Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
309 AM 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...
Guidance offers reasoable clustering days 3-4 (Thu-Fri), with
forecast spread/uncertainty increasing significantly over the
weekend. Accordingly, a composite model/ensemble blend was used to
derive the WPC medium range surface fronts/pressures and 500mb
progs days 3/4 before transitioning to a blend of modestly
compatable ensemble means days 5-7.
Forecast spread from the Pacific to the west coast is particularly
egregious days 5-7 in a pattern with much below normal
predictabilty. System evolutions and stream interactions seem
quite sensitive to guidance data ingest and most models and
ensembles have shown less than stellar run to run variance. Trends
from at least the last 7 runs of the ECMWF have also shown a
distinct diurnal variance with 12 UTC runs on one page and 00 UTC
runs on quite another. This modeling phenomena has been
highlighted in this sensitive flow pattern, but I've noticed this
occurance in varied degrees over the past few years.
...Weather/Threats Highlights...
Satellite and guidance trends offer quite favorable support in
that an inland lifting ern Gulf of Mexico low and anomalously deep
tropical moisture on its east side will likely spread a
significant threat of excessive rains from the Southeast to up the
Appalachians and Mid-Atlantic/Northeast later this week 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 nrn Rockies/plains and Midwest
to the Northeast will support and periodically reinforce a
persistent and wavy frontal zones 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 CA 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 sfc boundary and dry line to fire some storm threat
through the period.
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
Schichtel