Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
149 PM EDT Mon May 14 2018
Valid 12Z Thu May 17 2018 - 12Z Mon May 21 2018
...Inundating Rainfall Threat for the East...
...Guidance and Predictability Assessment...
The guidance offers reasonable clustering on days 3-4 (Thursday
and Friday), with forecast spread and uncertainty increasing
significantly over the weekend and into early next week.
Accordingly, the WPC medium range progs used a mostly operational
GFS/ECMWF blend early, transitioning to a totally ensemble based
blend by the end of the period. This offers a forecast very close
to that of the previous shift.
The main area in question continues to be out in the Pacific and
along the west coast days 5-7. System evolutions and stream
interactions seem particularly sensitive to guidance data ingest
as the majority of models and ensembles continue to show
significantly high run to run variability. Particularly, the ECMWF
has shown distinct diurnal variance with 12 UTC runs and 00 UTC
runs on totally separate pages. Today and yesterdays 00z runs
indicate more general troughing with embedded vorts lingering
along or off the Pacific Northwest coast, while yesterday and
Saturday's 12z runs have shown a deep closed low meandering in the
west Pacific. While the GFS has shown a slightly better
consistency the past few runs, it's still less than stellar. It
appears spread with this system amongst the various ensemble
members of both the GEFS and EC is high as well.
...Weather/Threats Highlights...
Satellite and guidance trends offer quite favorable support for an
inland lifting eastern Gulf of Mexico low and anomalously deep
tropical moisture on its east side likely spreading a significant
threat of excessive rains from the Southeast to up the
Appalachians and Mid-Atlantic/Northeast later this week and into
this weekend. In this pattern, record high temps persist mid-later
week for the mid-lower MS valley just to the west of the main
rainfall area, with possible record warm mins farther east in the
humid airmass.
Overtop...energetic northern stream flow swinging across/through
the U.S./Canadian border from the northern Rockies/plains and
Midwest to the Northeast will support and periodically reinforce a
persistent and wavy frontal zone from the Rockies/Plains to the
Northeast. Hot and juicy warm sector conditions to the south will
offer stark contrast to the cool/continental air mass to the
north. Additional mid-latitude impulses running over this frontal
zone will further act to focus instability and moisture pooling
which will support multiple rounds of heavy rain and embedded
quasi-disorganized convection.
Meanwhile...the West will also be quite active as an amplified
mid-upper level trough and associated surface frontal system
slowly work inland from California midweek to the Great
Basin/Southwest Thu-Fri and into the Rockies by the weekend. The
greatest precipitation threat will be along and north of the upper
system track as enhanced by terrain/deformation. Weaker ejecting
shortwaves out of the Rockies into the plains will help initiate
scattered showers/storms ahead of this weakening upper low, which
will likely exit onto the Plains in weakened state late Sat into
next Sun. SPC does not show a risk area for severe weather through
the period, but says enough instability/moisture should meander
around the surface boundary and dry line to fire some sort of
storm threat.
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