Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Sat Jun 1 2024

Valid 12Z Tue Jun 04 2024 - 12Z Sat Jun 08 2024

...Excessive heat likely to continue over southern Texas, while
heat builds in the West for the latter half of next week...


...Overview...

As the medium range period begins Tuesday, expect an upper-level
shortwave to track across the north-central U.S. along with a
surface low pressure/frontal system, spreading rain and
thunderstorms to much of the Mississippi Valley. This shortwave
trough will meet a somewhat blocky pattern farther east and
slow/deepen over the east-central U.S. late week into the weekend,
with rain chances gradually shifting toward the eastern U.S. along
and ahead of the cold front. Meanwhile an upper ridge atop Mexico
and southern Texas Tuesday will expand to affect much of the
western U.S. midweek and beyond, producing potentially hazardous
heat. An eastern Pacific upper trough may come into the picture by
next weekend but should still be well offshore as of Saturday.


...Guidance/Predictability Assessment...

The dynamical and machine learning (ML) guidance broadly agrees on
the upper pattern transitioning to a fairly amplified and more
persistent ridge-trough configuration. The developing eastern
trough should be composed of leading southern Canada/northern
Plains dynamics and trailing Pacific energy streaming in
underneath. However there still a number of discrepancies for
important details such as the shape/orientation/location of the
ridge and trough.

First looking at the developing trough/upper low and trough,
recent ensemble mean runs have been trending deeper and better
defined with the upper low--which at least helps to increase
confidence in the fairly deep operational guidance and ML models.
There are differences in upper low track though, with the 00Z MLs
on average a little south (more into the Great Lakes) of the
00Z/06Z dynamical guidance versus some of the new 12Z dynamical
runs (especially the CMC which was previously fairly far south)
adjusting northward into southern Canada through next Saturday. An
additional complication introduced by the 00Z ECMWF and persisting
into the 12Z cycle (now including the 12Z GFS to some degree) is
the potential for trough elongation--yielding notably higher
heights aloft over the Southeast, whether in the form of much
weaker troughing or even ridging. The 00Z MLs were not very
supportive of the ECMWF/12Z GFS ideas.

Farther west, the 00Z MLs continued to support a ridge axis between
the western ECMWF/ECens and eastern GFS/GEFS/CMCens with perhaps a
partial tilt in the ECMWF direction. For the second day in a row,
6-hourly GFS runs have been steadily trending toward slower arrival
of eastern Pacific troughing by late in the period so such a
compromise still looks reasonable. Meanwhile the 00Z MLs also
recommended slower northward progress (somewhat closer to latest
CMC runs) of a southern stream upper low expected to be west of
Baja California for most of the period and possibly lift northward
by the end of the week. Most guidance comprising the favored blend
reflected the faster progression but the ML trends will be worth
monitoring.

Guidance considerations based on the 00Z/06Z guidance led to a
forecast update consisting of the ECMWF/GFS/CMC/UKMET in order of
more to less weight through the first half of the period, followed
by GEFS/ECens increasing incorporation reaching nearly half by
next Saturday.


...Weather/Hazards Highlights...

A vigorous shortwave and frontal system (anchored by southern
Canada low pressure) reaching the Dakotas into the Upper Midwest on
Tuesday will provide forcing and a focus for rain and
thunderstorms as instability abounds ahead of it. Smaller scale
shortwaves farther south should also track eastward and promote
scattered convection. A broad Marginal Risk remains in place for
the Day 4/Tuesday Excessive Rainfall Outlook along and a ways
east/west of the northern three-fourths of the Mississippi Valley.
Some locations within this area will be sensitive due to rainfall
from prior days. While the combination of guidance clustering and
QPF totals do not yet appear sufficient for one or more embedded
Slight Risk areas, there are two regions of interest which at least
are on the higher side of the Marginal Risk spectrum. One would be
over the the Upper Midwest with the northern tier front/upper
dynamics, while the second would be along and east/northeast of
the Missouri/Arkansas border where weak shortwave energy may
generate locally heavy convection. Higher resolution guidance may
help to refine the potential threats over the coming days.

Into Wednesday, convection will likely shift east ahead of the
cold front into the Great Lakes region and Ohio and Tennessee
Valleys, with a Marginal Risk depicted in the Day 5 ERO over those
areas with some modest adjustments from continuity per latest
guidance. By then there is notable spread for timing of a leading
wave/warm front near along the East Coast, leading to the greatest
uncertainty in specifics over the northern Mid-Atlantic. Faster
timing of the UKMET/CMC and some ML models would lead to greater
rainfall potential over the Mid-Atlantic relative to the GFS/ECMWF
and most of their ensembles. For both days, the forward motion of
the storms may be a limiting factor for flash flood potential.
After early Thursday, progression of the front will take rain
chances more to the East Coast/Northeast, with possible focus of
heavier amounts from the northern Mid-Atlantic into the interior
Northeast where the best forcing is. By the latter half of the week
there is increasing uncertainty over the shape of the supporting
upper trough. This will affect the speed of the front and in turn
intensity/duration of heaviest rainfall. Even behind the cold
front, there may be diurnally enhanced episodes of rainfall given
the persistence of upper troughing.

Expect some lingering precipitation over the Northwest on Tuesday
followed by a drier trend as upper ridging amplifies over the
region. Gusty winds are possible in the northern Rockies behind
the strong southern Canada/northern tier U.S. surface low on
Tuesday. Into late week, some return flow of moisture is possible
across the southern/central Rockies and Plains, with one or more
surface boundaries helping to focus showers and thunderstorms.
Teleconnections relative to the forecast large scale pattern, as
well as dynamical/ML models, support the general theme of rainfall
potential over this region but confidence is currently low
regarding details of coverage and amounts. Some scattered rainfall
will also be possible late in the week over parts of the Interior
West/northern Rockies, with details depending on the position of
the upper ridge axis and how much moisture/shortwave energy may
lift northward to the west of this axis.

Next week will remain hot across South Texas westward through the
Rio Grande Valley, where the combination of hot temperatures into
the 90s and 100s as well as humidity will keep heat indices in the
upper 100s, locally 110+. Temperatures nearing or exceeding 100F
may stretch across the southern High Plains through Tuesday but
should moderate somewhat by midweek behind a frontal passage.
Then the next big concern will be increasing heat across the West
as upper ridging builds in. Highs broadly will run 10-15 degrees
above average, with even warmer anomalies of 15-20+ degrees for the
central California valleys and portions of the Great Basin.
Temperatures should reach over 100F across the central California
valleys, and near/over 110F in the Desert Southwest. Scattered
daily record high minimum and maximum temperatures are possible.
Per the Climate Prediction Center, this hot pattern could continue
into early next week. As for other aspects of the temperature
forecast, some above normal temperatures are possible across the
northern tier ahead of the front advancing eastward from the
northern Plains. Cooler air behind the front may bring readings
down to near or moderately below normal levels over an increasing
area to the east of the Rockies or Plains during Thursday-Saturday.


Rausch/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 forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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
https://www.wpc.ncep.noaa.gov/#page=ovw