30 Day Narrative
FXUS07 KWBC 311901
Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Wed Jul 31 2019
30-DAY OUTLOOK DISCUSSION FOR AUGUST 2019
The updated monthly temperature and precipitation outlooks for August 2019 are
based on the latest dynamical model guidance, WPC temperature and precipitation
forecasts during the first week of the month, the CPC 6-10/8-14 day temperature
and precipitation outlooks, Weeks 3-4 CFS and ECMWF model forecasts, and
influences from current soil moisture conditions. The Madden-Julian Oscillation
(MJO) is forecast to remain incoherent during the next couple of weeks.
Therefore, the MJO is unlikely to affect the mid-latitude circulation pattern
or modulate tropical cyclone development across the East Pacific and Atlantic
basins through early August.
The GFS and ECMWF model solutions remain consistent that a strong 500-hPa
blocking ridge at the high latitudes of the Northern Hemisphere retrogrades
west and becomes anchored over the Davis Strait at the beginning of August.
Downstream of an amplifying 500-hPa ridge over Alaska, an amplified upper-level
trough is likely to persist over central and eastern North America during early
to mid-August. Since below-normal temperatures are likely to accompany this
high amplitude trough during the first two weeks of the month, the coverage of
increased chances for below normal temperatures was expanded to include more of
the Great Plains and Corn Belt. Enhanced odds for below normal temperatures
forecast across the central Great Plains is also consistent with the likelihood
for enhanced rainfall during August. Cold air advection is not expected to
spread south into the southern Great Plains early in the month. Enhanced odds
for above normal temperatures are forecast for this region with the highest
odds across south Texas and the lower Rio Grande Valley. Above normal
temperatures are also favored for the Gulf Coast, Florida, parts of the
Northeast, and the Desert Southwest which is consistent with temperature tools
throughout much of the month and long-term trends. Probabilities for above
normal temperatures were increased across the Pacific Northwest and Alaska
based in part on anomalous warmth early in the month. Also, sea surface
temperatures remain much above normal surrounding Alaska.
An amplifying upper-level trough, a nearly stationary surface front, and
anomalous low-level moisture support heavy rainfall from the central Rockies
east to the central Great Plains and Ozarks region through early August.
Multiple daily runs of the CFS model have featured increased probabilities for
above normal precipitation in this same region for August. The highest
probabilities for above normal precipitation are focused across the lower
Missouri River Valley where heavy rainfall (locally more than 5 inches) is
forecast during the first week of the month. Anomalous northerly flow is
expected to persist across the Great Lakes and eastern Corn Belt through early
August when little to no rainfall is forecast. This relatively dry start to the
month elevates the odds for below normal precipitation for these areas. Model
solutions are in good agreement that monsoon rainfall is at least slightly
enhanced across parts of the Desert Southwest north to the Great Basin. Ridging
aloft is likely to result in little to no rainfall from the middle Rio Grande
north to central Texas during early August. Based on this dry start to the
month and consistent dry signal among the daily CFS model runs, enhanced odds
for below normal precipitation are forecast for parts of Texas. An amplified
upper-level ridge during early to mid-August increases chances for below normal
precipitation across the Alaska Panhandle and southeast mainland Alaska, while
the CFS model maintains enhanced odds for above normal precipitation across
western and northern mainland Alaska.
During the final week of July, the passage of a robust Kelvin wave contributed
to an increase in convection across the tropical Atlantic. On July 31, a
tropical wave is moving across Puerto Rico and Hispaniola. This tropical wave
is forecast to track northwest through the Bahamas and is likely to enhance
rainfall across south Florida at the beginning of August. Enhanced odds for
above normal precipitation across south Florida are based on this heavy
rainfall in the short-term along with a consistent wet signal among daily CFS
model runs. Another tropical wave is currently located across the eastern
tropical Atlantic. There is an increasing chance that this wave becomes a
tropical cyclone during the first week of August as it approaches the Lesser
Antilles. The eventual track of this potential tropical cyclone is uncertain
beyond one week.
----------- Previous message (from July 18) is shown below ------------
The August 2019 temperature and precipitation outlooks are based on dynamical
model guidance including the North American Multi-Model Ensemble (NMME),
statistical tools, current soil moisture conditions, and potential influences
from modes of tropical variability. The Madden-Julian Oscillation (MJO)
weakened during early to mid-July due to interference with a strong equatorial
Rossby wave crossing the West Pacific and a couple of Kelvin waves. Although a
remnant MJO may eventually reemerge over the Indian Ocean by the beginning of
August, the future evolution of the MJO is highly uncertain at this time. The
MJO is unlikely to influence the longwave pattern across the mid-latitudes, but
it can modulate tropical cyclone activity across the East Pacific and Atlantic
basins during August.
A longwave pattern change is likely to occur during late July. Model solutions
remain consistent with a retrogression of the pattern over North America.
During the final week of July, the GFS and ECMWF ensemble means indicate an
upper-level trough becoming centered over the east-central U.S. with anomalous
ridging extending north from the Pacific Northwest to Alaska. Dynamical model
output and long-term trends support increased chances for above normal
temperatures across much of the western U.S. and Alaska. The highest
probabilities for above-normal temperatures are forecast in parts of Alaska,
related to the anomalous upper-level ridging leading into the beginning of
August along with above-normal sea surface temperatures surrounding the state.
Uncertainty in the temperature outlook is highest across the central U.S. due
to weak or conflicting signals among the tools. However, below-normal
temperatures are slightly favored across the northern and central Great Plains
where above-normal precipitation is most likely to occur during August. Another
contributing factor for below normal temperatures is the moist topsoil since
the northern and central Great Plains have received 6 to 10 inches of rainfall,
locally more, since mid-June. Good model agreement supports increased chances
of above normal temperatures along the East Coast. Higher probabilities for
above-normal temperatures (above 50 percent) forecast for New England are also
related to long-term trends, while the NMME features probabilities of more than
60 percent across south Florida.
Enhanced odds for above-normal precipitation, albeit with limited
probabilities, across the northern and central Great Plains, northern and
central Rockies, Missouri River Valley, and much of mainland Alaska are based
largely on dynamical model guidance. Although a favorable 500-hPa height
pattern for enhanced monsoon rainfall is forecast for the Desert Southwest
during late July, its duration into August is unclear. Therefore, equal chances
(EC) for below, near, or above precipitation are forecast for this region.
There is a slight tilt in the odds for above normal precipitation forecast
across Colorado due to model guidance. Elsewhere, throughout the CONUS, EC is
necessary for the precipitation outlook due to limited skill in a half-month
lead during the summer. The precipitation outlook will be reassessed at the end
of July when a larger coverage for either above or below-normal precipitation
may be forecast.
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.
The next monthly outlook...for Sep ... will be issued on Thu Aug 15 2019
These outlooks are based on departures from the 1981-2010 base period.