Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Sat Apr 30 2022
Valid 12Z Tue May 03 2022 - 12Z Sat May 07 2022
...Overview...
A series of systems will produce episodes of precipitation from
the Northwest U.S. and northern-central Rockies (rain and high
elevation snow) through the central-southern Plains and much of
the East. A system tracking out of the Midwest early Tuesday
onward and another one tracking out of the West should be the last
in the series during the currently established pattern consisting
of discrete troughs/closed lows, while mean troughing aloft
setting up near the West Coast late week into the weekend will
likely bring a couple frontal systems and associated precipitation
focus into the West. The most active weather should be with the
system emerging from the West by midweek with the Storm Prediction
Center indicating a threat of severe weather over the southern
Plains and vicinity and WPC showing an experimental medium range
"Slight Risk" for excessive rainfall over parts of the region as
well, with runoff issues potentially enhanced due to wet ground
from rainfall expected in the short range time frame. The midweek
threat signal is stronger than average as a closed upper
system/lead upper diffluence combine with increasing inflow of
Gulf moisture into a wavy front. Rainfall will continue spreading
into/through the East later in the week.
...Model Guidance Evaluation...
Guidance through the 00Z/06Z cycles offered significant forecast
challenges in multiple respects. Every day or so the models keep
changing the specifics of flow aloft over eastern Canada into the
eastern U.S., leading to oscillations in strength/track of low
pressure tracking out of the Midwest early Tuesday onward. Latest
clustering suggests somewhat less interaction between Canada
troughing and the Midwest shortwave, generally yielding a
weaker/southward surface low and then by early Thursday a wave
offshore from the Northeast. Meanwhile the pronounced spread
continues for the upper low tracking out of the West, with the
GFS/GEFS mean on the fast side and the ECMWF slowest. From
Thursday onward the 00Z ECMWF mean and CMC/CMC mean offered a
compromise closest to what has historically been the best starting
point in this type of timing discrepancy, especially involving a
closed low, which is a two-thirds lean toward the slower side of
the envelope. General preferences ultimately leaned closest to the
00Z CMC/CMC mean and ECMWF mean. With the mean trough settling
near the West Coast, there is a general theme of a leading
shortwave reaching the coast Thursday and then another feature
arriving around Saturday. However models/ensembles have varied
wildly for amplitude of the leading shortwave. Some GFS runs/GEFS
ensembles have been quite amplified while ECMWF runs/ECMWF
ensembles have been trending noticeably flatter (with the CMC/CMC
mean a compromise). Compared to the very amplified 00Z run, the
06Z and then 12Z GFS runs have made a pronounced flatter trend
closer to the consensus.
Guidance preferences led to starting with a 06Z/00Z operational
model composite for the first half of the period, employing
slightly less than typical weight of the GFS component. Then the
forecast transitioned to a model/ensemble mean blend including
more CMC than ECMWF along with remaining input consisting of the
00Z ECMWF/CMC means and 06Z GEFS mean. New 12Z model trends
(including a faster adjustment in the ECMWF) remain supportive of
slower timing than the GFS for the system emerging from the West,
but faster than the 00Z ECMWF.
...Sensible Weather/Threats...
Over the West, the upper low forecast to track from near southern
Idaho through the central Rockies Tuesday-Wednesday and associated
surface reflection will spread rain and higher elevation snow
across the northern-central Rockies and High Plains during that
time. This system could produce some locally enhanced totals
depending on exact details. Then an amplified upper trough and
surface frontal system forecast to reach the West Coast and
progress inland next Thursday-Saturday should bring a renewed
precipitation focus into the Pacific Northwest and then farther
eastward. The southern extent of precipitation will depend on the
upper trough's precise amplitude which has been quite uncertain so
far, but latest trend suggest that most moisture should be
confined to the northern half of the West. The upper
dynamics/surface front could reach far enough east to start
producing at least scattered rainfall over the northern High
Plains by next weekend. The Pacific Northwest may see additional
precipitation by next Saturday with another front.
Over the central/eastern U.S., the initial rainfall focus will be
with a system tracking out of the Midwest early Tuesday onward.
This low and associated fronts should produce the highest totals
over the lower Great Lakes and Ohio Valley with somewhat lighter
amounts extending eastward through the East Coast. The next system
in the series will likely emerge over the Plains during the day
Wednesday and lead to another episode of potentially heavy
rainfall, with the WPC experimental Excessive Rainfall Outlook
showing a Slight Risk over/near parts of the south-central Plains
for Wednesday into early Thursday. The Storm Prediction Center is
also monitoring severe threats over and just east of the southern
Plains Wednesday-Thursday. Rain should continue pushing eastward
from the Plains through the rest of the week with some pockets of
moderate to heavy amounts. System details by late in the week are
still fairly uncertain but currently the most likely axis of
highest rainfall totals extends from the Midwest through the
Mid-Atlantic. Over the central/east-central U.S., rain that falls
prior to this final system could increase sensitivity to added
rainfall. Fortunately the guidance is consistent in keeping this
system's moisture south of the far northern Plains where soil
conditions are the wettest.
The system initially crossing the West will support highs 10-15F
below normal over the northern Rockies into Great Basin on
Tuesday. Then this system's progression will reinforce the chilly
pattern over the central Plains/Midwest (highs up to 10-15F below
normal Tuesday-Thursday) before temperatures may finally moderate
toward the end of the week. Warmer readings will cross the West
mid-late week with best potential for highs 10-15F above normal on
Wednesday-Thursday. A gradual increase of clouds/precipitation
will bring highs down to moderately below normal levels over the
Northwest Friday-Saturday. The northern two-thirds of the East
will see above normal temperatures into midweek, followed by a
gradual cooling trend that will expand coverage of near to
somewhat below normal highs late in the week.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, 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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml