Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Sun Jun 17 2018
Valid 12Z Wed Jun 20 2018 - 12Z Sun Jun 24 2018
...Heavy rain threat for southern Texas and the north-central
U.S...
...Pattern Overview and Guidance/Predictability Assessment...
Energy from a short range closed low over the Western U.S. ejects
eastward into the Plains by the start of medium range period
(Wed-Sun) and continues tracking eastward into the Mississippi
Valley by late week. Meanwhile, strong ridging over the Pacific
Northwest shifts eastward as a shortwave trough enters the
Northwest by Friday. Across the eastern states, the northern
periphery of ridging over the East/Southeast should be eroded by
northern stream cyclonic flow over the Great lakes and the
Northeast, which should suppress the oppressive heat and humidity
expected early this week.
Overall, the models and ensembles continue to show a reasonably
similar large scale pattern over the next week, with smaller
mid-scale embedded features still offering a bit of forecast
spread. There are minor timing and intensity differences regarding
the western U.S. shortwave, with the latest run of the ECMWF
showing a much faster progression into the Pacific Northwest coast
than the deterministic GFS or the ensemble means. The other area
of concern is with a upper level tropical weakness lifting into
the Texas Gulf Coast and lingering across the Rio Grande Valley
mid-week. The operational ECMWF and UKMET both show a defined low
somewhere near far South Texas, while the GFS and ensemble means
would support something weaker. Regardless, this poses a
heavy/excessive rainfall threat for portions of the Texas Coast
and far south Texas.
This cycle of the WPC medium range suite was mainly derived from a
blend of the latest runs of the GFS/ECMWF with their ensemble
means, with more weighting towards the ensembles by the end of the
period. This blend provides a good middle ground solution to all
systems mentioned above and de-emphasizes the less predictable
smaller scale features which still remain highly uncertain. This
also maintains good continuity with the previous shift.
...Weather Highlights/Threats...
A main and persistent/wavy frontal boundary remains
quasi-stationary this week from the Plains to the Ohio Valley and
Mid-Atlantic. Expect periods of locally excessive convection to
focus along this boundary and in the warm sector aided by early
period tropical moisture influx from the Gulf of Mexico. The best
potential for heavy to excessive rainfall however should be over
southern Texas, due to a potential tropical wave/disturbance now
being followed by NHC. As the trough and front shifts slowly
eastward into later week, the heaviest rainfall may increasingly
spread along the surface boundary as enhanced by any potential
wave developments/convective system propagations into the
Midwest/Ohio Valley states and Mid-Atlantic.
Santorelli
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4