Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1222 PM EDT Mon Sep 30 2019
Valid 12Z Thu Oct 03 2019 - 12Z Mon Oct 07 2019
...Central U.S. heavy rain threat diminishes late this week but
could return/extend eastward by the weekend and early next week...
...Record warmth continues for the Southeast into Thursday...
...Overview and Guidance/Uncertainty Assessment...
The trend toward less amplified and more seasonable mean flow
aloft will continue during the period with height anomalies
becoming fairly modest over the lower 48. Within this pattern, an
upper low crossing the Great Lakes on day 3 Thu (energy originally
associated with the deep trough/low now over the West) will bring
fairly vigorous low pressure across the Northeast while the
trailing cold front suppresses the heat over the South/East. Then
a shortwave progressing eastward from the West Coast will support
a front that crosses the western/central states and mostly likely
reaches near the Appalachians by next Mon. The greatest
uncertainty in the forecast arises by days 6-7 Sun-Mon, with
respect to details of eastern Pacific/western North American flow
and resulting influence on shortwave energy that comes in behind
the initial West Coast trough. Details of this evolution will
determine central-eastern U.S. frontal progression, potential
waviness, and rainfall extent/intensity during that time. Another
question mark will be with the evolution aloft over/near the Gulf
of Mexico with guidance not particularly consistent thus far.
For days 3-5 Thu-Sat the primary guidance difference involved the
00Z-06Z GFS runs being a little flatter and faster than consensus
with the West Coast to central North America trough. An average
of operational models looked good for the system crossing the
Northeast. Therefore this part of the forecast emphasized
operational model input but with somewhat less GFS weight than
usual to account for the western trough consensus.
By days 6-7 Sun-Mon solutions diverge for the amplitude and
longitude of northeastern Pacific troughing, in turn affecting
downstream ridging as well as trough energy farther east over the
lower 48. The one common theme in the D+8 multi-day mean charts
is a strong positive height anomaly center near the central
Aleutians. Teleconnections relative to this feature show the
northeastern Pacific trough seen in most guidance, but at most
just a hint of a western ridge/eastern trough--not nearly as
extreme as seen in the 00Z ECMWF (with recent runs not very
consistent either). Among remaining guidance the GFS/GEFS
mean/CMC mean are somewhat faster than the ECMWF mean/CMC with the
northeastern Pacific trough, but solutions do not cluster the same
way for the central-eastern U.S. trough. For now the
teleconnection guidance recommends a conservative approach to the
overall pattern and emphasis on the ensemble means. The
GEFS/ECMWF means each have merit based on telecons, the former for
bringing some negative height anomalies into western Canada and
the latter for its modest western ridge.
...Weather Highlights/Threats...
Great Lakes through Northeast low pressure Thu-Thu night will
bring focused rainfall to areas near/north of the low track.
Rainfall along the trailing front will diminish except for some
activity persisting over the southern Rockies/High Plains where
the front will stall for a day or so. The system emerging from
the West and into the Plains should bring scattered rain/high
elevation snow to northern parts of the West late this week and
then support some areas of locally moderate to heavy rain over the
central U.S. by this weekend. The forecast becomes more uncertain
by Sun-Mon given poor agreement regarding upper level flow details
that will determine frontal timing/waviness and how much moisture
may flow northward from the Gulf of Mexico. At the very least
there is the potential for some areas to see significant rainfall
but at this time confidence is very low for specifics. Locations
along the eastern half of the Gulf Coast may also see periods of
enhanced rainfall with moisture around the periphery of Gulf of
Mexico low pressure and near a potential upper level weakness/low.
Record warmth over and a little north of the Southeast should
continue into Thu before temperatures finally moderate. Expect
unseasonably cool readings over the Northwest (10-20F below
normal) to persist late this week and then trend closer to normal
first over the Pacific Northwest and then eventually over the
northern Rockies.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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