Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Apr 19 2019
Valid 12Z Mon Apr 22 2019 - 12Z Fri Apr 26 2019
...Overview and Guidance Preferences...
Expect two pronounced streams aloft to prevail across North
America next week. A fairly slow-moving southern stream will
feature a trough/upper low that should promote a heavy rainfall
threat over the southern Plains/Mississippi Valley, while a
progressive northern stream across southern Canada and northern
U.S. will contain shortwaves whose scale and speed will lead to
below average predictability. Questions of whether any stream
interaction may occur at some point during the forecast period
produce further uncertainty in forecast details.
Over the past day the southern stream upper trough (with embedded
closed low at times) drifting from the western U.S. into the
Plains/Mississippi Valley has shown the best relative stability
over the past day. That said, there has still been a recent
tendency toward greater southwestward digging of energy within the
trough during the early part of the week before the system
continues eastward. There is still considerable uncertainty over
specifics of individual shortwaves so overall confidence in the
exact forecast remains fairly low. At the very least the dominant
cluster of guidance suggests the 06Z GFS is too quick to shear the
upper trough later in the week.
Model/ensemble spaghetti plots become chaotic across the North
Pacific early in the period and this spread continues quickly
eastward with time. Guidance continuity has also been poor for
individual shortwaves across southern Canada and northern U.S.,
with significant corresponding variability in progression of
surface waves/fronts across the northern and eastern U.S. A
slower/stronger shortwave in latest guidance has resulted in a
better defined wave tracking from the Midwest into New England
during Mon-Wed. For a time this adjustment slows down frontal
progression into the East but with the wave's passage the front
pushes back down toward the previous cycle's consensus--but well
southward of what yesterday's 00-06Z guidance was advertising.
The other major change in latest guidance is faster progression of
North Pacific flow across North America. As a result the
overwhelming majority of current 00-06Z guidance brings a front
much farther southeast into the northern states versus previous
cycle. By the end of the period models/ensembles show completely
opposite solutions over the northeastern Pacific, the ECMWF/ECMWF
mean with troughing and the CMC/CMC mean plus the GFS to some
extent with ridging. The upper low over northern Canada
teleconnects to cyclonic flow over the northeastern Pacific so it
seems reasonable to lean away from the ridge solutions.
The upper low departing from the Northeast continues to display
significant timing differences. The 00Z GFS continues to be on
the fast side while the 06Z version actually becomes slower than
the ECMWF after early Tue. An average among the 06Z GFS and 00Z
ECMWF/CMC looks reasonable at this time.
Based on the current array of guidance the updated forecast
started with a blend of the 06Z GFS/00Z ECMWF/00Z CMC for the
early week forecast, followed by a gradual increase of the 06Z
GEFS/00Z ECMWF means to 70 percent total weight by next Fri.
...Weather Highlights/Threats...
The most prominent weather threat next week should be with heavy
rainfall from widespread showers/thunderstorms across the southern
Plains and eventually into the Lower Mississippi Valley from Mon
night/Tue onward. There will be multiple ingredients to support
this event: the upper system tracking out of the Southwest, a
leading slow-moving wavy front, and a multi-day feed of low level
moisture from the Gulf of Mexico. Ensemble guidance shows best
potential of highest rainfall totals during the period over Texas
and Oklahoma. Early in the week expect precipitation over the
eastern Great Basin and central-southern Rockies. Highest
elevations in Colorado may see some snow. Meanwhile a wave
tracking from the Midwest to New England may focus meaningful
rainfall near its track. Any snow should be confined to areas
north of the Canadian border except perhaps for the northern tip
of Maine. A front moving into/across the Northwest and onward
across the northern tier states will be accompanied by mostly
light rain and possibly a little very high elevation snow.
The western U.S. will see a broad area of plus 10-20F temperature
anomalies with some of this warmth extending into the northern and
eventually central Plains. A cold front moving into the Northwest
will bring a cooling trend to that part of the West during the
latter half of the week. Daily records appear most likely for
warm lows over California and the Interior West Wed-Thu but a few
record highs may be possible as well. Parts of the East will see
a period of above normal temperatures in the warm sector of the
Midwest to New England wave Mon-Wed. On the other side of the
spectrum the central-southern Plains will be the primary focus for
below normal highs during the period.
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