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
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.
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.