Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
216 AM EDT Tue Oct 16 2018

Valid 12Z Fri Oct 19 2018 - 12Z Tue Oct 23 2018

...Pattern Overview...

Reinforced upper troughing will persist in the eastern half of the
CONUS while ridging is forecast to linger over the Pacific
Northwest. A series of fairly weak Canadian fronts will swing
through the Great Lakes to maintain below average temperatures and
spark off some rain/snow showers especially in favored lake-effect
areas. The strong upper ridge over Florida will only slowly weaken
by next week and get squashed into Cuba. On its west side will be
a stalled front off the Texas coast which will keep a daily chance
of rainfall in the forecast, especially southern Texas. Finally, a
lost upper low near Southern California will seek an exit route
through the Great Basin early next week, helping to wring out some
light showers.


...Guidance/Predictability Assessment...

An operational model consensus continues to represent individual
features well for Fri-Sun. After that time model runs begin to
show greater shortwave differences among each other and/or in
consecutive runs, favoring a trend toward the ECMWF ensemble mean
and GEFS mean. Pacific front will attempt to move into
Washington/Oregon late next Tuesday but the ensembles differ
quicker/slower (ECMWF/GEFS, respectively). ECMWF and its ensembles
have had a tendency to break down the western ridge too quickly at
that time range but a shift northward in the positive height
anomaly could allow for an opening into the Pac NW as quick as the
ECMWF ensemble mean (perhaps not as quick as the 12Z/15 ECMWF).
For now, a modest middle ground sufficed.


...Weather Highlights/Threats...

Above average temperatures are favored in the west outside the
influence of the upper low, generally 5-10F above average.
Troughing in the east and cloud cover over Texas will keep
temperatures below average there but with a little variation
around frontal passages. Some areas will see near record warmth
(WA/OR and FL) with more widespread cool temperatures and chilly
afternoon highs around the Lakes into the Northeast (40s/50s).

Expect highest 5-day rainfall totals over portions of Texas and
near the western Gulf Coast, in the vicinity of an inverted trough
and to the northwest of a stationary boundary in the western Gulf.
Some of this southern Plains moisture may extend northeastward
along a cold front crossing the East during the weekend. The
lingering upper low in the Southwest will support rain and
possibly high elevation snow showers mostly focused on/along the
higher terrain while a stationary front across Florida will keep
daily shower/storm chances in the forecast. Over the Great
Lakes/Northeast lake-effect precipitation will start anew this
weekend behind a cold front and then again late in the forecast.
Coastal WA/OR will see rain develop and expand inland starting on
Monday and continuing on Tuesday, depending on the speed of the
front.


Fracasso/Rausch


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