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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0700Z Jun 14, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Fri Jun 14 2024

Valid 12Z Mon Jun 17 2024 - 12Z Fri Jun 21 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...

...Overview...

A strong upper ridge forecast to build over parts of the East
will likely 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/Northeast. Meanwhile strong dynamics ejecting from the
Northwest and associated surface development/fronts early in the
week should produce one or more areas of significant precipitation
between the northern Rockies and Midwest. The lingering western
U.S. mean trough aloft may retrograde to the West Coast by late in
the week. Beneath the upper ridge that should extend back to the
southern Plains/Rockies by the latter half of the week, the
combination of upper level impulses and abundant deep moisture will
bring a potential for areas of heavy rain along and near the
western half of the Gulf Coast.

...Guidance/Predictability Assessment...

Early in the period, the primary guidance difference of note is
that latest GFS runs are a bit on the slow side versus most other
solutions with the strong upper system crossing the Northwest
during Monday-Tuesday. This results in the model pulling its heavy
QPF axis farther west than consensus in the vicinity of the
northern Rockies.

Meanwhile guidance agrees that the center of the strengthening
eastern U.S. upper ridge should trace a path from near the western
Carolinas early Monday into the northern Mid-Atlantic/southern New
England by early Wednesday. Then consensus shows expansion of upper
ridging back into the southern half of the Plains/southern Rockies
but with increasing spread for where the strongest part of the
ridge will be (ranging between the Ohio Valley and western Atlantic
by Friday). Most of the 12Z ECMWF-initialized machine learning (ML)
models show the upper high over or a little south of the 12Z ECMWF
mean location over the East as of early next Friday while the new
00Z GFS/CMC have it over the western Atlantic. The mean trough over
the West should weaken and retrograde a bit mid-late week as the
upper ridge expands, with low predictability for shortwave details
after the vigorous early week system ejects. By next Friday the ML
models support a solution closest to the GEFS/ECens means, showing
a little better definition for the West Coast trough than some of
the 12Z/18Z dynamical model runs. Predictability is typically
fairly low for the upper impulses retrograding underneath the
ridge, and while there is a fairly good consensus for the area of
enhanced deep moisture retrograding across the western half of the
Gulf of Mexico/Gulf Coast, QPF solutions are much more diverse.

The updated forecast blend based on 12Z/18Z guidance started with
an operational model composite for the first half of the period
followed by gradually increasing 18Z GEFS/12Z ECens mean weight so
that the means were 60 percent of the blend by the end of the
forecast next Friday.

...Weather/Hazards Highlights...

Strong dynamics ejecting from the Northwest will promote Plains
surface development (ultimately tracking into Canada) during the
first part of the week. From Monday into early Tuesday, a leading
northern tier front will likely stall ahead of the evolving surface
wave, with guidance showing a fairly steady area of anomalous
moisture over the Upper Midwest. While there is still some
model/ensemble spread for the axis of heaviest rainfall, the setup
looks favorable enough to introduce a Slight Risk area in the Day 4
Excessive Rainfall Outlook. The proposed location over northern-
central Minnesota and some nearby areas represents an intermediate
approach among the current guidance spread. Meanwhile there is a
Marginal Risk across much of Montana where the upper system may
produce some areas of heavy rainfall--with some snow also possible
in highest elevations of the northern Rockies.

By the Day 5 time frame Tuesday/Tuesday night, expect central U.S.
convection to start expanding southward along the cold front
anchored by the surface wave tracking into Canada. Currently there
is enough spread to warrant only a Marginal Risk area from the
Upper Midwest into the central Plains but improved clustering in
the near future would merit one or more embedded Slight Risk areas.
A lingering Marginal Risk area is also in place over northeastern
Montana/northwestern North Dakota near the path of the upper
system.

Guidance generally agrees that an area of deep tropical moisture
should shift westward across the western half of the Gulf of
Mexico, bringing a potential for some heavy rainfall to locations
along the central and western Gulf Coast next week. Operational
models differ considerably for QPF details with ensemble
probabilities also somewhat diffuse, while ML models show a bit
more coherence for rain potential along the Louisiana and eastern
Texas coasts. For now prefer to maintain a Marginal Risk for Days
4-5 while awaiting better agreement regarding QPF signals.

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. This heat
wave will likely extend beyond Friday. While the pattern details
were obviously different, the axis of greatest temperature
anomalies has some similarity to the 1994 heat wave that was
observed around the same part of June. In contrast to the eastern
heat, the deep upper trough crossing the Northwest early next week
will bring a couple days of highs 10-20F below normal to parts of
the West and northern High Plains. After this system departs,
expect the West to trend warmer, reaching above normal by late
week. Moist easterly flow should bring a cooler trend to the
southern High Plains by 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