FXUS02 KWBC 261859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Fri Apr 26 2024

Valid 12Z Mon Apr 29 2024 - 12Z Fri May 03 2024

...Potentially severe thunderstorms and heavy rain causing flash 
flooding are forecast to continue in the Lower Mississippi Valley 
Monday...


...Overview...

Upper troughing will support a surface frontal system with 
widespread thunderstorms ahead of it across the central U.S. into 
Monday. Heavy rain is likely to cause flash flooding in the Lower 
Mississippi Valley in particular, while severe weather may also be 
a threat. The upper pattern looks to become more zonal after the 
early part of the week, with increasing uncertainty in 
shortwave/embedded upper low details and surface specifics from the
Pacific eastward. For the Lower 48 there is at least a general 
theme of a leading system followed by another frontal system 
affecting the central-eastern U.S. by the latter half of the week,
with both serving to focus areas of showers and thunderstorms.
There is still a lot of uncertainty over the Northwest by late next
week though. Near to above normal temperatures should prevail over
much of the East into at least the southern half of the Plains
through Intermountain West, while the Northwest and possibly other
northern tier areas may see one or more days of below normal highs.


...Guidance/Predictability Assessment...

Recent variability/trending and spread in the guidance leads to
below average confidence in some important aspects of the forecast
next week. Through the 00Z/06Z cycles, there were essentially two
primary layers of uncertainty. The first involves the trough/upper
low(s) over the Northwest U.S. and British Columbia as of early
Monday. Over the past day or so the GFS/ECMWF and their ensemble
means have trended toward greater progression of this feature whose
energy could ultimately split into two pronounced upper troughs 
and/or lows. In contrast, the 00Z UKMET/CMC-CMCens clustered with 
what had been the prior consensus of holding the initial feature 
back near the Pacific coast while awaiting input from upstream 
Pacific energy. 00Z ECMWF-initialized machine learning models (MLs)
agreed upon the more progressive theme, thus recommending 
exclusion of the 00Z UKMET/CMC from the forecast. However the MLs 
suggested that the upper energy could be more consolidated into one
upper low (located near the northern High Plains as of early 
Wednesday) versus what the GFS/ECMWF were showing.

Beyond this discrepancy, guidance diverged even more with respect 
to Pacific/western U.S. flow later in the week. ECMWF runs and the 
00Z GFS along with the GEFS/ECens means brought a trough/possible
compact upper low into the Northwest (with a lot of spread among 
individual ensemble members), but a slight majority of the MLs
actually built a ridge over the West Coast by late week due to
greater troughing over the Pacific near 140-150W. The 06Z GFS 
trended that way too, deflecting the late week upper low farther 
north into Canada.

In light of guidance differences, ML suggestions, and continuity,
the updated forecast used a blend of 00Z and 12Z/25 ECMWF runs and
the 00Z/06Z GFS early in the period followed by a 40-50 percent
weight of the 06Z GEFS/00Z ECens means with lingering weight of
those GFS/ECMWF runs mid-late period. This solution yielded only
minor nudges from continuity in this lower predictability regime.

In the new 12Z guidance, the UKMET/CMC have trended significantly
toward other dynamical guidance for progression of the initial 
Northwest U.S./British Columbia feature. This still leaves a
difference between the dynamical solutions and more consolidated
MLs. The CMC/UKMET also favor higher heights along the West Coast
by late week while the GEFS mean still brings in a modest trough.


...Weather/Hazards Highlights... 

Ample moisture and instability will be present across much of the 
central U.S. into Monday ahead of a Plains upper trough and
associated surface frontal system. The most likely focus for the 
heaviest convection and rainfall is across the Lower Mississippi 
Valley, where a Slight Risk is maintained in the Monday/Day 4 
Excessive Rainfall Outlook. This may be a higher-end Slight Risk if
the heaviest model QPFs of 3-5 inches with rain rates over 2 
inches per hour end up verifying. By Tuesday the upper-level 
support will likely weaken, so overall expect lower rainfall 
amounts and rain rates. However, some additional convection could 
develop within an area of persistent above-climatology deep moisture,
which could lead to flash flooding concerns especially if storms 
develop over areas that saw excessive rainfall amounts the previous
day. A Marginal Risk remains a reasonable starting point to cover 
this threat on Tuesday/Day 5. Meanwhile, the models are starting 
to show more similar ideas for flow aloft to the north, leading to 
a front stalling over the Plains (with moisture increasing
especially by Tuesday night) while awaiting upstream 
troughing/surface waviness. This has led to an increasing signal 
for locally heavy convective potential, currently with the best 
focus over or near eastern Kansas into western/northern Missouri. 
Thus the updated Day 5 ERO has introduced a Marginal Risk area over
this region. There continues to be potential for locally heavy
rainfall over other parts of the Plains/Mississippi Valley given
recent behavior of guidance. By midweek and beyond, additional 
rounds of rain and thunderstorms are likely for the 
central/southern Plains and Mid/Lower Mississippi Valley (perhaps 
extending into nearby areas) as moisture surges ahead of a wavy 
frontal system.

Elsewhere, one or more shortwaves/upper lows will lead to rounds 
of precipitation across the northern tier. In the Northwest, some 
higher elevation snow is possible with this activity, with the most
widespread coverage on Monday before snow levels may rise even 
further. Mainly rain is expected farther east across the northern 
Plains to Midwest/Great Lakes/Ohio Valley and into the 
Northeast/Mid-Atlantic at times. 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. At the same time, these trends are indicating less
precipitation than previously forecast over the Pacific Northwest
by late in the week.

With the exception of the cooler West Coast and Northwest 
especially Monday-Tuesday, the bulk of the lower 48 will have near 
to warmer than average temperatures next week. An upper ridge over 
the East Monday-Tuesday should help produce temperatures around 
10-20F above normal, with temperatures in the 80s possibly as far 
north as the Great Lakes region. Scattered daily record highs and
perhaps more numerous record warm lows are possible. Temperatures 
are likely to gradually moderate but stay above average in the East
until late next week, depending on frontal progression. Meanwhile,
parts of the central U.S. will also see above normal temperatures 
with anomalies around +10F Monday-Wednesday before cooling. The 
Intermountain West to Rockies on the other hand should gradually 
warm and see its greatest warm anomalies Thursday-Friday. However 
note that recent guidance spread and variability yield lower than 
average confidence in the temperature forecast over the West,
especially by the latter half of next week.


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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