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FXUS02 KWBC 312023

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Wed May 31 2023

Valid 12Z Sat Jun 03 2023 - 12Z Wed Jun 07 2023

...Broad multi-day thunderstorm and local heavy rain threat
continues from the Northern Rockies to the Southern Great Plains...


A strong upper ridge/high developing over the Upper Midwest and
south-central Canada this weekend will persist into next week,
supporting prolonged above normal temperatures from the northern
Plains into middle/upper Mississippi Valley through midweek. Broad
upper low energy over the Intermountain-West through the southern
High Plains will weaken this weekend, but then a well-advertised
southern stream Pacific low should track into southern California
on Monday. These features will maintain periods of convection over
the Rockies and parts of the central/southern Plains, gradually
expanding to the Sierra Nevada, through the weekend. Expect above
normal temperatures over the northwestern states through at least
midweek, though a front ahead an ejecting Pacific upper low should
provide a brief reprieve near the Pacific Northwest coast during
the weekend. Downstream from the central upper ridge, guidance
continues to vary though a consensus is moving toward anomalous
low pressure over the Northeast developing this weekend and
persisting into or through midweek which would bring cool and
wetter conditions to New England and perhaps the northern

...Guidance Evaluation/Predictability Assessment...

The 12Z GFS has backed off a bit from its recent outlier (and
ignored in the initial model blend today) solution of a stronger
central ridge that allowed a cutoff low to track across the
Northeast this weekend and over the Midwest by Sunday. The 12Z
consensus of global deterministics are to contain this upper low
to the Northeast this weekend through Monday with divergence then
about how strong the reinforcing shortwave trough is with the 12Z
CMC much stronger, developing a second low, and the 12Z UKMET now
digging much farther south to the southern Appalachians while the
GFS and ECMWF limit the progression to the Mid-Atlantic (with the
GFS being farther south of the two). By next Tuesday-Wednesday,
the latest ensemble means suggests the low/trough will be anchored
over southern New England or the northern Mid-Atlantic. Meanwhile,
the National Hurricane Center continues to monitor a disturbance
over the Gulf of Mexico associated with a southern tier upper
trough that looks to shift east to the Atlantic this weekend with
the motion dependent on upper feature details to the north.

The forecast farther westward is less chaotic in most respects.
Guidance has been consistent in weakening initial shortwave
energy/one or more upper lows over the West over the course of the
weekend. There have been some recent trends toward a little
farther eastward extent of height falls ahead of the upper low
ejecting from the northeastern Pacific, with corresponding effects
on the leading cold front and Pacific Northwest temperatures for a
time. The majority cluster of guidance still holds up well for the
upper low to track into southern California next week, with most
recent cycles having one or two stray solutions that ultimately
trend closer to the mean.

Based on the array of guidance from this morning, the forecast
progs were based on a non-06Z GFS consensus that increasingly
favors the 00Z ECENS mean through time.

...Weather/Hazards Highlights...

The pattern keeps a persistent threat for showers and
thunderstorms over parts of the Rockies and Plains during the
period, with diurnally favored showers/storms also likely to
become more common with time from the Sierra Nevada-southern
Cascades into the Great Basin as the southern stream Pacific upper
low approaches/reaches southern California Monday. Gradual
weakening of upper impulses/lows initially over the West/High
Plains should lead to somewhat lower maximum rainfall totals
Sunday/Monday than in the shorter term, but localized flash flood
potential will likely continue given wet ground conditions from
recent or soon to occur rainfall. The Days 4-5 Excessive Rainfall
Outlooks (covering Saturday through Sunday night) depict Marginal
Risk areas across locations from the northern Rockies into parts
of the southern Plains to reflect the persistent moisture/overall
pattern that could still produce some areas of locally heavy
rainfall. It will likely take into the shorter range time frame to
resolve areas of more pronounced focus/confidence for any embedded
Slight Risk areas. The Day 5 outlook incorporates the Sierra
Nevada into the overall Marginal Risk area with moisture expected
to increase across the region.  Meanwhile the Day 4 outlook
continues to have a Marginal Risk area for some lingering heavy
rainfall focused over the lower Florida Peninsula. Expect the
potential for some showers/storms with a couple cold fronts
dropping south over the Great Lakes/Northeast during the period,
with a potential trend toward diurnally favored pockets of
rainfall in the Northeast as upper troughing deepens next week.
Most of any moisture still crossing Florida during the weekend
should progress offshore thereafter, though still with some
lingering uncertainty given the guidance spread for the pattern
farther north.

The area of well above normal temperatures from the northern
Plains through Upper/Middle Mississippi Valley into the central
Appalachians as of Saturday should get steadily trimmed away on
the eastern side as upper troughing develops over the East with
increasing coverage of near to somewhat below normal readings. The
weekend should start with decent coverage of highs 10-20F above
normal, with some daily records possible, followed by most plus
10F or greater anomalies getting confined to the northern Plains
by next Wednesday. The Northwest will be the other focus for very
warm temperatures, with highs 10-20F or so above normal especially
Monday-Wednesday. An ejecting Pacific upper low/leading front
should temper the warmth during the weekend over western
Oregon/Washington though. Clouds and periods of rainfall should
keep highs 5-15F below normal over the southern half of the
Rockies/High Plains for most of the period and the upper low
reaching southern California Monday will also bring a cooling
trend there for early next week.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

- Heavy rain across portions of the Southeast, Sat, Jun 3.
- Flooding possible across portions of the Northern Plains.
- Flooding occurring or imminent across portions of the Northern
Rockies/Great Basin.
- Flooding likely across portions of the Central Rockies, and the
Northern/Central Great Basin.
- Much above normal temperatures across portions of the Pacific
Northwest and the Northern Great
Basin, Mon-Tue, Jun 5-Jun 6.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are