Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Tue Jun 15 2021
Valid 12Z Fri Jun 18 2021 - 12Z Tue Jun 22 2021
...Record to dangerous heat continuing through the week across
much of the Interior West...
...Extreme heat for the Southwest/California through the end of
the week...
...Tropical system may bring heavy rainfall to Central Gulf Coast
later this week and weekend...
...Overview...
The anomalously strong upper ridge over the Four Corners region
will continue the dangerous and record heat wave for much of the
Southwest to Interior West into the end of the week while on
Friday the East will be in the latter part of a brief period of
drier/cooler air as an upper trough moves over the western
Atlantic. The Four Corners ridge will then steadily weaken and
drift southward while mean troughing sets up over the central U.S.
As this trough deepens expect low pressure to track through and
northeastward from the Plains Sunday onward. This low pressure and
trailing front as well as a leading wavy stationary/warm front
should produce increasing coverage and intensity of rain and
thunderstorms. Meanwhile a potential tropical system could bring
heavy rainfall hazards to portions of the central Gulf Coast and
nearby areas from Friday through the weekend. The National
Hurricane Center is now advertising a high likelihood for
increased development and organization of that system over the
next few days.
...Guidance/Predictability Evaluation...
The full envelope of surface low tracks for the possible tropical
system is still broader than desired. The average of GFS/ECMWF
runs is still a bit faster than 24-36 hours ago, but the past two
CMC runs have been much slower than their predecessors and the
UKMET continues to be on the southwestern side of the spread. Over
the past day the GEFS mean has gained some definition, suggesting
a little improvement in clustering, while its track now more
closely mimics the operational run that is somewhat on the eastern
side of the envelope. Thus far the last couple ECMWF runs are on
their own in maintaining as strong of a system as they show after
landfall. After Sunday the progression of any remaining surface
reflection will become increasingly dependent on the details of
northern stream flow. Given the ongoing spread and varying trends
over the past few days, the updated forecast made only modest
nudges from continuity for strength and track.
Away from the tropical system, there is decent agreement with the
general evolution of the upper pattern but various differences
with details. A blend approach resolves continued shortwave
discrepancies for the southern Canada into northern tier U.S.
shortwave that pushes a front through the northeastern quadrant of
the lower 48 with the western part stalling over the
Plains/Rockies. Convective outflows still provide some uncertainty
over precise frontal position. Then there are typical differences
with how incoming energy amplifies into the developing central
U.S. mean trough aloft Sunday-Tuesday. Recent GFS/ECMWF runs have
trended deeper with the core of this trough, leading to a deeper
and more northeastward surface low that will interact with the
leading central/eastern U.S. front.
Guidance comparisons for the Gulf system as well as flow to the
north ultimately led to a starting blend incorporating the 18Z-12Z
GFS/18Z GEFS mean and 12Z ECMWF/ECMWF mean through the period.
More weight went to the operational runs early and slightly more
to the means late, while awaiting more continuity to gain
confidence in the operational ECMWF/GFS solutions at that time
frame. Also the blend removed the 12Z GFS by day 7 Tuesday as its
northern tier trough began to differ more from consensus.
...Weather Highlights/Hazards...
Dangerous and record breaking heat wave for much of the western
U.S. will continue through this week with some of the most extreme
temperatures over the Southwest. Expect daily temperatures 15F to
20F above normal with afternoon readings well into the 100s and
110s for the Southwest, California Central Valley, and portions of
the Interior West. To the north readings will be in the 90s. The
combination of extreme daily temperatures and duration, as well as
being rather early in the season for such an event, will make this
a dangerous heat wave for sensitive groups. Excessive heat
warnings/watches are in place for many areas of the Southwest and
the California Central Valley. Temperatures over most of
California and the Southeast will gradually moderate with time
into early next week but areas from the northern Great Basin into
the Northwest should continue to see highs 10-20F above normal
into next week--but any daily records should be fairly isolated.
Some areas from the central/south-central Plains into the Midwest
should see highs up to 10-15F above normal on Friday before
trending cooler with frontal passage. The central U.S. should see
a broadening area of modestly below normal readings early next
week under an amplifying upper trough. The East will start out on
the cool side Friday morning and then rebound to or a bit above
normal, though clouds/rain reaching parts of the South may lead to
below-normal highs during the weekend.
A potential tropical system is forecast to drift northward from
the Bay of Campeche later this week and approach the central Gulf
Coast by Friday into this weekend. The National Hurricane Center
is monitoring for increased organization and development but
regardless, the system will be capable of producing heavy rainfall
over portions of the Central Gulf Coast and this could spread
inland over the Southeast this weekend. Farther north, a leading
front crossing the northeast quadrant of the lower 48 followed by
a system expected to track through the Plains/Upper Great Lakes
may serve to focus multiple areas of locally heavy rain within a
broad area from Plains/Midwest into the Ohio Valley/Great Lakes.
Details in convective complexes and potential interactions from
the tropical system to the south lower forecast confidence quite a
bit so it may take a few days to resolve any potential heavy rain
threat there.
Rausch/Taylor
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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