Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue Oct 23 2018

Valid 12Z Fri Oct 26 2018 - 12Z Tue Oct 30 2018

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

Mid-latitude flow amplifying across the Mid-South will ingest the
upper-level remains of Willa which aids in the development and
strengthening of a surface low lifting out of the northeastern
Gulf of Mexico. The negative tilt of the shortwave will further
aid in the deepening process as the cyclone lifts poleward along
the Gulf Stream toward the offshore waters of the northeastern
U.S. Trailing this feature should be another disturbance tracking
southeastward across the Ohio/Tennessee Valley regions over the
weekend. The combination of these systems will maintain a broad
negative height anomaly in the vicinity of the northeastern U.S.
into early next week. Farther upstream, the pattern is forecast to
remain progressive which forces the mean upper ridge across the
western U.S. to advance toward the Rocky Mountain chain by
Monday/October 29. This will ultimately allow further troughing to
invade the immediate West Coast by early next week while broad
ridging re-establishes its presence over the eastern Pacific.

Multi-day ensemble low plot comparisons have shown a decrease in
spatial spread, particularly as the 12Z ECMWF members have begun
to cluster. However, the ECMWF ensemble solutions continue to be
the quicker amongst the global ensemble datasets. While the models
disagree on forecast track, the intensity does seem to shut off by
Saturday with pressures eventually rising toward Sunday as the
wave lifts up toward coastal New England. Regarding the amplified
flow pushing toward the Tennessee River Valley, multi-cycle
ensemble spaghetti plot comparisons have shown a strengthening
trend, particularly with operational solutions. Uncertainty exists
in how long to hold onto the northeastern U.S. mean trough but it
appears it should stick around until Day 7/October 30. Farther
west, while height falls track into the Pacific Northwest/British
Columbia coasts, the ECMWF solutions continue to favor a slower
progression. Ensemble means afford a more stable approach given
the level of uncertainty exhibited from run to run. This approach
supports a mean upper ridge across the eastern Pacific while a
shortwave embedded within mid-level northwesterly flow will take
shape over the Intermountain West next Tuesday.

The preference focused heavily on the output of the 18Z/12Z GFS
and 12Z ECMWF early on before adding their respective means into
the picture in a rather linear fashion. Generally reduced the
operational portion of the blend by 20 percent each day which
allowed for a heavily ensemble-based forecast for next week. Would
say confidence is around average as a whole but still a lot of
uncertainty revolving around the coastal low along the Eastern
Seaboard.


...Weather Highlights/Threats...

Primary heavy rainfall impacts should focus from the coastal
Carolinas northward across the Mid-Atlantic and into New England.
Of course any displacement of the low from the current forecast
would move the area of higher impacts. Given the time of year,
this will be a liquid precipitation event for many although enough
cold air damming inland should afford a snowfall threat over the
higher terrain of New England. The other region of active weather
will be the Pacific Northwest given the semi-permanent onshore
flow slamming into the Olympics and Cascades. The typical rain
shadow is evident in the rainfall forecast with a secondary maxima
inland over the Upper Intermountain West.

Regarding temperatures, areas east of the Great Plains will remain
cool throughout much of the period. This will particularly be the
case along the Eastern Seaboard given the combination of abundant
cloud cover, heavy rainfall, and lower 1000-500 dm thicknesses. At
times, readings should sit around 10 to 15 degrees below
climatology suggesting highs in the 50s in many locations.
However, these same factors should diminish nocturnal cooling
effects with expected lows around average. Across the western
U.S., warm conditions will prevail higher heights sitting over
California into the Central Great Basin. Widespread 80s are likely
for highs across the Desert Southwest with even a few pockets of
lower 90s possible.


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