Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Sat Apr 20 2019
Valid 12Z Tue Apr 23 2019 - 12Z Sat Apr 27 2019
...Overview and Guidance Preferences...
Guidance spread and variability for details of flow across the
Pacific and extending into/across the northern Lower 48 and
southern Canada continue to keep confidence fairly low for many
aspects of the forecast. The one part of the forecast that has
better confidence is the slower moving southern stream trough that
progresses from the southwestern U.S./northwestern Mexico into the
southern Plains Tue-Thu. There are still some differences and
run-to-run adjustments in this time frame but a strong signal
persists for associated heavy rainfall over the southern Plains
and likely extending at least into the Lower Mississippi Valley.
After early Thu the character of northern stream flow will become
increasingly important to the timing/progression of the southern
trough.
The latest forecast update reflects a blend of the past two
12-hourly ECMWF/6-hourly GFS runs along with minority input from
the 06Z GEFS/00Z ECMWF means during the Tue into Thu time frame.
The rest of the forecast eliminates the GFS runs, holds onto
ECMWF/ECMWF mean/GEFS mean input, and introduces some WPC
continuity.
The preferred blend reflects gradually improving consensus toward
fairly slow departure of the initial New England upper low, but
not as slow/amplified as the 00Z CMC becomes. Behind this low the
blend remains close to continuity (an intermediate solution) for
southeastern Canada shortwave energy--depicted in the guidance as
anything from a weak open wave to a concentrated closed upper low.
Thus far models and means have held onto or even furthered a bit
yesterday's faster trend for the front crossing southern Canada
and the northern tier of the U.S. By late next week prefer to
stay closer to ECMWF-based continuity which reflects greater
stream separation over the East versus the aggressive northern
stream amplification (and western Atlantic or New England storm
development) depicted in the GFS/CMC runs. Evolution of recent
systems would also favor more stream separation.
Farther westward one favorable trend in the guidance is better
clustering among GFS/ECMWF runs and their means toward
northeastern Pacific cyclonic flow during the latter half of the
period--as supported by teleconnections relative to the upper low
over northern Canada. Based on this cluster/teleconnection
relationship, ridging in the 00Z CMC mean looks rather suspect.
The forecast looks as uncertain as ever farther south over the
eastern Pacific. The 00Z ECMWF and some ECMWF ensembles quickly
pull off energy from a shortwave around 140-145W by Thu while a
greater proportion of models/ensembles bring more of the shortwave
energy toward the West Coast. Thus a more even blend of GFS/GEFS
and ECMWF-based guidance may be more suitable over this region at
least into day 6 Fri.
...Weather Highlights/Threats...
The system forecast to affect the southern Plains Tue-Wed should
produce widespread showers and thunderstorms, with
models/ensembles showing a strong signal for heavy rainfall
potential over Texas and Oklahoma. Some convection may be strong
to severe as well. The Storm Prediction Center currently
indicates best chances for severe weather in the early Wed-early
Thu time frame. Check latest SPC products for further
information. Expect scattered to numerous showers and
thunderstorms to accompany the system eastward into the Lower
Mississippi Valley and Southeast later in the week but increasing
uncertainty in system strength and timing reduce confidence in
duration and intensity of rainfall to the east of the Plains.
Current preference for a somewhat slower solution would maintain
decent potential for locally heavy rainfall into portions of the
Gulf Coast and Southeast.
Waviness extending from the Midwest/Great Lakes into New England
and associated front(s) early in the period will provide a focus
for rainfall over parts of the East. Still expect snow to be
confined to locations north of the Canadian border except perhaps
for northern Maine. A front pushing into the Northwest and then
continuing eastward may generate some rainfall but most activity
should be in the light-moderate range. Precipitation may persist
for a time over parts of the Interior West/Rockies as the western
part of the front stalls. Some moisture may reach the Northwest
late in the period but timing/extent are uncertain given the wide
spread in guidance for details of flow aloft at that time.
There will be a broad area of above average temperatures with plus
10-20F anomalies over the West, with some of this warmth extending
into the northern and then central Plains. Daily records
(especially for warm lows) are most likely on Wed-Thu and some are
possible on Fri as well. The front pushing into the Northwest
will produce a cooling trend over northern parts of the West.
Below normal highs will prevail over the southern Plains
especially on Tue-Wed. Parts of the northern tier should trend a
little below normal for highs by Fri-Sat.
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