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Official 90-day Outlooks are issued once each month near mid-month at 8:30am Eastern Time. Please consult the schedule of 30 & 90-day outlooks for exact release dates.

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    0.5mn MJJ 2019
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    0.5mn May 2019


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HOME> Outlook Maps>Seasonal Forecast Discussion
 
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.
$$

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