Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Wed Jul 31 2024


30-DAY OUTLOOK DISCUSSION FOR AUGUST 2024

The updated monthly temperature and precipitation outlooks for August 2024 are 
based on official forecasts and outlooks from WPC (week-1), CPC (week-2, weeks 
3-4), and dynamical model guidance from the GEFS and European ensemble suite. A 
weighted GEFS-CFSv2 30-day forecast for both temperature and precipitation is 
also used, where weighting is based on recent 500-hPa height anomaly 
correlation scores. The updated outlooks also consider the latest soil moisture 
anomalies, coastal sea-surface temperatures (SSTs), 14-day precipitation 
deficits, initial conditions, climatology, long-term trends, and expected 
tropical cyclone activity in the Atlantic and East Pacific basins.

The updated temperature outlook for August 2024 favors above-normal 
temperatures across much of the contiguous U.S. (CONUS) and most of the 
northeast half of the Alaska Mainland. Maximum probabilities exceed 60 percent 
over the Middle and Southern Atlantic coast states, and 70% over central 
portions of the Interior West. This broad expanse of favored warmer-than-normal 
temperatures is supported by the 30-day European ensemble forecast, 30-day 
weighted GEFS-CFSv2 temperature forecast, and is consistent with most of the 
dynamical model guidance and official temperature outlooks noted in the first 
paragraph. The 30-day European ensemble forecast depicts 500-hPa heights to be 
above-normal across practically all of the CONUS, with the larger height 
departures predicted over the West and Northeast. This leaves the general 
vicinity of the Mississippi Valley open to a weak mean trough and expected 
cooler surface temperatures. This weak trough, and the fact that this portion 
of the Lower 48 states has seen a large amount of temperature and precipitation 
variability over the past month, are the main reasons underpinning the forecast 
of Equal Chances (EC) of below-, near-, and above-normal temperatures. This is 
also the basis for reducing the probabilities for above-normal temperatures 
relative to the rest of the CONUS. Below-normal temperatures are favored over 
most of the southwestern half of the Alaska Mainland, the Alaska Peninsula, and 
the eastern Aleutians. This is supported by the last 9 runs of the CFSv2 model, 
anomalously cold SSTs near the western and northern coasts of Alaska, the 
30-day European ensemble forecast, and the weighted GEFS-CFSv2 temperature 
forecast. Some of these tools, but not all, support above-normal temperature 
chances over the northeast half of the Mainland. In areas where temperature 
tools were at odds, or where temperature signals were weak, EC is indicated.

The updated precipitation outlook for August 2024 favors above-normal 
precipitation for the eastern Dakotas, Upper Mississippi Valley, Great Lakes 
region, Atlantic coast states, the Gulf Coast area, southern Texas, and into 
the Four Corners region. This is based on some of the objective tools and 
considers a very active hurricane season in the Atlantic basin that is 
forecasted. Relatively warm SSTs are observed along most of the Gulf and 
Atlantic coasts, which would benefit any tropical cyclones passing over them. 
The latest extended-range (out to 2 weeks) runs of the European ensemble and 
GEFS depict a significant chance of a tropical system approaching the Southeast 
coast sometime around August 8th and 9th. If this scenario materializes, 
portions of the Southern and Middle Atlantic coasts could easily receive enough 
rain to exceed their monthly normal precipitation for August. Over the vicinity 
of the Upper Mississippi Valley and Great Lakes region, the favored 
above-normal precipitation, which is predicted by many (but not all) of the 
tools, is consistent with the idea of a weak monthly mean trough over or just 
upstream of this region. Weak to moderate mid-level ridging centered over the 
northwestern CONUS favors a slight tilt in the odds towards above-normal 
precipitation for portions of the southern Four Corners states, in the deeper 
easterly flow south of the mean ridge axis. Therefore, a slightly enhanced 
monsoon is favored across this portion of the Southwest. This same ridge, along 
with negative soil moisture anomalies, favors below-normal precipitation across 
much of the northwestern quarter of the CONUS, with a southeastward extension 
across south-central portions of the Great Plains and MIssissippi Valley, and 
into western Tennessee. Elsewhere across the CONUS, EC is favored. For Alaska, 
above-normal precipitation is favored across most of the state, with the 
exception of EC over Southeast Alaska and the western Aleutians. This is 
primarily based on the 30-day European ensemble, and to a lesser extent, the 
GEFS-CFSv2 precipitation forecast, the last several runs of the CFS, and 
extended-range precipitation outlooks.

—------- The previous monthly discussion for August is shown below —---------

The August 2024 monthly temperature and precipitation outlooks are based on 
various climate factors. These include the phase of ENSO, the Madden-Julian 
Oscillation (MJO), dynamical and statistical model guidance, and boundary 
conditions such as coastal sea-surface temperatures (SSTs) and recent soil 
moisture anomalies. Historical temperature and precipitation trends were also 
considered, as were recent observations and the latest Weeks 3-4 temperature 
and precipitation outlooks which extend through the 9th of August.

Currently, ENSO-neutral conditions are observed but La Niña is favored to 
emerge during August-September-October (ASO) 2024 (70% chance) and persist into 
the Northern Hemisphere winter (79% chance) during November-December-January 
(NDJ) 2024-25. The latest weekly Niño 3.4 index was +0.3 deg C. Mostly 
near-normal SSTs prevailed across the near-equatorial eastern and east-central 
Pacific, while temperatures below the surface in these regions were 
below-normal. Convection was near-normal to slightly above normal across 
Indonesia and the vicinity of the Date Line. Collectively, these 
ocean-atmosphere indicators reflected ENSO-neutral. The MJO signal showed 
little eastward propagation this past week between the Indian Ocean and 
Maritime Continent regions, and the OLR plots displayed considerable 
disorganization. Impacts from both ENSO and the MJO for the month of August are 
expected to be minor.

The North American Multi-Model Ensemble (NMME) and Copernicus Climate Change 
Service (C3S) multi-model, also known as the International Multi-Model 
Ensemble, and their constituent model inputs served as the dynamical basis for 
the monthly outlooks. The Constructed Analog on Soil Moisture Tool (CA-SMT), 
ENSO composites and regressions, and historical trends served as the 
statistical basis for the outlooks. Three consolidation tools were used that 
skill-weighted and calibrated the dynamical models (NMME-CON), the statistical 
models (STAT-CON), and a Final-CON which consolidated the other two CONs.

Boundary conditions (coastal SSTs and recent soil moisture anomalies) were also 
considered in the construction of the monthly temperature and precipitation 
outlooks for August 2024. SSTs were below-normal near the coasts of northern 
and western Alaska, mostly near-normal near the West Coast, and above-normal 
along the Gulf and Atlantic coasts. Recent areas of relatively low soil 
moisture include the Ohio and Tennessee Valleys, the Central and Southern 
Appalachians, portions of the Mid-Atlantic region, the Carolinas, the Rio 
Grande Valley of Texas, and large portions of the northwestern Lower 48 states. 
In contrast, recent areas of relatively high soil moisture include from the 
north-central states to the central Great Lakes region (due to frontal activity 
and thunderstorm clusters), and eastern Texas and much of the Lower Mississippi 
Valley, primarily due to the passage of former Hurricane Beryl on July 8-9. 
NOAA’s Atlantic Seasonal Hurricane Outlook calls for a well-above average 
season this year, which puts the Gulf and East Coasts at an increased risk of 
impacts from tropical storms and hurricanes.

The temperature outlook for August 2024 favors above-normal mean temperatures 
over nearly all of the contiguous U.S. (CONUS). This is supported by the three 
consolidation (CON) tools, CFS, and the C3S. A majority of the tools, including 
the NMME PAC, depict a relative weakness and/or break in the favored warmth 
across portions of the Great Plains and Mississippi Valley, which is consistent 
with significant climate variability observed so far this summer. Maximum 
probabilities favoring above-normal temperatures exceed 70% over portions of 
the north-central Rockies, supported by much of the model guidance and the 
expected late-season influence of dry soils. This is a relatively dry time of 
the year for this area. For the Southwest, the C3S and its inputs are confident 
of significantly above-normal temperatures and below-normal total 
precipitation. Depending upon the speed of the anticipated transition of 
ENSO-neutral to La Niña later this summer, some of the favored dryness across 
the Southwest may be related to the typically dry La Niña footprint over this 
region.  With other model solutions not being quite as confident as the C3S, 
suggesting lower probabilities for above-normal temperatures would be prudent 
across the Southwest. Historical trends also support elevated probabilities for 
warmer-than-normal temperatures over this region and the southern High Plains 
during August. Across the north-central states, odds are only slightly tilted 
towards above-normal temperatures due to several factors. These factors include 
adequate model and tool support, the expectation of significant variability 
continuing in the future, and the anticipated continued presence of very moist 
soils. Over the eastern CONUS, there is plenty of dynamical model support for 
the favored above-normal temperatures, with limited statistical support. 
Relatively warm water surrounds the Florida Peninsula and enhances the chances 
for anomalous warmth. Odds surpass 60% over an area centered on the Central 
Applachians, in part due to low soil moisture. Near the immediate West Coast, 
Equal Chances (EC) of below-, near-, and above-normal temperatures are favored 
in August, attributed to the influence of the marine layer.

For Alaska, odds are tilted towards below-normal temperatures over southwestern 
sections of the state, largely due to the nearby Bering Sea and its unusually 
cold water for this time of year. The favored below-normal temperatures are 
also backed by a regression tool which regresses temperature anomalies against 
the standardized Niño 3.4 index for the July-September season (with the central 
month, August, being our target period). For the remainder of the state, a 
subjective consensus of models and tools suggests a weak tilt towards 
above-normal temperatures for the northeast portion of the Mainland and EC 
elsewhere.

The precipitation outlook for August 2024 favors above-normal precipitation for 
the southeastern third of Texas, much of the Lower Mississippi and Tennessee 
Valleys, southern Ohio Valley, the Southeast, Southern Appalachian mountains, 
and from much of the Mid-Atlantic region to southern and central New England. 
This is supported in large part by the three CON tools, CA-SMT, CFS, and C3S. 
Maximum probabilities for above-normal precipitation chances surpass 50% for 
most of the Gulf Coast and Southeast coast states. The last 9 runs of the CFS 
model favor above-normal precipitation over much of the same general area. This 
anticipated relatively wet pattern is also deemed judicious given NOAA’s 
seasonal hurricane outlook which favors a very active Atlantic hurricane season 
in 2024. In contrast, below-normal precipitation is favored for much of the 
western half of the CONUS. Most of the support for the relatively dry outlook 
comes from the three consolidation tools, C3S, CFS, Meteo-France model, and the 
UKMO model. For several of the last 9 daily CFS runs, wetter-than-normal 
conditions have been predicted by the CFS model over the northwestern CONUS, 
which is a significant deviation from most other tools and was therefore 
discounted. Minimal probabilities favoring below-normal precipitation amounts 
are indicated across most of Arizona and New Mexico, given the early and 
consistent start to the Southwest monsoon and potential backdoor cold front 
activity that often affects New Mexico in particular.

In Alaska, there is a tilt in the odds towards above-normal precipitation for 
the southwestern Mainland, the Alaska Peninsula, and the eastern Aleutians. 
This has the support of many of the C3S inputs, including the German and 
Canadian models (DWD and CMCC, respectively), to name a few. The latest 3 runs 
of the CFS also lend some support to the favored above-normal precipitation 
chances across southwestern Alaska. Elsewhere across Alaska and the Lower 48 
states, EC is favored.

FORECASTER: Anthony Artusa

The climatic normals are based on conditions between 1991 and 2020, following
the World Meteorological Organization convention of using the most recent 3 
complete decades as the climate reference period.  The probability anomalies
for temperature and precipitation based on these new normals better represent
shorter term climatic anomalies than the forecasts based on older normals.

The next monthly outlook...for Sep ... will be issued on Thu Aug 15 2024

These outlooks are based on departures from the 1991-2020 base period.
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