171
FXUS02 KWNH 030701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Mon Jun 3 2024

Valid 12Z Thu Jun 06 2024 - 12Z Mon Jun 10 2024

...Hot temperatures across the West may be hazardous for the latter
half of the week...

...Overview...

Broadly, a western upper ridge and eastern upper trough pattern
will set up later this week through the weekend, though troughing
may approach the West Coast early next week. The ridge will produce
potentially hazardous heat across much of the West through the end
of the week. Meanwhile some rain is possible in the East on
Thursday and perhaps continuing through the weekend in the
northeastern U.S. underneath the trough. Additionally, the forecast
pattern should support increasing coverage and intensity of
showers/storms over the central or south-central Rockies and
Plains as systems ride the ridge through much of the forecast
period.


...Guidance/Predictability Assessment...

While model guidance is generally agreeable in showing the western
ridge/eastern trough pattern in some fashion, differences in the
details will impact sensible weather across the lower 48. The first
concern is the depth of the trough extending south and east from
the anchoring upper low near the Great Lakes. Dynamical and
AI/machine learning (ML) models have differed in the low/trough
position and depth, namely with many ML models indicating a farther
south centroid of the upper low and thus the resultant trough`s
southern extent compared to dynamical models. Aspects of the
pattern affecting the upper low include ridging to the north and
east over Hudson Bay/northern Ontario (a stronger ridge pushing the
upper low farther south) and the ultimate evolution of a
developing west-central Canada upper low that still shows
considerable spread in the guidance. Continued to favor a slightly
deeper low than some of the more shallow guidance like the UKMET,
but not to the southern extent of the ML models. For this forecast,
that favored the 12Z GFS as well as the ensemble means. Spread and
thus uncertainty continue to grow with what becomes of the trough
into early next week, but at least the newer 00Z GFS and ECMWF seem
somewhat in line with each other (00Z CMC farther east with the
upper low).

Ridging in the West through late week is agreeable in the model
guidance, though of course the eastern side will be dependent on
the trough`s shape. Models show a southern stream upper low
near/west of Baja California opening up as it approaches the
Southwest, with some expected detail differences. Much farther
north, upper low development in the northeast Pacific Friday-
Saturday will extend troughing south that approaches the West Coast
early next week. Differences with this have plagued the forecast,
some with timing of the eastward shift of the trough but especially
as models vary with dropping energy southward to form upper lows
somewhere in the eastern Pacific. The new 00Z model suite generally
shows a faster trend of the trough approaching the Northwest and
less chance for diving energies until perhaps Monday, but still
show some variations.

The WPC forecast favored a blend of the deterministic models early
in the forecast, with the 12Z GFS heavier weighting than the 18Z,
and gradually increased the proportion of ensemble means to half by
the end of the period.


...Weather/Hazards Highlights...

On Thursday, the upper trough and the surface cold front will
shift into the Eastern Seaboard and lead to rain and thunderstorms
there. Most guidance is still not too enthusiastic about heavy
rainfall potential, but available moisture/instability ahead of a
unseasonably strong upper low may be enough to produce some fairly
intense rain rates. Thus the Day 4/Thursday ERO depicts a Marginal
Risk area for the interior Northeast near the best forcing and
lower flash flood guidance. Removed the northern Mid-Atlantic from
the Marginal Risk as rain totals have sufficiently lowered there.
Uncertainties persist regarding the precise evolution and track of
the approaching upper low/trough which will affect the speed of the
surface front and in turn intensity/duration of heaviest rainfall.
Even behind the first cold front, there may be diurnally enhanced
episodes of rainfall in the northeastern U.S. into late week and
next weekend given the persistence of upper troughing. Held off
from issuing any Marginal Risk there for Day 5/Friday as rain
amounts look to decrease compared to Thursday, but there may be a
nonzero risk of flooding concerns especially if heavy rain falls in
similar areas as the previous day.

Farther west, some return flow of moisture (including perhaps
Pacific moisture advecting around the upper ridge) is possible
across the southern/central Rockies and Plains, with one or more
surface boundaries helping to focus showers and thunderstorms.
Placement and track of this convection each day is rather uncertain
based on the shape of the ridge. Scattered showers and storms may
begin Thursday but the signal for their existence and intensity is
too low to delineate any ERO risk. By Day 5/Friday, storms should
increase in coverage and magnitude in the central U.S., potentially
producing heavy rain. For a first take at the Day 5 ERO, have a
Marginal Risk delineated for Kansas/Oklahoma and clipping
surrounding states. This is somewhat broad due to the uncertainty
in placement--not all areas within may see heavy rainfall--but it
may be able to lessen in size in future forecasts if models
converge. Additional rounds of rain are likely in south-central
parts of the Plains and Mississippi Valley into the weekend, with
rain chances also increasing for the southern/central Rockies and
Front Range. Some scattered rainfall is possible late in the week
over parts of the Interior West/northern Rockies, likely increasing
in coverage early next week as troughing approaches.

The western ridge will promote notably hot temperatures across the
West especially for the latter part of this week. Highs broadly
will run 10-20 degrees above average and locally 20+ degrees above
average in the Great Basin. High temperatures above 110F are
forecast for the Desert Southwest, with 90s and 100s stretching
into the central California valleys to the Great Basin to much of
Texas. Scattered daily record high minimum and maximum temperatures
are possible. The higher anomalies look to shift into the
Northwest for the weekend with some minor moderation of the
temperatures in the Southwest. Meanwhile, most areas east of the
Rockies should trend to slightly below normal behind the cold
front. Exceptions to this trend will be Texas (especially southern
areas) and Florida where highs several degrees above normal may
lead to some daily records.


Tate


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



$$