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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1857Z Jul 19, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Fri Jul 19 2024

Valid 12Z Mon Jul 22 2024 - 12Z Fri Jul 26 2024

***Heat wave continues across the West, and showers/storms with
 heavy rainfall expected from the Four Corners to portions of the
 East***

19Z Update: The 12Z model guidance suite remains in above average
agreement on the synoptic scale through the end of next week, with
a multi-deterministic model blend working well through Wednesday.
The GFS does become more progressive with the trough over the Great
Lakes region by Friday, and is also stronger with the Western U.S.
ridge during that time. Some ensemble means were included by
Wednesday night, and gradually increased to about 30-40% by the end
of next week. The main uncertainties involve the timing and
coverage of individual MCS events that will be the main drivers for
heavy convective rainfall across the south-central U.S. next week.
The NBM appeared to be both too light and also delayed with
convective initiation across the Four Corners region with the
monsoonal moisture in place, so more of the deterministic ECMWF and
GFS was incorporated to boost expected QPF. The previous forecast
discussion is appended below for reference. /Hamrick
------------------------


...General Overview...

Latest guidance shows an amplified pattern that will be fairly
slow to evolve with time. A strong ridge extending from the
Southwest U.S. into western Canada as of early Monday will produce
dangerous and potentially record-setting heat into next week, with
the Northwest seeing only gradual cooling as a Northeast Pacific
upper low wobbles into western Canada. More persistent ridging over
the Southwest will accompany monsoon conditions across the
southern Rockies and vicinity. Meanwhile, a wavy front will be on
the leading side of Great Lakes into southern Plains mean troughing
aloft, leading to multiple days of rain/thunderstorms with areas
of heavy rainfall from the southern Plains into the Mid-Atlantic
and parts of New England. An area of tropical moisture will pass
through Florida early next week to enhance daily shower and
thunderstorm activity, followed by the Atlantic upper ridge
building into the Southeast, peaking in strength around Wednesday-
Thursday.

...Guidance/Predictability Assessment...

Averages of latest dynamical guidance and machine learning (ML)
models compare fairly well to each other for the western ridge
through the period and Northeast Pacific upper low that reaches
into British Columbia after midweek (along with the trough to the
south). There are some detail differences with these features but
these have low predictability several days out in time.

Regarding the Great Lakes into southern Plains trough, there is
reasonable agreement that energy dropping south from Canada by
Tuesday-Wednesday should begin to eject an embedded weak upper low
over the Midwest as of early Monday. Then some more noticeable
differences arise with the overall trough after midweek. The 12Z
ECMWF strayed to the slow/southwestern side of the spread with the
core of its trough, while the GFS has been leaning a bit on the
faster side among the remaining more progressive models/means.
Interestingly, by next Friday most of the 12Z ECMWF-initialized
ML models become fairly progressive as well, and with somewhat more
amplitude than depicted in the latest GFS runs. There have been a
couple cases in the past few months when the MLs ultimately were
too amplified with forecast Northeast troughing so trends one way
or the other will be worth monitoring. The new 00Z ECMWF has
adjusted somewhat faster, now comparing better to the means and CMC
by next Friday. By late next week the cluster that is faster than
the 12Z ECMWF and less amplified than the MLs with the upper trough
(with corresponding surface evolution/progression) represents the
best intermediate solution.

With the 12Z UKMET not comparing well to consensus for some details
from the Upper Midwest northeastward after Monday, the first part
of the forecast based on 12Z/18Z guidance incorporated the
GFS/ECMWF/CMC. Then the latter half of the period gradually
increased 18Z GEFS/12Z ECens weight, reaching 50 percent total by
next Friday, while phasing out 12Z ECMWF input due to its
questionably slow Midwest upper trough.

...Weather/Hazards Highlights...

Expect dangerous heat over the West to extend into next week with
high temperatures reaching the 90s and 100s and warm overnight lows
providing only limited relief. The interior Northwest/far northern
Rockies should see the greatest coverage of major to extreme Heat
Risk values (along with daily record highs) on Monday, followed by
gradual cooling moving in from the Pacific Northwest as an upper
low eventually moving into British Columbia helps to lower heights
aloft. Elsewhere temperatures in the West should remain well above
average next week aside from perhaps some moderation over the Great
Basin toward the end of next week. Some of the heat will extend
into Montana/northern High Plains. The prolonged period of dry
heat could also result in enhanced wildfire danger.

Monsoonal moisture will linger over the Four Corners region and
promote daily episodes of showers/thunderstorms. Isolated to
scattered instances of flash flooding will be possible, especially
near steep terrain and burn scars. The Days 4-5 Excessive Rainfall
Outlooks covering Monday-Tuesday continued to show Marginal Risk
areas, with the Day 4 outlook continuing a Slight Risk area over
parts of northern New Mexico and south-central Colorado given
sufficiently favorable guidance signals, persistence from the Day 3
time frame, and already wet ground conditions, along with
vulnerable burn scar areas.

A southern Plains into Northeast wavy front will provide a multi-
day focus for areas of heavy showers and thunderstorms next week.
The southern Plains part of the boundary may dissipate by midweek
but another front pushing south from the northern tier should
maintain a mean front over the eastern half of the country into
late week. With ample moisture and instability in place, locally
heavy rainfall will be possible and may cause instances of flash
flooding, especially in urban and poor drainage areas where
convection repeats/trains. During Day 4/Monday, guidance is
signaling the best potential for heavy rainfall over parts of
south-central/east-central Texas, and the existing Slight Risk
area has been expanded some over this region. A broad surrounding
Marginal Risk area remains in place from the southern Plains and
central Gulf Coast to the Mid- Atlantic. The Day 5/Tuesday ERO
continues a broad Marginal Risk area from the southern
Plains/central Gulf Coast into the northern Mid-Atlantic/southern
New England, with guidance suggesting that frontal waviness and
associated moisture will lift northeastward over the eastern U.S.
The southern portion of the existing Marginal across the Southeast
and southern Mid-Atlantic was trimmed back some, while the northern
portion was extended slightly more. There is also a Marginal Risk
area on Tuesday over parts of the Upper Midwest where shortwave
energy dropping south from Canada may produce locally heavy
convection.

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