Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
334 PM EDT Thu Mar 21 2019
Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019
...Overview and Guidance Assessment/Preferences...
Today's guidance continues the recent general theme aloft of
strong mean ridging over western Canada and/or Alaska, periodic
downstream troughing, and features progressing from the eastern
Pacific into/across the Lower 48. Recent model/ensemble runs have
displayed some significant differences/trending with individual
western-central U.S. features that continue eastward and affect
the forecast over the eastern half of the country. Guidance has
been a little better behaved for a fairly deep eastern Pacific
trough/embedded upper low that should reach the West by Wed-Thu
but typical amplitude/timing differences still exist.
The forecast over the eastern half of the Lower 48 depends on
specifics of a compact upper low over the north-central Plains and
Interior West energy as of early day 3 Sun, and perhaps to some
degree Canadian troughing that dips into the Great
Lakes/Northeast. Over the past day there have been some trends
toward more emphasis on the Plains feature and less on the western
energy. This has led the ECMWF/CMC and their means in particular
to trend faster with associated low pressure crossing the
mid-lower latitudes of the central/eastern states during Sun-Tue.
The adjustment brings the guidance cluster closer to the prior
GFS/GEFS scenario that was fairly progressive. However by early
Tue the 00Z ECMWF/ECMWF mean may have overshot the trend just a
bit, favoring partial inclusion of the 12Z/20 run as part of a
consensus blend that includes the 06Z GFS and 00Z UKMET/CMC into
Tue. The 00Z GFS was slightly faster. It remains a close call as
to whether the Canadian/Northeast U.S. trough may influence the
southern stream system. Recent trends for the amplitude of this
trough are mixed.
A blend/consensus approach looks reasonable for depicting the
evolution of the amplified trough/embedded upper low settling over
the eastern Pacific during the first half of next week and then
opening up with progression inland. The overall upper low
contains multiple small shortwaves whose scale results in low
predictability for those details. By Wed-Thu the models and
ensembles have varied somewhat with amplitude/timing as trough
energy progresses into the West. Recent ECMWF runs and their
ensembles have tended to be on the faster side of the overall
spread in the past day and the 00Z ECMWF looks particularly fast
in bringing height falls into the northern Plains by Thu. The
ECMWF mean has trended slower over the past 24 hours and the 06Z
GFS is a bit slower than the 00Z run so preferences for this
feature late in the period tilt more toward the latest ECMWF
mean/06Z GEFS mean/06Z GFS and 12Z/20 ECMWF.
Downstream from an upper high expected to close off over eastern
Alaska and the Yukon, there is decent agreement that an amplifying
trough over central Canada will push a cold front into the
northern Plains around day 7 Thu. There have been enough spread
and variability for the character of the upper ridge to suggest
some future guidance adjustments are possible over the northern
U.S. and southern Canada during the latter half of the week.
...Weather Highlights and Hazards...
One or more waves progressing across the eastern half of the
country will produce areas of mostly rain Sun-Tue, with
thunderstorms possible from the southern half of the Plains into
the Southeast. Some rainfall could be locally moderate/heavy but
recent faster trends suggest that any intense activity should be
fairly brief. At the same time the faster progression of moisture
may limit any snow in the northern part of the moisture shield to
farther north latitudes than previously anticipated--due to a
higher proportion of moisture crossing the East ahead of the
colder air pushing southeast from Canada.
Mostly during Sun-Sun night an initial western U.S. shortwave will
generate mountain snow/valley rain over the Great Basin and
northern-central Rockies. Some of this rain/snow may extend into
the northern Plains. The system settling into the eastern Pacific
during the first half of next week will likely bring multiple
surges of moisture into the West Coast and extending into favored
terrain farther inland over northern/central parts of the West.
The first round of enhanced precipitation should commence by Sun
night-Mon. By Thu there is some uncertainty over the exact
character of upper flow along the West Coast, lowering confidence
in how much precipitation may persist over the West Coast states
at that time. Guidance is persistent in showing best potential
for highest precip totals from northern California/extreme
southwest Oregon into the Sierra Nevada. The combination of
events will provide less extreme but still significant totals over
the northern/central Rockies.
System progression will lead to variable temperatures over many
areas during the period. Expect warmest anomalies to be for
morning lows from the Rockies into Plains/Mississippi Valley
Sun-Mon and then for max/min readings from the Interior
West/Rockies into the Plains/Upper Mississippi Valley Tue-Thu.
There may be fairly broad coverage of plus 10-20F anomalies over
the central U.S. by next Thu. Parts of the east should see
temperatures at least 10F below normal for a brief time around
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
- Heavy precipitation across portions of California, the Central
Great Basin, the Pacific Northwest, and the Southwest, Mon-Thu,
Mar 25-Mar 28.
- 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.
- Flooding likely across portions of the Northern/Central Plains,
the Upper/Middle Mississippi Valley, and the Great Lakes.
- Much above normal temperatures across portions of the
Northern/Central Plains, and the Upper/Middle Mississippi Valley
Wed-Thu, Mar 27-Mar 28.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: