Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Fri May 03 2019
Valid 12Z Mon May 06 2019 - 12Z Fri May 10 2019
...Overview...
The pattern evolution expected for next week may lead to
additional heavy rainfall over parts of the central U.S. that have
already received significant accumulations recently. Unsettled to
active weather will also be possible across other portions of the
Lower 48 for at least part of the period. Guidance suggests flow
aloft will amplify toward the latter half of next week, with a
building eastern Pacific ridge helping to develop a western U.S.
trough. Flat ridging aloft should settle over the East
Coast/western Atlantic by next Wed-Fri. The other major feature
of note will be an upper low that should reach California at the
start of the week and then open up/eject northeastward over and
beyond the Rockies.
...Guidance Evaluation/Predictability Assessment...
One of the less confident aspects of the forecast involves
shortwave energy progressing across southern Canada and the
northern U.S. from day 3 Mon onward. Among 12-18Z guidance, the
GFS/GEFS mean were on their own in being slower and stronger with
the upper dynamics--leading to surface low pressure tracking
through the Upper Midwest/Upper Great Lakes and wrapping up over
Canada. Remaining solutions were flatter and faster in varying
ways, resulting in a more suppressed wavy front over the Northeast
quadrant of the U.S. next Tue-Thu. Continuity/clustering have not
been very good over the past couple days, plus either scenario is
plausible from the multi-day mean point of view. Preferred to
blend toward the majority scenario, which also had support from
the 18Z FV3 GFS. Most of the new 00Z solutions still favor the
non-GFS idea with the GEFS mean showing a partial trend in that
direction as well.
Within the amplifying western U.S. trough there have been signals
for an embedded closed low by the latter half of the week but with
a fair amount of spread for location. Based on the trough axis in
the ensemble means and average of the past couple ECMWF runs (the
most compatible guidance), it appears the most likely path of an
upper low would be over or just southwest of the Great Basin.
The upper low moving through the Southwest will open up and eject
northeastward as the western trough begins to amplify. The
ejecting shortwave will likely produce a rainfall-enhancing
frontal wave tracking through and beyond the Plains mid-late week.
Clustering has been fairly good though the new 00Z GFS becomes
slower than most other solutions after early Tue.
Based on above considerations the first half of the period started
with an operational model blend (including two ECMWF runs given
the lack of confidence with northern stream details and associated
surface forecast). Then the forecast rapidly adjusted to a blend
of the 18Z GEFS/12Z ECMWF means with minority input from the two
ECMWF runs through day 7 Fri and the 18Z GFS through day 6 Thu
(after which time the GFS ejected part of the western trough
faster than desired).
...Weather Highlights/Threats...
Portions of the Plains and Mississippi Valley should see the
highest potential for heaviest total rainfall during the 5-day
period, especially across southern areas. A wave embedded within
a north-central U.S. front should promote one area of enhanced
rain/thunderstorm activity over the Plains/Midwest during the
first half of the week with lesser amounts of moisture extending
farther east. Meanwhile moist flow from the Gulf will intersect a
front initially draped near Gulf Coast, and then the ejecting
Southwest U.S. feature will bring a surface wave and trailing
front into the southern Plains. The threat for heavy rainfall may
persist over/near the southern Plains through Fri as the southern
Plains front likely decelerates/stalls as it becomes parallel with
flow aloft while low level moisture continues to feed in from the
Gulf. The combination of fronts/waves may bring some significant
rainfall amounts to the Northeast/Mid-Atlantic but likely with
lower totals than expected over the central U.S.
The western states will see a broad area of precipitation (from
California through the Great Basin/northern Arizona and into most
of the Rockies) in association with the upper low tracking through
the Southwest early in the week and the upper trough/possible
embedded upper low developing over the West later in the week.
Some of the precipitation may be in the form of snow over high
elevations of the northern-central Rockies. Expect highest precip
totals to be over favored terrain of the Great Basin and central
Rockies. Snow potential could extend into the extreme northern
Plains early in the week.
The greatest temperature anomalies during the period should be
with cool air pushing south from the northern High Plains, leading
to decent coverage of highs 10-20F below normal by Wed-Thu.
Highest warm anomalies, of similar magnitude, should be for
morning lows in the warm sector from the eastern/southeastern
Plains to Mid-Atlantic Tue onward and for highs over the Pacific
Northwest.
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