841
FXUS02 KWBC 111750
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 PM 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...

The guidance shows good enough agreement for a general model blend
(with WPC continuity) Tuesday and Wednesday which includes a
system into the East and northern stream energy skirting the
Northeast, as well as the next system dropping into the interior
West/northern tier. Beyond Wednesday though, there is a lot of
uncertainty with the details of interaction between this northern
tier energy and another shortwave across the Southwest as it
reaches the southern Plains around Thursday. The GFS continues to
show a little bit stronger ridging building out West which pushes
and northern stream energy faster and holds the southern stream
energy back more than the other guidance which shows more
interaction/mean troughing moving through the Central states. The
GFS is also farther north with energy/closed low into western
Canada. Compared to the latest ensemble means and ECMWF-initialized
ML models, prefer to stick closer to the ECMWF and CMC for now,
plus increasing WPC Continuity, which shows a little more troughing
eventually across the Northwest (owing also to additional energy
into the Northwest late week). There is a lot of run to run
inconsistencies in most of the models too forecast confidence is
still low for the latter half of the period especially. The blend
for Thursday and beyond stuck closer to the ensemble means and WPC
continuity with a little bit of the deterministic ECMWF and CMC
just for a little added system definition. These details are going
to have important implications for the rainfall pattern across the
CONUS mid to late week.


...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 near
to 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.

Santorelli/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

















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