252
FXUS02 KWNH 300721
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 AM EDT Thu May 30 2024

Valid 12Z Sun Jun 02 2024 - 12Z Thu Jun 06 2024

...General Overview...

An active flow pattern over both central/eastern portions of the
country and from the Pacific across the northern tier U.S. and
southern Canada (with multiple frontal systems) should favor
multiple episodes of showers and thunderstorms from the weekend
into next week. The Southwest should remain mostly dry, while
southern Texas should see persistent hot and humid conditions under
the northern periphery of an upper ridge anchored over Mexico. A
developing ridge over the southern half of the West by mid-week
will bring widespread above average temperatures and potentially
hazardous heat.

...Guidance/Predictability Assessment...

Initial uncertainty into the medium-range period not surprisingly
revolves around the timing of multiple shortwaves passing from the
Plains/Mississippi Valley eastward towards the Midwest/Ohio Valley
before shearing off towards the East Coast. The 12Z ECMWF/UKMET,
supported by the 12Z ICON and suite of EC machine learning (ML)
models, lie in the middle of the envelope of all solutions showing
an initial wave lifting into the Ohio Valley with the 12Z CMC a
bit faster and the 18Z GFS the most different, racing ahead the
other solutions in eastward position/movement and with a secondary
wave to its southwest. Timing is a bit more evenly distributed
with another rather progressive wave following in its wake, with
the cluster of solutions likely providing a good compromise. Began
with a multi-model blend slightly favored towards the
ECMWF/UKMET/CMC
over the GFS given the stronger initial differences. Attention
then turns west as shortwave energy ejects from a deep closed low
south of Alaska, traversing an eastward path close to the
international boarder, before a broad longwave ridge builds over
the West. The deterministic guidance falls into three camps with
respect to both features, with the ridge in the ECMWF taking a more
western position anchored towards the West Coast, the GFS further
east centered more over the Intermountain West, and the CMC in
between, a bit closer to the ECMWF but less amplified with respect
to both the ECMWF and GFS with the ridge in general. Further east,
the GFS and ECMWF are more aggressive with the lead shortwave
energy digging southward over the Midwest/Great Lakes while the CMC
is similarly less amplified further east with a more northerly
wave track. The 12Z ECens/GEFS means are similar to their
respective deterministic counterparts with both the ridge and
northern wave, though with a lesser degree of amplification. The
12Z ICON was more similar to the 12Z ECMWF, with the suite of EC ML
models once again providing a range of solutions encompassing the
deterministic/ensemble mean solutions. Given this range of
solutions, opted for an even blend of the deterministic and
ensemble mean guidance for this latter period until there is more
movement towards one of the solutions. The overall pattern,
specifically the notable ridging in the West, is still relatively
well agreed upon, with a heat wave expected over the region but
with differences on extent with respect to time and coverage.


...Weather/Hazards Highlights...

Periods of showers and thunderstorms are expected broadly across
central and eastern portions of the country heading into next week
with multiple upper-level waves and accompanying surface frontal
systems encountering a humid airmass. Both the uncertainty at this
range with respect to the timing of the waves and their generally
progressive nature make it a more difficult task to narrow down
areas of potentially locally heavier rainfall and flash flood
concerns. The updated day 4 and 5 Excessive Rainfall Outlooks
(EROs) contain multiple Marginal Risks where guidance shows the
best overlap of higher QPF values. For day 4 (Sunday), this
includes portions of the Southern Plains and Lower Mississippi
Valley as well as the Upper Midwest in the wake of an initial
shortwave lifting through the Ohio Valley and a second approaching
wave/frontal system passing from the Plains towards the Mississippi
Valley. Some better agreement in the deterministic guidance for
potentially more widespread heavier rainfall worthy of a Slight
Risk was noted from Minnesota into Wisconsin, however the subtle
and progressive nature of the upper-level wave still casts some
doubt on the timing and extent of rainfall, backed up by low
ensemble probabilities. Some moderate to heavy rainfall will also
be possible in the Ohio Valley as the lead wave lifts through. An
additional round of storms is expected day 5 (Monday) ahead of the
frontal system pushing further into the Great Lakes and southward
into the Middle Mississippi Valley and Central/Southern Plains,
with another Marginal Risk introduced for the latter region.
Another upper-level system is expected to traverse the
international
border and eventually dig southeastward into the Midwest. This may
bring areas of moderate to heavy rainfall to the Upper Midwest
Tuesday and Ohio Valley/Lower Great Lakes by Wednesday, though with
greater uncertainty. Additional weak upper- level energies from
the Southern Plains to the Southeast will also keep up the
potential for additional storms.

An upper-level wave and accompanying plume of Pacific moisture
will bring heavy rainfall to the Pacific Northwest on Sunday. A
Marginal Risk is included on the day 4 ERO along the
Washington/Oregon
coasts and inland to the Cascades. The warm moist air should keep
snow limited to only the highest mountain peaks. The warmer
temperatures may also help enhance snowmelt locally, adding to the
risk for some isolated flooding concerns. While streamflows are low
locally, another Slight Risk may be necessary here if the forecast
QPF amounts continue to increase. The precipitation will spread
inland over the northern Rockies Monday with the chance for locally
heavy amounts over 1 inch, and a Marginal Risk introduced.
Precipitation chances will linger into midweek for the Pacific
Northwest, though should decrease each day as ridging builds in
more broadly over the West.

Forecast high temperatures will start above average by 10-15
degrees late this weekend into early next week over portions of
the High Plains as well as New England, with generally near-average
temperatures in between with daily bouts of storms for the
Plains/Mississippi Valley/Southeast. One exception will be south
Texas westward through the Rio Grande Valley, where the combination
of hot temperatures into the 90s and 100s as well as humidity will
keep heat indices in the upper 100s, locally 110+. Highs will
initially remain below average over the Pacific Northwest as the
Pacific system passes through the region. The big story
temperature-wise will begin to develop by mid-week as ridging
builds over the West. Highs broadly will be running 10-15 degrees
above average, with even warmer anomalies of 15-20+ degrees for the
central California valleys and portions the Great Basin. Forecast
highs are into the 100s from west Texas into the Desert Southwest
and central California valleys. Unfortunately, the Climate
Prediction Center has highlighted much of this region with at least
a Slight Risk of Excessive Heat with the expectation this heat
wave will continue beyond the current forecast period into at least
next weekend.

Putnam


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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