Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Thu Oct 25 2018

Valid 12Z Sun Oct 28 2018 - 12Z Thu Nov 01 2018

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

Amplified flow will remain fixed over the continental U.S.
accompanied by broad longwave troughing from the Mississippi River
eastward. Within this negative height anomaly, a lead shortwave
should be in the process of weakening while lifting through the
New England coast. In its wake, sharp height falls are forecast to
sweep through the Mid-South on Sunday night spawning an additional
area of low pressure over the Delmarva Peninsula by the following
morning. Mean troughing will remain over the northeastern U.S.
until Tuesday before mid-level southwesterly flow ensues ahead of
the next trough to conclude the period. Shifting focus to upstream
locations, progressive mid-latitude flow will usher in lowering
heights toward the Pacific Northwest/British Columbia coast on
Sunday. A full-latitude trough should continue to makes its way
eastward toward the central U.S. by mid-week while a closed low
may pinch off over the Four Corners region. By Day 7/November 1,
the ultimate synoptic configuration will favor ridges over the
eastern Pacific and western Atlantic with broad longwave troughing
sandwiched in between.

Regarding the model guidance, a general quicker trend is evident
via multi-cycle ensemble spaghetti plot comparisons. Regarding the
evolving wave with the secondary height falls tracking across the
Mid-South, there are less surface low stamps over the Delmarva
Peninsula region on Monday morning suggesting a delay in surface
development. By Tuesday morning, many operational models support
the wave crossing very near the 40N/70W benchmark, albeit in a
somewhat fractured state. In its wake, global models agree on
establishing high pressure over the eastern U.S. by mid-week while
a slow moving frontal zone tracks through the central states.
Intermittently solutions will show a more amped up wave tracking
across the middle of the country, most notably the 12Z GFS.
However, these particular scenarios do not show up very often as
the better forcing is expected to stay closer to the international
border with Canada. There is definitely increasing confidence of
an upper low pinching off near the Four Corners with this trend
showing up a few runs ago in the ECMWF ensembles. Looking back
toward the eastern Pacific, strong ensemble clustering exists with
the mean ridge setting up into the start of November.

The forecast preference was focused heavily on the 18Z GFS/12Z
ECMWF solutions as well as their ensembles (including the 12Z
NAEFS mean). Linearly reduced operational components of the blend
each day as the spread increased in time. And did select the 18Z
GFS cycle over its previous run because of the overly developed
central U.S. wave next week.


...Weather Highlights/Threats...

Regarding precipitation, the most active area will be over the
Pacific Northwest given the semi-permanent onshore flow regime
setting up. This will of course be augmented during the passage of
any notable shortwaves, especially the one arriving late this
weekend. Several inches of rainfall are likely across the Olympics
and Cascades given favorable upslope trajectories with snow
expected over the higher elevations extending into the interior
mountainous terrain. Across the northeastern U.S., it will be
intermittently wet over the region with snow possible back over
the Appalachians. A heavy stripe of precipitation will be exiting
northern Maine on Sunday morning as a wave exits the region while
broad cyclonic flow and additional forcing keeps showers in the
picture into early next week. Regarding that next trough crossing
the central U.S. by mid-week, poleward moisture return should
allow rainfall to break out along the slow-moving frontal zone.
While likely focusing over the Arklatex region, uncertainty
remains rather great and will be driven by where instability is
more prevalent.

In terms of temperatures, it should remain cool over large
sections of the eastern U.S. given the lower heights traversing
the region. Forecast anomalies should be around to 10 to 15
degrees below climatology, particularly over the Ohio Valley into
the northeastern U.S. through early next week. Eventually numbers
should return closer to average for late October given
mid-southwesterly flow becoming established. Out to the western
U.S., temperatures will be around 10 to 15 degrees above average,
particularly over the Central Great Basin into the Rockies given
the ridging aloft. As heights lower from Tuesday onward, a cool
down is expected to shift eastward toward the central U.S. for
much of next week.

This will be my final discussion after 12 years at WPC (formerly
HPC). My adventures will take me to Sacramento, California where I
will be working at their local forecast office starting in
mid-November!


Rubin-Oster


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes 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
https://www.wpc.ncep.noaa.gov/heat_index.shtml