Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1133 AM EDT Mon Apr 22 2019
Valid 12Z Thu Apr 25 2019 - 12Z Mon Apr 29 2019
...16Z Update...
Guidance somewhat trended in opposite directions over the
Southeast with respect to the exiting closed low (weakening to an
open wave) with the 00Z ECMWF and many of its ensemble members
quicker than the GFS/UKMET while the 00Z Canadian was mostly in
between. Opted to rely on the previous forecast as it sat nicely
in between the quicker/slower camps given no clear reason to
change as of yet. Next system out of the Gulf of Alaska into
southwestern Canada has mostly trended weaker in the guidance but
the GFS remained stronger than the rest, which resulted in
(likely) too much ridging into the Plains next Sun/Mon compared to
the ensemble consensus. Upper low approaching California and
weakening on Sunday remains rather well-forecast in the past two
model/ensemble cycles though the ECMWF has had better continuity.
Again, previous forecast with only very minor modifications proved
to be a best starting point, but with a few changes to later
periods in the temperature forecast based on the latest ECMWF
ensembles and National Blend guidance.
Previous discussion is below...
Fracasso
...Overview and Guidance Preferences...
Upper-level flow across the CONUS is not expected to change
significantly during the extended forecast period. A progressive
northern stream and less progressive southern stream, similar to
the pattern seen in recent weeks, is expected to persist.
Models/ensembles do hint at potential northern stream flow
amplification from the North Pacific into Canada by next weekend,
however. Development of a broad Rex block extending from Hawaii to
Alaska is expected, which will reinforce upper troughing centered
west of Hudson Bay. Conceptually, this setup seems to favor
potential southward transport of polar air into central Canada and
the north central U.S., and this bears out when teleconnections
associated with the Pacific Rex block are examined.
Models handle a southern stream trough initially across the
Southern Plains on day 3 (Thu) similarly through Friday, before
solutions begin to diverge on timing. Timing has been an issue
with this system in the guidance for several days, largely hinging
on the degree of separation between northern/southern streams and
amplification of northern stream shortwave energy crossing the
Great Lakes on Fri. In general, the trend has been toward more
separation of streams and a slower progression of the trough/upper
low across the Southeast through Sat. Farther west, timing of
northern stream shortwave energy digging into the northwestern
U.S. on Sat is a problem, with models showing a fair amount of
variability. In general, a multi-model deterministic blend
including the 12Z ECMWF/UKMET and 18Z GFS was used as a basis for
the forecast during days 3-5 (Thu-Sat).
After a few days of waffling, models/ensembles seem to have
largely settled on the idea of another cutoff southern stream
system approaching California Sat-Sun. The ECMWF/GFS are quite
similar on timing of the feature, at least through Sun, which also
has support from the respective ensemble means. By Mon,
differences emerge, with confidence quickly decreasing. The
culprit seems to once again be significant northern stream
differences. The GFS has been persistent in digging an amplified
upper trough into the Pacific Northwest by Sun-Mon, while the
ECMWF/CMC show a more subdued feature with weaker height falls.
Ensembles show a fair amount of disbursement, suggesting low
confidence in this feature. The northern stream evolution by Mon
also affects the southern stream feature, with a more amplified
trough in the Northwest kicking the southern stream wave inland
more quickly, and a less amplified trough keeping more stream
separation and an upper low lingering across the Southwest for
longer. The pattern in recent weeks would suggest that the idea of
more stream separation is more likely, but the broader northern
stream flow amplification described above suggests that the answer
may be less simple. Thus, leaned heavily toward ensemble means
(12Z ECENS/18Z GEFS) by days 6-7 (Sun-Mon), representing a
compromise solution across the western U.S.
...Weather Highlights/Threats...
Widespread showers and thunderstorms are expected from the lower
Mississippi Valley to the Southeast Thu-Fri in association with
the southern stream shortwave/upper low. Models suggest that at
least locally heavy rainfall is possible. The frontal boundary
associated with this system is expected to stall across the
Florida Peninsula over the weekend and linger into early next
week, which should keep scattered to numerous showers and
thunderstorms a possibility, along with locally heavy rainfall.
Farther west, rain and mountain snow will be possible across the
Rockies Thu night into Fri as shortwave energy passes overhead.
Precipitation should spread into the Northern/Central Plains Fri
night into Sat in association where a couple stationary frontal
boundaries will add to the lift generated by the upper-level
shortwaves. This precipitation should spread east into the
Midwest/Great Lakes by Sun, with colder air on the northern
fringes potentially leading to some chance for snows from northern
Minnesota east to northern Wisconsin and the Upper Peninsula of
Michigan. The arrival of additional northern stream energy should
bring another round of precipitation to the Rockies on Sun and
into the Central/Northern Plains Mon.
Temperatures are expected to be well above average from California
east to the central Rockies Thu-Fri, with highs expected to reach
10 to 20 deg F above average for many areas. These temperatures
anomalies may spread east into the Southern/Central High Plains
for Sat. Meanwhile, a cooler polar air mass should begin to spread
into areas from the northern Rockies to the Upper Midwest north of
a surface front next weekend, with high temperatures at least 5 to
10 deg below average expected. The flow across central Canada by
next weekend will play a significant role in temperatures across
the north central U.S., with some model solutions much colder than
currently forecast.
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