Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Sun Jul 21 2024

Valid 12Z Wed Jul 24 2024 - 12Z Sun Jul 28 2024

***Western U.S. heat wave to become more pronounced over the
 northern High Plains while moderating elsewhere, and
 showers/storms with heavy rainfall expected from the Four Corners
 to portions of the East***

19Z Update: The 12Z models maintain very good synoptic scale
agreement through the end of the week, and guidance has improved
with the shortwave/upper low crossing southern Canada. The GFS had
been a progressive solutions on earlier runs, but is now more in
line with the model consensus. The NBM still appeared too light
with convective QPF across many areas, including the Desert
Southwest and also across portions of the Gulf Coast region, so
these values were increased with incorporation of the deterministic
GFS and ECMWF. A slight Risk area will be introduced for the Day 5
period across portions of the Carolinas and into southern Virginia.
By next weekend, the CMC becomes more amplified with the trough
exiting the East Coast and stronger with a trough across the
Central Plains, but relatively comparable elsewhere, and the ECMWF
is slower for the trough to exit the East Coast. Given the good
overall agreement even through next weekend, the use of the
ensemble means was only increased to about 30% by that time, with
the majority composing deterministic guidance and a little of
previous WPC continuity. The previous forecast discussion is
appended below for reference. /Hamrick
------------------------

...General Overview...

Latest guidance continues to show a steady transition toward a more
typical summertime mean pattern after starting out fairly amplified
at the start of the period early Wednesday. Expect the strong
ridge initially over the West (producing hazardous heat from the
West into the northern High Plains) to become weaker and more
suppressed as a Pacific upper low tracks across southwestern Canada
and residual troughing sets up along the West Coast. Monsoonal
moisture will contribute to daily episodes of showers/storms over
the Four Corners states into the Great Basin. Some of this
moisture may eventually interact with a front pushing into the
northern Plains as Canadian dynamics continue eastward. Meanwhile,
one or more wavy fronts will be on the leading side of Great Lakes
into southern Plains mean troughing aloft, leading to multiple
days of rain/thunderstorms with areas of heavy rainfall from the
southern Plains into the Mid-Atlantic and parts of New England.
The Great Lakes and Northeast should eventually trend drier late
week as the northern part of the trough moves eastward, but a more
persistent upper level weakness over the Plains may continue to
generate episodes of rainfall farther south. Atlantic upper
ridging should build into the Southeast for a time, peaking in
strength around Wednesday-Thursday. Southeast ridging may rebuild
by next Sunday.

...Guidance/Predictability Assessment...

The most prominent forecast issue continues to involve the mid-late
week trough crossing the Great Lakes and Northeast. Through the
12Z/18Z cycles, the array of guidance remained similar to the past
couple days with the ECMWF/ECens mean on the slow side and GFS runs
fairly fast and open with the UKMET/CMC in the middle. The CMCens
mean tilted to the slower side as well but ECMWF-initialized
machine learning (ML) models have consistently been in the middle
to faster part of the spread--at least favoring somewhat of a lean
away from the slow 12Z ECMWF. The new 00Z ECMWF has arrived with a
significantly faster/open trend. Farther west, as the western
Canadian upper low continues eastward and possibly opens up later
next weekend, the 18Z GFS strayed a bit south/southeast of
consensus with the upper low, bringing greater height falls across
the northern tier U.S. Thus the 18Z GFS could be overdone with the
southward extent of the leading cold front. The new 00Z GFS looks
better in that regard. The overall array of guidance supported an
intermediate solution among 12Z/18Z operational models early in the
period and then a transition toward approximately half models/half
means late in the period.

...Weather/Hazards Highlights...

Dangerous heat over the West/northern Rockies should extend at
least into midweek with high temperatures reaching the 90s and
100s and warm overnight lows providing only limited relief. After
decent coverage of plus 5-15F anomalies for highs on Wednesday,
some cooling will likely move in from the West Coast (bringing
highs within a few degrees on either side of normal) as moderate
upper troughing sets up near the coast. Meanwhile, forecasts
continue to show the most anomalous heat shifting into
Montana and the northern High Plains during mid-late week. In
particular during Wednesday-Thursday the experimental HeatRisk
index shows fairly broad coverage of major risk of heat-related
impacts. A few daily records will be possible as well. The
prolonged period of dry heat over the West extending into this week
could also result in enhanced wildfire danger. In contrast, the
cloudy and wet pattern over portions of the South and East will
tend to keep highs below normal from Texas east-northeastward, with
greatest coverage of negative anomalies Wednesday-Friday (and
coolest versus normal over Texas/Louisiana). Southern and eastern
Texas may remain below normal into the weekend.

Monsoonal moisture will continue to support episodes of diurnally
favored convection over the Four Corners region into the Great
Basin, with some shifting of coverage over the course of the period
based on pattern evolution. Isolated to scattered instances of
flash flooding will be possible, especially near steep terrain and
burn scars. The Day 4 Excessive Rainfall Outlook valid on Wednesday
has expanded the Marginal Risk area northwestward based on guidance
signals and favorable combination of anomalous moisture and
instability (especially in the GFS) and proximity of a wavy front.
The Day 5 ERO for Thursday reflects guidance consensus of moisture
pushing eastward given southwesterly mean flow aloft. Potential
remains for embedded Slight Risk upgrades entering the short range
period depending on how guidance clusters relative to each other
and with sensitive burn scar areas/regions with wettest ground
conditions. Toward late week through the weekend, some of this
moisture may interact with the front progressing across the
northern Plains to produce some areas of focused rainfall.

The upper trough extending from the Great Lakes into southern
Plains and one or more leading wavy fronts will support a broad
corridor of locally heavy shower and thunderstorm potential from
the southern Plains northeastward next week. The best guidance
signals in terms of coherence and rainfall magnitude (with ample
moisture and instability) persist over southern/southeastern Texas
on Wednesday, although the latest trend has been for a more
southward progression and closer to the coast. Thus the planned
Day 4 ERO update will be scaled back some on the northern extent
of the existing Slight Risk area depicted over this region. The
surrounding Marginal Risk area extends northeastward through the
Mid-Atlantic and southern New England, although the overall risk
area has been made a little more narrow to account for latest model
trends.

The Day 5 ERO valid on Thursday depicts a broad Marginal
Risk area from southern/eastern Texas through the South and into
the Northeast, with persistence of the corridor of enhanced
moisture from Day 4 and potential interaction of the Great Lakes
system (with its separate Marginal Risk area on Day 4) with this
moisture over the Mid-Atlantic and into the Northeast. A Slight
Risk area is now planned from the upstate of South Carolina to the
greater Richmond metro area of Virginia, with this region expected
to have multiple rounds of heavy rainfall in the days leading up to
this. Expect the Great Lakes/Northeast to trend drier late week
into the weekend while showers and thunderstorms of varying
intensity will likely persist into the weekend over the southern
tier.

Rausch


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