####018004590####
FXUS02 KWBC 091905

PMDEPD



Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

305 PM EDT Thu May 9 2024



Valid 12Z Sun May 12 2024 - 12Z Thu May 16 2024



...Excessive Rainfall/Flooding Threat for the Southern Plains, 

Gulf Coast and Southeast...





...Guidance/Predictability Assessment...



The latest models and ensembles show reasonably good agreement 

that by the start of the period on Sunday, the pattern should trend

more progressive featuring a series of troughs/shortwaves 

migrating from the Central to Eastern U.S. as upper level ridging 

tries to build back in over the West. There was good enough 

agreement in the guidance for a general blend of the deterministic 

guidance the first half of the period as troughing shifts out of 

the Northeast and another potent shortwave moves through the 

Central Plains to Lower/Middle Mississippi Valley. By Day 

6/Wednesday, there are some timing differences with this shortwave 

as it shifts into the Mid- South and Southeast which has critical 

implications on resulting QPF across this region. There are also 

some more noticeable differences with an amplifying shortwave from 

central Canada into the Plains, and how strong ridging builds back 

in across the West. Over the past couple of guidance runs, the CMC 

remains the strongest with that ridge, while the GFS and ECMWF (to 

varying degrees) are a little weaker due to stronger shortwave 

energy British Columbia next Thursday. There is a lot of run to run

inconsistencies between models too, so it seems prudent at this 

point the blend trends increasingly more towards the ensemble means

late in the period to mitigate/smooth these uncertainties. Did 

continue to include some ECMWF just for a little extra system 

definition. The approach maintained good continuity with the 

previous WPC forecast as well.





...Weather/Hazards Highlights...



A general split flow pattern will persist over the lower 48 states

into next week. This pattern will be highlighted by an amplified 

southern stream upper trough slated to eject slowly but steadily 

east- northeastward from the Southwest Sunday and through the Mid- 

South/South to offshore the Mid-Atlantic/East Coast next Thursday. 

This system and associated lead wavy front will increasingly tap 

and feed inland a return of deep Gulf moisture and instability 

under favorable upper trough and right entrance region upper jet 

support to fuel a potentially significant heavy rainfall and 

runoff/flooding period. The WPC Day 4/Sunday Excessive Rainfall 

Outlook (ERO) depicts a Marginal Risk area spread 

south/southeastward from the central-southern Plains to the lower 

Mississippi Valley. Within this, a Slight Risk area remains in 

place for Southeast Texas into Louisiana given moist 

soil/streamflow sensitivities from recent heavy rainfall. The 

overall risk should shift east on Day 5/Monday as highlighted by 

the ERO with a marginal risk across the Middle/Lower Mississippi 

Valley into the Gulf Coast/Southeast. An expansive slight risk 

exists from Southeast Texas into the Florida Panhandle. Heavy rain 

and a threat of runoff issues is also expected to spread across the

Southeast through next midweek and possibly build back into the 

southern/central Plains as additional shortwave energy moves into 

the region.



Meanwhile, several amplified upper troughs and surface systems are

set to work over the northern tier of the nation from the 

Northwest and north- central U.S. through the Northeast and focus 

mainly modest moisture and precipitation. 



Santorelli/Schichtel





Additional 3-7 Day Hazard information can be found on the WPC

medium range hazards outlook chart at:

https://www.wpc.ncep.noaa.gov/threats/threats.php



WPC medium range 500mb heights, surface systems, weather grids, 

quantitative precipitation forecast (QPF), excessive rainfall 

outlook (ERO), winter weather outlook (WWO) probabilities, heat 

indices, and Key Messages can be accessed from:



https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif

https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif

https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html

https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml

https://www.wpc.ncep.noaa.gov/#page=ero

https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 

https://www.wpc.ncep.noaa.gov/heat_index.shtml

https://www.wpc.ncep.noaa.gov/#page=ovw















$$

####018006981####
FXUS21 KWNC 091909

PMDTHR

US Hazards Outlook

NWS Climate Prediction Center College Park MD

300 PM EDT May 09 2024



SYNOPSIS: Early in week-2, a transient area of forecast mid-level low pressure 

sustains the risk of heavy precipitation and possible flooding for many parts 

of the southeastern contiguous U.S. (CONUS) and Mid-Atlantic.  Upstream, 

building mid-level high pressure over the lower Four Corners is expected to 

bring unseasonably warm temperatures, renewing the risk of excessive heat 

conditions for parts of the Rio Grande Valley. Combined with drier than normal 

conditions and strengthening surface pressure, there is an increased risk of 

high winds which may elevate wildfire potential over the southwestern CONUS. 

Over Alaska, cooler than normal conditions are forecast as snowmelt and river 

ice breakup season is underway.  



HAZARDS



Slight risk for heavy precipitation for portions of the Lower Mississippi and 

Tennessee Valleys, Southeast, Appalachians, and Mid-Atlantic, Fri-Sat, May 

17-18.



Slight risk for excessive heat for parts of the Southern Plains, Sat-Tue, May 

18-21.



Slight risk of high winds for parts of the Four Corners and western High 

Plains, Fri-Tue, May 17-21. 



Flooding possible for parts of the Southern Plains, Lower Mississippi Valley 

and Southeast. 



DETAILED SUMMARY



FOR SUNDAY MAY 12 - THURSDAY MAY 16: 

https://www.wpc.ncep.noaa.gov/threats/threats.php



FOR FRIDAY MAY 17 - THURSDAY MAY 23: Since yesterday, the week-2 hazards 

perspective mostly remains on track, though there are a few new developments 

and discrepancies in the latest model guidance adding to more uncertainty in 

the updated outlook.  Late in week-1, both the GEFS and ECMWF ensembles 

continue to feature anomalous 500-hPa ridging extending from the eastern 

Pacific into the western CONUS, with a fairly large coverage of anomalous 

troughing east of the Rockies. Within this negative height anomaly coverage, 

two shortwave disturbances have become more evident in the GEFS and ECMWF 

ensembles; one situated over the Tennessee Valley, and a secondary vorticity 

maximum associated with the possible remnants of a cutoff low upstream over 

northern Mexico. In the wake of a frontal system that remains favored to bring 

enhanced precipitation over the Southeast and Mid-Atlantic towards the middle 

of next week, this secondary feature is expected to renew the risk of heavy 

precipitation over parts of the southern and eastern CONUS heading into week-2. 

The return of potentially heavy precipitation is supported in both the 

uncalibrated and calibrated Probabilistic Extreme Tool (PET) guidance, thus the 

corresponding slight risk area is continued over southeastern CONUS, and 

extended through May 18. This area is expanded northward further into the 

Mid-Atlantic in the updated outlook where the uncalibrated ECMWF depicts 

greater than a 20% chance for 3-day precipitation amounts exceeding an inch. 







A possible flooding hazard is also added in the outlook extending from eastern 

Texas and Louisiana (where some areas have registered more than 10 inches of 

rainfall in the past two weeks), eastward into parts of western Georgia and the 

Florida Panhandle where heavy precipitation is favored during week-1. Any 

additional precipitation during week-2 may trigger flash flooding as well as 

river flooding within the highlighted region, even in areas where precipitation 

deficits are currently registered.  







As the secondary shortwave weakens and lifts out, both the GEFS and ECMWF 

favorincreasing mid-level heights over the Interior West and Great Plains, with 

a maximum height center amplifying over the lower Four Corners and northern 

Mexico. However, the ensembles are divided in regards to the pattern evolution 

both upstream and downstream. While the GEFS shows a sharper trend towards 

developing a trough over the eastern Pacific and West Coast, the ECMWF is 

virtually absent of this mid-level feature and maintains a widespread coverage 

of positive height departures which spread northward across the West Coast and 

into southwestern Canada. As a result of the stronger ridging over western 

North America, the ECMWF shows anomalous troughing redeveloping over the 

Midwest by the middle of the period, translating into predominantly cooler 

conditions than the GEFS over many parts of the central and eastern CONUS later 

in week-2. Notably, the ECMWF PET and uncalibrated tools depict the possible 

return of heavy precipitation over the Central Plains and Middle Mississippi 

Valley tied to the deepening trough aloft. No corresponding hazards are posted 

given no support in the GEFS PET, and significant differences among the models 

at this lead, but this troughing potential will continue to be monitored moving 

forward. 







Despite these differences in the evolution of the height pattern, however, the 

potential for excessive heat still remains a concern across the southwestern 

and south-central CONUS given model consensus of the amplifying ridge center 

over the lower latitudes of North America.  There is fair agreement in GEFS and 

ECMWF PETs indicating 20-30% chances for maximum temperatures exceeding the 

85th percentile and 100 degrees F along the Rio Grande during the middle of 

week-2 , therefore a slight risk for excessive heat remains posted for the 

region (May 18-21). Similar heat signals are also evident further west across 

the Sonoran Desert of California and southern Arizona, though maximum 

temperatures are not expected to exceed hazard criteria over this part of the 

country for this time of year. The excessive heat potential will continue to be 

closely monitored over the Desert Southwest, lower Four Corners, and Southern 

Plains as these regions were plagued with excessive heat conditions for much of 

the warm season last year.  With warmer and drier than normal conditions 

favored, a slight risk of high winds is also posted for parts of the Desert 

Southwest, Southern Rockies, and Southern Plains (May 17-21). This is supported 

in the GEFS and ECMWF PETs and raw ensembles depicting an anomalous surface 

high near the Front Range of Colorado which could induce episodes of high winds 

and elevate the wildfire risk. 



 



No hazards are issued for Alaska. Snowmelt season is underway and frozen rivers 

are beginning to break up, leading to the potential for river flooding related 

to ice-jams. Currently, there are no indications of impending major river 

break-ups or serious threat of ice-jamming so no flooding-related hazards are 

posted at this time. Caution will continue to be exercised as river break-up 

can be unpredictable and local conditions can change quickly.



FORECASTER: Nick Novella 



$$