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Extended Forecast Discussion
 
(Latest Discussion - Issued 2032Z May 27, 2022)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
432 PM EDT Fri May 27 2022

Valid 12Z Mon May 30 2022 - 12Z Fri Jun 03 2022

...Potential for heavy precipitation next week from the northern
Rockies to Upper Midwest and then southward through the Plains...

...Weather Pattern Overview...

The medium range period begins Monday with a rather amplified
upper level pattern for late May, consisting of an anomalously
deep trough with embedded low in the West and ridging over much of
the East (though suppressed in the Northeast by an eastern Canada
upper low). This configuration will allow for potentially heavy
precipitation to spread from the northern Rockies into the
northern Plains and Upper Midwest. Higher elevation snow is
possible in the Rockies while heavy rain and severe weather are
threats in the Plains to Midwest along with warmer than normal
temperatures. As next week progresses, the mean flow aloft should
flatten quickly as the main western upper low lifts into
south-central Canada and consolidates with separate central Canada
energy to yield a fairly deep upper low. The highest precipitation
chances should shift into the central/southern Plains and across
the East along and ahead of the cold front that pushes eastward
from the Plains. Toward the end of next week an upper trough may
bring a front and some associated moisture into the Northwest.


...Model Guidance Evaluation/Preferences...

Latest guidance through the 00Z/06Z cycles has generally followed
through on recent trends toward faster ejection of the main
western U.S. upper low through the northern tier and then stronger
interaction with central Canada energy to yield a rather deep
upper low and vigorous surface system just southwest of Hudson Bay
by midweek. The deeper evolution ultimately leads to somewhat
faster progression of the front that begins to push eastward from
the Plains. Even with the faster trend, other 12Z models suggest
the new GFS may be too fast with the northern U.S./southern Canada
system by early Tuesday. Details of a separate weaker northwestern
U.S. upper low shearing out as the mean pattern rapidly flattens
continue to be somewhat ambiguous, and will affect frontal
wave/rainfall specifics to the east of the Rockies. Outside of the
UKMET, new 12Z models have trended stronger with this feature
through Tuesday-Wednesday. Meanwhile there is decent agreement in
principle that energy underneath the eastern U.S. upper ridge may
try to consolidate into a closed low somewhere near the Southeast
coast/Florida/northern Bahamas by midweek. However by day 7 Friday
recent GFS/GEFS mean runs have developed a deeper and more
expansive trough covering the Southeast U.S. and eastern Gulf
relative to most other guidance. The 12Z GEFS mean trended a bit
weaker with its day 7 upper trough though.  On the northern side
of the eastern ridge, guidance is gradually consolidating toward
an episode of fairly strong cyclonic flow around a far eastern
Canada upper low. This leads to increasing potential for the
associated cold front reaching New England early in the week to
push as far south as the northern Mid-Atlantic as a backdoor front
before eventually returning north as a warm front. Consensus is
reasonable for upper ridging to retrograde from the
East/Mississippi Valley and then settle over the southern
Plains/northern Mexico, connecting with the ridge that builds over
the Interior West/Rockies. For the upper trough approaching the
West Coast late in the week, GFS runs show a little more
southeastward amplitude versus the majority at times but
ultimately model/mean differences are within typical error ranges
for forecasts that far out in time. Guidance comparisons led to
using a 00Z/06Z model blend early in the period followed by
transitioning to a model/mean mix, with slightly more weight to
the 00Z ECMWF/ECMWF mean versus the 06Z GFS/GEFS mean (with
lingering 00Z CMC input) due to aforementioned issues over/near
the Southeast.


...Sensible Weather/Threat Highlights...

Early next week, the upper low emerging from the West along with
areas of low level upslope flow should combine to generate
moderate to heavy precipitation over the northern Rockies and High
Plains. Higher elevations of the northern Rockies particularly in
southern Montana to northern Wyoming could see significant late
season snow as the anomalously deep system leads to colder than
average conditions. Farther east, rain and thunderstorms are
threats across the northern Plains into the Midwest along a wavy
front that should initially be slow to move. Along this front, a
fairly strong surface system developing over the northern tier
into southern Canada during the first half of the week may help to
focus rainfall as well. Severe weather and heavy rain are both
possibilities with this convection. As the upper pattern flattens,
lower amplitude troughing will likely move slowly eastward across
northern parts of the central and eastern U.S. and push a cold
front ahead of it into the East back into the central/southern
Plains. This front will serve as a focus for convection as it
presses east across the Ohio Valley to the Eastern Seaboard and
south across the Plains and Mississippi Valley for the latter half
of the workweek. Some moderate to heavy rainfall is possible
especially across parts of the central U.S. but still with lower
than average confidence for the areal details and amounts.
Elsewhere, Florida can expect some scattered diurnally favored
convection, aided by energy aloft to the south of the eastern U.S.
upper ridge. This convection may become more widespread by late
week. Also the Northeast may see mostly light rainfall along a
backdoor front for the first half of the workweek before the
better focus for rainfall around Thursday just ahead of the main
cold front. A front expected to push into the Northwest late in
the week may bring areas of light rainfall.

The eastern U.S./Mississippi Valley upper ridge will promote above
average temperatures by 10-20F from the Plains to the Midwest/Ohio
Valley and the Northeast/Mid-Atlantic Monday-Wednesday, with some
daily record high maximum and minimum temperatures possible. The
backdoor front reaching New England and possibly as far south as
the northern Mid-Atlantic would bring a cooler trend to these
areas though. Meanwhile upper troughing will bring very cool
temperatures to areas from the Intermountain West through northern
Plains early in the week with highs forecast to be 10-20F or below
normal. The flattening pattern aloft will push the initial Plains
cold front eastward while leading to a moderating trend for below
normal temperatures over the central U.S., with mostly
single-digit negative anomalies by Thursday-Friday. The East will
see shrinking coverage of warm temperatures after Wednesday. The
West Coast and into the Great Basin could flip back to warmer than
normal temperatures by around 5-10F Wednesday and beyond, followed
by modest cooler trend on Friday along the immediate West Coast
with the arrival of a Pacific front.

Rausch/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

Hazards:
- Heavy precipitation across portions of the Northern
Plains/Rockies, Mon, May 30.
- Heavy rain across portions of the Central Plains, the Middle
Mississippi Valley, and the Southern
Plains, Tue-Wed, May 31-Jun 1.
- Heavy rain across portions of the Upper Mississippi Valley, and
the Northern Plains, Mon, May 30.
- Severe weather across portions of the Great Plains, and the
Middle/Upper Mississippi Valley, Mon,
May 30.
- Flooding occurring or imminent across portions of the
Central/Northern Plains, and the Upper
Mississippi Valley.
- Flooding likely across portions of the Central Plains.
- High winds across portions of the Central Plains/Rockies/Great
Basin, the Southern
Rockies/Plains, and the Southwest, Mon, May 30.
- Enhanced wildfire risk across portions of the Southern Rockies,
the Central/Southern Plains, and
the Southwest, Mon, May 30.

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

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