Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Apr 16 2020


30-DAY OUTLOOK DISCUSSION FOR MAY 2020

The May 2020 temperature and precipitation outlooks are based primarily on 
dynamical model guidance, statistical tools, and current soil moisture 
conditions. ENSO is not a factor in the monthly outlooks since it is predicted 
to remain neutral. Since the beginning of April, a large-scale area of enhanced 
convection propagated rapidly eastward across the global tropics from the 
Maritime Continent to the Western Hemisphere. As of April 15, the anomalous 
upper-level divergence and associated enhanced convection are overspreading 
Africa. Although a coherent Wave-1 pattern of enhanced (suppressed) convection 
is observed over the Western Hemisphere (Indian Ocean and Maritime Continent), 
this anomalous convection is likely more related to a convectively coupled 
Kelvin wave due to its fast eastward propagation. Based on the latest dynamical 
model forecasts of the RMM index, Kelvin and equatorial Rossby waves are 
expected to contribute to anomalous tropical convection heading into May. 
Therefore, the Madden-Julian Oscillation (MJO) is unlikely to influence the 
mid-latitude circulation pattern. Weekly sea surface temperature (SST) 
anomalies, centered on April 8 are running as much as 2.5 degrees C above 
normal in parts of the Gulf of Mexico. Since SSTs (as of April 13) are near or 
slightly above 27 degrees C in the southern Gulf of Mexico, the future 
progression of Kelvin waves will have to be closely monitored for an early 
season tropical cyclone later in May into the beginning of June. 

During the final week of April, the GFS, Canadian, and ECMWF ensemble means 
feature a less amplified longwave pattern compared to early to mid-April. Also, 
there is large spread among ensemble members which is not surprising due to the 
lower amplitude. The increased chances of above normal temperatures forecast 
across the western CONUS, Gulf Coast, and most of the East Coast are based on 
the North American Multi-Model Ensemble (NMME) and is also consistent with 
long-term trends during the spring season. The largest probabilities (60 
percent or higher) of above normal temperatures are forecast across central and 
south Florida due to unanimous support from the dynamical models, long-term 
trends, and much above normal SST anomalies surrounding the Florida Peninsula. 
Although the NMME indicates increased chances of above normal temperatures 
across the Great Lakes,  equal chances (EC) of below, near, or above normal 
temperatures are forecast for this region since the ECMWF model features below 
normal temperatures during early May. Soil moisture currently ranks above the 
90th percentile from the Missouri River Basin south to the Ozarks and Tennessee 
Valley. This high soil moisture content is likely to have a cooling effect on 
temperatures for these areas during May. The EC forecast for temperatures 
across the Great Plains, Mississippi Valley, and Corn Belt is consistent with 
these ongoing soil moisture conditions along with the calibrated NMME.

The dynamical models are in good agreement and feature increased chances of 
above normal precipitation across the central Rockies, much of the Great 
Plains, and Southeast. This wet pattern is also supported by the CFS and ECMWF 
models (initialized on Apr 13) with their depiction of an upper-level trough 
over the Rockies and High Plains during early May. Despite the good model 
agreement, the probabilities of above normal precipitation are tempered due to 
predictability in the monthly precipitation outlook at this time lead. Also, 
precipitation becomes more convective and less predictable later in the spring. 
The NMME supports increased chances of below normal precipitation across the 
coastal Pacific Northwest. Modifications to the precipitation outlook are 
likely with the updated release on April 30. 

Above normal temperatures are most likely across Alaska, due to good agreement 
among the dynamical model guidance. In addition, the CFS and ECMWF models 
feature above normal temperatures during early May. Probabilities of above 
normal temperatures were reduced across northern mainland Alaska, due to a deep 
snowpack for this time of year. The only notable signal among the precipitation 
tools is a slight lean towards below normal precipitation across the Aleutians 
and Alaska Peninsula during May. 

FORECASTER: Brad Pugh

The climatic normals are based on conditions between 1981 and 2010, 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.

An updated monthly outlook... for May will be issued on Thu April 30 2020

These outlooks are based on departures from the 1981-2010 base period.
$$