Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Thu Jun 22 2023

Valid 12Z Sun Jun 25 2023 - 12Z Thu Jun 29 2023

...Prolonged heat wave continues across Texas and southeastern New
Mexico and will expand northeast next week...


...Overview...

The overall pattern next week will be fairly slow-moving, as a
couple of shortwaves/lows should combine to produce upper
troughing across the East and allow for additional rain and
thunderstorms, while another upper trough moves into the West. In
between, an upper high centered over Texas will continue to bring
persistent excessive heat to the southern Plains and into the
Lower Mississippi Valley.


...Guidance Evaluation/Predictability Assessment...

Across the East, early in the week/forecast period, most guidance
showed good agreement with the overall pattern including a compact
upper low tracking across the Midwest/Great Lakes while additional
troughing gets absorbed and the combined energy leads to an upper
trough over the eastern U.S. by around Tuesday. The 12Z UKMET
seemed to be an outlier with the details of the Midwest low,
tracking across the region more slowly and keeping a closed low
farther south of other guidance into Tuesday. The newer 00Z UKMET
looks to be somewhat more in line with other guidance. By
Wednesday there continue to be some differences with the
position/timing of the trough gradually moving east, with GFS runs
on the faster side and the ECMWF on the slower side. A blend of
the 12/18Z guidance along with the ensemble means seemed to give a
good middle ground position.

Farther west, good confidence remains for the hot upper high to
persist across Texas. There has been some spread by the latter
half of the period on the centroid of the high's exact position,
as 12/18Z GFS and CMC runs track east while the ECMWF is held back
west. The ensemble means were more agreeable and show placement in
the middle of this spread. Meanwhile ridging extending to the
north of the high will be affected by the trough coming into the
West. Models show fairly good consistency in a positively tilted
trough with its axis near the West Coast around Sunday-Monday. The
trough should push east while weakening as the week progresses,
with increasing spread in timing and strength along with some
signal for energy to split and race east across the north-central
U.S., with low confidence. Thus a blend of the 18Z GFS/12Z
ECMWF/12Z CMC at first and increasing amounts of the 18Z GEFS and
12Z EC ensemble means with time worked well with these features
and across the CONUS for this forecast.


...Weather/Hazards Highlights...

At the start of the period Sunday, a low pressure system aloft and
at the surface could cause some rain to linger across the Upper
Midwest, with locally heavy amounts possible given the good
dynamical support. Farther south, shower and storm chances will
likely also extend southward across the Mississippi Valley along
and ahead of the frontal system, and areas of the Mid-South may
see some heavy rain rates in potential mesoscale convective
systems near a dissipating warm front. Meanwhile, there may again
be a somewhat focused axis of rainfall across the northern
Mid-Atlantic region into the Interior Northeast on Sunday, with
some flooding concerns over areas with low Flash Flood Guidance.
All these areas have Marginal Risks of excessive rainfall in place
for Sunday-Sunday night. As the systems consolidate with time,
convection looks to overspread much of the East on Monday, with
severe weather a possibility too. A large Marginal Risk has been
introduced for much of the East as there may be locally heavy rain
with these scattered to widespread storms in an unstable
environment. Embedded Slight Risk(s) are possible as well in
future cycles if model guidance agrees on areas of focus for heavy
rain, especially if those areas have been greatly impacted by the
recent and short range heavy rains. By Tuesday and Wednesday, the
front should push slowly eastward with drying behind the front,
while additional rounds of rain and thunderstorms continue for the
Eastern Seaboard ahead of it. Meanwhile the next system into the
West will also allow scattered showers and storm chances to return
across parts of the Northwest and into the northern Rockies and
Plains next Monday-Wednesday.

In terms of temperatures, there is really no end in sight for the
excessive heat that has plagued particularly Texas/southeastern
New Mexico in recent days. In fact, into next week, temperatures
over 100F and heat indices much higher will continue expanding
east into the Lower Mississippi Valley. Several record
temperatures could be set or tied, as temperatures nearing or
exceeding 110 could again occur in western and southern Texas. The
Desert Southwest starts to see temperatures well over 100F next
week too, though not really anomalous for this typically hottest
time of the year there. As mean upper ridging expands farther
north, highs in the 90s are likely across the southern two-thirds
of the Plains and Mississippi Valley, with current forecasts
showing temperatures exceeding 100F into Oklahoma/Kansas/Arkansas
by Wednesday-Thursday. Meanwhile the West should see slightly
cooler than normal temperatures particularly for highs with the
bouts of upper troughing aloft. The East looks to be within a few
degrees of average for early summer, with a trend toward below
average by around 5-10 degrees for the Great Lakes region.


Tate


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, experimental excessive rainfall
outlook, 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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml