Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
442 PM EDT Wed Mar 20 2019
Valid 12Z Sat Mar 23 2019 - 12Z Wed Mar 27 2019
...16Z Update...
The array of 00Z/06Z guidance yields a forecast rationale similar
to previous cycle--a consensus approach for days 3-4 Sat-Sun and
then emphasis on the 00Z ECMWF/CMC and NAEFS/ECMWF means by days
5-7 Mon-Wed. GFS/FV3 GFS runs and GEFS means are still on the
faster side of the spread with the shortwave crossing the southern
half of the Lower 48 (though the UKMET hedges in that direction by
Mon). At the same time the GFS/GEFS mean are more amplified with
flow dropping into the Great Lakes by Mon, leading to more phasing
between streams than the majority scenario. From late Mon onward
the 00Z ECMWF has trended back to the north with its eastern U.S.
surface low to agree better with ECMWF/CMC means. The average of
members within the CMC/ECMWF cluster has adjusted a tad faster
from continuity and this is reflected in the updated forecast.
Over the eastern Pacific/West Coast there is reasonable consensus
into day 5 Mon. After that time the majority of guidance
generally suggests the upper low off the coast will be more
consolidated than depicted by the 06Z GFS/FV3 GFS. The 00Z GFS is
closer to consensus. Including the NAEFS mean in the blend
accounts for the possibility that the ECMWF/CMC cluster could be
slightly fast to bring the upper trough inland next Wed, given the
overall blocky nature of the evolving western North American
pattern at that time.
Models and ensembles are stable with the overall signal for one or
more episodes of heavy precipitation focused over/near northern
California and Sierra Nevada during Mon-Wed. Some of this
moisture and/or the leading system crossing the West during the
weekend will spread rain and higher elevation snow over remaining
parts of the northern half or more of the West. Spread and
variability in model guidance keep confidence on the low end of
the spectrum for coverage/intensity/timing of precipitation from
the Plains to East Coast, as well as for details of any snow that
may fall in the northern fringe of the moisture shield.
Clouds/precip will contribute to above normal morning lows (by
10-20F in some cases) from parts of the West into the Plains
during the weekend and central/southern Plains into lower
Mississippi Valley early next week. Highs will tend to be near to
below normal over the West during the weekend. The Interior West
and Rockies/Plains should warm up again by next Tue or Wed. On
the other hand below normal readings will likely push
southeastward from the northern Plains/Great Lakes during Mon-Wed.
Rausch
Previous Discussion:
...Overview and Guidance Assessment/Preferences...
A split flow pattern is expected to persist through the medium
range. Anomalous upper ridge axis extending from Alaska and
northwest Canada into the north central U.S. is expected to
persist, with downstream mean troughing centered east of Hudson
Bay - making for highly amplified, if not blocked northern stream
flow. Meanwhile, the stream will be fairly progressive across the
CONUS, with shortwaves spaced a couple days apart as energy ejects
from an upstream mean trough across the north central Pacific.
Days 3-5
In general, a multi-model blend of the latest deterministic
guidance served as a good forecast starting point during days 3-5
(Sat-Mon). Model solutions still differ to some degree on the
position of a deep surface low near/east of Maine Sat morning, but
solutions are generally well-clustered around the ensemble means,
indicating that a consensus approach is likely best. As the Sat
system quickly pulls away, another shortwave should traverse the
broader mean trough, diving into the Great Lakes/Northeast
Sun-Mon. Solutions have trended more amplified with this wave over
the past several model cycles, and a consensus approach is also
favored here at this time. Farther west, models show good
agreement that a Pacific shortwave should reach the West Coast
early Sat, then cross the Four Corners Sun, reaching the
Southern/Central Plains by Mon. At least through Sun, models
handle this feature similarly, with differences beginning to
emerge by Mon as the system moves into the Plains. Nonetheless,
there is at least modest consensus that a surface low should
develop across the Southern Plains Sun night-Mon.
Days 6-7
The forecast gets quite tricky by next Tue-Wed, particularly with
respect to the southern stream system moving from the Plains
toward the East Coast. The GFS and ECMWF have both shown
significant difficulty handling this system. The ECMWF has been
highly variable from run-to-run, while the GFS has consistently
been a fast outlier, only seeming to get faster with each run. A
sizable number of ECENS and CMCE members have shown good
clustering for a few runs now around a surface low tracking from
the Southern Plains into the Tennessee Valley Mon night-Tue (a
solution starkly different from the 12Z ECMWF which seemed to be
an outlier in the context of its ensemble). The 12Z CMC/GEM global
and NAVGEM were the only deterministic solutions representing this
significant cluster, which is also reflected in the ECENS mean. By
Tue night-Wed, energy appears likely to transfer to a new surface
low along the Southeast U.S. coast, in a "Miller-B" scenario. This
process seems to be best represented by the ECENS and CMCE means,
with the 12Z ECMWF solution also in the ball park (if you ignore
its outlier solution for the Gulf Coast low). Finally, farther
west, consensus remains relatively good that strong height falls
should reach the West Coast Tue-Wed as a significant trough moves
onshore. While timing among models is similar, there are
structural differences that remain, leading to the conclusion that
an ensemble approach for that system is likely best. Given these
considerations, the forecast for days 6-7 was based heavily on the
ECENS and NAEFS ensemble means, with components of the CMC/GEM
global and NAVGEM also included, given their aforementioned
handling of the central/eastern U.S. system.
...Weather Highlights and Hazards...
Showers and thunderstorms are expected to develop across the
Southern and Central Plains Sat-Sun. A leading upper shortwave Sat
and a surface warm front/dry line will initially focus convection
across these areas. The potential exists for some severe weather
as well as locally heavy rainfall threats. Please refer to
products issued by the SPC for further information on the severe
weather threat. By Mon-Tue, showers and thunderstorms should
spread east into the lower/mid-Mississippi Valley as well as the
Ohio/Tennessee Valleys and the Southeast. Development of the
surface low along the Southeast U.S. coast Tue night-Wed would
also likely bring an enhancement of rainfall to portions of the
Carolinas and southern Mid-Atlantic. Ensemble probabilities
suggest some potential for wintry weather on the northern
periphery of this system Tue-Wed, from portions of the upper Ohio
Valley to the Mid-Atlantic and Northeast. Confidence in the extent
of cold air in place by that time is very low, and only low-end
probabilities were shown at this time on the Day 7 Winter Weather
Outlook. Farther west, rain and mountain snows are expected across
the Great Basin/Rockies with the passage of the shortwave Sat-Sun.
A more significant area of precipitation could spread into
northern California by next Mon-Tue as the stronger upper trough
approaches, with some signal for a heavy precipitation event in
the model guidance.
Ryan
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 California, the Central
Great Basin, and the Pacific Northwest, Mon-Wed, Mar 25-Mar 27.
- Heavy rain across portions of the Lower Mississippi Valley and
the Southern Plains, Sat-Sun, Mar 23-Mar 24.
- Heavy rain across portions of the Middle/Lower Mississippi
Valley, the Central/Southern Appalachians, the Tennessee Valley,
the Mid-Atlantic, the Southeast, and the Ohio Valley, Mon-Tue, Mar
25-Mar 26.
- Heavy snow across portions of the Mid-Atlantic and the Central
Appalachians, Tue-Wed, Mar 26-Mar 27.
- Heavy snow across portions of the Northeast and the Great Lakes,
Sat, Mar 23.
- Severe weather across portions of the Southern Plains, Sun, Mar
24.
- Flooding possible across portions of the Northern/Central
Plains, the Upper/Middle Mississippi Valley, and the Great Lakes.
- Flooding occurring or imminent across portions of the
Northern/Central Plains, the Mississippi Valley, the Tennessee
Valley, the Great Lakes, 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