Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu Apr 16 2020
Valid 12Z Sun Apr 19 2020 - 12Z Thu Apr 23 2020
...Excessive Rainfall/Severe Weather Events to Again Threaten the
South Sunday-Monday and Wednesday-Thursday...
...Guidance/Predictability Assessment...
Model timing and smaller scale differences seem best mitigated
with primary reliance on a forecast strategy using ensemble means
and manually applied adjustments. However, the main events/threats
seem to overall offer average to above average predictability due
to their relatively larger scale nature. The WPC medium range
product suite was primarily derived from a blend of the GEFS/ECMWF
ensemble means and the National Blend of Models. Applied greatest
weighting in this blend to the ECMWF ensemble mean that offers a
less progressive southern stream flow consistent with flow
separation and WPC continuity. This solution tends to be more
progressive than the ECMWF and less progressive than the GFS.
...Weather/Hazard Highlights...
The medium range period next week again appears quite busy with
the potential for two back to back heavy convective rainfall
events over the Southern U.S.. The first should focus Sunday into
Monday as shortwave trough energy moves from the Southern Plains
on Day 3/Sunday to off the Mid-Atlantic coast day 4/Monday. Moist
return flow out of the Gulf of Mexico and instability will fuel
widespread showers and thunderstorms from the Lower Mississippi
Valley to the Southeast U.S. with increasing confidence in the
potential for a heavy rainfall event. SPC also shows a significant
threat for severe weather. Overall, there is very good agreement
on the large scale pattern/evolution of the shortwave, with
smaller scale less predictable differences in the details
remaining.
Upper level ridging from the West will make its way eastward
following the first system, promoting a period of above normal
temperatures into the Plains states. Meanwhile, another possible
closed low/amplified trough should send light to moderate
precipitation with a frontal boundary into the Southwest/Southern
Rockies Monday into Tuesday. Lead convection is also slated to
fire across the s-central Plains in this pattern. As the front
reaches the Southern Plains on Wednesday and lead instability and
Gulf moisture rebounds, the stage may be reset for another
possible heavy rainfall/runoff and severe weather threat for parts
of the Southern Plains and across the Deep South. Thereās decent
agreement at this time frame on the large scale pattern, but
continued run to run variability in the details of the
deterministic solutions.
Meanwhile, there will also be possible shortwave energy to move
more in earnest into the Pacific Northwest by next Thursday.
Expect light to moderate showers and higher elevations snows to
accompany this system across the Northwest.
Santorelli/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