Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sun Jun 13 2021

Valid 12Z Wed Jun 16 2021 - 12Z Sun Jun 20 2021

...Record to dangerous heat continuing through the week across
much of the Interior West and through midweek over the northern
Plains...

...Extreme heat possible for the Southwest/California through the
end of the week...


...Overview...

A very strong upper ridge centered over the Four Corners states
will remain a dominant feature in the pattern through the period
but should gradually weaken and drift a bit southward by next
weekend.  Long-term mean troughing over eastern North America will
consist of a leading East Coast trough that departs after Thursday
followed by moderately amplifying energy over western-central
parts of southern Canada and northern tier U.S. (and possibly some
trailing flow from Canada).  This amplifying energy will push a
front across the northern half of the lower 48 through the period,
with the trailing part of the front likely stalling over the
central Plains/Rockies.  Pacific flow should bring another
shortwave/upper low and leading frontal system into western Canada
and northwestern U.S. next weekend.  The western upper ridge will
support a hazardous heat wave event over parts of the Interior
West/Rockies and extending into California during the latter half
of the week, with a broad area of temperatures that should
approach/exceed daily record values.  Some of this heat will
extend into the northern Plains through Wednesday followed by a
moderating trend.  The upper ridge may weaken enough by Sunday to
bring temperatures below record levels, albeit still well above
normal.  In the Gulf of Mexico there is continued potential for a
disturbance to form near the Bay of Campeche and move northward
late in the week and weekend, but with a large degree of
uncertainty in its evolution.  The National Hurricane Center
continues to monitor this area in its outlooks.


...Guidance/Predictability Evaluation...

Based on guidance through the 12Z/18Z cycles, the updated forecast
started with an operational model blend for day 3 Wednesday
followed by removal of the 12Z UKMET and in another couple days
the CMC while ultimately trending to an even weight of the 12Z
ECMWF/ECMWF mean and 18Z GFS/GEFS mean by day 7 Sunday.

Within the initial East Coast trough there are still
low-predictability shortwave uncertainties that will take
additional time to resolve.  Some models hint at enough flow
separation to close off a small-scale upper low for a brief time
somewhat south of the main closed low forecast to track just north
of Maine.  The 12Z UKMET held the upper trough west of consensus
while the new run compares better to other guidance.

A blend approach worked well to resolve detail spread for the
southern Canada/northern tier shortwave that ultimately settles
over eastern North America.  By late week GFS runs stray a bit to
the strong side with the upper trough and southward with the
leading cold front.

Guidance spread for North Pacific flow reaching western Canada and
the northwestern U.S. by late week/weekend is less dramatic than
it was 24 hours ago but some differences persist.  The 12Z UKMET
became exceedingly fast after Wednesday and then the CMC strayed
on the fast side as well.  By day 7 Sunday an average between the
faster 12Z ECMWF and slower 18Z GFS for the arriving trough/upper
low compared reasonably well to the GEFS/ECMWF means in principle.
 The new 00Z UKMET/CMC are somewhat less extreme than their 12Z
runs and are close to the past couple ECMWF runs through late week.

Today's guidance still signals the potential for tropical
development to emerge from the Bay of Campeche and track northward
over the Gulf of Mexico.  The prevailing trend over the past day
or more has been slower (with the CMC continuing to be on the
faster side of the operational model envelope) and in some cases
westward.  Ensemble means are still fairly ill-defined due to
individual member spread.  Needless to say there is still
considerable uncertainty.  The updated manual forecast reflected
only a very slight westward nudge of the surface system and
extrapolation yielded an approach to the west-central Gulf Coast
next weekend about a day later than was the case 36 hours ago.


...Weather Highlights/Hazards...

Ahead of the northern tier cold front on Wednesday, expect a broad
area of temperatures 10-20F above normal from California into the
northern Plains.  Continued progression of the front east of the
Rockies will bring a pronounced cooling trend to the central U.S.
after Wednesday/Thursday.  The Southwest/California and the Great
Basin should maintain similar anomalies into the weekend with the
Northwest also rebounding to well above normal levels late in the
week.  The combination of an arriving Pacific front and gradual
weakening of the Four Corners ridge may finally start to reduce
the coverage of plus 10F and greater anomalies by next Sunday. 
Before then expect multiple days with numerous locations seeing
temperatures approach or exceed daily record highs/warm lows.  The
combination of extreme daily temperatures and duration, as well as
being rather early in the season for such an event, should make
this a dangerous heat wave for sensitive groups.  Excessive heat
warnings are in place for many areas of the Southwest next week
where temperatures may climb well into the 110s in the lower
deserts with low temperatures only in the 80s and 90s.

The upper trough near the East Coast should reach far enough
eastward to allow for a drier trend over the northern half of the
region mid-late week though shortwave energy/Atlantic surface
waviness could still produce a little rain near the southern
Mid-Atlantic.  At the same time somewhat more rain may fall over
Florida while a stalling/weakening front near the Gulf Coast may
help to focus diurnally enhanced showers and thunderstorms.  Areas
near the Gulf Coast should continue to monitor forecasts/National
Hurricane Center discussions over the coming days for the latest
information regarding any potential tropical cyclone development
by the latter half of the week into the weekend.  Regardless of
the strength of any possible system, the northward flow of
tropical moisture would bring the threat of heavy rainfall to
portions of the southern tier.  The cold front pushing eastward
from the northern Plains will produce some rain and thunderstorms
from the Midwest eastward/southeastward into the eastern U.S. 
Some of this activity could be locally heavy but it may take into
the short range time frame to determine any areas where it may be
relatively more likely.  Areas of rain may also be possible along
the stalling front back over the central Plains/Rockies.

Rausch/Fracasso


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