Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 AM EDT Mon May 06 2019
Valid 12Z Thu May 09 2019 - 12Z Mon May 13 2019
...Active springtime pattern for parts of the Lower 48...
...Overview...
Significant precipitation will be in the forecast across portions
of the southern and eastern U.S. during the period. A storm
system reaching the Midwest by the start of the period early Thu
and tracking northeast with time will bring one area of enhanced
rainfall across the Great Lakes/Northeast late this week, along
with a threat of strong to severe convection in the warm sector.
Meanwhile there is still a pronounced signal for heavy rainfall
potential from eastern Texas through the Lower Mississippi Valley
and to some degree extending farther northeastward. This threat
corresponds to guidance continuing to show the Midwest storm's
trailing front stalling near and northeast from the western Gulf
Coast, with some degree of low level Gulf inflow providing
moisture while impulses progress along in southwesterly flow
aloft. At the same time upper troughing over the West will bring
multiple days of precipitation to locations from central-southern
California into the southern half of the Rockies with highest
totals over areas with favorable terrain enhancement.
...Guidance Evaluation/Predictability Assessment...
The majority of guidance shows a similar theme for the evolution
of mean flow aloft, an eastern Pacific ridge building into the
Northwest states while split flow prevails to the southeast--a
mean trough settling over east-central Canada into the Upper
Midwest/Great Lakes and a developing upper low tracking near the
western U.S.-Mexico border. The relative agreement in mean flow
conceals significant differences/uncertainty in some day-to-day
details.
Greatest differences over the past couple days have involved
northern steam flow, both within the mean trough over southern
Canada and northern U.S. as well as North Pacific flow heading
toward/into North America. The most noticeable multi-day trend
has been for stronger ridging to build over the Northwest and to
persist for a longer time. Uncertain details of North Pacific
flow rounding the ridge continue to cause difficulty in resolving
the timing and amplitude of a potential surface low/frontal system
dropping from Canada into the northern U.S. late week onward.
Based on the full array of guidance GFS runs may be somewhat
strong/fast with the system but the 12Z ECMWF/ECMWF mean are
slowest/weakest. The old 00Z ECMWF was somewhat closer to other
guidance. Across this expanse of northern latitudes a
blend/compromise approach provides reasonable continuity while
reflecting latest trends for the Northwest ridge aloft.
Guidance still shows a complex evolution taking place along and to
the south of the California coast, as Interior West energy pushes
southwest to interact with an offshore upper low moving eastward
along 30N latitude. There will be various ways that this
evolution could ultimately stray from consensus but so far the
majority/intermediate solution has been consistent in showing the
center of the upper low reaching near the southern Arizona border
by Sun. Given the ridge building over the Northwest, suspect the
ECMWF/ECMWF mean may be too quick to open up/eject the feature
after Sun. The new 00Z GFS/GEFS mean maintain support for slower
timing versus ECMWF-based solutions by Mon.
For the storm tracking northeast from the Midwest, UKMET runs are
notable for being on the strong/northwest fringe of the model
spread. Otherwise consensus has trended a bit weaker from prior
cycle but to a degree that is generally within the noise range.
Most solutions continue to show a triple point wave off New
England by early Sat. Along the trailing front that will become
the focus for heavy rainfall over the South, some uncertainty in
waviness/frontal position arises toward Sun-Mon in response to
details of the upper trough to the north.
For the full forecast domain a general model blend early in the
period trending toward a blend among the 18Z GFS/GEFS mean and 12Z
ECMWF/ECMWF mean provided a reasonable starting point for
representing the desired consensus or intermediate solution for
features of interest. This approach provided fairly good
continuity while awaiting any possible adjustments over areas of
greatest uncertainty.
...Weather Highlights/Threats...
The storm tracking northeastward from the Midwest Thu-Fri will
produce an area of rainfall that may be heavy at times over the
Great Lakes/Northeast as well as thunderstorms (some strong to
severe) in the warm sector from the southeastern Plains/Lower
Mississippi Valley into the Ohio Valley/Lower Great Lakes, and
possibly extending farther eastward with time. Consult Storm
Prediction Center products for latest info on the severe weather
forecast. In the northern part of the precip shield there may be
localized areas of snow at times, with best potential (though with
fairly ow probability in absolute terms) over the western UP of
Michigan and extreme northern Wisconsin.
The favorable setup of a stalling front, low level Gulf inflow,
and impulses aloft carried by southwesterly mean flow, are
maintaining the threat for heavy rain across parts of the South.
The best potential for highest 5-day totals still exists over
eastern Texas through Louisiana with significant totals also
extending farther northeastward. Guidance continues to show the
possibility for at least 5-10" accumulations at some locations.
The front may drift a little southeastward late in the period to
tone down the heavy rainfall threat.
From Fri onward a system dropping out of Canada may bring mostly
light to locally moderate rainfall to some locations from the
northern Plains into the Great Lakes. Thus far guidance has not
been sufficiently consistent or similar to provide meaningful
confidence in timing/coverage of precipitation.
Within the area of terrain-enhanced precipitation from
central-southern California eastward into the Rockies, the best
potential for higher elevation snow will be centered over the
Rockies of Colorado Thu into Fri before heights aloft rise over
that area. There is still uncertainty over details of the upper
low that may track along the southwestern U.S./northern Mexico
border but the favored average of guidance during the weekend into
early next week yields a fairly consistent forecast for the time
being. Rainfall may increase in coverage/intensity over the
southern High Plains early next week as the feature approaches.
The upper ridge building into the Northwest will bring multiple
days of well above normal temperatures--generally plus 10-20F
anomalies and maybe locally higher. Some daily records for
highs/warm lows will be possible. On the other hand much below
normal highs (minus 15-25F anomalies) will push down through the
central/southern High Plains late this week and slowly moderate
thereafter. To a less extreme extent the Southwest will see cool
daytime highs as well. The East will be warm late this week ahead
of the Midwest system, followed by a trend toward more normal
readings.
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