Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Sun Apr 28 2019
Valid 12Z Wed May 01 2019 - 12Z Sun May 05 2019
...Strong to Severe Thunderstorms with Heavy Rains and Flash
Flooding/Flooding Potential Will Threaten the Southern Plains and
Lower/Middle Mississippi Valley Wednesday into Thursday...
...16Z Update...
The overnight model guidance maintained its pattern of showing
troughing over the western U.S. midweek with shortwaves moving
into the central U.S. on day 4/Thu, and a general model blend
worked well there. By Fri and into the weekend, model spread has
been high regarding an upper low tracking across the eastern
Pacific toward California. Fortunately, the 00Z and 06Z GFS/GEFS
have now come more in line with the EC, namely, this southern
stream low remains split from the northern stream flow throughout
the period, whereas older GFS runs combined the streams and moved
the merged trough quickly across the West. The GFS suite remains
quicker than the EC with the upper low track, but has been
trending slower, and there is much better agreement than
previously. However, questions remain downstream across the CONUS
by the weekend--the 00Z EC/EC mean trended toward more zonal flow
in the central and eastern CONUS, whereas the GFS/GEFS indicated a
somewhat more amplified pattern with ridging in the central U.S.
and troughing in the East. Tried to keep some amplification to the
flow by using some WPC continuity from the night shift by Sat/Sun.
Widespread thunderstorms remain likely in the central U.S.,
continuing from the short range period into Wed/Thu. See the
previous discussion for more details.
Tate
...Previous Discussion...
...Overview and Assessment of Model/Ensemble Guidance and
Predictability...
A large closed upper circulation will retrogress and settle over
western Canada this week. This occurs as a series of northern
stream troughs, height falls, and precipitation swaths dig into
the Northwest then eastward over the U.S. northern tier. There is
potential for some degree of phasing with a southern stream flow
of impulses/height falls working underneath during this period
from the eastern Pacific inland over the U.S. southern tier. The
GFS/GEFS and to a lesser extent the FV3 have tended to be on the
progressive side of the full envelope of solutions with recent
ECMWF runs and to a lesser extent ECMWF ensembles and the
UKMET/Canadian lie on the slower side of the forecast envelope.
The WPC medium range product suite was primarily derived from
composite solutions including the National Blend of Models
increasingly toward ECMWF ensemble mean guidance over time. This
is with consideration of the overall blocky flow upstream over the
Pacific, uncertainty with embedded feature timing and phasing in
two main stream flows, and the inherently slow nature of closed
lows to eject into the flow.
...Weather Highlights/Threats...
With the troughing in the West early in the period, temperatures
will be cold enough to support snow mainly in the Northern/Central
Rockies, though late season light snows are possible in the
Northern/Central Plains. A resultant lead and wavy frontal system
will progress mid-late week over the central then eastern U.S.
Deepening inflow/moisture from the Gulf of Mexico consistent with
ultimate frontal wave developments will feed into convectively
active warm sector meso-boundaries as well as a stark baroclinic
zone overtop. Strong to severe thunderstorms with heavy rains and
flash flooding/flooding potential will focus/threaten the Southern
Plains and Lower/Middle Mississippi Valley Wednesday into
Thursday. Flooding remains possible as particularly the
Mississippi Valley has had flooding concerns for weeks, and with
the convective nature of the precipitation, flash flooding is
possible as well. The bulk of activity will progress across the
East and South into Friday before shifting offshore by next
weekend as new upstream northern and southern stream trough
energies/height falls and modest precipitation likely works back
into the West in continued split flow.
Schichtel
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