Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
142 AM EDT Sat Apr 20 2019
Valid 12Z Tue Apr 23 2019 - 12Z Sat Apr 27 2019
...Overview and Guidance Preferences...
Upper-level flow during the extended period will be largely
separated into a progressive northern stream across Canada and the
U.S. northern tier, and a much less progressive southern stream,
with the potential/tendency for systems to close off aloft. Models
continue to struggle with this flow pattern, with predictability
reductions resulting from timing/amplitude differences within each
stream further amplified by potential interaction (or lack
thereof) between the streams. As a result, forecast
confidence/predictability quickly decreases to below average by
the latter portion of the forecast period.
Models have settled on a slower progression of a compact upper
low/surface low pressure system away from the northeastern U.S.
Tue-Wed, and solutions now handle this feature comparably. The
evolution/progression of an upper low initially across the Four
Corners region on day 3 (Tue) remains the most significant problem
for this forecast period. Deterministic solutions handle this
feature similarly through early Wed as the feature elongates and
moves into the Southern Plains. Guidance has overall trended
slightly quicker with the progression of this system than the
ECMWF showed at this time last night, but remains slower than some
of the much more progressive solutions shown previously by the
GFS. Differences continue to grow by Thu-Sat, with the GFS
continuing to favor a more dominant/amplified northern stream and
a much more progressive southern stream wave, quickly crossing the
Southeast by Thu night. The ECMWF continues to favor more
separation between streams, keeping a closed upper low across the
lower Mississippi Valley/Southeast through Fri-Sat. Given
observation of previous similar systems over recent weeks,
continue to favor more separation between streams, and a solution
more along the lines of the ECMWF and ECENS ensemble mean. Farther
west, there is general agreement among the guidance that northern
stream height falls should approach the Pacific Northwest by
Fri-Sat, with timing/structural differences among deterministic
solutions by that time favoring an ensemble approach. The same
holds true for another round of southern stream energy potentially
nearing California by Sat.
Given these considerations, the forecast during days 3-5 (Tue-Thu)
was based on a multi-model (ECMWF/GFS/CMC) deterministic blend,
with a shift to majority weighting of ECENS/NAEFS ensemble means
during days 6-7 (Fri-Sat).
...Weather Highlights/Threats...
The system crossing the Southern Plains Tue-Wed is expected to
produce widespread showers and thunderstorms, with models
indicating the potential for heavy rainfall from central Texas
north into Oklahoma. Scattered to numerous showers and
thunderstorms are expected to accompany the system eastward into
the lower Mississippi Valley and Southeast Thu-Fri, but any signal
for heavy rainfall is much less clear in the guidance given
increased spread on the specific evolution of the system. Assuming
the slower/more amplified solutions currently favored verify,
would expect the continued potential for at least areas of locally
heavy rainfall across portions of the Gulf Coast and Southeast.
Above average temperatures are expected through much of the period
across portions of the western U.S. as upper ridging builds in
behind the southern stream trough. High temperatures are forecast
to be 10 to 20 deg F above average from California to the Great
Basin on Tue, with warm temperatures progressing east into the
Rockies and portions of the Central Plains Wed-Thu. The arrival of
a cold front by later in the week should bring a return to near or
slightly below normal temperatures to the Pacific Northwest and
northern Great Basin.
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