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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1445Z Oct 12, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1045 AM EDT Fri Oct 12 2018

Valid 12Z Mon Oct 15 2018 - 12Z Fri Oct 19 2018

...Impressive record cold for the central/southern Plains early
next week...


...Pattern Overview...

An upper low near Baffin Island/Davis Strait will slowly translate
southeastward as upper ridging in the Northeast Pacific breaks
down. This will allow troughing initially over the Great Lakes to
split between a northern and southern portion with the former
exiting much quicker than the latter. Across Florida, subtropical
ridging will hold in place, preventing a cold front from slipping
southward to Lake Okeechobee.


...Guidance/Predictability Assessment...

A multi-model deterministic blend (including the ECMWF/GFS/UKMET)
served as a starting point for the forecast during days 3-5
(Mon-Wed). The GFS remains a bit quicker than the ECMWF/UKMET with
shortwave energy crossing the Great Lakes on Mon, but has moved
closer to the slower solutions relative to yesterday and is now
usable. Consensus remains high that energy will separate from the
trailing end of the central U.S. trough and cutoff/linger across
the Southwest, and the described blend represents this process
well. By days 6-7 (Thu-Fri) consensus suggests that the energy
previously cutoff over the Southwest will begin to slowly drift
eastward toward the central U.S. Spread among the deterministic
guidance/ensemble means with respect to this is relatively low,
although a number of individual ensemble members (particularly in
the GEFS/NAEFS) show slower solutions which are not necessarily
out of the question given the broad flow amplification in place.
Boosted weighting of ECENS/NAEFS ensemble means in the forecast
during days 6-7, with a majority ensemble means by day 7 to
account for somewhat increased spread in the Southwest/central
U.S. as well as with additional northern stream shortwave energy
crossing central Canada.


...Weather Highlights/Threats...

Well below average temperatures (likely breaking record low minima
and maxima over many stations and states) will sink southward
behind a strong cold front late this weekend into next Monday.
Highs in the 40s are forecast for most of the central Plains into
Texas even to the Big Bend which will be about 25-35 degrees below
average. Some locations may break their record low daytime high
temperature by 5-10 degrees even where records go back over 100
years. The cold will start to relent after Monday as it settles to
the Gulf Coast and Lower Mississippi River Valley. The cold front
will have less bite farther east from the Great Lakes to the
Northeast/Mid-Atlantic where temperatures will only be a few
degrees below average. To the south, Florida will maintain above
average temperatures thanks to upper ridging.

Precipitation may be heavy from Texas northeastward as the upper
low behind future ex-Sergio meanders eastward through the Four
Corners region and moisture from the western Gulf flows northward
atop the front. The rain axis will extend to the Mid-Atlantic but
in lessened amounts. The rest of the lower 48 behind the front
will be largely dry with some showers around the Lakes (some snow,
some rain). Widespread snow (some lower elevation rain) will exit
the Southern Rockies on Monday with some snow accumulation
thereafter as the upper low swings through.


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