Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Wed Apr 3 2019
Valid 12Z Sat Apr 6 2019 - 12Z Wed Apr 10 2019
15Z Update: Similar to yesterday, there remains considerable
model spread across the eastern half of the U.S. regarding the
structure and evolution of the low pressure system crossing the
Great Lakes region and its associated frontal boundaries.
Teleconnections with the positive anomaly over Greenland is
supportive of a trough over the western Pacific and also over
Quebec, Canada. The FV3 GFS was substituted for the operational
GFS for this forecast as part of the overall blend given larger
run-to-run differences with the operational GFS. The UKMET and
CMC begin exhibited differences from the ensemble means as early
as this weekend regarding the trough over the central U.S. and
Midwest states. Given lower than average confidence for the
latter half of the forecast period, the WPC forecast remained
fairly close to previous continuity while also taking into account
the EC mean, GEFS mean, FV3, and ECMWF.
Hamrick
...Overview...
Expect the continuation of a moderately progressive pattern
dominated by Pacific flow from the weekend into the middle of next
week. There two largest scale features of note will be western
U.S./Rockies energy that should lead to a trough reaching the East
by next week and an upstream trough most likely moving into the
West next Tue-Wed. Between these two features, fast flow will
bring multiple impulses across the Pacific Northwest. Guidance
has been consistent in highlighting an area from the
northern/central West Coast eastward into the Rockies for
significant precipitation. The upper trough progressing into the
East will spread a broad shield of moisture across the eastern
half of the country but with a lot of uncertainty over intensity
and duration of precipitation at any particular location.
...Guidance Assessment/Preferences...
The details are as uncertain as ever for evolution of the overall
trough forecast to progress eastward from the western U.S./Rockies
Sat onward. At the start of the period this trough should consist
of a combination of southern stream energy reaching the southern
Rockies/northern Mexico along with strong northern stream
shortwave energy pushing through the Northwest. Operational
models as a whole are signaling a decent potential for an embedded
closed low to form by Sun-Mon but with tremendous north-south
spread for the path of such a low. Solutions among latest runs
valid 12Z Mon range between the Canadian border (12Z ECMWF and 00Z
FV3 GFS) and central/south-central Plains into Mississippi Valley
(recent GFS runs and 00Z UKMET). The old 00Z/02 ECMWF run in fact
had a compact upper low tracking even farther south. The wide
spread in guidance required earlier than usual inclusion of a
significant percentage of the ensemble means to provide stability
for the deterministic forecast--even if confidence in the exact
ensemble mean scenario is not particularly high either. Based on
the ensemble mean forecast a complex area of Plains low pressure
should consolidate as it tracks into/across the Great
Lakes-Northeast during the first half of next week. Low track
would differ significantly if some of the operational runs were to
verify.
Over the eastern Pacific/Western U.S. during Mon-Wed, faster/more
amplified trends in the GFS/GEFS mean over the past day have
narrowed the guidance spread for the trough expected to move into
the West. Through the 12-18Z cycles the GFS/GEFS mean still
seemed to be in the trending process, while teleconnections
relative to prominent height anomaly centers over the western
Pacific (negative) and Northeast Atlantic/Greenland (positive)
have been promoting height falls reaching the southern half of the
West. Thus the forecast tilted more toward the ECMWF
mean/ECMWF/NAEFS mean. The 12Z CMC was on the extreme side of the
envelope for trough amplitude. The new 00Z GFS/GEFS mean appear
closer to forecast preference. Also worth noting, guidance
rapidly diverges for details of fast North Pacific flow by next
Mon and these differences migrate into the northeastern Pacific by
Tue-Wed. Issues across those areas may also affect the forecast
for the West at that time.
...Weather Highlights/Hazards...
Within the favored area for significant rainfall and higher
elevation snow from the northern/central West Coast into the
Rockies, highest totals will likely occur over northern California
and western Oregon/Washington where the Sat-Mon period should
feature strong flow aloft with multiple embedded shortwaves.
Predictability in small-scale details is low but there is
potential for one or more narrow bands of very focused precip. By
Tue-Wed the upper trough moving into the West will shift the focus
for rain/snow farther inland--most likely across the north-central
Intermountain West and Rockies. Some of this moisture may reach
the central Plains as well. Eastern slopes of the Rockies could
see activity enhanced by low level upslope flow around midweek.
Strengthening low level flow of moisture from the Gulf of Mexico
ahead of the upper trough emerging from the West/Rockies will
spread a broad area of precipitation from portions of the Plains
through the East during the period. There is at least a moderate
degree of consensus that areas from the eastern half of Texas
through the Lower Mississippi Valley and possibly into the
southern Appalachians will see a threat of heavy rainfall. There
are indications that the Northeast may see significant amounts as
well. Otherwise the wide disparity in guidance for specifics of
upper level/surface evolution will make it difficult to have much
confidence in determining areas that will see highest precip
totals. Parts of New England may see wintry weather with this
system.
Over the West expect modestly below normal daytime highs from the
West Coast into northern Rockies. The Southwest/southern Rockies
into Great Basin will see a brief period of warmth Sun-Tue (some
plus 10F or greater anomalies) before upper troughing expands the
coverage of below normal temperatures over the West by midweek.
Most of the West should see above normal mins during the period.
Farther eastward expect an area of much above normal min temps to
progress from the Plains through the East Sat-Tue. Plus 15-25F
anomalies may challenge daily record warm low values at some
locations. Anomalies should be less extreme for highs. Cool flow
behind the system crossing the East should bring some modestly
below normal highs into the northern half of the central/eastern
states by next Wed.
Rausch
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
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