673
FXUS02 KWBC 131845
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Mon May 13 2024

Valid 12Z Thu May 16 2024 - 12Z Mon May 20 2024

...Another southern tier heavy rainfall threat during the latter
half of the week...


...Overview...

Latest guidance forecast spread has decreased valid for Thursday
and Friday, bolstering confidence to a degree. Even so and into
longer time frames, guidance overall continues to show a recent
issue of showing considerable difficulty in resolving important
details of flow progressing around and downstream from a gradually
reorienting/retrograding eastern Pacific upper ridge. One or more
southern stream features may add to the already above average
uncertainty in the forecast. The most common theme is for lower 48
flow aloft to be flat to broadly cyclonic from the multi- day mean
perspective, with a leading complex of shortwave energy pushing
along one or more surface waves/fronts with accompanying rain
across the central- eastern U.S. Thursday-Saturday. The trailing
front could linger along the Gulf Coast into the weekend. Highest
totals should be along the southern tier which will be sensitive to
additional rain due to prior heavy rain events.

...Guidance/Predictability Assessment...

The WPC medium range product suite was primarily derived from now
slightly better clustered GFS/ECMWF/UKMET/Canadian model guidance
for Thursday and Friday. A composite tends to mitigate lingering
system variances, with detail seemingly consistent with individual
system predictability.Switched preference fully to more compatible
and run to run consistent GEFS/ECMWF ensemble means for Saturday
to next Monday amid quick forecast spread growth. Overall, this
forecast strategy acts to maintain good WPC product continuity.

...Weather/Hazards Highlights...

It generally remains the case that during the Thursday-Thursday
night time frame of the Day 4 Excessive Rainfall Outlook, there is
still a good signal for significant convective development from the
southern Plains into the lower Mississippi Valley/central Gulf
Coast in association with emerging Plains-Southwest shortwave
energy and surface waves/fronts. There are still typical
differences in specifics for heaviest rainfall emphasis, with the
GFS/GEFS still generally farther north than most other dynamical
model/ensemble solutions and ML models. The Day 4 Slight Risk area
extending from just east of central Texas through much of Louisiana
into southwestern Mississippi reflects the best overlap among the
majority scenario. Consensus continues to show a pronounced surge
of moisture into this region with adequate instability as well. An
eventual need for an upgrade to a Moderate Risk is possible and
will depend on additional rainfall in the short term period and how
guidance clusters going forward. A front farther north may help to
focus less extreme but still locally heavy rainfall (given
potential for sufficient moisture/instability), which is accounted
for by a Marginal Risk area that extends into parts of the south-
central Plains and into parts of the Midwest. Latest trends favor
a skinnier threat area for today`s ERO update.

As the southern tier convection likely continues eastward during
the Day 5 period (Friday-Friday night), the Slight Risk area
likewise extends eastward to cover an area centered over the east-
central Gulf Coast. However, frontal wave/orientation also suggests
potential for repeat rains held back through Louisana to monitor.
Again the specifics of short-term rainfall will play a role in how
sensitive this region will be to added rainfall by Day 5. The
surrounding Marginal Risk area accounts for some potential of a
convective axis hanging back even further to the west (most
pronounced in GFS runs). Farther north, some areas of locally heavy
rainfall will be possible across the Ohio Valley/Mid-Atlantic as
shortwave energy/surface features continue eastward. However the
specifics for mass fields and associated convection do not appear
coherent enough yet to depict a risk area.

Portions of the East Coast should see some rain continue for at
least part of the weekend while a front lingering along the Gulf
Coast may help to focus additional showers and thunderstorms
through the weekend as well. Elsewhere, parts of the Rockies may
see a little precipitation, possibly snow at highest elevations in
northern areas. The Plains could see at least a modest rebound in
rainfall around Saturday-Monday depending on specifics of shortwave
energy aloft. A weak upper trough/low approaching California could
promote localized diurnal rainfall in the Sierra Nevada.

An eastern Pacific upper ridge extending into the West should
support above normal temperatures over most of the West Coast
states and parts of the Great Basin into late week, with some
areas seeing plus 10-15F or so anomalies. Thereafter, lowering
heights aloft should promote a moderating trend, with northern
areas trending closer to normal while California/Nevada remain
somewhat above normal. The Florida Peninsula and South Texas should
see several days with highs up to 5-10F above normal, in a warm
pattern to the south of a couple fronts expected to focus upon
reaching more northern parts of these states. Some daily records
for highs/warm lows will be possible. These temperatures may bring
heat indices as high as 105-110 degrees for south Florida and
upwards to 115 degrees for south Texas, highlighting a threat of
hazardous heat. Otherwise, areas to the east of the Rockies will
tend to be near normal or moderately above normal, with some daily
variability due to system progression.

Rausch/Schichtel


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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