988
FXUS02 KWBC 301818
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
218 PM EDT Thu May 30 2024

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

...General Overview...

Progressive flow pattern across the northern/middle CONUS through
early next week before ridging in the west/troughing in the east
becomes dominant through midweek. A trough crossing the Pacific
Northwest Monday before developing into a closed low over the
northern Plains Tuesday. Frontal systems ahead of this wave favor
repeating episodes of showers and thunderstorms over the
central/eastern portions of the CONUS. The Southwest remains dry,
while hot and humid conditions persist over southern Texas under
the northern periphery of an upper ridge anchored over Mexico. This
ridge expands over the western U.S. by midweek, bringing
widespread and potentially hazardous high temperatures.


...Guidance/Predictability Assessment...

Decent agreement in the 00Z ECMWF/06Z GFS with the main synoptic
feature which is the Northwest to northern tier trough into low
which allowed their dominance in the morning model blend through
Day 5. The 00Z UKMET is much more zonal/less amplified with the
leading trough from the low that settles off BC allowed it to be
removed from the blend by Day 4. The 00Z ECMWF seemed the most
reasonable with both the low development over the northern Plains
Tuesday and the subsequent ridging in the west, so it becomes the
dominant entry into the blend (as well as QPF) by Day 5. The 06Z
GFS is more progressive with the northern tier low/trough and is
inching more toward the ECMWF with the western ridge through
midweek, but remains less dominant with that ridge by Day 7, so it
was less included in the morning model blend.


...Weather/Hazards Highlights...

Periods of showers and thunderstorms are expected broadly across
central and eastern portions of the country Sunday night/Monday
with multiple shortwave troughs and accompanying surface frontal
systems encountering a humid airmass. Uncertainty with the timing
of the waves and their generally progressive nature continue to
warrant broad brushes for the Marginal Excessive Rainfall Outlooks
(EROs) in Days 4 and 5 over the central sections of the CONUS. For
Day 4 (Sunday), this includes the Upper Midwest to central Plains
as well as the southern Plains to the central Gulf Coast. An
additional round of storms is expected Day 5 (Monday) ahead of the
frontal system over the Middle Mississippi Valley and
Central/Southern Plains where the Marginal Risk was expanded over
the Ozarks.

An upper-level wave and atmospheric river into the Pacific
Northwest brings heavy rainfall Sunday before tapering off on
Monday. Marginal Risks are included on the Days 4/5 ERO along the
Washington/Oregon coasts and inland to the Cascades. The warm moist
air will keep snow limited to the higher mountain peaks with
enhance snowmelt below the snow level, adding to the risk for some
isolated flooding concerns. While streamflows are currently low
locally, a Slight Risk may be necessary here if focused QPF
increases. The precipitation will spread inland over the northern
Rockies Monday with the chance for locally heavy amounts over 1
inch, with the Marginal Risk maintained there for Day 5.
Ridging then builds in broadly over the West with a dry pattern
beginning midweek.

This upper-level trough is expected to shift east to the northern
Plains Tuesday before digging 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
details retain some uncertainty. Additional weak upper-level
energies from the Southern Plains to the Southeast will also keep
up the potential for additional storms.

Forecast high temperatures will start above average by 10-15
degrees early next week over portions of the Plains then over the
northern tier through New England, with generally near-average
temperatures with daily bouts of storms for the eastern Plains/
Mississippi Valley/Southeast. South Texas westward through the Rio
Grande Valley is 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+. Below normal temperatures persist early
next week over the Pacific Northwest as the system passes through
the region. Then the next big story temperature-wise will begin to
develop by midweek 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. 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.


Jackson


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


$$