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Extended Forecast Discussion
(Latest Discussion - Issued 0700Z Jul 21, 2024)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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***

...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/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. Thus the Day 4 ERO made little change to the Slight
Risk area depicted over this region. The surrounding Marginal Risk
area extends northeastward through the Mid-Atlantic and southern
New England, with some embedded Slight Risk areas possibly
emerging as guidance refines the most likely regions for heavy
rainfall potential in the shorter term. The Carolinas and southern
Virginia may be one such area of focus, given wet ground from
recent rainfall. 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/Northeast. Again there will
likely be some embedded Slight Risk areas to emerge once guidance
signals become more coherent. 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.


Additional 3-7 Day Hazard information can be found on the WPC
range hazards outlook chart at:

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: