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000
FXUS02 KWNH 260701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM 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 by Tuesday-
Wednesday, but uncertain shortwaves in the northern stream should
continue to push frontal systems across the Lower 48 with
additional showers and thunderstorms. More amplified troughing may
come into the West by late next week, ahead of which near normal to
warmer than normal temperatures will be the norm across much of
the country from the Intermountain West eastward.


...Guidance/Predictability Assessment...

Model guidance is reasonably agreeable at the start of the period
Monday, with compact upper and surface lows in the north-central
U.S. and upper ridging in the East while additional energy comes
into the Northwest. By Tuesday-Wednesday, though the overall flow
becomes more zonal, the northern stream jet shows some notable
differences in the timing of those potent shortwaves coming into
the northwestern to north-central U.S., leading to some out of
phase differences. There are generally two camps for these
features. The 12/18Z GFS and 12Z ECMWF, in a departure from their
previous runs, indicate some energy feeding faster into the
Northwest while some energy is held back over the Pacific. Most of
the ECMWF-based AI/machine learning models are generally similar to
this 12Z trend. However, some guidance like the CMC, UKMET, and
the AI FourCastNet stick with a solution like the older GFS/ECMWF
runs in holding most energy back and creating a deeper upper low in
the eastern Pacific. The 06Z GFS-based AI Graphcast was also in
this second camp, but the 12Z run flipped to more progressive
initial energy. Ensemble members vary somewhat but do generally
stick with their parent models more than preferred. The incoming
00Z CMC and UKMET still indicate more energy hanging back west than
the GFS/EC camp, but are not as deeply aggressive with the eastern
Pacific upper low as their 12Z runs. Overall, evidence was more
compelling to prefer the GFS/ECMWF type of solution, even though
this was a new trend. Models do still indicate troughing coming
into the West by Friday with ridging ahead of it in the central
U.S. and some troughing to the east of that, but with amplitude and
exact positioning dependent on earlier forecast problems. Thus the
WPC forecast was based on a blend heavily favoring the 12/18Z GFS
and 12Z ECMWF, but gradually reducing the deterministic runs in
favor of the GEFS and EC ensemble means especially by Day 7 given
increasing model spread.


...Weather/Hazards Highlights...

Ample moisture and instability will be present across much of the
central U.S. into Monday ahead of the upper trough and a 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 is forecast to
weaken, so overall lower rainfall amounts and rain rates are
expected. However, some additional convection could develop, which
could lead to flash flooding concerns especially if storms develop
over areas that saw excessive rainfall amounts the previous day. A
Marginal Risk will serve as a starting point to cover this threat
on Tuesday/Day 5. There is also some potential for convection
farther north from generally the central Plains into the Mid-
Mississippi and Ohio Valleys. Model guidance is not agreeable with
where any heavier rainfall might occur and these elements may be
progressive anyway, precluding any ERO risk area there at this
point. By midweek and beyond, additional rounds of rain and
thunderstorms are likely for the central/southern Plains and
Mid-/Lower Mississippi Valley as moisture surges ahead of frontal
systems again.

Elsewhere, multiple shortwaves 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.

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 are
possible. Temperatures are likely to gradually moderate but stay
above average in the East until late next week. 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.


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



$$