Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT Tue Mar 31 2020


30-DAY OUTLOOK DISCUSSION FOR APRIL 2020

The updated outlooks for April are largely similar for the eastern CONUS and 
Alaska, but reflect substantial changes for the western CONUS. The outlooks 
make use of shorter term forecasts from WPC, the Week-2 guidance from CPC, 
Week-3/4 guidance from CFS and ECMWF, and monthly guidance tools based on model 
combinations. The outlooks also reflect a small bit of contribution from MJO, 
as the signal has emerged over the Maritime Continent. Some small, MJO-related 
signals that meet significance testing thresholds, indicate a mid-month flip in 
temperatures over the eastern CONUS. The AO is predicted to be positive through 
the first half of the month, and there is little reason to forecast a major 
change for the second half of the month, so progressive solutions are favored.

With respect to temperatures, model outlooks have strong signals for the first 
2-weeks of the month for below normal temperatures over the western CONUS and 
above normal temperatures over the eastern CONUS. Those signals are likely to 
moderate in the latter half of the month. ECMWF guidance for the second half of 
the month is more progressive than that in the CFS, which maintains a more 
amplified pattern and colder temperatures for much of the CONUS. The official 
outlook is warmer than CFS guidance, based on guidance through Week-2 and the 
uncertainty from the ECMWF during the latter half of the month. Across Alaska, 
a predicted cold start to April favors below normal temperatures for the month 
over southeastern Alaska and the Alaska Panhandle. Sea surface temperatures 
have risen quickly while sea ice extent has dropped rapidly during March over 
the Bering Sea, so that would favor above normal temperatures for the western 
Alaska.

For precipitation, the first week of April is forecast to be quite wet for 
portions of central and southern California and the southern Plains, where some 
locations are predicted to receive more precipitation in one week than what 
defines the bound for the lower tercile. Model outlooks for the latter half of 
April differ significantly for the Southeast, with the most overlap in signals 
for above normal precipitation from the lower Mississippi Valley to the 
Southern Appalachians, pulled away from the Gulf Coast.  The Ohio Valley and 
and Lower Great Lakes are an area of high uncertainty, as the month is forecast 
to start try, then turn slightly wetter through Week-2, followed by a period of 
elevated uncertainty. Predicted mean ridging south of Alaska would steer storms 
northward and into central mainland Alaska, favoring above normal precipitation 
for northern Alaska, and below normal precipitation for southern mainland 
Alaska and the Alaska Panhandle.

**************************************************************************
**** Previous discussion from the mid-month outlook for April follows ****
**************************************************************************


The April outlooks for temperature and precipitation reflect the latest model 
guidance, likely impacts from trends, tropical intraseasonal oscillations, 
antecedent soil moisture, current and likely snowpack, as well as sea ice near 
Alaska.  Over the western CONUS, the forecast is highly uncertain with initial 
model output from earlier in the month depicting conflicting signals with more 
recent runs, and statistical tools depicting weak signals. The increased 
uncertainty is reflected in the increased coverage of equal chances, compared 
to many other CPC monthly outlooks.

With respect to forcing factors for the outlook, trends would favor above 
normal temperatures at most locations, while high soil moisture would weakly 
favor below normal temperatures over the Great Plains. The MJO is forecast to 
emerge over the Indian Ocean and progress to the Maritime Continent by the 
beginning of the month, but the connection from the tropics to the mid-latitude 
is muted in April, relative to a winter.  Forecasts models predict a strong 
circumpolar flow in the mean, which would also mollify tropically teleconnected 
impacts, so the MJO was considered, but not any significant component for the 
April outlooks.  

Snowpack (as assessed by NOHRSC) is below normal for much of the Great Plains 
and Northeast, which would favor above normal temperatures. Across the west, 
snowpack is generally lower than normal over the Southern Rockies, with closer 
to normal values in the Central Rockies. Late season storminess is predicted in 
the 6-14 day time frame, which could add significant snowpack to the West, 
though the impacts to temperature from snowpack are small, the potential for 
rapid changes just before the valid period does increase the uncertainty. 

Model outlooks from the NMME suite reflected generally above normal 
temperatures, though some models had indications of below normal temperatures 
in the Northern Rockies and Northern Great Plains. More recent model output has 
significantly cooler solutions from the West Coast to the Great Plains, with 
the east remaining warm, so the uncertainty lead to low coverage in the 
temperature outlook from the West Coast to the Northern and Central Rockies and 
across the Central Great Plains. Recent dry conditions are not likely to abate 
in southern Texas, favoring above normal temperatures. Across the eastern 
CONUS, model outlooks and trends favor above normal temperatures, while 
anomalously low snowpack (with the exception of areas around the Upper Great 
Lakes) enhances odds over the Northeast.

Across Alaska, trends favor above normal temperatures for much of the state, 
though ice coverage was near median in early March, so it's higher coverage 
than many of the past few years, so probabilities for above normal temperatures 
are muted along the west coast of Alaska. Snowpack has recovered from a 
multiyear drought over southeastern Alaska and the Alaskan Panhandle, so that 
agrees with model forecasts favoring below normal temperatures for those areas.

The precipitation outlook is more uncertain than the temperature outlook as 
trends are not monotonic and weak in the spring. Model guidance and trends 
favor above normal precipitation from the southeast to the Middle Mississippi 
Valley, while the same predictors favor below normal precipitation across 
portions of the Pacific Northwest. A strong circumpolar flow could keep an 
active storm track focused into Alaska, which would favor above normal 
precipitation for central and northern Alaska, with below normal precipitation 
for the Alaska Panhandle and Southeast Alaska. Model guidance largely supports 
that pattern, so the official outlook reflects those patterns.

FORECASTER: Matthew Rosencrans

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 May ... will be issued on Thu Apr 16 2020

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