Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1131 AM EDT Sat Oct 13 2018

Valid 12Z Tue Oct 16 2018 - 12Z Sat Oct 20 2018

...Cooler than average temperatures for much of the lower 48 next
week...


...Pattern Overview...

An upper low near Baffin Island/Davis Strait will meander in place
as upper ridging in the Northeast Pacific pushes into western
Canada and flattens. This will favor positively tilted troughing
from the Four Corners region (where an upper low will wobble
around) to the Great Lakes/Northeast where a couple cold fronts
will swing through. Across Florida, subtropical ridging will hold
in place, maintaining warm weather but with a daily chance of
rain/storms as a front stalls over the peninsula.


...Guidance/Predictability Assessment...

Began the medium range forecast with a multi-model deterministic
blend (ECMWF/GFS/UKMET) during days 3-5 (Tue-Thu), as models
showed generally good consensus with respect to the upper low
lingering across the Southwest and with amplifying shortwave
energy crossing the Great Lakes, pushing a surface front toward
the East Coast during the time period. During days 6-7, the main
point of contention among the guidance becomes how much
erosion/flattening of the western North American ridge occurs as
shortwave energy traverses the northern periphery of the ridge.
The 00Z ECMWF was the most aggressive deterministic solution with
flattening the ridge, resulting in an overall deamplification of
the upper flow across North America relative to the GFS/CMC. At
this point, preferred to compromise, but leaned slightly toward
the more amplified solutions given the persistent amplified flow
regime across much of the northern hemisphere. The day 6-7
(Fri-Sat) forecast was based on increased ECENS/GEFS mean
weighting along with continued minority components of the
ECMWF/GFS.


...Weather Highlights/Threats...

Well below average temperatures on Tuesday (likely breaking record
low minima and maxima over many stations from KS/CO southward to
TX/LA) will start to relent but still continue into Wednesday
across much of Texas where highs will struggle to reach the 60s
away from the coast. On the warm side, parts of Florida may
tie/break record highs south of the front where upper ridging will
be centered.

Precipitation may be heavy on Tuesday over northern Texas on the
cool side of the front but between the upper low to the west and
the upper ridge to the east. The rain axis will extend
northeastward to the Mid-Atlantic but in lessened amounts. The
rest of the lower 48 behind the front will be largely dry with
some rain/snow showers around the Lakes.


Ryan/Fracasso


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