####018009354####
FXUS02 KWBC 271859

PMDEPD



Extended Forecast Discussion

NWS Weather Prediction Center College Park MD

259 PM 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 should amplify

over the Intermountain West Wednesday-Thursday and provide support

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

heavy rain and flash flooding among the possible hazards. This

convection will likely shift gradually eastward into late week. 

Precipitation including some higher elevation snow is possible 

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

rather low confidence in the details at this time.





...Guidance/Predictability Assessment...



While guidance runs since the 12Z/26 run appear to have resolved

most of the prior divergence for the initial upper low energy (most

likely comprising two separate centers) along the western 

U.S.-Canadian border, there is still a lot of spread for what

becomes of the western half of the feature as well as upstream

details.



Recent trends, including from ECMWF-initialized machine learning 

models (MLs), support fairly amplified upper troughing over the 

West by Wednesday-Thursday. However from that point both dynamical

guidance and the MLs diverge for timing and shape as the feature 

ejects from the West, affecting the surface evolution. The 06Z GFS

appeared to be somewhat on the fast side of the spread but

otherwise the envelope was broad enough to preclude discounting

many solutions. 12Z model runs continue to differ by at least a day

for timing of the upper system by next Friday, with corresponding

effects on the Plains into Great Lakes surface development that

most guidance is showing. Farther east the one item of note is that

UKMET runs are notably aggressive with shortwave energy crossing

New England around Thursday when consensus shows more upper

ridging.



There is still considerable spread for a compact upper low expected

to reach western North America by Thursday. The new 12Z GFS/CMC

runs maintain their theme from prior runs in showing a track into

Washington state while the ECMWF is well north over British

Columbia. The UKMET flipped from the GFS cluster in the 00Z run to

shearing it out north of the ECMWF in the 12Z run. The GEFS/ECens

means not surprisingly weaken the feature as it arrives but would

imply a somewhat farther north track than the GFS cluster. The 00Z

MLs were unanimous with a British Columbia track. Preference leans

to the ECMWF/ML cluster. Interestingly the guidance gets a little 

closer together by Friday as the ECMWF/ML cluster drops the upper 

low southeast to around the Canadian border. What happens with this

feature may ultimately have some influence on details of flow to 

the east.



Significant differences continue as another upper trough (with

possible low) approaches the Northwest. The 06Z GFS was on the

eastern side of the spread in bringing a deep upper low into

Washington state. Meanwhile the MLs on average show an intermediate

idea between the deep and sometimes eastward GFS runs versus the

slower/flatter ECMWF and ECens mean runs. The MLs are actually

remarkably agreeable in showing the associated surface front

somewhat offshore the Pacific Northwest as of early Saturday. As

expected, GEFS/EC/CMC ensemble members show considerable spread for

this and aforementioned features.



In order to accommodate considerations for the various features of

interest, the updated forecast started with a 00Z/06Z operational

model composite early in the period. Then the blend eliminated the

UKMET by Thursday, followed by splitting GFS/ECMWF input between

consecutive 6-/12-hourly runs and incorporating 20-40 percent total

of the 06Z GEFS/00Z ECens means.





...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 (including the new 12Z

version) 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. Some isolated moderate to locally heavy

rain may be possible along/north of the Texas and Louisiana coast

but what signal exists remains diffuse enough not to merit a risk

area (maintaining the prior cycle's reduction that favored an

earlier Marginal Risk area). By Wednesday, the upper flow (with 

troughing to the West and a southern stream shortwave) will become 

more supportive of more widespread 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 

locations. The Day 5/Wednesday ERO update incorporated latest

guidance to yield only mild adjustments for the established Slight

Risk area extending from parts of Texas into southern Iowa. Within

this risk area, there may be somewhat greater potential between

Kansas and Iowa given proximity to the stalling surface front.

There is a little more spread farther south. This overall region

will be sensitive due to wet antecedent conditions and potential

for embedded upgrades will be possible depending on details of

prior convection and how guidance consolidates for amounts/location

of heaviest rainfall. Expect a slow trend of the 

rain/thunderstorms eastward through the rest of the week,

corresponding to progression of the surface front.



Elsewhere, one or more shortwaves and 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

(with associated wavy surface reflection) and some instability in 

place could lead to heavy rainfall causing isolated flooding 

concerns, so the Day 4/Thursday ERO maintains the previously

introduced Marginal Risk area centered in upstate New York with 

its footprint stretching a bit into the northern Mid-Atlantic and 

western New England. Meanwhile the Northwest and into the northern

and central Rockies could get rounds of precipitation through the 

week, including higher elevation snow. There is still some

potential for one or more areas of locally enhanced totals but

resolving specifics for important details of the supporting upper 

and surface pattern continues to be a challenge. A front nearing

the Pacific Northwest coast may bring some precipitation to the 

region by next Saturday.



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 upper 

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. 

Temperatures over the West may rebound to near normal or moderately

above normal levels by late week, but confidence remains lower 

than average given continued guidance spread for upper flow 

details.





Rausch/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



























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