Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Tue Apr 30 2019


30-DAY OUTLOOK DISCUSSION FOR MAY 2019

The May 2019 outlook is updated at the end of the month considering the latest 
forecast guidance for the month of May, including dynamical model forecasts for 
May average temperature and accumulated precipitation from the NCEP Climate 
Forecast System (CFS) and Global Ensemble Forecast System (GEFS), as well as 
shorter range forecasts for week 1 and 2 from the NCEP GEFS, ECMWF, and 
Canadian (ECCC) models, and guidance for weeks 3 and 4 from the CFS, ECMWF and 
JMA models as well as the Subseasonal Experiment (SubX), a suite of 
experimental and operational models, including the GEFS and ECCC models. The 
background climate state is similar to that during the release of the prior May 
outlook issued mid-April. El Nino conditions continue across the equatorial 
Pacific, and are forecast to persist, likely into boreal summer. Equatorial 
convection is enhanced near and just west of the International Date Line, and 
upper-level anticyclonic circulations are observed to the north and south of 
the equator. An MJO event developed recently, with enhanced convection 
progressing eastward from the Indian Ocean to the Maritime Continent in the 
last week. Dynamical models predict the progression of enhanced convection 
across the Western Pacific in the next couple of weeks, where it may alter the 
current El Nino influenced background state. 

The temperature outlook indicates a larger area of enhanced probabilities of 
below normal than the previous May outlook for much of the central CONUS east 
of the Rockies. The area of likely below normal extends eastward to the western 
Great Lakes, across the northern tier, and from the southern Rockies to western 
Texas in the south. Enhanced probabilities of above normal temperatures 
continue in the outlook for much of the eastern CONUS, while likely below 
normal temperatures for the Northeast early in the month lead to reduced 
probabilities for May above average temperatures for the region. Dynamical 
model forecasts for increased probabilities of below normal temperatures over 
the central CONUS and above normal temperatures over the Southeast in week 2 
appear to be influenced by an active MJO over the Western Pacific. Above normal 
temperatures continue to be most likely for Alaska and the northwest CONUS, 
under predicted ridging, with the highest probabilities over western and 
southern coastal areas of Alaska, where above average sea surface temperatures 
persist. Decadal trends and dynamical model forecasts indicate a slightly 
enhanced probability of above normal temperatures extending into parts of the 
Southwest, west of the Four Corners region. 

The precipitation outlook indicates substantially enhanced probabilities of 
above normal from northeastern Texas into eastern Oklahoma and Kansas, as well 
as the states of Arkansas and Missouri, where heavy precipitation is predicted 
in dynamical model forecasts for the first week of May, and above normal 
precipitation is likely throughout the month. The precipitation outlook for 
above normal precipitation over much of the central CONUS and along the Gulf 
Coast for May appears consistent with the current El Nino base state as 
modified by MJO activity over the western Pacific. Dynamical model forecasts 
predict enhanced probabilities of above normal precipitation for May from parts 
of Central and Southern California eastward into the Central Rockies, and 
across the Central and Southern Plains to the Mississippi Valley ahead of 
troughing near the southwestern CONUS. The outlook indicates slightly enhanced 
probabilities of above normal precipitation extending eastward to the 
Appalachians and Florida, where dynamical model forecasts indicate below normal 
precipitation in the first couple days of May and during the second half of the 
month, but precipitation in model forecasts for week 2 increase the chances of 
above normal for total monthly accumulated precipitation. The outlook indicates 
equal chances of above and below normal precipitation for the Atlantic Coast 
from Georgia to Maine. Dynamical model forecasts for May from the CFS and GEFS 
continue to indicate enhanced probabilities of below normal precipitation for 
the southern Alaska Panhandle and for much of the Pacific Northwest, under a 
predicted mid-level ridge, while above normal precipitation is likely for much 
of western Alaska, ahead of a predicted trough over the Aleutians. 

The previous discussion for the May 2019 outlook, issued April 18, follows 
below:
______________________________________________________________________________

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: Dan Collins

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.

The next monthly outlook...for Jun ... will be issued on Thu May 16 2019

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