Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1145 AM EDT Fri Jun 14 2019
Valid 12Z Mon Jun 17 2019 - 12Z Fri Jun 21 2019
1545 UTC Update...
Relatively progressive flow across the U.S. northern tier is
expected to persist through much of the medium range. By late next
week a somewhat more amplified flow pattern will take hold aloft
as an upper trough amplifies across the Pacific Northwest/northern
Rockies with additional wave amplification across the Great
Lakes/Northeast by that time. Earlier in the period, a series of
upper shortwaves will traverse a persistent frontal boundary
extending from the central U.S. to the Mid-Atlantic/Northeast,
resulting in several waves of low pressure along the front. Models
offered a range of solutions with the timing of these waves, and
given their scale predictability is only moderate at best. A blend
of the ECMWF and GFS seemed to offer a reasonable solution through
days 3-5 (Mon-Wed). The GFS seemed a bit more closely aligned with
consensus with respect to the aforementioned frontal waves
Tue-Wed, and thus that solution was weighted a bit more relative
to the ECMWF. By days 6-7 (Thu-Fri) models agree on a broad upper
trough amplification in the Northwest, but differ on the details
of the feature and smaller scale waves embedded within it. Farther
east, solutions varied on timing and amplitude of the trough
amplifying across the Great Lakes/Northeast by that time. Thus,
weighting of ECENS/GEFS ensemble means was boosted to comprise a
majority of the forecast by days 6-7.
Ryan
Previous Discussion (issued at 0447 UTC)...
...Overview and Guidance/Predictability Assessment...
Latest guidance indicates a persistent southeast Canada trough
with confluent flow south in the the MS Valley and Oh Valley to
the northeast and mid Atlantic; allowing a frontal system to
progress across these areas.
Consensus still shows robust strengthening of the Pacific ridge
from late weekend into early next week. Teleconnections from an
anomalously strong high in the northeast Pacific lead to more
energetic troughing over parts of the northwestern-north central
U.S. next Tue-Fri 21 Jun.
Prefer a blended approach giving more weight to the ensemble means
of the 12z ECMWF and 18z GEFS Ensemble Means, with less weighting
to the operational 12z GFS and 12z ECMWF.
The 12-18z GFS varied as to which northern stream wave amplified,
with the 12z run amplifying a wave over the northern Plains Thu 20
Jun and the 18z run weakening the northern Plains wave but
amplifying the wave crossing the Great Lakes to southeast Canada
by 12z Fri 21 Jun. The 00z run has shifted the Fri morning low 21
Jun south into NY State. Given the large run to run swings and
teleconnection signal of a low amplitude Great Lakes trough, using
the mean was preferred.
Farther west the greatest forecast problem involves the specifics
of energy expected to amplify into a trough drifting east from the
northern Rockies into the northern Plains next Wed-Fri 21 Jun.
The 12z ECMWF was more intense with the 500 mb low with more ECMWF
ensemble members maintaining an open wave, so blending with the
ECMWF Mean mitigated differences with the trough timing/intensity.
Given the ECMWF and GFS indicate an elongated trough with
potential for multiple circulation centers over the northern
Rockies and northern Plains and the depiction of the 2 models
changed considerably from run to run, more weighting was given to
the Ensemble Means of the 18z GEFS and 12z ECMWF.
...Sensible Weather Highlights/Threats...
The expected pattern evolution at the surface and aloft continues
to be favorable for multiple episodes of convection from the
southern half of the Plains across the lower-mid MS Valley into
the Ohio Valley/Tn Valley/ Appalachians. However there is still
considerable uncertainty for some of the day-to-day details. The
activity across this area will likely extend into the
Northeast/Mid-Atlantic with locally moderate-heavy rainfall
possible.
Over the West the best concentration of showers/thunderstorms
should be over the northern-central Rockies Mon-Tue. During
Tue-Thu expect showers/storms to spread into the northern
Plains/Upper Mississippi Valley with the system forecast to affect
those regions. The Florida peninsula should see diurnal
convection through the period.
Over the West expect broad coverage of above normal temperatures
Mon in CA/OR/interior WA/ID with some pockets of greater than plus
10F anomalies. Readings should then trend cooler as jet energy
reaches the Northwest Tue/Wed, leaving interior California/Nevada
as the primary area of warmth heading into midweek. Areas across
the northern Plains will tend to see below normal highs by as much
as 10 degrees Wed 19 Jun through Fri 21 Jun.
Petersen
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml