Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Fri Mar 19 2021
Valid 12Z Mon Mar 22 2021 - 12Z Fri Mar 26 2021
...Overview...
Guidance continues to show a broad mean trough aloft settling over
the lower 48, between a strong eastern Pacific ridge and western
Atlantic mean ridge that will become anchored by a strengthening
upper high over Cuba and the Bahamas by mid-late week. Ahead of
the mean trough expect an active weather pattern over the central
and eastern U.S. with multiple areas of heavy rainfall potential
over the course of next week. Pacific features digging into the
western side of the mean trough will bring periods of rain and
mountain snow to locations from the Pacific Northwest into a
majority of the Rockies. Finally, the upper trough with possible
embedded low forecast to be over the East Coast to start the week
should gradually progress to the east/northeast, pushing along
associated western Atlantic low pressure. Models and ensembles
continue to agree on the large scale pattern while differing on
some meaningful timing and detail issues.
...Model Guidance/Predictability Assessment...
The system initially off the East Coast continues to provide a
challenge for the guidance and predictability is still tempered by
the fact that the upper support essentially becomes embedded
within the larger scale western Atlantic mean ridge. GFS runs are
persistent in being more offshore than other guidance due to
extending the upper trough farther eastward, while remaining
solutions still vary considerably. At least the new 12Z UKMET has
adjusted southeastward toward the general consensus in contrast to
its previous two northward runs that greater influence on New
England. Preference remains close to continuity as yielded by a
multi-model compromise.
The next system of note is a vigorous shortwave ejecting out of
the Rockies on Monday and reaching into the Upper Midwest by
midweek, supporting surface low pressure that tracks northeast
from the southern High Plains. Trends over the past day or so have
been toward a more concentrated evolution by day 4 Tuesday and
then toward the middle of recent spread by day 5 Wednesday (by way
of the deep GFS trending a little weaker and earlier ECMWF runs
becoming stronger). There is still a fair amount of timing spread
with the GFS/GEFS consistently on the fast side and the UKMET
lagging.
Guidance seems to be in a gradual process of trying to converge
for the next system digging through the West. The most common
theme is that the energy will form a closed low just south of the
Four Corners by early Wednesday and then open up as it ejects to
the east and northeast, encouraging a surface wave to lift
northeastward from the western Gulf Coast late in the week. Among
solutions through the 06Z cycle, the primary cluster appeared to
center over the 00Z ECMWF mean, an average of the past two ECMWF
runs (00Z faster, 12Z/18 run slower), and 00Z CMC/06Z parallel
GFS. Operational GFS runs have been faster with somewhat more
stream separation than indicated by consensus. New 12Z GFS/CMC
runs are now close to each other by 12Z next Friday and a bit
faster than the aforementioned majority/preferred cluster. The new
12Z ECMWF nearly duplicates its 00Z run.
Upstream Pacific energy should dig into the West during the latter
half of next week, with a general model/mean blend providing a
reasonable starting point and good continuity. Stream separation
that occurs over the North Pacific early in the period provides
some lingering uncertainty over specifics (especially in the 12Z
UKMET and to a lesser degree in the CMC).
Guidance preferences led to an early-period blend that leaned
mostly away from the GFS for the western Atlantic and Plains/Upper
Midwest systems, followed by a transition to an average among the
past two ECMWF runs, 00Z CMC/06Z GFSP, and more 00Z ECMWF mean
than 06Z GEFS mean.
...Weather/Threats Highlights...
The developing mean trough aloft with periodic reinforcement will
bring below normal temperatures over much of the West into the
Rockies during most of next week. Expect coolest temperatures to
be Monday-Wednesday with highs 10-15F below normal over parts of
the Great Basin and central-southern Rockies. Many areas will see
a moderating trend after midweek but another cold front may
reinforce minus 5-10F anomalies for highs from the Northwest into
central Rockies. Incoming shortwaves/leading surface fronts will
bring periods of rain and mountain snow to the Pacific Northwest
and extending eastward/southeastward through the West into the
Rockies as the shortwaves amplify. Most of this activity should be
of light to moderate intensity but some localized heavier areas
will be possible.
The upper dynamics ejecting from the Rockies will support a
central Plains through Upper Midwest system early-mid week. This
system may produce a focused area of enhanced snowfall over or
near the Colorado Rockies (the majority likely to be just before
the medium range period early Monday). Lighter snow is possible on
the northwest fringe of the precipitation shield from the central
High Plains northeastward on the cold side of the surface low
track. Farther south, improving Gulf inflow should promote areas
of heavy rainfall from the central Plains/Midwest into the Lower
Mississippi Valley and Gulf Coast with lighter rainfall extending
northeastward. Recent guidance has been highlighting the potential
for highest rainfall totals near the central/east-central Gulf
Coast around midweek as the front trailing from the Midwest system
decelerates/stalls while awaiting the next western U.S. shortwave.
This latter ejecting western energy should generate a late week
system that lifts northeastward over the east-central U.S.,
spreading areas of enhanced rainfall over a greater portion of the
East. Latest trends for the western Atlantic system have lowered
the rainfall amounts for some locations along the East Coast from
Monday onward. Some areas may still see a period of brisk winds as
supported by the gradient between the system and high pressure to
the north.
Warmest temperature anomalies will progress from the
Midwest/western Great Lakes into the Northeast over the course of
next week. These areas should see one or more days with highs of
10-20F above normal. Morning lows will be somewhat more above
normal with greater southward extent of plus 10F and greater
anomalies over the eastern half of the country, aided by the moist
flow from the Gulf. High temperature anomalies of plus 5-10F over
Florida later in the week are less impressive in absolute terms
but readings could be warm enough to approach daily record values
for some sites with relatively shorter periods of record.
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
Hazards:
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the
Mid-Atlantic, the Southern Appalachians, the Southeast, the
Southern Plains, and the Ohio Valley,
Tue-Thu, Mar 23-Mar 25.
- Heavy rain across portions of the Central/Southern Plains, and
the Upper/Middle Mississippi
Valley, Mon, Mar 22.
- Heavy snow across portions of the Central Great Basin, the
Pacific Northwest, the Northern
Rockies, and the Northern Great Basin, Mon, Mar 22.
- Heavy snow across portions of the Central/Southern Rockies, the
Central Great Basin, and the
Central/Southern Plains, Tue, Mar 23.
- Flooding possible across portions of the Central Plains, the
Middle Mississippi Valley, the
Southern Appalachians, the Ohio Valley, and the Tennessee Valley.
- Flooding occurring or imminent across portions of the
Central/Southern Plains, the Middle
Mississippi Valley, the Ohio Valley, and the Tennessee Valley.
- Flooding likely across portions of the Central Plains, the
Middle/Lower Mississippi Valley, and
the Ohio Valley.
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