NOAA Outlook for May: Warmer than Average for the Southwest, Cooler for Great Lakes Region

The Climate Prediction Center just released their monthly weather outlook for May. According to the CPC, May sees the best chances for a warmer than average month across the Southwest, while odds tilt cooler across the Great Lakes.

The weather outlook summary for May 2020 by the CPC is attached below: 

30-DAY OUTLOOK DISCUSSION FOR MAY 2020 
 
The updated monthly temperature and precipitation outlooks for May 2020 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, and climate linkages to current soil moisture. The 
enhanced phase of the Madden-Julian Oscillation (MJO) propagated east across 
Africa and the Indian Ocean during mid to late April. Despite the coherent, 
ongoing MJO and the likelihood of continued eastward propagation through early 
May, it is unclear how much influence the MJO will have on the mid-latitude 
circulation pattern. If the MJO continues to shift eastward over the Western 
Hemisphere, the large-scale environment would become favorable for tropical 
cyclone (TC) development in the East Pacific later in May and perhaps the Gulf 
of Mexico heading into June. Please refer to CPC’s Global Tropics Hazards for 
updates on the MJO along with TC development forecasts. 
 
Dynamical model forecasts are in excellent agreement for this time of year and 
feature a high amplitude ridge (trough) over western (eastern) North America 
during the first two weeks of May. This highly amplified ridge aloft increases 
chances of above normal temperatures throughout the West extending to the 
central and southern Great Plains. The largest probabilities (70 percent or 
more) of above normal temperatures are forecast across the Southwest due to 
anomalous heat likely to begin the month and also long-term trends . Rapidly 
drying topsoil supports those large probabilities extending east to the 
southern High Plains. An area of increased chances of below normal temperatures 
was added to the Great Lakes, eastern Corn Belt, and parts of the Northeast due 
to a persistent and highly amplified upper-level trough early in the month. 
Week 3-4 tools such as the CFS and ECMWF (initialized on Apr 27) suggest that 
temperatures across these areas would moderate or even transition to 
warmer-than-normal during the latter half of May. Equal chances of below, near, 
or above normal temperatures extend from the northern Great Plains southeast to 
the southern Appalachians and Carolinas where temperatures are expected to be 
highly variable during the month. Although the amplified upper-level trough 
over eastern North America is expected to result in a period of near or below 
normal temperatures across Florida during early May, above normal temperatures 
remain the most likely outcome for the entire month, especially the southern 
half of the Peninsula. 
 
The high amplitude ridge/trough pattern over North America, forecast during 
early May, is a complicating factor in the revised precipitation outlook. This 
longwave pattern is a relatively dry one for the CONUS, while the outlook 
(released on April 16) featured a broad area with elevated probabilities of 
above normal precipitation throughout much of the Great Plains, Mississippi 
Valley, and Southeast. Since there is likely to be at least a week of mostly 
dry weather across the northern half of the Great Plains and parts of the 
Southeast, the coverage of elevated probabilities of above normal precipitation 
was reduced. The Weeks 3-4 tools (as of Apr 27) along with the CFS monthly 
continue to support either near or above normal precipitation across the Gulf 
Coast States. The slightly elevated chances of above normal precipitation, 
centered across the Ozarks to the middle Mississippi Valley, is associated with 
predicted rainfall at the beginning of the month. During the second week of 
May, model guidance continues to trend towards a wetter solution from the upper 
Rio Grande Valley north into New Mexico as return flow from the western Gulf 
spreads northwest. Given that these areas are relatively dry this time of year, 
above normal precipitation is slightly favored for parts of the Big Bend of 
Texas and New Mexico. A wet start to the month and many of the precipitation 
tools support increased chances of above normal precipitation across the 
Northeast. Given that the first half of the month is likely to be quite dry 
with anomalous northerly flow, below normal precipitation is most likely across 
the Great Lakes and northwest Corn Belt. To the north of the dry climatological 
areas of the Southwest, below normal precipitation is favored across the Great 
Basin due in part to the persistent, upper-level ridging early in the month. 
 
Although near to below normal temperatures are forecast to prevail across 
Alaska at the beginning of May, the high amplitude ridge is likely to result in 
the development of above normal temperatures by the second week of the month. 
Once this upper-level ridge establishes itself, it is expected to continue 
through at least mid-May. This persistent ridge aloft and good agreement among 
Weeks 3-4 tools support increased chances of above normal temperatures across 
Alaska. This evolving longwave pattern also elevates probabilities of below 
normal precipitation across western mainland Alaska.