Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 31 2024

Valid 12Z Mon Jun 03 2024 - 12Z Fri Jun 07 2024

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


...Overview...

The early part of next week will feature progressive shortwaves
and frontal systems in the northern two-thirds or so of the lower
48, leading to precipitation chances in the Northwest on Monday and
possibly multiple rounds of convection in the central and eastern
U.S. through midweek. The upper pattern looks a little more blocky
farther east, with a narrow ridge over the eastern Great
Lakes/Canada and a North Atlantic mean low near the Canadian
Maritimes. Meanwhile very hot and humid conditions should continue
across Texas (especially southern areas) under the northern
periphery of an upper ridge anchored over Mexico, to the south of
the storm track. By the latter half of the week, expect a more
amplified and persistent pattern to develop, as upper ridging
expands from Texas into the Interior West (expanding the coverage
of potentially hazardous heat), while an upper trough axis develops
downstream in the East.


...Guidance/Predictability Assessment...

The primary synoptic-scale questions arise by mid-late week as the
amplified upper ridge-trough pattern takes shape. By early
Wednesday
the past couple days of dynamical and ECMWF-initialized machine
learning (ML) models have been indicating a better ridge developing
over California/Nevada and somewhat lower heights over the southern
Plains versus what GFS/GEFS runs have been advertising. However the
12Z UKMET/CMC have nudged somewhat in the GFS direction. Later in
the week, the ML models vary in their details but on average seem
to favor a compromise between the western ridge axis of the ECMWF
and eastern axis of the GFS/GEFS. The 06Z and 12Z GFS runs have
been steadily raising heights over the West Coast versus the 00Z
run so this compromise still looks good. Details of this ridge will
also affect how much moisture could begin to lift into the West
from lower latitudes, with the GFS more aggressive in that regard.
Farther east, the ML models ultimately cluster a bit east/northeast
of the 00Z ECMWF for the upper low that may reach Ontario or
vicinity by late in the week. Only one of five ML models supported
anything remotely like the 00Z CMC that dropped its upper low into
the Tennessee Valley. The new 12Z CMC adjusted well northward by
late week but is still a bit on the southeast side relative to
most other solutions for the upper low.

With the 00Z UKMET showing flatter flow than consensus across the
northern tier, the updated blend based on 00Z/06Z guidance
incorporated the 06Z GFS-GEFS/00Z ECMWF and a little 00Z CMC for
about the first half of the period (GEFS helping to tone down some
GFS track issues across southern Canada). Then the forecast
transitioned to split ECMWF/ECens mean input and phase out the CMC,
ultimately yielding a compromise with a very slight tilt to the
ECMWF/ECens relative to the GFS/GEFS by next Friday.


...Weather/Hazards Highlights...

An upper-level wave and atmospheric river coming into the
Northwest will bring heavy rainfall continuing into Monday. In the
Day 4 Excessive Rainfall Outlook, Marginal Risk areas remain in
place for flooding concerns with the heavy rain. Snow levels may be
lowering some by Monday in the Cascades and northern Rockies as
the shortwave moves overhead, but there is still some concern for
rain at elevations that still have a snowpack, possibly increasing
the flooding threat. Latest guidance shows some precipitation
continuing into Tuesday, but with a less pronounced moisture feed
and less southward extent. Thus there are no risk areas over the
Northwest in the Day 5 ERO. Expect drier conditions mid-late week
as upper ridging builds.

An initial low pressure/frontal system in the central U.S. and the
next one coming in from the West, plus more diffuse shortwaves
farther south, will lead to rounds of showers and thunderstorms
over a broad portion of the central and eastern U.S. The focus for
potentially heavier rain is rather uncertain for Monday as
instability abounds in widespread areas, but there still seems to
be a general model and ensemble signal for Missouri/Arkansas and
surrounding states to maintain a Marginal Risk in the Day 4 ERO. A
vigorous frontal system (anchored by southern Canada low pressure)
reaching the northern Plains by around late Monday night may also
produce locally heavy rainfall over North Dakota, but for now this
activity appears sufficiently isolated and progressive to preclude
any risk area in the ERO. By Day 5/Tuesday, the focus for
convection is a little more clear as instability pools ahead of the
aforementioned front. A Marginal Risk is still in place across
northern and central parts of the Mississippi Valley Tuesday. Some
locations within this area will be sensitive due to rainfall from
prior days. Current detail/timing differences suggest it may take
until closer to the event to determine whether there may be any
merit to an embedded Slight Risk area.

Continued progression of the Plains cold front should shift the
best shower/thunderstorm focus into the eastern third of the
country by mid-late week. Some areas of moderate to heavy rainfall
will be possible but there is still considerable uncertainty over
timing and location of highest rainfall totals. Additional weak
upper-level energies from the Southern Plains to the Southeast
will also keep up the potential for additional storms. A period of
low level upslope flow behind the front reaching the southern
Plains by Wednesday could also eventually help to develop some
convection over parts of the southern Rockies/Plains. Also of note
from the hazards perspective earlier in the week, the northern
Rockies may see high winds on Tuesday behind the strong southern
Canada/northern tier U.S. system.

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 early in the week but
should moderate somewhat by midweek behind a frontal passage. Some
above normal temperatures are possible across the northern tier at
times. But the next big concern will be increasing heat across the
West as upper ridging builds in. Highs broadly will be running
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 reaching over 110F in the Desert
Southwest. Per the Climate Prediction Center, this hot pattern
could continue at least into next weekend.


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