Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
133 AM EDT Fri Apr 19 2019
Valid 12Z Mon Apr 22 2019 - 12Z Fri Apr 26 2019
...Overview and Guidance Preferences...
Upper-level flow across North America 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. Overall, models have struggled with a
similar flow pattern for the past week or two, and this certainly
seems to be the case going forward as well. Predictability
reductions resulting from timing/amplitude differences within each
stream are further amplified by potential interaction (or lack
thereof) between the streams. As a result,
confidence/predictability at the beginning of the extended
forecast period is average at best, and quickly decreases to well
below average by later in the period.
A multi-model deterministic blend (including the 12Z
ECMWF/UKMET/CMC and 18Z GFS) was favored during days 3-4
(Mon-Tue). Deterministic solutions and ensemble members vary with
respect to how quickly a compact upper low initially across the
Mid-Atlantic region on Mon will lift out, but a consensus
solutions close to the ensemble means was preferred. A trend
toward a somewhat slower progression of this feature Mon-Tue was
noted among much of the guidance and ensemble members over the
past couple runs. This process is related to potential interaction
with progressive northern stream energy crossing north of the
Great Lakes on Tue, which also shows timing/amplitude differences
among the guidance. Farther west, models are in relatively good
agreement that another slow-moving southern stream system should
cross the Four Corners region Mon-Tue, with differences (at least
early on) confined to how amplified the feature becomes.
From day 5 (Wed) onward, model spread quickly grows with respect
to the southern stream feature approaching the southern/central
plains by that time. The precise timing/amplitude of the feature
(which remains uncertain) will play a significant role in whether
or not any interaction occurs with progressive northern stream
flow and ultimately how quickly the feature moves eastward. A
significant trend was noted in the ensemble guidance toward a
slower/deeper southern stream system with less northern stream
interaction. This has been a trend among the guidance with a
couple recent similar systems, so given the trend was inclined to
lean more heavily toward that idea, as shown by the 12Z ECMWF and
ECENS mean. Additionally, models generally agree on another round
of Pacific shortwave energy approaching the West Coast by Thu or
Fri, although given the time range, solutions vary widely on the
nature and timing of the feature. What does seem at least a bit
more clear is that a strong upper-level jet should be accompanied
by height fall and a surface cold front pushing into the
northwestern U.S. by Thu-Fri. Given these considerations, the
forecast during days 5-7 was trended heavily toward ensemble means
(the ECENS mean in particular), with some continued weight from
the deterministic ECMWF as well.
...Weather Highlights/Threats...
The southern stream system slowly moving from the Southwest into
the central U.S. is expected to produce widespread showers and
thunderstorms with potentially heavy rainfall. The most
widespread/heaviest rains should be across the southern/central
plains by Mon night/Tue, and spreading east toward the Gulf Coast
and lower Mississippi Valley by Wed-Thu. Meanwhile, well above
average temperatures are expected across portions of the western
U.S. from Tue onward, with high temperatures expected to be 10-20
deg F above average for some areas. The warm temperatures should
gradually spread east toward the central U.S. by later next week,
with a cold front bringing a return to near normal temperatures
for the Pacific Northwest.
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