818
FXUS02 KWBC 101900
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Mon Jun 10 2024

Valid 12Z Thu Jun 13 2024 - 12Z Mon Jun 17 2024


...Heat shifts from the Southwest into parts of the central and
eastern U.S. during the period...

...Heavy rain likely for Florida through late week and perhaps the
central Gulf Coast early next week...


...Guidance/Predictability Assessment...

Model and ensemble forecast spread has overall decreased over the
past few guidance cycles over much of the lower 48 with a series of
northern and southern stream upper systems through late week. A
trough sneaking across the northeastern quadrant of the CONUS seems
well handled, though a bit faster with its associated surface
front than in the previous cycle. Meanwhile, a southern stream
upper low starting off Thursday just offshore of southern
California is forecast to move northeast across the Four Corners
region Friday into the central High Plains Saturday and become a
shortwave embedded within the mean flow, riding north of a warm
upper ridge.

Southern stream troughing/energy across Florida as well as a
frontal boundary and a highly anomalous moisture axis lead to
reasonable agreement in the wet pattern there, with expected detail
differences. The upper and surface energies look to sneak west
early next week and direct moisture somewhere into the central Gulf
Coast. Model guidance varies with placement for QPF, which will
partially depend on whether a surface low forms to focus moisture
on the eastern side (Florida Panhandle), versus more broad
southerly flow may promote a western (Louisiana) QPF axis. Tropical
system development is not currently forecast by the National
Hurricane Center, but heavy rain is likely regardless.

Another medium to larger scale difference is into early next week
across the Northwest. Both dynamic and AI/machine learning models
have varied with handling troughing from the northern Pacific into
the Northwest, with many but not all models splitting energy into
two upper lows. These variations will affect QPF and other sensible
weather coming into the Northwest and will have to be refined with
time.

Accordingly, the WPC medium range product suite was primarily
derived from a composite blend the 06 UTC GFS and 00 UTC
ECMWF/Canadian/UKMET for Thursday- Saturday. Opted for smaller
input from the models in lieu of more consistent GEFS/ECMWF
ensemble means at longer time frames amid growing uncertainties.
The blending process tends to mitigate variance by limiting detail
consistent with individual feature predictability. This also acts
to maintain good WPC product continuity that remains generally in
line with a similar blend of latest 12 UTC guidance.


...Weather/Hazards Highlights...

A protracted period favorable for heavy rains should continue
through later week for the Florida Peninsula as well above normal
tropical/Caribbean moisture pools along and south of a
wavy/stalling front with aid from upper level impulses/energy back
through an unsettled Gulf of Mexico. There remains high confidence
in heavy rainfall in Florida with locally heavy rainfall rates, but
lower confidence on placement of any extreme convective amounts on
any particular day. Storm total rainfall (including what falls
before the medium range period even starts) of 7-10 inches, with
locally higher amounts to 15 inches, is forecast. WPC Excessive
Rainfall Outlook Slight Risk areas remain in place for the central
to southern Florida Peninsula Thursday and Friday. Heavy rain may
last into Saturday across Florida, but as the weekend progresses,
moisture influx should increase for the central Gulf Coast and
vicinity, with model spread on placement of heaviest QPF once
again.

Leading and dynamic northern stream upper trough energy approach
and passage is expected to fire convection along/ahead of a
wavy/pooling surface front from the Midwest through the Northeast
later this week. Marginal Risk areas remain in place from the
vicinity of Chicago Thursday through New England Friday as the
parallel upper flow and convective axis may lead to some repeat
cells and local runoff issues. With slightly faster frontal timing
in this forecast cycle, shifted both Marginals a bit south of their
previous positions from overnight. The Storm Prediction Center is
also keying in on a severe weather threat for Thursday in parts of
the Midwest.

An increasing precipitation threat across much of the central and
northern Plains emerges late week into the weekend, as amplified
southern stream troughing ejects northeastward into the region from
the Southwest to focus lift/instability. A Marginal Risk remains
in place for Day 5/Friday from the southern High Plains into the
central Plains, except for the Sandhills of Nebraska that are not
sensitive to heavy rain.

As for anomalous temperatures, an amplified mean upper ridge
slated to slowly shift from the West/Southwest to the south-central
U.S. and parts of the East will continue to produce much warmer
than average temperatures. The highest temperature anomalies of
10-20F above normal are forecast for the Great Basin to central
Rockies/Plains on Thursday with lesser anomalies into the East. The
Northwest should gradually cool down to below normal this weekend
underneath upper troughing, while the Desert Southwest moderates
just a bit from above normal temperatures nearing 110F to near
normal 100s. Meanwhile portions of the central and eastern U.S.
should remain warmer than average into the weekend. More
specifically, south Texas can likely expect high heat indices at
times. The Southeast is forecast to see above normal temperatures
into the weekend before clouds/precipitation chances expand there
early next week. But meanwhile, anomalous heat is likely to build
in the Mississippi to Ohio Valleys early next week as the upper
ridge shifts east.


Tate/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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