708
FXUS21 KWNC 121808
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 12 2026

SYNOPSIS: A pair of forecast mid-level high pressure centers over the eastern
and western contiguous U.S. (CONUS) results in a more favorable pattern for
unseasonably warm temperatures, with possible extreme heat conditions being
felt over parts of the Mid-Atlantic, Southeast, and California early in week-2.
Surface low development in the lee of the Rockies supports potentially heavy
precipitation over the south-central CONUS, with the highest risk now timing
off into week-1. Meanwhile, cyclonic mid-level flow over the southwestern CONUS
may also bring episodes of high winds across coastal California and Oregon.

HAZARDS

Slight risk of extreme heat for portions of the Mid-Atlantic, and Southeast,
Wed-Thu, May 20-21.

Slight risk of extreme heat for the Central Valley of California, Wed-Thu, May
20-21.

Slight risk of heavy precipitation for portions of the Southern Plains, Lower
Mississippi, Tennessee and Ohio Valleys, Wed-Thu, May 20-21.

Slight risk of high winds for much of coastal California and Oregon, Wed-Fri,
May 20-22.

DETAILED SUMMARY

FOR FRIDAY MAY 15 - TUESDAY MAY 19:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY MAY 20 - TUESDAY MAY 26: As previously advertised, a major
pattern shift looks to be well underway during week-1. Dynamical models favor
the development of amplified ridging over much of the eastern CONUS, troughing
over portions of the Great Plains and Interior West, with another anomalous
500-hPa ridge center nudging eastward from the northeastern Pacific into the
West Coast. By the start of week-2, both the GEFS and ECWMF ensembles are in
fair agreement depicting a temporary bridging between the two 500-hPa ridge
centers over the eastern and western CONUS, consisting of rising heights over
the Interior West and Great Plains, followed by a breakdown of this pattern
towards the end of next week. It is at this forecast lead where the ensemble
mean solutions begin to diverge considerably in the latest guidance. The ECMWF
favors more of a progressive evolution of the pattern, featuring more ridging
shifting over the western CONUS from the eastern Pacific, with more downstream
troughing east of the Rockies. In contrast, the GEFS is more steadfast with the
amplified ridge over the eastern U.S. ridge, while favoring more longwave
troughing upstream across the West. As a result, there is significant
uncertainty in the hazards outlook beyond day 10 (May 22) due to these
mid-level features being completely out of phase over the CONUS in the various
models.



Tied to the amplified ridge across the East at the start of week-2, a slight
risk of extreme heat remains posted (May 20-21) over portions of the
mid-Atlantic and the Southeast where Probabilistic Extreme Tools (PETs)
maintain increased chances for maximum daytime temperatures exceeding the 85th
percentile climatologically and 90 deg F.  Both raw and calibrated temperature
tools continue to show much of the anomalous warmth peaking late in week-1,
before the ridging aloft deamplifies over the eastern U.S. in both the GEFS and
ECMWF by the middle of next week.



Due to the predicted larger and slightly stronger coverage of positive height
departures overspreading the West Coast compared to yesterday, a slight risk of
extreme heat is also posted (May 20-21) over the Central Valley of California
which is supported by warm trends in the latest temperature tools.  Although
the GEFS is comparatively weaker with the heat potential than the ECWMF, both
PETs show at least 20% chances of maximum temperatures exceeding the 85th
percentile, with support in the raw tools indicating similar chances for actual
temperatures exceeding 100 deg F.  Based on historical heat risk data and
classifications defined at https://www.wpc.ncep.noaa.gov/heatrisk/, such
temperatures surpass orange and red level thresholds over this part of the
country during May. If spring heat is realized over these highlighted areas,
such temperatures may adversely impact unsuspecting populations without cooling
or hydration systems in place, and also those who are not yet acclimated to
this level of warmth.



Across the Great Plains, surface low development and an accompanying frontal
system continues to be favored, potentially bringing heavy precipitation to the
south-central CONUS late in week-1. Much of the surface return flow to enhanced
moisture advection into the Plains and Mississippi Valley is favored to ease
heading into week-2, with the mid-level heights also favored to rise. Daily
mean precipitation totals from the GEFS, ECMWF, and Canadian ensembles now show
the highest amounts timing off, though raw and calibrated probabilistic
guidance continues to indicate increased chances for 3-day amounts exceeding an
inch focused over the Southern Plains and Lower Mississippi Valley, albeit
lower chances compared to yesterday. Based on the drier trend in the tool and
the less favorable synoptic pattern, the moderate risk of heavy precipitation
is discontinued in the updated outlook, however a slight risk remains posted
and valid through May 21 due to some of the frontal activity potentially
lingering into early week-2. Flash flooding is possible in some areas with
anticipated rainfall during week-1 and the beginning of week-2, especially in
some areas in the Deep South that are currently experiencing river flooding.




Given the transient nature of the pattern over the Great Plains, with anomalous
troughing lifting out and mid-level heights beginning to rise, the risk of high
winds is expected to diminish.  Although there continues to be marginal signals
in the PETs for winds speeds exceeding the 85th percentile, there is not enough
forecast confidence that wind speeds will exceed hazard thresholds based on the
favored surface pattern and uncalibrated wind guidance among the GEFS and
ECMWF, thus the slight risk of high winds is removed over the central CONUS.
However, the slight risk of high winds remains posted over portions of coastal
California and Oregon where ensembles generally show cyclonic mid-level flow
underneath the anomalous ridge center over the northeastern Pacific as well as
periods of strengthening pressure gradients in the surface guidance.



In Alaska, the typical peak of the river ice breakup season is approaching, and
ice jam flooding can occur this time of year with little or no notice as
conditions on frozen rivers can change quickly. Please check with the Alaska
Pacific River Forecast Center for the latest conditions and advisories.

FORECASTER: Nick Novella

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