Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sat May 04 2019
Valid 12Z Tue May 07 2019 - 12Z Sat May 11 2019
...Overview...
Various details remain uncertain but guidance is continuing to
suggest that a strong eastern Pacific ridge will promote western
U.S. troughing, while a broad/flat ridge will likely be oriented
near or just offshore the East Coast. An ejecting Southwest U.S.
upper low Tue onward and associated surface wave/frontal system,
followed by slow movement/possible stalling of the trailing front
as it becomes aligned with southwesterly flow aloft, will produce
a heavy rainfall threat over parts of the Plains and Mississippi
Valley. Some affected areas will likely already have wet ground
thus elevating flash flood concern. A less extreme portion of
this moisture should extend into the East, while the ejecting
Southwest upper low and subsequent larger scale troughing should
generate terrain enhanced precipitation over parts of the West.
...Guidance Evaluation/Predictability Assessment...
In the Tue-Wed time frame there had been considerable uncertainty
over details across the northeastern quadrant of the Lower 48 into
Canada. Starting with yesterday's 06Z GFS there has been better
agreement in principle on the more suppressed scenario which the
majority cluster had shown 24 hours ago. There has still been
somewhat of a southward trend with the northeastern front dropping
into the Mid-Atlantic.
As the ejecting Southwest U.S. upper low reaches the Rockies and
beyond, guidance displays typical divergence for timing--due in
part to differences that develop within the amplifying western
trough. Overall prefer an intermediate timing at the moment, at
least leaning away from the slow side of the spread (latest GFS
runs) as the feature is carried along by progressive southwesterly
flow aloft. On the other hand slower trend of the 00Z CMC leaves
the UKMET as the fastest solution. Over the past day guidance has
trended stronger for the surface wave reaching the Midwest by
early Thu, reflecting better definition of the upper system. Also
worth noting, recent GFS runs are stronger than other models with
a leading eastern U.S. wave at the surface and aloft Wed-Thu.
The rest of the forecast involves more broadly dispersed
models/ensemble members for specifics. GFS/FV3 GFS runs are among
the more western or southwestern solutions for the upper low that
may close off within the western U.S. trough aloft. On the other
hand the 12Z ECMWF mean is one of the most aggressive to knock
down the upstream ridge toward the end of the period, leading to
faster trough progression. The 12Z ECMWF is similar with the
trough in spite of holding onto a little more ridge upstream.
There is a general signal for some erosion of the ridge though.
Toward the end of the period a compromise among the faster (12Z
ECMWF/ECMWF mean) and slower (18Z GEFS mean/old 00Z ECMWF)
solutions would provide a reasonable starting point--and is close
to the 12Z CMC mean.
Separate troughing (with possible embedded low) over southern
Canada and extending into the northern tier of the Lower 48 will
provide added forecast challenges, especially during the latter
half of the period. Thus far guidance has not been very agreeable
or consistent with the associated wave(s)/front(s) reaching the
northern U.S.--maintaining low confidence in any specific solution.
For about the first half of the period a multi-model blend
emphasizes the most common elements of guidance. Then the
forecast shifts toward a blend of the 18Z GEFS/12Z ECMWF means
(more of the former due to aforementioned issue with the eastern
Pacific ridge) and past two ECMWF runs.
...Weather Highlights/Threats...
Continue to expect a threat of heavy rainfall over the
central/southern Plains and Mississippi Valley. Less extreme but
still significant rainfall may extend into the the Ohio/Tennessee
Valleys and Northeast/Mid-Atlantic but with lesser confidence on
specifics. The central/north-central Plains activity will likely
take place during the first half of the period. A longer duration
of heavy rain/thunderstorms will be possible farther south--in
advance of the system emerging from the West into the Plains and
then late week into the weekend as persistent low level flow from
the Gulf interacts with a front that may stall for a time over the
Lower Mississippi Valley/western Gulf Coast. The wave reaching
the East late in the week will provide the primary focus for
enhanced rainfall over that part of the country.
Across the West, anticipate varying intensity of precipitation
from California through the Great Basin/northern Arizona and into
most of the Rockies as the upper low tracks through southern areas
early in the week and the upper trough/possible embedded upper low
develop over the West later in the week. Some of the
precipitation should be in the form of snow over high elevations
of the northern-central Rockies. Depending on exactly how
features evolve and time of day, some snow could extend into the
north-central High Plains. Highest precip totals are most likely
over favored terrain within the Rockies from Wyoming to northern
New Mexico. Some locations farther west may see significant
amounts as well.
Chilly air pushing southward over the Plains may bring highs down
to 15-25F below normal over north-central/central High Plains on
Wed-Thu, with a broader area of below normal readings over the
central U.S./Rockies and Southwest during most of the period. The
system tracking out of the Southwest will bring warmth into the
East later in the week with low temperatures in particular 10F or
greater above normal. The Pacific Northwest should see multiple
days with highs 10-20F above normal.
Rausch
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