973
FXUS02 KWBC 130659
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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 continues to show 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...

Given the ongoing difficulties that guidance has been having for
important details at the surface and aloft, the updated forecast
using a 12Z/18Z operational model composite to start on Thursday
quickly transitioned to about half models/half means by Saturday
and 75 percent means thereafter with remaining minority input from
the 18Z GFS/12Z CMC.

While ensemble/model spread has been narrowing for the recently
problematic area of the northeastern Pacific/western North America
during mid-late week, individual solutions still differ in
important ways and these differences become increasingly
exaggerated with time downstream. Besides that, 12Z ECMWF-
initialized machine learning (ML) models differed from each other
in some ways as well and at times had some common ideas different
from the dynamical guidance. Of particular note, there was minimal
support for the 12Z ECMWF that tracked an upper low from the
Midwest through northern Mid-Atlantic mid-late period so that
solution was not used after the first part of the forecast. The new
00Z ECMWF compares better to other guidance. Meanwhile the
ensemble means and to a greater extent ML models suggest the latest
GFS/UKMET runs may be slow/west with upper troughing reaching into
the Northwest by Friday- Saturday. Guidance is signaling
additional trough energy could arrive into or near western North
America by the end of the period Monday but with a lot of spread.
Beyond that, the new 00Z GFS amplifies its mid-period trough a lot
more over the West versus other models/means by Sunday-Monday.
There are additional uncertainties with details of leading
shortwave energy crossing the eastern half of the country
Thursday-Saturday as well as for one or more southern stream
features.

...Weather/Hazards Highlights...

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 generally farther
north than most other dynamical model/ensemble solutions and ML
models. The planned 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
and is a modest westward shift from continuity. 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
Midwest.

As the southern tier convection likely continues eastward during
the Day 5 period (Friday-Friday night), the Slight Risk area
likewise shifts eastward to cover an area centered over the east-
central Gulf Coast. 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 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 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 stall upon reaching the far
northern part of the state. Some daily records for highs/warm lows
will be possible. Combined with high dew points, these
temperatures 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 near normal or moderately above normal, with some daily
variability due to system progression.

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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