Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 15 2018
Valid 12Z Fri May 18 2018 - 12Z Tue May 22 2018
...An excessive rainfall threat over the East and active Western
to Central U.S. weather upstream...
...Guidance and Predictability Assessment...
Guidance offers reasonable clustering days Friday and Saturday,
but forecast spread and uncertainty increases significantly later
weekend and into early next week. Accordingly, the WPC medium
range progs were primarily derived from a GFS/ECMWF/UKMET blend
early, then quickly transition to an ensemble blend.
...Weather/Threats Highlights...
Anomalously deep moisture from the Gulf of Mexico and western
Atlantic will likely spread a significant threat of excessive
rains from the Southeast to up the Appalachians and
Mid-Atlantic/Northeast later week into the weekend. In this
pattern, record high temps persist mid-later week for the
mid-lower MS valley west of the main rainfall area, with record
warm mins possible for the Mid-Atlantic in the humid airmass.
The West will meanwhile also be quite active as an amplified
mid-upper level trough and frontal system slowly work over the
Great Basin/Southwest Fri and Rockies by the weekend. Widespread
precipitation will develop along and north of upper system track
as enhanced by terrain lift and deformation. Ejecting shortwaves
out of the Rockies into the plains will help initiate scattered
showers/strong convection ahead of this weakening upper system,
that then further shears over the east-central U.S.
Overtop...Energetic nrn stream flow also digs to the U.S.to also
periodically reinforce a persistent and wavy frontal zone from the
Plains to the Northeast. Hot and moist warm sector conditions to
the south will offer stark contrast to the cool/continental air
mass to the north. Instability and moisture pooling along the
front will support multiple rounds of heavy rain with embedded
convection.
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
Santorelli/Schichtel