Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 AM EST Sun Feb 10 2019

Valid 12Z Wed Feb 13 2019 - 12Z Sun Feb 17 2019

...Overview...

Models/ensembles continue to show broad agreement on anomalous
upper ridging persisting from the North Pacific across Alaska
during the medium range, with a persistent downstream mean trough
axis just off the Pacific Northwest, and a subtropical ridge
attempting to build into the south central and southeastern U.S.
Pacific shortwave energy will round the ridge axis near Alaska and
then dive southward toward the U.S. West Coast throughout the
period, before moving east across the Rockies and central U.S. A
sharp thermal gradient across the central U.S. between the large
scale trough/ridge will support rapid cyclogenesis as these
features arrive, with the low pressure systems moving
northeastward toward the Great Lakes/New England. Despite broad
agreement on this general pattern, forecast confidence becomes
quite low during the mid to latter portion of the medium range as
guidance struggles to deal with the degree of separation/phasing
between northern/southern stream energy across the central U.S. by
Thu-Fri. Additional major differences emerge in the wake of this
system and ahead of the next Pacific shortwave, when models cannot
even agree on whether broad cyclonic or anticyclonic flow will
prevail, with major implications for the next incoming shortwave
by late in the week, and its evolution as it moves inland.


...Guidance/Predictability Assessment and Preferences...

Models were initially reasonably well-clustered with the low
pressure system crossing the Great Lakes on Wed, and the triple
point low developing along the New England coast. some timing
differences emerge as this low pulls northeast away from New
England on Thu, but no clear outlier solutions were evident.
Farther west, a complex low pressure system in the eastern Pacific
should move onshore on Wed and into the Four Corners/Rockies on
Thu ahead of an amplifying shortwave. Model solutions showed some
timing and slight structural differences with this feature, but a
consensus solution is likely best. Thus, the forecast during days
3-4 was based on a multi-model consensus blend, including the
ECMWF/CMC/GFS/FV3.

From day 5 (Fri) onward is when big problems emerge in the
forecast such that it was a struggle to find much in the way
consensus. By the time the amplified Pacific shortwave reaches the
central U.S. Fri, solutions differ substantially as to how deep
this feature will be (closed vs. open wave) and whether it will
interact/phase with northern stream energy. This results in a
range of potential tracks for the surface cyclone anywhere from a
Midwest/Great Lakes track (GFS/FV3/CMC) to a southern plains/Ohio
Valley track (ECMWF/UKMET). Ensemble scatter plots reveal a
bimodal distribution of solutions for the surface low Fri-Sat that
breaks down along the described scenarios. A slight trend was
noted among recent model runs toward keeping the two streams less
phased for longer, so the rapidly deepening solutions shown by
some previous GFS/FV3 runs seem less likely, but it is also
unclear whether keeping the streams separate as long as the
UKMET/ECMWF is likely. Thus, a middle ground solution was
preferred (relatively close to the 00Z GFS). As this system
reaches the eastern U.S. by Sat, differences in timing/amplitude
persist - likely stemming from the disparate solutions on Fri.
These differences continue all the way into day 7 (Sun) with some
solutions keeping a deep low across eastern Canada while others
quickly eject the system into the North Atlantic.

Additionally, as alluded to in the overview, models struggle with
the nature of the upper-level flow across the western/central U.S.
in the wake of the low pressure system. The GFS and CMC build an
upper ridge axis with broad anticyclonic flow, while the ECMWF
keeps a stream of strong Pacific into the western U.S., which is
able to quickly amplify across the central U.S. in a broad region
of zonal to even slightly cyclonic upper flow. Ensembles show
little reduction in spread (and perhaps even a slight increase)
across recent cycles by days 6-7 on these aspects of the large
scale flow. Given the persistent mean trough axis near/off the
Pacific Northwest, was a bit skeptical to allow strong height
falls pushing well inland as shown by the 12Z ECMWF. The GFS has
perhaps shown a bit more consistency for at least a few runs but a
look back several runs does reveal a more ECMWF like scenario.
Thus, opted to lean heavily on ensemble means by days 6-7, and
forecast confidence is quite low.


...Weather Highlights/Hazards...

The low pressure system affecting New England on Wed is expected
to produce wintry precipitation across much of New England, with
heavy snow possible on Wed for portions of northern Maine. The
next deepening low pressure system moving from the central U.S.
toward the Great Lakes Fri-Sat could produce an area of wintry
weather north of the low track, across portions of the
Midwest/Great Lakes on Thu and perhaps portions of New England on
Fri-Sat. As mentioned above, confidence in the track of this
second system is low, as are any potential winter weather impacts.
Farther south, this second system is expected to produce
widespread showers and thunderstorms as Gulf of Mexico moisture
streams northward into the deepening surface low. Locally heavy
rains will be possible from portions of the Southeast/Tennessee
Valley regions to the Mid-Atlantic. Meanwhile, the West Coast will
also remain active with the mean upper trough offshore
continuously ejecting shortwave energy toward the coast. Heavy
rain and mountain snows are expected to continue through the
extended forecast period, with areas from California north to
Oregon likely seeing the most widespread heavy precipitation.

The pattern will continue to favor very cold temperatures from the
Pacific Northwest to the northern Rockies and northern plains
through the week. High temperatures are forecast to be 15 to 30
deg F below average across many of these areas through the
forecast period. Meanwhile, areas from the southern/central plains
to the Eastern Seaboard will see near to above average
temperatures varying by day with the warmest temperatures ahead of
the late week low pressure system, when highs may reach 10 to 15
deg above average for some areas.


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