Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1143 AM EDT Sat Sep 28 2019
Valid 12Z Tue Oct 01 2019 - 12Z Sat Oct 05 2019
...Heavy rainfall possible from the Southern/Central Plains to the
lower Great Lakes by the middle of next week...
...Anomalous warmth to persist over the central to eastern U.S.,
while very chilly conditions moderate somewhat in the northern and
northwestern states ...
...Overview and Guidance/Uncertainty Assessment...
Model guidance this evening continues to indicate a deep upper low
over the northwestern U.S. will eject eastward toward the Great
Lakes during the medium-range period. This will lead to a more
progressive pattern across the northern tier states by the end of
next week as a strong upper ridge gradually weakens over the
Southeast. Meanwhile, another upper trough moving toward British
Columbia is forecast to amplify as it reaches the Pacific
Northwest late next week. A wavy front is forecast to gradually
move across the central and eastern U.S. next week, with a swath
of heavy precipitation possible from the central Plains to the
lower Great Lakes in the middle of the week.
A blend of the 00Z ECMWF/GFS/UKMET was used as a forecast starting
point through much of the forecast period, with most emphasis
among these three solutions placed on the ECMWF, which was
consistently well-centered in the ensemble spread at least through
days 5-6 (Thu-Fri). The 06Z GFS was much faster than the guidance
consensus with the eastward progression of shortwave energy across
the Northern Plains/Upper Midwest next Wed-Thu, and this solution
was discarded. Model consensus was relatively good with the next
amplifying trough expected to reach the West Coast Thu, with
variation among solutions largely confined to the amplitude of the
feature. As this feature moves inland from the northern Rockies to
the Northern Plains Fri-Sat, the ECMWF was the most well-centered
solution within the ensemble spread, and showed good agreement
with the ECENS/NAEFS means. Spread increases by late in the
forecast period (Sat) as another upper trough approaches the
Pacific Northwest with model run-to-run variability also
increasing. Thus, weighting of ensemble means was boosted
substantially by day 6 (Fri) and especially day 7 (Sat).
...Weather Highlights/Threats...
A wavy surface front will focus precipitation from the
Southern/Central Plains to the lower Great Lakes by midweek. Flow
around the western periphery of the strong ridge over the
southeastern U.S. will draw tropical moisture northward where it
will interact with the frontal boundary. The potential exists for
a few inches of rain in a band from the Central Plains to the
lower Great Lakes during the middle of the week, with flash
flooding a possibility. Meanwhile, deep-layer easterly flow and an
influx of deeper tropical moisture will promote an increase in
showers and thunderstorms over Florida.
Very chilly conditions over the western U.S. into Northern Plains
are expected to moderate somewhat by early next week. On the other
hand, many locations from the Gulf Coast to New England will be
averaging 10-20F above normal with numerous daily records expected
to be broken into late next week.
Ryan/Kong
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