Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Mon May 06 2019
Valid 12Z Thu May 09 2019 - 12Z Mon May 13 2019
...Multi-day heavy rainfall threat for eastern/southeastern Texas
and the Lower Mississippi Valley later this week...
...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
(southeast Texas northeastward to the Midwest). Meanwhile there
is still a pronounced signal for heavy rainfall potential from
eastern/southeastern Texas through the Lower Mississippi Valley
and to some degree extending farther northeastward Thu through the
weekend. 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. Modest snow at the end of the short range over
Colorado will continue into the start of the medium range period
before diminishing.
...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 (with another attendant weak upper low
rounding its base). 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 00Z ECMWF/ECMWF mean continued
to be slowest/weakest. Across this expanse of northern latitudes
a blend/compromise approach provides reasonable continuity while
reflecting latest trends for the Northwest ridge aloft, with a
reliance on the previous shift that fell nicely between the spread
of quicker/slower solutions (GFS/ECMWF) somewhat near the 00Z
Canadian at times before it became slower than the ensemble
consensus. Low confidence for that system given its history.
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. Deterministic models generally agree on sliding the
farther west upper low eastward underneath the forming upper low
over coastal CA Friday then into NW Mexico and SE AZ early
Saturday, providing another mechanism for precipitation. The 00Z
ECMWF/ECMWF mean may be a bit too quick to open up/eject the
feature after Sun given the ridge building over the Northwest
(GEFS mean was stronger with the upper ridge into the OR coast Sat
and slower with the speed of the upper low into NW Mexico).
For the storm tracking northeast from the Midwest, UKMET runs were
still 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 as the system progresses out to sea.
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. These will likely have to wait until the
short range to be resolved but the overall picture should stay the
same.
For the full forecast domain a general model blend early in the
period trending toward a blend among the 06Z GFS/GEFS mean and 00Z
ECMWF/ECMWF mean along with continuity provided a reasonable
starting point for representing the desired consensus or
intermediate solution for features of interest. Some adjustments
were made to fine tune some details where the consensus blend
appeared too washed out. 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. They currently highlight an area of potential severe
weather on Thursday from eastern/southeastern Texas northeastward
to the Midwest (IL/IN/OH). In the northern part of the precip
shield there may be localized areas of snow at times, with best
potential (though with fairly low 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.
Precipitable water values near 2 inches (+2 to +3 sigma) and
strengthening/incoming subtropical jet (less ensemble agreement)
will provide an environment conducive for heavy rain. 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(s) 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.
Timing remain uncertain that far out in the period.
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 with some areas approaching 90F.
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 given the extensive
cloud cover and scattered rain. The East will be warm late this
week ahead of the Midwest system, followed by a trend toward more
normal readings.
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
Hazards:
- Heavy precipitation across portions of the Southern Rockies and
the Southwest, Fri, May 10.
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the Middle Mississippi Valley, the Southern
Appalachians, the Southeast, the Southern Plains, and the Ohio
Valley, Thu-Sun, May 9-May 12.
- Heavy rain across portions of the Great Lakes, the Mid-Atlantic,
the Northeast, and the Central Appalachians, Thu-Fri, May 9-May 10.
- Heavy snow across portions of the Southern Rockies, the Central
Rockies, the Southern Plains, and the Central Plains, Thu, May 9.
- Severe weather across portions of the Lower Mississippi Valley,
the Tennessee Valley, the Southern Plains, the Middle Mississippi
Valley, the Great Lakes, and the Ohio Valley, Thu, May 9.
- Flooding possible across portions of the Central Plains, the
Central Appalachians, the Northern Plains, the Great Lakes, the
Middle Mississippi Valley, the Upper Mississippi Valley, and the
Southern Plains.
- Flooding occurring or imminent across portions of the Central
Plains, the Lower Mississippi Valley, the Northern Plains, the
Great Lakes, the Middle Mississippi Valley, the Upper Mississippi
Valley, the Southern Plains, and the Ohio Valley.
- Much below normal temperatures across portions of the Southern
Rockies, the Central Rockies, the Southern Plains, and the Central
Plains, Thu-Fri, May 9-May 10.
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