Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
830 AM EDT Thu Apr 18 2019


30-DAY OUTLOOK DISCUSSION FOR MAY 2019

The May 2019 outlook is informed by various forecast tools, both dynamical and 
statistical, and is issued against a tropical background state that is largely 
dominated by ENSO. El Nino conditions remain in place across the equatorial 
Pacific, and are forecast to persist into boreal summer. Forecast tools that 
contribute to the 0.5-month lead May 2019 outlook also vary in terms of their 
characteristic time scales. Medium range dynamical models now extend into very 
early May, and the the latest Weeks 3-4 guidance covers nearly the first half 
of the month. These are in addition to guidance initialized during early April 
for the entire month of May, which will tend to emphasize interannual 
variability. 

A good starting place for the May outlook is the calibrated NMME guidance. This 
guidance is augmented by uncalibrated (but non-biased) forecast guidance from 
the ECMWF and more recent runs of the CFS. Long-term trends play a role as 
well. Given the time of year, antecedent soil moisture conditions play a role 
by affecting the partitioning of incident solar radiation between sensible and 
latent heating.

The temperature outlook consists of above-normal temperatures favored in an 
arc-like region from parts of the West Coast, across the northern tier, and 
over much of the eastern CONUS. Above-normal temperatures are strongly favored 
over much of Alaska. Inspection of the forecast circulation for the CFS, for 
example, reveals an ENSO-like wave dispersion featuring positive 200-hPa height 
anomalies centered near and southeast of Hawaii, a weakness in above-normal 
heights extending across parts of the eastern Pacific and southwestern North 
America, and an anomalous 200-hPa ridge over west-central Canada extending 
southeastward to the eastern CONUS. This general structure of the upper-level 
height field is apparent in the latest Weeks 3-4 guidance and is broadly 
consistent with ENSO regressions, but shifted 10-20 degrees longitude eastward 
over the Pacific-North America domain. While this circulation pattern is 
sufficient to explain the overall temperature pattern, there are some other 
factors at play. High-amplitude soil moisture anomalies over much of the 
central CONUS extending into the Midwest have a negative impact on temperature 
during the warm season, and lead to a reduction in above-normal probabilities 
relative to model guidance over parts of the Upper Midwest and Plains. 
Below-normal temperatures are more likely over parts of the Southern Plains, 
where forecast above-normal rainfall and wetter-than-normal soils support the 
model consensus. Equal chances is depicted over a large region encompassing 
much of the Southwest and parts of the Great Basin. Dynamical models are not in 
good agreement here as the long-term warming trends and a relatively cool 
interannual signal are destructively interfering.

The precipitation outlook is consistent with the aforementioned circulation 
pattern; above-normal rainfall is more likely near and east of the anomalous 
cyclonic upper-level circulation. This extends over much of the interior West, 
High Plains, and Southern/Central Plains. There is an eastward extension at 
very modest probabilities across the Gulf Coast and Deep South consistent with 
ENSO and the dynamical model consensus. There is a small region extending from 
the Pacific Northwest to the Alaska panhandle where below-normal precipitation 
is more likely. This is likewise based on the model consensus and consistent 
with long-term trends. There was some consideration of depicting below-normal 
rainfall over parts of the Northeast and Great Lakes in accordance with the 
latest Weeks 3-4 guidance and recent CFS runs. However, this is a low-skill 
region in the hindcast and contrary to long-term trends, so equal chances is 
depicted over the northeastern quarter of the CONUS.

FORECASTER: Stephen Baxter

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 Tue April 30 2019

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