FXUS07 KWBC 301901
PMD30D
Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Tue Apr 30 2024


30-DAY OUTLOOK DISCUSSION FOR MAY 2024

The May 2024 Temperature and Precipitation Outlooks are updated considering 
current Weather Prediction Center (WPC) Outlooks for week 1, Climate Prediction 
Center (CPC) Extended Range and Subseasonal Outlooks, dynamical model guidance 
for week 2 and weeks 3-4, soil moisture, background climate states, and recent 
Climate Forecast System version 2 (CFSv2) monthly forecasts of temperature and 
precipitation. Currently, equatorial sea surface temperatures (SSTs) are above 
average across the central and east-central Pacific Ocean and El Niño 
conditions are observed. However, tropical Pacific atmospheric anomalies are 
weakening and a transition from El Niño to ENSO-neutral is likely by April-June 
2024, with the odds tilted toward La Niña developing by June-August 2024. 
Despite the weakening of El Niño some lingering impacts are expected into May. 
The Madden Julian Oscillation (MJO) weakened during mid to late April, but 
dynamical model forecasts favor a re-emerging MJO across the eastern Indian 
Ocean, with propagation across the Maritime Continent and Western Pacific 
forecast during early May. Some weak impacts due to MJO are possible in early 
May, though teleconnections tend to be weak in spring.



CPC 6-10 Day Outlooks (early May) feature a 500 hPa trough over the Bering sea 
and Alaska and down the west coast, while ridging is forecast over the central 
and eastern Contiguous United States (CONUS). This pattern is persistent but 
less amplified in the CPC 8-14 Day Outlook period (through mid-May). By weeks 
3-4, CFSv2, GEFSv12, and ECMWF 500 hPa predictions generally favor ridging over 
the western CONUS and troughing over the east, with differences in placement of 
ridge or trough axes and strength of the height pattern. This indicates a 
fairly transient pattern expected throughout the month of May, with troughing 
moving eastward and becoming less certain in terms of placement and amplitude 
by the end of the month.



The updated Monthly Temperature Outlook remains similar to the previously 
released Outlook over the eastern CONUS where above normal temperatures are 
favored, though there are some changes to the spatial extent. Ridging over the 
eastern CONUS through mid-May supports the tilt toward above normal 
temperatures, and good model agreement exists over parts of the Southwest and 
Texas as well as the Great Lakes and parts of New England, thus probabilities 
remain similar to the previously released Outlook. Though short term forecasts 
through mid-May are overall warm over the eastern CONUS, model runs for week 
3-4 begin to introduce troughing and cooler temperatures by the end of the 
month stretching from the Central Plains to the Mid-Atlantic. However, the 
below normal temperature probabilities are weak due to uncertainty in the 
forecasted mid-level height pattern. Given the warm beginning and middle of the 
month, but weaker and uncertain below normal probabilities by the end of May, 
33 to 40% probabilities are maintained for much of the region. The largest 
change from mid-month is to the northwestern edge of the above normal 
probabilities. Shorter term forecasts favor below normal temperatures from the 
west coast overspreading through the Central and Northern Plains from early to 
mid-May beneath favored troughing that is forecasted to shift eastward with 
time. Given the forecasted cooler start to the month over the west that could 
stretch into the central and north central CONUS by mid-month, the western edge 
of above normal temperature probabilities is shifted eastward compared to the 
initial release. The forecasted cooler start and transition to above normal 
temperatures by the end of the month supports removal of the above normal 
probabilities over the Northwest in favor of Equal Chances of Above, Near, and 
Below Normal Temperatures (EC). Near normal is maintained over southern 
California and parts of the the Southwest where earlier North American 
Multi-Model Ensemble (NMME) and Copernicus Model Suite (C3S) forecasts were in 
good agreement, but is shifted toward the coast where we might see mixed 
temperatures throughout May due to the potential for marine layer intrusions. 
Over Alaska, below normal temperatures are indicated over the southwest where 
EC was previously indicated due to persistent below normal sea surface 
temperature (SST) anomalies and better model agreement in updated runs.



Several changes are made in the updated May 2024 Precipitation Outlook. Given 
the transient  pattern forecast throughout May, the Precipitation Outlook is 
relatively uncertain, and areas of EC are indicated where signals are weak and 
inconsistent in forecasts throughout May. EC is now favored in place of the 
weak tilt toward below normal precipitation previously indicated over northern 
Washington, Idaho, and Montana due to the potential for above normal 
precipitation in the beginning of May, and weak and inconsistent signals 
throughout the remainder of the month despite the expected below normal El Niño 
influence. A weak tilt toward below normal precipitation is still forecast over 
the Southwest where early to mid-May forecasts tilt toward below normal along 
with CFSv2 monthly forecasts, but probabilities are damped considering Weeks 
3-4 Outlooks that indicate EC or above normal precipitation over parts of the 
region. Above normal precipitation is indicated from the Central and Southern 
Plains eastward to the Mid-Atlantic coast, but tilts toward EC along coastal 
areas of Gulf States where forecasts vary throughout the month of May, and 
models show weak or inconsistent signals. Below normal precipitation 
probabilities are favored over Peninsular Florida where below normal forecasts 
are more consistent in models, and where the CPC Week-2 Hazards Outlook 
indicates Rapid Onset Drought Risk. A weak tilt toward above normal 
precipitation is depicted over eastern North Dakota, Minnesota, and the western 
Great Lakes due to forecasted early to mid-May above normal precipitation and 
consistent signals in most recent CFSv2 runs. Precipitation probabilities over 
Alaska remain similar to the mid-month Outlook. 



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*****              Previous mid-month discussion below            
***** 
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El Niño conditions are currently observed, and equatorial sea surface 
temperatures (SSTs) are above average across the much of the central and 
eastern equatorial Pacific Ocean, reaching 0.9 degrees Celsius above normal in 
the Niño3.4 region, 0.6 degrees Celsius above normal in the Niño3 region, but 
have decreased to -0.2 degrees Celsius below normal in the Niño1+2 region. 
Above average SSTs have weakened across most of the equatorial Pacific Ocean in 
the last 4 weeks, and tropical Pacific atmospheric anomalies are weakening as 
well. Given the weakening signals in the equatorial Pacific, a transition from 
El Niño to El Niño Southern Oscillation (ENSO)-neutral conditions is likely by 
April-May-June 2024. The Madden Julian Oscillation (MJO) has continued to 
weaken recently and though there has been eastward propagation in the past 
week, the amplitude has dropped. Dynamical model forecasts of the Realtime 
Multivariate MJO (RMM) index through close to the end of April are weak and 
incoherent due to an emerging low-frequency feature over the western Indian 
Ocean which appears to be interfering with the MJO. While we may see some 
lingering impacts to temperature and precipitation due to El Niño, the 
influence is expected to weaken throughout the next few months. Moreover, the 
weak amplitude of the MJO does not support much of a teleconnection response 
over the Contiguous United States (CONUS). As such, the May 2024 Temperature 
and Precipitation Outlooks are mainly supported by the lingering influence of 
El Niño, local SST anomalies, influence of soil moisture anomalies, dynamical 
model guidance from the North American Multi-Model Ensemble (NMME), Copernicus 
model suite (C3S), and the Climate Forecast System version 2 (CFSv2), as well 
as statistical models that include the influence of trend and ENSO. The Week 
3-4 Outlook for the early part of May was also considered, as was the expected 
evolution from the Week 2 forecast period. 



The May 2024 Temperature Outlook favors above normal temperatures over the 
Northwest, much of the eastern half of the CONUS, and southeastern Alaska. A 
relatively small region of near normal temperatures is indicated over parts of 
California, Arizona, Nevada, and Utah. The highest probabilities of above 
normal temperatures, 50 to 60 percent, are located over Washington, the Great 
Lakes, northern New England, southeastern New Mexico and parts of southwestern 
Texas, the southern tip of Florida, and southeastern Alaska. Probabilities are 
enhanced over these regions given good consistency among tools on above normal 
probabilities. Decadal trends also support above normal probabilities over the 
northwest and northern New England. Anomalously warm Great Lakes temperatures 
provide additional support for above normal probabilities over the region. 
Lower than normal soil moisture along with forecasted below normal rainfall in 
dynamical models, and strong probabilities for above normal temperatures in 
recent CFSv2 forecasts of May temperatures adds to confidence over the 
Southwest. Signals were more mixed over western Alaska given cooler SST 
anomalies and sea ice, but NMME and C3S favor a transition to warmer 
temperatures over the southeast part of the state. Near normal and equal 
chances of above, near and below normal temperatures (EC) are indicated over 
southern California and the Four Corners where there are observed coastal below 
normal to neutral SSTs, near normal temperature forecasts from NMME and C3S, 
and a below normal CFSv2 temperature forecast. While tools tilt toward above 
normal temperatures over the Gulf States (particularly coastal regions), 
partially supported by warmer Gulf of Mexico SSTs, high amounts of recent 
rainfall over eastern Texas and Louisiana have resulted in high soil moisture, 
and as such probabilities are still above normal but damped. EC is favored 
where models were inconsistent and/or where there was a lack of support from 
statistical tools. 



There is more uncertainty in the May 2024 Precipitation Outlook than the 
Temperature Outlook, as evidenced by lower overall probabilities and larger 
areas of EC. Models favor above normal precipitation over parts of the central 
and southern CONUS, including parts of the Mid-Atlantic, Middle Mississippi and 
Ohio Valley regions. Some lingering influence from El Niño further supports the 
tilt toward above normal precipitation over the Southeast, as well as enhanced 
soil moisture. Though El Niño can lead to below normal precipitation over the 
Great Lakes, Middle Mississippi Valley, and Ohio Valley, this influence is 
overshadowed by above normal precipitation trend. EC is indicated over the 
Great Lakes where models had weak or inconsistent signals and trend and El Niño 
influence are opposite. However, over the Mississippi and Ohio Valley regions 
C3S and NMME favored above normal rainfall and thus a weak tilt toward above 
normal is indicated. Dynamical models and dry soil moisture led to the favored 
probability of below normal precipitation over parts of the southwest, which is 
also indicated in the Seasonal Outlook for May-July 2024. However, this is at 
odds with the recent Weeks 3-4 Precipitation Outlook for early May that tilted 
weakly toward above median precipitation. We expect below normal precipitation 
over the region by mid- to late- May. Tools were again mixed over the 
northwest, but influences from El Niño and trend tilt the odds toward below 
normal. Finally, over Alaska, above normal precipitation is favored over the 
southwest where there has been recent above normal precipitation, with a weak 
tilt toward below normal precipitation over southeastern Alaska due to expected 
influence from El Niño.

FORECASTER: Johnna Infanti

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 Jun ... will be issued on Thu May 16 2024

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