Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Sun Mar 21 2021
Valid 12Z Wed Mar 24 2021 - 12Z Sun Mar 28 2021
...Wednesday-Thursday heavy rain threat most likely centered over
parts of the South...
...Overview...
Through late in the week two vigorous shortwaves within a large
scale mean trough with an axis over the Plains/west-central U.S.
will support fairly strong surface systems and a very warm/wet
pattern over much of the eastern half of the country. Meanwhile
incoming energy will produce areas of rain and mountain snow
extending from the Pacific Northwest into much of the Rockies
along with below normal temperatures. By next weekend guidance is
suggesting that upper ridging over the eastern Pacific will start
to build into the West and bring a warmer/drier trend for the
region, but consensus is lacking for the shape/strength of the
ridge as well as for evolution of the downstream trough. The
central-eastern U.S. should see additional precipitation and
moderately warm temperatures ahead of a third system next weekend
but a slightly flatter trend to the pattern will likely lead to
lower coverage of significant rainfall and moderate the warm
anomalies.
...Model Guidance/Predictability Assessment...
With guidance finally coming together for the system tracking out
of the Upper Midwest from early Wednesday onward, the primary
forecast issues that remain are for the timing and evolution of
strong energy expected to eject northeastward from Arizona/New
Mexico along with associated low pressure, followed by the
combined details of upstream shortwave energy reaching the
western/central U.S. and Pacific ridging that should build into
the West.
Guidance is continuing to have difficulty in resolving how quickly
the upper low just south of the Four Corners on early Wednesday
ejects northeastward, and how strong/closed the feature will be as
it continues through/beyond the Plains. Over the past 24 hours
most guidance has generally held onto the slower timing introduced
by yesterday's 12Z cycle (after consensus had trended somewhat
faster for the prior two or three runs). Within the latest
envelope the 00Z ECMWF is slowest and most closed with latest CMC
runs just a little faster and also closed aloft/deep at the
surface. GFS runs already become faster and/or more open with the
upper feature versus most guidance by Thursday. Recent parallel
GFS (GFSP) runs have generally been a tad slower than operational
runs. Recent persistence of the slower progression closer to valid
time provides support to lean somewhat in the slower half of the
spread and a tad slower than continuity. However the slow 00Z
ECMWF may not be ideal either as the system is carried along by
southwesterly mean flow aloft. The old 12Z ECMWF would be closer
to representing current preference.
Upstream the general theme of recent trends has been toward some
combination of a stronger ridge moving into the West along with a
somewhat weaker/less amplified version of the western-central U.S.
trough. Individual models/ensembles also display varied ways in
which energy on the western fringe of the late week western trough
may pull off an upper low near or offshore the West Coast.
Parallel GFS runs are more reflective of recent consensus though
could be a bit deep relative to ongoing trends.
Based on forecast preferences, the operational model blend early
in the period used the 06Z GFSP for the GFS input. Then ECMWF
input split the past two runs and trended toward half 06Z GEFS/00Z
ECMWF mean weight to yield conservative adjustments to account for
recent trends over the lower 48 from late week through the weekend.
...Weather/Threats Highlights...
Lingering precipitation (rain and/or snow) is likely on the north
side of the surface low exiting the Upper Midwest on Wednesday.
South of this however, the interaction of moist Gulf inflow with
the trailing front--which should stall for a time while awaiting
ejection of upstream dynamics from the West and southern
Plains/Gulf Coast wave development--is likely to produce heavy
rainfall over parts of the South through Wednesday-Thursday.
Recent slower trends by the average of guidance have adjusted the
northern part of the heaviest rainfall axis a bit westward, now
from the central Gulf Coast northward into the Tennessee Valley.
Other surrounding areas could see significant rainfall as well and
the surface/upper system could be strong enough to produce another
band of enhanced precipitation on the northwest side of the
surface low track. System strength will also determine how much if
any snow may occur in the northwest fringe of the moisture shield.
Greater progression of rainfall should lead to lower totals over
areas closer to the East Coast. Some warm sector convection may be
strong/severe and future Storm Prediction Center outlooks will
provide more information on this threat as guidance becomes more
agreeable for important details.
Before helping to generate heavy rainfall downstream, the upper
system initially south of the Four Corners will produce a period
of enhanced snowfall over the southern Rockies around Wednesday.
The next upper trough moving into the West will bring a mid-late
week episode of mostly light-moderate rain/mountain snow to the
Pacific Northwest and areas to the east and southeast through the
Rockies. Some precipitation (rain and/or snow) should then spread
across the northern tier as low pressure continues toward the
Great Lakes. Rainfall will likely expand over the East again ahead
of this low pressure and trailing front. A warm front may enhance
amounts over some southern areas while lighter activity should
prevail farther north.
Above normal temperatures over the eastern half of the country
will be the most extreme mid-late week, in the warm sector ahead
of the leading system tracking out of the Upper Midwest on
Wednesday and trailing system that may reach into the Great
Lakes/Northeast late in the week. Plus 10-25F anomalies for
highs/morning lows will be most common from the Great Lakes/Ohio
Valley into the Northeast/Mid-Atlantic. Moist flow from the Gulf
will also promote a broad area of very warm lows farther
southward. Some locations in the East may see lows close to daily
record warm values. Most of the East will see a cooler trend next
weekend but should stay moderately above normal. The upper ridge
strengthening over Cuba mid-late week and persisting into the
weekend will promote very warm temperatures over the Florida
Peninsula from Thursday onward, possibly bringing high
temperatures close to daily records at sites with a relatively
shorter period of record.
Areas from the western U.S. into Rockies/High Plains will see
below normal temperatures mid-late week followed by a warming
trend. The most extreme anomalies of 10-20F below normal for highs
are likely over the central-southern Rockies and High Plains on
Wednesday. Pockets of highs 10F or so below normal are possible
over parts of the Great Basin/central Rockies into Friday. The
arrival of the Pacific ridge aloft should bring highs up to 5-15F
above normal from the West Coast states into northern Rockies by
Sunday.
Rausch/Santorelli
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, 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