Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1052 AM EST Tue Feb 12 2019
Valid 12Z Fri Feb 15 2019 - 12Z Tue Feb 19 2019
16z update... The newest cycle of guidance continues to show
fairly good agreement on the large scale, with smaller scale
details/systems remaining low predictability and confidence. Given
this assessment, opted to use about 25 percent previous shift
continuity in the blend, along with the latest runs of the
GFS/ECMWF and their respective ensemble means. This resulted in a
forecast very close to the previous shift with minimal changes
needed. See previous discussion below for additional details on
the overall pattern, guidance evaluation, and sensible weather
impacts.
Santorelli
Previous Discussion...
...Overview...
Models/ensembles continue to show strong consensus that a Rex
block pattern across the central/north Pacific, extending from
near Hawaii to Alaska, should persist through the next week. This
will leave the flow regime across the CONUS largely quasi-zonal.
Teleconnections relative to the strongest 500 hPa positive height
anomaly center across the North Pacific continue to favor a
positively-tilted mean trough near the Pacific Northwest with weak
positive height anomalies from the southern Plains east into the
Southeast, and a broad region of modestly cyclonic upper-level
flow across much of the CONUS. Shortwave energy ejecting from the
northeastern Pacific every couple days will move across the CONUS,
undergoing gradual amplification in the broad cyclonic flow. Model
guidance suggests that the dominant storm track should gradually
shift southward over the next week, with a much more active
southern stream taking shape by early next week.
...Guidance/Predictability Assessment and Preferences...
Despite relatively good agreement among the guidance on the large
scale flow regime, the smaller scales remain highly chaotic with
low predictability. The system crossing the central/eastern U.S.
on day 3 (Fri) has shown continued volatility in model solutions
over the past several runs, with varying degrees of interaction
between northern/southern stream shortwave energy. The trend over
the past 12 hours has been overwhelmingly toward a more phased
system with low pressure moving northeastward much more quickly,
with a frontal wave developing across the lower Mississippi Valley
in response to additional shortwave energy. Given the time range
under consideration, one would expect the degree of run-to-run
variability to be quickly decreasing; but models have demonstrated
difficulty for days with their handling of this system, and would
prefer to see another run of more stable solutions before forecast
confidence increases. Farther west, additional Pacific shortwave
energy is expected to cross the Southwest and approach the central
U.S. by Sat. Given the intense frontal zone lingering near the
Gulf Coast, this wave seems likely to generate a frontal wave by
day 5 (Sun) although model solutions vary widely as to the details
of any frontal wave (or whether one occurs at all). The GFS
continues to develop the strongest wave, with the ECMWF weaker.
The WPC forecast during days 3-5 was based on a blend of the 12Z
ECMWF/18Z GFS, along with their respective ensemble means, and a
small component of forecast continuity to temper the dramatic
adjustment slightly with the northern stream system days 3-4.
Meanwhile, a more amplified upper trough appears set to move into
the Southwest on Sun, crossing the Four Corners region on Mon and
reaching the southern/central plains by Tue. While model/ensemble
consensus is initially good with this system, differences emerge
by the time the shortwave reaches the central U.S., with differing
levels of northern stream influence resulting in varying surface
evolutions. One thing in common among the guidance that is
different from the system late this week is that the southern
stream feature appears more dominant. Given the persistent and
intense baroclinic zone across the Gulf of Mexico/Gulf Coast by
that time with the subtropical ridge building off the Southeast
U.S. coast, this at least opens the door to the development of a
significant frontal wave somewhere in the vicinity of the Gulf by
next Tue. Confidence in the specifics of any such feature is quite
low at this point, however. The WPC forecast during days 6-7 was
based heavily on a blend of the ECENS/GEFS ensemble means, with
minority components of their respective deterministic solutions.
...Weather Highlights/Hazards...
Areas from central/northern California to western Oregon and
Washington will see continued rounds of precipitation (rain and
snow varying with elevation) through Sat before at least somewhat
of a lull by late in the weekend/early next week as the main upper
trough shifts inland. Another relatively weak shortwave/frontal
system arriving by Mon may produce another round of precipitation
for the Northwest. The low pressure system expected to cross
Ontario/Quebec on Fri may produce areas of wintry weather across
the interior Northeast in a region of warm-advection
precipitation. Farther south, a frontal wave crossing the
Southeast Fri-Sat will produce widespread moderate rains and
perhaps a few thunderstorms along the Gulf Coast. The next
southern stream frontal wave crossing the Southeast Sun-Mon looks
to produce a more significant area of precipitation from the lower
Mississippi Valley to the southern Mid-Atlantic, with an area of
heavy rainfall possible across the Tennessee Valley and the
southern Appalachians/foothills. Model solutions vary as to the
degree of cold air north of the system, but given the rather tight
thermal gradient depicted in much of the model guidance across the
region, it seems likely that at least some areas on the northern
portion of the precipitation area may see some potential winter
weather.
Below average temperatures will persist and become more widespread
across the western/central U.S. through the medium range. The core
of the cold air late this week will initially be from the northern
plains to the Upper Midwest, with temperatures 15 to 25 deg F
below average. By Sun-Mon as the upper trough move into the West
and heights fall across much of the western/central U.S. expect
high temperatures of 15 to 30 deg below average across much of the
western and central U.S., continuing through the end of the
forecast period (Tue).
Ryan
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found 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