Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1206 PM EDT Fri May 03 2019
Valid 12Z Mon May 06 2019 - 12Z Fri May 10 2019
...Overview...
Much of the southern and central U.S. will remain in a mostly
unsettled and wet pattern as the flow aloft amplifies toward the
latter half of next week allowing a longwave trough to become
reinforced over the western U.S. An upper level low approaching
the California will track through the southern Great
Basin/Southwest through the extended period and will northeastward
over and beyond the Rockies. This may lead to additional heavy
rainfall over parts of the central U.S. that have already received
significant accumulations recently, which will keep the risk for
flash flooding elevated during this period. Flat ridging aloft
should settle over the East Coast/western Atlantic by next
Wednesday-Friday.
...Guidance Evaluation/Predictability Assessment...
Model guidance have struggled a bit with a shortwave feature that
will track south/southeast from the southern tier of Canada into
the northern U.S. states. As noted from the previous forecast, the
GFS/GEFS mean were 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. The
rest of the deterministic and means depicted flatter and faster
flow in varying ways, resulting in a more suppressed wavy front
over the Northeast quadrant of the U.S. next Tuesday-Thursday. The
latest runs show a slight improvement with the details of this
feature, however there is uncertainty.
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. Clustering was better with
the latest guidance and is suggesting that it will track
southwest/south of the Great Basin; the ejecting shortwave will
likely produce a rainfall-enhancing frontal wave tracking through
the Southern High Plains and into the Mississippi Valley and
beyond by midweek.
The preferred blend for the beginning of the extended forecast
consisted of the ECWMF and its ensemble mean, the GFS, UKMET and
the Canadian model. Weighting of the operational models decreased
with time as the weighting of the ECWMF ensemble mean and GEFS
Means increased.
...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.
Campbell/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