211
FXUS02 KWBC 291859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Wed May 29 2024

Valid 12Z Sat Jun 01 2024 - 12Z Wed Jun 05 2024

...General Overview...

A leading Mississippi Valley shortwave and trailing weaker
impulses, plus an active flow pattern from the Pacific across the
northern tier U.S. and southern Canada (with multiple frontal
systems) should favor multiple episodes of showers and
thunderstorms over the Plains/east-central U.S. from the weekend
into next Wednesday, along with a wet pattern over the Pacific
Northwest--especially with a fairly strong frontal system at the
start of the week. Most of the Southwest and the Northeast 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 and a developing ridge over the southern half
of the West by next Wednesday may begin to raise the potential for
hazardous heat over that part of the country.

...Guidance/Predictability Assessment...

The updated forecast started with a blend of operational 00Z/06Z
models followed by some GEFS/ECens input by the latter half of the
period. Early in the period, the primary differences of note were
with the initial Mississippi Valley shortwave. 00Z ECMWF-
initialized machine learning (ML) models looked more like the 00Z
UKMET/CMC that were somewhat more concentrated in appearance,
versus the more northward-sheared GFS/ECMWF, and the new 12Z ECMWF
looks more like the ML cluster. Then, over the past 24-36 hours
guidance has come into somewhat better agreement on stronger
emphasis of a trough reaching the Pacific Northwest around Monday
and eventual weakening of leading shortwave energy. There is still
typical spread for how the vigorous Monday system evolves farther
east into midweek, as well as for details of upper ridging expected
to start building over parts of the West. By early next Wednesday
the ML models generally favor more ridging over the southern half
of the West like the ECMWF/ECens, but with less northward amplitude
than the operational run. The latest GFS is somewhat east with its
ridge at that time.

...Weather/Hazards Highlights...

Numerous showers and thunderstorms are likely to remain ongoing
across portions of the Southern Plains and eastward to the central
Gulf Coast region going into the weekend, with multiple weak
disturbances aloft interacting with a warm and humid air mass.
Meanwhile most guidance shows a leading better defined shortwave
reaching near the Mississippi Valley as of early Saturday, with
potential for an associated surface wave and weakening frontal
system. Latest guidance does still does not appear to cluster well
enough for an embedded Slight Risk yet, but various solutions are
now evenly enough distributed to support one broad Marginal Risk
area in the Day 4 Excessive Rainfall Outlook to cover these
potential areas of convection. Locations currently with wet soil
conditions and/or experiencing heavy rain in the Days 1-3 period
will have added sensitivity to additional rainfall by Day 4.

Going into the Day 5 period Sunday, a more organized storm system
approaches the Dakotas and Upper Midwest, and guidance continues to
suggest that convection will be widespread/heavy enough to merit a
Marginal Risk here as a starting point. Fairly wet soil/above
average streamflows over the Upper Midwest add to the potential for
a Slight Risk upgrade as details become better resolved closer to
the event. Some strong to severe thunderstorms are also likely
with favorable parameters in place. The potential exists for
scattered QPF maxima of 1-3 inches during this time period for some
of these areas that get repeated rounds of convection. Meanwhile
the Day 5 ERO depicts a Marginal Risk area parts of the South with
the combination of lingering moisture, instability, and shortwave
impulses. There is also an increasing likelihood of a late season
heavy rain event across northwestern Oregon and into western
Washington on Sunday, potentially an atmospheric river event, with
a Marginal Risk area maintained over this region. GFS/ECMWF runs
indicate precipitable water values may reach at least 3-4 standard
deviations above normal for a time, with best agreement for
heaviest totals along the coast. Significant rainfall (and possibly
some very high elevation snow) may also extend into the Cascades
but with less model consensus.

Beyond early Monday, precipitation should linger over the
Northwest, including the northern Rockies, but with somewhat
lighter totals and a drier trend moving up from the south as upper
heights rise somewhat. Parts of the Plains and east-central U.S.
should continue to see episodes of showers and thunderstorms
early-mid week as the Upper Midwest front continues into the Great
Lakes and another front reaches the central U.S.

Expect coverage of above normal temperatures to increase after
Saturday. Best potential for plus 10-15F anomalies and perhaps
locally higher is likely to be over the High Plains Sunday-
Tuesday, the Northeast by Monday-Wednesday with persistent flow
around the northern periphery of Atlantic high pressure, and over
California/Nevada into southern Oregon by next Wednesday as upper
ridging builds overhead. The combination of heat and humidity will
likely lead to hazardous heat index values over southern Texas
through the period, and the heat developing over the California
Central Valley may become hazardous by next Wednesday. This looks
like the beginning of a longer term heat event over the West during
the week 2 period per Climate Prediction Center forecasts. As for
exceptions to the warmth, clouds and rainfall will promote below
normal highs for multiple days over the Pacific Northwest while
locations from the Ohio Valley into Mid-Atlantic will see a cool
start to the day on Saturday.

Rausch/Hamrick


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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