Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1206 PM EDT Fri Oct 05 2018

Valid 12Z Mon Oct 08 2018 - 12Z Fri Oct 12 2018

...Central U.S. Excessive Rainfall Threat/Active Western U.S.
Pattern...
...Separate Areas of Tropical Moisture Possibly Arriving into the
Gulf Coast and Southwest/Southern Rockies...


...Pattern Overview and Guidance/Predictability Assessment...

To commence the period, highly amplified flow with anomalous
height centers will encompass the country. Relative to
early/mid-October climatology, the deep upper low over the
southwestern U.S. should feature 500-mb height departures around 3
to 3.5 sigma while the western Atlantic 594-dm ridge will feature
anomalies around 2 sigma. Models continue to advertise the
sprawling upper low should eject northeastward toward the Central
Plains by Tuesday/Wednesday inducing a modest area of low
pressure. Notable deepening is anticipated as the cyclone lifts
toward the Upper Midwest and crossing the international border
into Ontario. The southern extension of height falls should
dislodge the eastern U.S. ridge placing it out into the Atlantic.
Shifting focus back to the western U.S., replenishment of the mean
upper trough is expected as lower heights descend from British
Columbia. Unlike the preceding upper low, the next surge of
shortwave energy will dig through the Central Great Basin before
the longwave trough gradually drifts eastward in time. In its
wake, ensemble means suggest an upper ridge settling over the
eastern Pacific although operational model runs support some
shortwave features intruding on this configuration.

Considering tropical developments, what is currently Hurricane
Sergio will continue its westward progression through the weekend
before making a quick north and then northeast turn back toward
North America. At this point, the remains of Sergio and its
moisture fluxes would reach Baja California and eventually the
southwestern U.S. Farther east, an area of disturbed weather
currently being monitored by the National Hurricane Center is
likely to lift into the Gulf of Mexico early next week. During the
next 5 days, a 60 percent chance of tropical cyclogenesis is noted
in the latest outlook.

Model spread is a bit higher than usual during the period which
suggested below average confidence in the forecast from Day
5/Wednesday onward. Beginning with the upper low ejecting out of
the Four Corners, previous runs of the GFS continue to be quicker
with the trough and wave progression. Its solution appears to be
on its own but all of the guidance suggest cyclogenesis unfolding
perhaps 12 to 18 hours later. Ensemble low plots are scattered
about along the north-central U.S. with the 00Z ECMWF being
significantly displaced to the west relative to the better
ensemble clustering, particularly by Day 6/Thursday. Considering
the mean longwave trough position, the past couple of GFS runs are
decidedly quicker although the 06Z cycle did slow down a bit. The
past few cycles of the ECMWF and 00Z CMC are more aligned favoring
a frontal passage along the I-95 corridor by the end of the
period. Moving down to the tropics, while there is reasonable
clustering with Sergio's movement toward Baja California, models
are struggling with the Gulf system. The 00Z UKMET is quick and
deep taking a strong low into the western Florida panhandle while
the 00Z CMC has stronger operational model support back toward the
central Gulf Coast. However, as usual, its solution is deeper than
most others available, generally aligned with the 06Z FV3-GFS. The
ensemble low spread is rather high with the ensemble means
actually sitting off to the east near the Florida panhandle.
Eventually many forecasts outside of the GFS favor stalling the
cyclone given a slower mid-latitude trough progression.

Given the uncertainty looming throughout much of the period,
decided to keep a large chunk of overnight continuity in the mix
while taking an even split of operational and ensemble guidance
for the remainder of the blend. This includes the 06Z GFS and 00Z
ECMWF/CMC with the following ensembles: 06Z GEFS and 00Z
ECMWF/NAEFS means.


...Weather Highlights/Threats...

Heavy rainfall across the Southern/Central Plains should continue
to pose an issue, particularly given the compromised state of
soils over the region. This multi-day heavy rainfall event will
continue from this weekend into the following week which suggests
flash flooding is possible over many locations the next several
days. However, the frontal zone is forecast to be on the move
shifting the focus of moderate to locally heavy rainfall to the
Mississippi River Valley with a better focus near the surface low
moving into the north-central U.S. With substantial cold air in
place, wintry precipitation is likely over sections of the western
Dakotas extending back into the Northern Rockies. In addition to
the mid-latitude system threats, the tropics will also provide
additional issues. Given the current track of Sergio, appreciable
moisture advancing into the Desert Southwest and Southern Rockies
will raise the probabilities for rainfall by late next week. And
across the central to eastern Gulf Coast, substantial moisture
coupled with energetics with the low itself could spread heavy
precipitation to the region by mid/late next week.

With the amplified flow in place, rather marked temperature
differences will be likely as readings around 20 to 30 degrees
below climatology are expected across the Central/Northern Rockies
into the adjacent Great Plains. Over the mountain locations,
overnight lows should plunge into the teens and 20s for multiple
nights. Farther east, given the mild air in place south and east
of the frontal zone, temperature anomalies around 10 to 15 degrees
from average are anticipated, especially across the Ohio Valley
and Great Lakes region. Even larger departures from climatology
are noted for the overnight minima with widespread daily records
likely anywhere within the vast warm sector. This is in response
to the humid conditions which will limit the amount of radiational
cooling occurring each night.


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