661
FXUS02 KWBC 150658
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Jun 15 2024

Valid 12Z Tue Jun 18 2024 - 12Z Sat Jun 22 2024

...Early season heat wave from the Midwest to the East Coast...
...Heavy rainfall threat along the central-western Gulf Coast and
parts of the northern tier into central Plains...


...Overview...

Guidance continues to show a strong upper ridge building over the
East through midweek and then gradually dropping a little southward
and elongating to cover the south-central Plains and southern
Rockies by the end of the week. This ridge will support a broad
area of hot and dry weather next week, with the greatest anomalies
and potential for daily records extending from the Ohio
Valley/Lower Great Lakes into the northern Mid-Atlantic and
Northeast. Between this ridge and mean troughing along or just
inland from the West Coast, the area from the northern half of the
Plains into Upper Midwest may see multiple episodes of heavy
rainfall from a combination of a persistent wavy mean front,
anomalous moisture, and upper level impulses. To the south of the
upper ridge, upper level energy and abundant deep moisture will
bring the potential for a period of heavy rain along and near the
western half of the Gulf Coast.

...Guidance/Predictability Assessment...

Most aspects of the guidance are fairly agreeable during the first
couple days or so of the period, aside from smaller scale detail
differences. By mid-late period there is more notable spread among
the dynamical guidance or between the operational models and
12Z ECMWF-initialized machine learning (ML) models. One difference
that arises by midweek is for the western Gulf evolution, with the
new 00Z CMC a little north for its surface reflection versus other
dynamical solutions, while most ML models show a track farther
north than the dynamical guidance (with corresponding
northward/northeastward shift of rainfall emphasis). By late week
into the weekend, a number of operational model runs have been
stronger with another upper weakness crossing the Gulf Coast
region. This is in contrast to most ML models that maintain
stronger ridging a little farther south than the operational
cluster (over the southern Plains and/or Southeast by next
Saturday) and thus suppress any weakness traveling below the ridge.
The 12Z GFS, to some degree the 00Z GFS, and latest ensemble means
were closest to the ML ideas.

Early-period model consensus favored a blend of 12Z/18Z models for
the forecast through midweek, followed by transitioning GFS input
from the 18Z run to the 12Z run and increasing weight of the 18Z
GEFS and 12Z ECens/CMCens means with time. Total ensemble mean
weight reached 75 percent by next Saturday.

...Weather/Hazards Highlights...

Locations from the central Plains through the Upper Midwest will be
one area of focus for heavy rainfall potential during the Days 4-5
Excessive Rainfall period covering Tuesday through Wednesday night.
During Day 4 expect convection to expand southward along a cold
front anchored by a northern tier into Canada surface wave, with
the latest majority guidance cluster favoring some southward
expansion of the existing Slight Risk area, now planned to extend
from parts of Minnesota/Wisconsin into eastern Nebraska/far
northeastern Kansas. By Day 5 the trailing part of the cold front
may stall and eventually lift northward while abundant moisture
persists over the region. There is more spread for convective
details (the 00Z GFS showing more of a dry push from the north
versus most other solutions), yielding only a broad Marginal Risk
area at this time. The wavy front may remain nearly parallel to
upper flow through the rest of the week, maintaining the potential
for additional episodes of heavy rainfall from the northern half of
the Plains into the Upper Midwest from Thursday onward.

Guidance continues to show an area of deep tropical moisture and
some degree of upper level energy shifting westward across the
western half of the Gulf of Mexico and vicinity during the Days 4-5
ERO period. There are still significant differences for specifics
of associated heavy rainfall, but a composite of dynamical/machine
learning guidance has at least provided enough of a signal to
introduce a Slight Risk area along the western two-thirds of the
Louisiana coast into eastern Texas coast on Day 4, and along the
entire Texas coast on Day 5. The southern part of the Texas coast
has drier antecedent conditions but rain rates may still be high
enough to cause some runoff issues. Rainfall along and inland from
the western Gulf Coast should trend less heavy later in the week
while upstream energy underneath the elongating central-eastern
U.S. upper ridge could lead to some rebound in rainfall over
Florida/western Gulf region but with low confidence in the details.

The strong upper ridge building over the East next week will tend
to produce the greatest temperature anomalies from the Midwest and
Ohio Valley/Lower Great Lakes into the northern Mid-Atlantic and
Northeast, with multiple days of readings 10-20F above normal. This
would translate to highs well into the 90s, along with lows in the
upper 60s to upper 70s providing little overnight heat relief.
Daily records will be possible within the above areas. With some
typical detail differences, the axis of greatest temperature
anomalies in the current forecast shows some similarity to the 1994
heat wave that was observed around the same time in June. Some
flattening of the northern part of the ridge by the end of the week
may make the heat a little less extreme over New England, while the
central Plains may trend a little hotter (but with single-digit
positive anomalies). In contrast to the eastern heat, lingering
upper troughing will support highs 10-20F below normal over parts
of the Interior West/northern High Plains on Tuesday with
moderately cool readings over the northern-central High Plains
Wednesday. Then rising heights aloft will bring a warmer trend to
most of the West, with the region likely seeing highs 5-12F above
normal by Friday-Saturday. Moist easterly flow should lead to two
or three days of below normal highs over the western Gulf Coast
and southern High Plains by mid-late week.

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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