Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sun May 05 2019
Valid 12Z Wed May 08 2019 - 12Z Sun May 12 2019
...Busy springtime pattern for much of the Lower 48...
...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. The Storm Prediction
Center has outlined an area over the southern Plains to
Missouri/Arkansas on Wednesday and then from the ArkLaTex
northeastward to the Midwest on Thursday for the potential for
severe weather. 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. Marginally cold air at higher elevations
supports the chance for snow Wed-Fri as the lead upper trough
moves through.
...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/northwestern Mexico as of early day 7 Sun--slower than the
00Z GFS but faster than the 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. Spread favors a blended approach with increasing
ensemble mean emphasis at later times.
A consensus approach has held up fairly well for the storm
tracking from the Plains into Midwest Wed-Thu, but the latest
00Z/06Z GFS runs were displaced a bit south/east of the
ECMWF/UKMET/Canadian consensus early Thu into Fri. Preferred this
non-GFS blend given the better ensemble clustering to its north
but the GFS runs do come back into agreement as the low moves into
Canada. The forecast after early Thu becomes dependent on more
variable central Canada into northern U.S. upper trough details
where the GFS was quicker than the rest with lead height falls
preceding the system, which may tarnish its forecast thereafter.
Overall trend still points to a stronger/farther northward low
pressure reaching into Canada while a triple point wave may break
off just east of New England.
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, but there
remains modest spread in track/intensity of this front.
The first half of the forecast reflected the most common ideas (or
average depending on the system) among the 00Z
ECMWF/UKMET/Canadian. The rapidly increasing detail uncertainty
over some areas led to increasing emphasis on the 06Z GEFS/00Z
ECMWF means by next weekend along with the previous forecast.
...Weather Highlights/Threats...
The storm tracking northeast from the central Plains Wed onward
will produce various hazards. These include:
1) heavy rainfall potential over the Midwest into the Great Lakes
with meaningful rain likely extending into parts of the Northeast
2) thunderstorms with an embedded severe threat from the southern
Plains into Lower Mississippi Valley that extends into the Ohio
Valley/Lower Great Lakes as the system moves northeastward.
Consult Storm Prediction Center products for latest info on the
severe weather forecast.
3) mountain snow over Colorado Wed/Thu with several inches likely
at higher elevations above 7000-8000ft but even some snow to
6000ft possible. More localized pockets of snow in the northern
fringe of the precip shield are possible with best probabilities
(albeit low in absolute terms) over the western UP of Michigan and
northeastern Wisconsin but also over western Nebraska early in the
period.
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 (please consult the latest day 4-7 winter
weather outlook probabilities).
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 especially Friday from Olympia southward
to Portland and Salem (mid to upper 80s).
Fracasso/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