Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
128 AM EDT Wed May 15 2019
Valid 12Z Sat May 18 2019 - 12Z Wed May 22 2019
...Guidance Assessment and Weather Highlights/Threats...
...System moving from the Plains Sat/Sun to Upper Ms Valley/Great
Lakes Mon/northeast Tue...
The latest deterministic models (18 UTC GFS and 12 UTC
ECMWF/UKMET/Canadian)
have good clustering of the surface low forecast to be in Nebraska
Sat morning, with gradual weakening of this low followed by
development of a second low 00z Sun over KS that moves north to
near the NE/IA border by 12z Sun morning. The slow frontal movement
leads to potential for multiple rounds of showers/storms and
locally heavy rain from the southern to the central Plains. See
the Storm Prediction Center outlooks for the probability of severe
convective weather.
The WPC product suite primarily was derived from a composite
ensemble mean solution with greater weighting on the ECMWF
ensemble mean as its position is close to the multi-model
consensus 12z Sun. By 12z Mon 20 May, excellent overlap of the 12z
ECMWF Ensemble Mean and 18z GEFS Mean with the low pressure center
supports a mean solution. Greater weighting on the means remains
through mid week as the models/means divergence as the system
crosses into the Upper MS Valley/Great Lakes/northeast and into
the coastal and offshore waters by the middle of next week.
...Developing western US Mean trough over the weekend into the
middle of next week...
The models show a negatively tilted upper trough poised to move
into the western US over the weekend and remain in place through
the middle of next week. There is potential for a closed low to
develop at the base of the trough in the southwest but the 12z
ECMWF/18z GFS/12z Canadian global models are hundreds of miles
apart on the low track/position by the middle of next week,
lending decreasing confidence to the forecast. The 12z ECMWF was
on the southeastern edge of the suite of solutions in the southern
Plains with its 500 mb low position, so more weighting to the
better clustered 12z ECMWF/18z GEFS Ensemble Means was used
instead.
Much of the western US to the northern high Plains will turn
colder, with persistent areas of 10-20 egress below normal for
high temperatures expected in the central to northern Great Basin
to the northern High Plains. As a result of cold temperatures,
terrain enhanced snows are possible in the ranges in the Pacific
Northwest/northern CA/Great Basin/northern to central Rockies.
Downstream, the persistence of western cyclonic circulations
induces a mid level anticyclone over the lower MS Valley to the
southeast. Underneath the upper ridge, much of the lower MS Valley
eastward across the southeast, including the Carolinas, turn more
summer like with above normal temperatures this weekend into the
middle of next week. A stalled front leads to the persistence of
showers/storms Sat over the FL Keys.
Petersen
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