####018007897####
FXUS02 KWBC 270731

PMDEPD



Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

331 AM EDT Sat Apr 27 2024



Valid 12Z Tue Apr 30 2024 - 12Z Sat May 04 2024



...Rounds of convection are likely in the central U.S. for much of

next week...





...Overview...



As the medium range period begins Tuesday, quasi-zonal upper flow

will be in place over much of the lower 48, but embedded shortwaves

could push frontal systems through the central and eastern U.S. and

cause some showers and thunderstorms. Mean troughing is forecast to

amplify in the Intermountain West Wednesday-Thursday and provide 

support for increasing convection over much of the central U.S., 

with heavy rain and flash flooding possible hazards. A gradual 

shift east with the convection is likely into late week. 

Precipitation including some higher elevation snow is possible 

across the Northwest into the northern/central Rockies, but with 

rather low predictability.





...Guidance/Predictability Assessment...



Recent model guidance is fortunately in better agreement than a 

day ago regarding energy across the northwestern U.S./British 

Columbia on Tuesday. There is a general theme of some energy 

developing a closed low and pressing east near the U.S./Canadian 

border Wednesday-Thursday while secondary energy causes a trough to

dig into the Intermountain West. Operational models and AI/machine

learning models show some variability in how separate or phased 

these features may be, but generally within typical spread for the 

medium range. There is also some model spread in how deep the 

trough may dig in the Intermountain West and pushing toward the 

Plains by Friday. The 18Z GFS was quite aggressive with a 

deeper/neutrally (rather than positively) tilted trough and a 

resultant surface low track farther west compared to other 

guidance. Thus for these features, preferred a model blend of the 

12Z ECMWF/GFS/CMC. The newer 00Z GFS looked more in line with other

guidance.



Upstream, there are even larger model differences across the 

northwestern U.S. by Thursday that lead to dynamical 

models/ensembles/ML models showing out of phase solutions. Early 

Thursday, most 12/18Z deterministic models indicated a ridge over 

the West Coast while they held an upper low farther north into 

western Canada compared to previous runs. Most of the 12Z ML models

also supported this ridge solution. On the other hand, the 

ensemble means persisted in indicating troughing atop the Northwest

more similar to the previous runs. Individual ensemble members 

show ample spread. Having to choose a camp, preferred the 

deterministic and ML type solutions with ridging over the 

Northwest, thinking that the ensemble means were lagging the trend.

The WPC model blend maintained more deterministic components than 

normal for the latter part of the period, especially the ECMWF. 

However, this may have been the wrong choice given the incoming 00Z

models generally show troughing (though think the 00Z CMC may be 

too aggressive with its compact closed low). There may need to be 

some changes made with frontal and precipitation timing in future 

forecasts because of this. By Friday-Saturday, there are additional

uncertainties with northern Pacific and Canadian flow affecting 

the West. Thus predictability and confidence are somewhat low for 

the pattern in the western and central U.S. after midweek. 





...Weather/Hazards Highlights... 



On Tuesday, lessening upper-level support should lead to lower 

rainfall amounts and rainfall rates in the central U.S. compared 

earlier and later in the week. However, some convection is still 

likely to occur in a moist and unstable environment. Currently the 

best chance for rain with isolated flooding potential is across the

south-central Plains to Mid-Mississippi Valley as a front stalls. 

Within that region there is some spread in location of locally 

heavy convective potential--GFS runs have been persistent in 

showing convection near the Red River while other models indicate 

local maxima farther north. A Day 4 Marginal Risk is in place in 

the ERO to cover these areas and will be refined with time. 

However, rainfall amounts have trended down across the board 

farther south in Texas and Louisiana, so a Marginal Risk that was 

in place there on what is now Day 4 was removed. By Wednesday, the 

upper flow( with troughing to the West and a southern stream 

shortwave) will become more supportive of more widespread and heavy

rain and thunderstorms for the central/southern Plains and 

Mid/Lower Mississippi Valley as moisture surges ahead of a wavy 

frontal system. Once again there is spread in the exact position of

the heaviest totals, but rain amounts of 3-5 inches are likely, 

and some areas of heavy rain are likely to overlap with 

current/short range heavy rainfall areas. A large Slight Risk will 

serve as a starting point for the Day 5/Wednesday ERO especially 

given the likely sensitivity of these areas due to wet antecedent 

conditions. Expect a slow trend of the rain/thunderstorms eastward 

Thursday and Friday as the front presses east. 



Elsewhere, one or more shortwaves/upper lows will lead to rounds 

of precipitation across the northern tier. On Tuesday, ample 

moisture with precipitable water values over the 90th or even 95th 

percentile will be in place across the northeastern U.S., with rain

and thunderstorms likely. A shortwave aloft moving through and 

some instability in place could lead to heavy rainfall causing 

isolated flooding concerns, so added a Marginal Risk centered in 

upstate New York with its footprint stretching a bit into the 

northern Mid-Atlantic and western New England on Day 4/Tuesday. 

Meanwhile the Northwest and into the northern and central Rockies 

could get rounds of precipitation, including higher elevation snow.

Guidance is still in the process of trending, but suggests the 

potential for a period of enhanced northern Rockies/Plains 

precipitation that will be worth monitoring. Timing and amounts of 

precipitation will depend on the eventual uncertain evolution of 

the upper and surface pattern. 



Temperatures across the south-central U.S. all the way to the 

Eastern Seaboard will be warmer than average by generally 10-15F on

Tuesday and Wednesday, while cooler than normal temperatures are 

likely for the northwestern quadrant of the lower 48. Then 

troughing and a cold front pushing from the Northwest should cool 

temperatures to near or below normal across the central U.S. by 

Thursday and reaching the east-central U.S. Friday and Saturday. 

There is a possibility of a warmup across the West late week, but 

the guidance spread is still yielding lower than average confidence

in the temperature forecast there.





Tate





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



















$$