Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Mon Sep 9 2019
Valid 12Z Thu Sep 12 2019 - 12Z Mon Sep 16 2019
16Z Update: There is overall good model agreement across the
continental U.S. for the end of the week. Some of the EC ensemble
members are supporting a deeper upper trough over the Upper
Midwest on Friday, and some of the CMC ensemble members are
indicating a stronger trough near the Pacific Northwest. Going
into the weekend, both the 00Z and 6Z runs of the GFS begin to
differ quite a bit from the model consensus, and then the GFS
becomes completely out of phase with the upper level pattern and
considered an outlier solution when compared to the ensemble
plots, even when compared to GEFS ensemble members. These
differences emerge from flow separation in association with an
upper level trough tracking east from Asia and across the northern
Pacific early in the forecast period. The GFS develops an upper
level low that becomes detached from the westerlies, whilst the
CMC/ECMWF/ensembles are more progressive. This results in an
upper level trough centered near the West Coast, in stark contrast
to the ridge depicted by the GFS. Similar to the previous
forecast, the WPC medium range forecast is derived primarily from
a deterministic blend of the ECMWF/CMC/UKMET through early
Saturday, and then more of the GEFS and EC means by Sunday and
Monday with no GFS used.
Hamrick
...Overview and Guidance Evaluation/Preferences...
An upper-level ridge is forecast to remain rather entrenched from
the lower Mississippi Valley to the Southeast through much of the
medium range. Meanwhile, a stream of relatively vigorous upper
shortwaves are forecast to traverse the CONUS northern tier.
Farther south, an easterly wave is forecast to approach Florida
and the coastal southeastern U.S. by the weekend, traversing the
southern periphery of the expansive upper ridge.
A blend of the 12Z ECMWF/CMC/18Z GFS was used for the forecast
initially during days 3-4 (Thu-Fri). An energetic upper-level
shortwave and surface low pressure system crossing the north
central U.S. were handled comparably by these solutions
(particularly the ECMWF and GFS) and a blend should work quite
well. Farther west, a leading shortwave off the Pacific Northwest
Thu, and a larger amplifying upper trough in its wake continued to
pose the most significant forecast problems this cycle. The
problems begin as soon as day 4 when the GFS starts to become
faster than consensus with the leading wave, and the differences
only grow into day 5 (Sat) as the GFS is much more amplified than
other guidance with the shortwave across the northern Rockies. In
the wake of the leading wave, the ECMWF/CMC both amplify a fairly
deep upper-level trough, bringing significant height falls into
the Pacific Northwest by next weekend, while the GFS keeps zonal
flow in place - a major difference. Given a relatively amplified
flow regime in place farther west across the North Pacific,
continue to prefer the deeper trough along the U.S. West Coast by
days 6-7 as shown by the ECMWF/CMC. Interestingly, the ECMWF had
previously shown a solution more like the GFS but has trended more
toward the CMC (which has been consistently more amplified) in
recent runs. Thus, the forecast during days 5-7 (Sat-Mon) was
based on a blend of the 12Z ECMWF/CMC and gradually increasing
ensemble means (ECENS/NAEFS) through time to account for
increasing spread.
...Weather Highlights/Threats...
Moderate to locally heavy rain will be possible across portions of
the Upper Midwest Thu-Thu night in association with a surface low
pressure system crossing the region. Farther south, scattered
showers and thunderstorms may produce locally heavy rains along
the cold front Thu-Fri from the Southern Plains to the
Mid-Mississippi Valley. Scattered showers and storms will move to
the East Coast states by next weekend along with the cold front.
The easterly wave approaching the Southeast and Gulf Coast states
by late in the week is expected to increase shower and
thunderstorm activity, with locally heavy rains possible.
Increasing rain, potentially moderate to heavy, will also be
possible across the mountain ranges of the Pacific Northwest by
late week as upper troughing progresses into the region.
Well above average temperatures are expected to persist across
much of the central and eastern U.S. through late this week. High
temperatures are forecast to be 5 to 15 degrees F above average
across a large area from the Ohio Valley to the lower Mississippi
Valley and the Southeast. Several record high temperatures are
possible each day, especially from the lower Mississippi Valley to
the Southeast where highs may soar well into the 90s, perhaps
approaching 100 degrees at some locations.
Ryan
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