Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat May 12 2018
Valid 12Z Tue May 15 2018 - 12Z Sat May 19 2018
...A wet and convectively active weather pattern...
...Pattern overview and guidance/uncertainty assessment...
An upper low over Hudson Bay Monday will slide eastward toward
southern Greenland by the end of next week which favors broad
cyclonic flow to its south along the US/Canadian border from the
Upper Midwest eastward to New England.
Upper ridging will slowly bridge atop an active northern Great
Basin upper low Tue/Wed. This low is expected to meander across
the Great Basin/Rockies into midweek, with weakening energies then
drifting into the plains to fire local convection.
Farther east, the west Atlantic subtropical ridge will continue to
exert influence from the lower Mississippi valley and mid/deep
south to the Mid-Atlantic as an active and wet upper low and
tropical moisture plume will be temporarily trapped in between
just west of Florida. This low will slowly lift northward into the
Southeast midweek and be slow to dissipate, but will bring
widespread and locally heavy downpours. In the region of
relatively weak upper flow between the western trough/upper low
and the southeastern ridge, expect a wavy surface front to linger
from the Rockies to the Mid-Atlantic through much of the period
and remain a focus for pooling moisture and periodic convection.
Some record high and low temperatures are possible in the warm
sector Tue/Wed from the lower/mid Mississippi valley to the Ohio
Valley and Mid-Atlantic Tue/Wed.
An amplified mid-upper level trough is expected to track from the
eastern Pacific inland into northern CA and then the south-central
Great Basin next Wed-Fri, with ejecting energy/height falls slated
to breach the Rockies/Plains Fri-Sat. which will offer the
potential for widespread precipitation along and to the north of
its path.
Accordingly, with models/ensembles continuing to show above
average consensus for much of days 3-5 (Tue-Thu), it seems
sufficient to justify use of a multi-model and ensemble blend as
mainly derived from the GFS/ECMWF and NAEFS/ECMWF ensembles.
Increase blend weighting toward ensembles days 6/7 consistent with
growing forecast spread/uncertainty.
...Sensible weather/threats highlights...
Scattered to numerous showers and thunderstorms will be possible
across a large area in the vicinity of the wavy surface frontal
boundary, from the Great Basin/central Rockies to the
Mid-Atlantic/Northeast. Several areas of potentially heavy
rainfall are possible, with the potential for multiple rounds of
convection. The specific areas most likely to receive heavy rains
will likely be somewhat difficult to pin down until the short term
when models begin to resolve mesoscale/convective processes.
Farther south, higher confidence exists as we near the event in
the likelihood for heavy rainfall across the Florida peninsula. An
upper low developing across the northeastern Gulf of Mexico will
advect very high PWs across the Florida (about +2 standard
deviations or around the 98th percentile), and multi-inch rainfall
totals are possible across several days with the potential for
especially urban flooding in the I-95 corridor from Miami to
Jacksonville.
Current indications among the guidance are that deeper moisture
may gradually advect farther north into much of the southeastern
U.S. as the upper low drifts northward, and locally heavy rains
will spread through the Southeast and perhaps into the southern
Mid-Atlantic by the end of the week. As a Canadian cold front
sinks southward to the Mason-Dixon line, precipitation may expand
along this boundary as well, setting up a potential modest/heavy
rain event for the mid-Atlantic to southern New England. Farther
west, precipitation will begin to increase across the West Coast
states by midweek as the aforementioned upper low approaches the
region and its attendant cold front moves ashore.
Schichtel
Associated WPC medium range forecasts of 500 mb heights, surface
systems, weather grids, quantitative precipitation forecasts, and
winter weather outlook snow/sleet 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