Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun May 05 2019
Valid 12Z Wed May 08 2019 - 12Z Sun May 12 2019
...Overview...
Expect active weather for one or more days of the period from
portions of the central/southern West through a significant part
of the central/eastern states. A storm emerging over the Plains
on Wed and tracking northeastward thereafter will produce a warm
sector severe weather threat and one area of potentially heavy
rainfall from the Midwest into the Northeast. Locations from the
eastern half of Texas through the Lower Mississippi Valley and
possibly into the Tennessee Valley will be of greater concern for
a significant multi-day heavy rainfall event. The front trailing
from Plains low pressure will generate initial convection and then
likely stall near and northeast from the western Gulf Coast
Fri-Sun, interacting with impulses embedded in southwesterly flow
aloft and a steady flow of Gulf moisture. At the same time upper
troughing over the West will promote terrain-enhanced
precipitation from parts of California into the Rockies.
...Guidance Evaluation/Predictability Assessment...
From the eastern Pacific into western North America, guidance is
still beginning to establish western U.S. troughing aloft by
midweek in response to a strong eastern Pacific ridge. However
later in the week most solutions are showing a trend toward the
ridge extending farther into southwestern Canada and the Pacific
Northwest. This leads to a farther southwest track of a potential
embedded upper low (advertised by some earlier GFS runs), with the
added complication that this energy could interact with a separate
southern stream upper low that starts the period around 130-140W
longitude. The best consensus/average among models and means thus
far would place the center of the upper low near southern Arizona
as of early day 7 Sun--slower than the 18Z/new 00Z GFS runs but
faster than the new 00Z CMC. Meanwhile after early Fri models and
ensemble members continue to become chaotic in depicting exactly
how northern Pacific flow will erode the eastern Pacific ridge.
At the very least the 12Z ECMWF mean has made a favorable
adjustment toward slower weakening of the ridge versus its run
from 24 hours ago. The wide spread favors a blended approach with
increasing ensemble mean emphasis.
A consensus approach has held up fairly well for the storm
tracking from the Plains into Midwest Wed-Thu. Faster solutions
from 1-2 days ago (CMC/UKMET and to some degree ECMWF) have
trended slower while the GFS has trended a bit faster from its
prior slow timing. The forecast after early Thu becomes dependent
on more variable central Canada into northern U.S. upper trough
details. Trends have been faster within this northern
stream--leading to a more phased evolution aloft and
stronger/farther northward low pressure reaching into Canada while
a triple point wave may break off east of New England. This
evolution was becoming evident in yesterday's 00Z guidance and
subsequent runs have been increasing the confidence in such a
scenario.
Aforementioned issues with incoming Pacific flow after Fri along
with model/ensemble spread and run-to-run variability do not lend
much confidence to details of trailing flow within and around the
central Canada/northern U.S. mean trough aloft. Best clustering
would have a Canadian wave anchoring a front that drops into the
northern U.S. during the latter half of the period.
The first half of the forecast reflected the most common ideas (or
average depending on the system) among the 12-18Z GFS and 12Z
ECMWF/CMC. The rapidly increasing detail uncertainty over some
areas led to increasing emphasis on the 18Z GEFS/12Z ECMWF means
by next weekend. The CMC and 18Z GFS contributions were phased
out before day 7 Sun.
...Weather Highlights/Threats...
The storm tracking northeast from the central Plains Wed onward
will produce various hazards. These include heavy rainfall
potential over the Midwest into the Great Lakes with meaningful
rain likely extending into parts of the Northeast, locally heavy
convection from the southern Plains into Lower Mississippi Valley,
and a threat of severe weather that may extend from the southern
half of the Plains into the Ohio Valley/Lower Great Lakes.
Consult Storm Prediction Center products for latest info on the
severe weather forecast. Localized pockets of snow are possible
in the northern fringe of the precip shield with best
probabilities (albeit low in absolute terms) over the western UP
of Michigan and northeastern Wisconsin.
Areas from eastern Texas through the Lower Mississippi Valley and
possibly farther northeast will see a continued threat of periods
of heavy rain into the weekend as low level Gulf moisture feeds
into the trailing front that likely stalls near and northeast from
the western Gulf Coast and southwesterly mean flow aloft carries
along convection-enhancing shortwaves. For the full five-day
period model/ensemble guidance shows very high probabilities of 2"
or greater accumulations over this area and models are suggesting
some 5-12" totals.
Over the western U.S. some details will likely keep changing as
consensus flow aloft evolves, but there is a consistent theme of
enhanced precipitation from California through the southern half
of the Rockies in association with the mean trough aloft.
Currently expect highest precipitation totals (rain and higher
elevation snow) over the Colorado/New Mexico portions of the
Rockies. Significant totals are also possible over Utah/northern
Arizona, while increased potential for an upper low to trace a
wider arc compared to previous days has raised amounts to some
extent over favored terrain in central/southern California. Some
of the Rockies snow could briefly extend into the High Plains
early in the period.
Chilly air behind the midweek Plains storm will bring a broad area
of minus 10-25F anomalies for highs through much of the central
U.S. The most extreme temperatures within that range should be
over the High Plains. On the other hand southerly flow ahead of
the storm will bring warmth into the East, with greatest anomalies
for morning lows (plus 10-20F). The Pacific ridge aloft building
into the Pacific Northwest will lead to multiple days of highs
10-20F above normal over that region--possibly approaching record
values at some locations.
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