Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
207 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...
...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 consistint with
ultimate frontal wave developments will feed into convectively
active warm sector meso-boundaries as well as a stark baroclinic
zone overtop. 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. 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. 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