274
FXUS02 KWBC 110705
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Sat May 11 2024

Valid 12Z Tue May 14 2024 - 12Z Sat May 18 2024

...Excessive rainfall/flooding threat reaching the Southeast by
Tuesday, followed by another potential episode of southern tier
heavy rain mid-late week...


...Overview...

The combination of Mississippi Valley through Mid-Atlantic low
pressure and a front to the north will spread a broad area of
rainfall across the East Tuesday-Wednesday, with some flash
flooding concerns over parts of the Southeast. Then a frontal
system and one or more supporting upper shortwaves will likely
spread another large area of rainfall across the central/eastern
U.S. with highest totals again possible over portions of the
southern tier. Behind this system, guidance is still having
difficulties in resolving exactly how flow separates within
central Pacific troughing around Tuesday-Wednesday plus details of
trailing northern stream energy. This leads to continued lower
than average confidence in flow details across the West and
eventually farther downstream from Wednesday onward.

...Guidance/Predictability Assessment...

Regarding the system crossing the East early the period, the
primary differences in guidance arise from about early Wednesday
onward as the surface low tracks away from the Mid-Atlantic. The
18Z GFS/12Z CMC in particular showed more involvement from eastern
Canada upper flow, pulling the surface low northward of other
guidance. 12Z ECMWF-initialized machine learning (ML) models
generally favored greater separation between Canadian flow and the
shortwave supporting the system of interest (along with surface
pressures not as deep as the deeper side of the dynamical model
spread). This scenario points toward the 12Z ECMWF and 12Z/18Z
ensemble means.

Near the West Coast, the guidance continues to diverge
significantly from Wednesday onward in response to differences in
how flow separates within central Pacific troughing around
Tuesday-Wednesday, with differences in trailing northern stream
energy also adding to the spread. 18Z/00Z GFS runs are slower to
lower heights over the West and then eastward across southern
Canada and the northern tier U.S. but at least they have trended
somewhat more toward the ECMWF/ECens/CMCens versus some earlier
runs that had a strong ridge over the Northwest. The 18Z GEFS mean
also trended flatter aloft but the new 00Z GEFS has reverted back
to a stronger ridge. MLs continue to offer fairly strong support
for the 12Z ECMWF and 18Z GEFS/12Z ECens-CMCens in principle,
leading to favoring those solutions from midweek onward. Farther
east, this cluster agrees fairly well in bringing a modest surface
wave close to the Southeast or Mid-Atlantic coast by next Saturday.
Confidence in surface details is fairly low by that time as
solutions differ a fair degree for how shortwave energy will be
distributed within the overall upper trough.

...Weather/Hazards Highlights...

As the system forecast to be near the Middle Mississippi
Valley/Lower Ohio Valley as of early Tuesday continues eastward,
the best signal for highest short-term rainfall rates during the
Day 4/Tuesday Excessive Rainfall Outlook continues to be over
parts of the Southeast. This region will have a favorable
combination of instability and anomalous moisture along with a
leading warm front lifting through early in period followed by a
cold front approaching from the west. Thus far there has been no
compelling need for much adjustment of the Slight Risk area
centered mostly over northern Florida and southern Georgia, or the
Marginal Risk area extending north into parts of the Ohio Valley
and Mid-Atlantic. Guidance signals are more diffuse within the
Marginal Risk area but there will be a favorable combination of
instability and moisture along with currently wet ground
conditions. Over the Northeast, a cold front may focus some
enhanced rainfall but guidance has been showing fairly minimal
instability, so there is still uncertainty as to whether short-
term rain rates will be high enough to pose a flash flood threat
and thus no risk area is depicted for now. Meanwhile a leading
stalling front and another front heading into the central Plains
may generate some isolated moderate to heavy rainfall but at this
time the magnitude and organization do not appear sufficient to
merit a risk area.

By Day 5/Wednesday, the primary focus for heavy rainfall potential
will return back to the southern half of the Plains. Model/ensemble
guidance is not yet in great agreement for some details, but a
decent number of ingredients appear to favor convective development
around midweek--instability and increasing moisture, along with
multiple surface features plus supporting dynamics aloft (and a
southern stream shortwave emerging from the Southwest). As a
starting point, the Day 5 ERO depicts a Slight Risk area centered
over portions of Texas, representing the best overlap of GFS/GEFS
and ECMWF/ECens signals plus currently wet ground conditions, with
a surrounding Marginal Risk extending into parts of the central
Plains. There is a lingering Marginal Risk over northern Florida as
the front reaching the area stalls.

Expect central U.S. rainfall to continue eastward after midweek,
reaching the East toward the end of the week. Heaviest rainfall
will most likely be across the southern tier which will be
sensitive to additional rainfall given prior events. Sensible
weather over the Northwest continues to be a question mark in
light of the divergence of guidance aloft. The most likely scenario
is currently for upper heights to decrease enough for some light
precipitation to reach northern parts of the Pacific
Northwest/Rockies around mid-late week while temperatures stay
modestly above normal.

Farther south, California and Nevada should be more consistently
above normal next week with some locations seeing multiple days of
plus 10-15F anomalies. Florida may warm up to a few degrees above
normal by mid to late week as a warm front lifts north of the state
Tuesday and trailing cold fronts likely stall over the far
northern part of the state. Combined with high dew points, this may
bring heat indices as high as 105-110 across southeast/southern
Florida, signaling the first hazardous heat days of the summer.
Otherwise, areas to the east of the Rockies will tend to be within
a few degrees of normal for highs while the eastern U.S. should see
somewhat above normal lows for most of the period.

Rausch


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















$$