expect a secondary precip maximum over
the northern Rockies. The system reaching the Northwest early in
the weekend may be accompanied by a period of brisk winds.
Episodes of lighter rain and higher elevation snow will extend
farther southeast across the West.
From late week into the weekend expect the warm air extending
across the eastern half of the country to produce widespread plus
10-30F anomalies and possibly greater especially for morning lows.
Daily records for highs/warm lows are possible. Expect warm
temperatures to continue into next week. Within the area of
colder air to the west, the most extreme anomalies will be over
the northern Plains where readings could be as low as 20-35F below
normal. Northern parts of the West could see readings drift
toward 10F or so below normal by early next week while most of the
remainder of the West will see temperatures generally 5-10F or so
below normal.
Rausch
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