513
FXUS21 KWNC 101932
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 10 2026

SYNOPSIS: A developing area of mid-level low pressure and an unseasonably
cooler air mass east of the Rockies is expected to reduce the risk of
persistent extreme heat across the lower Great Plains and Mississippi Valley,
but also possibly bring first frost and/or freezes to the northern tier of the
CONUS during the week-2 period. Another round of enhanced tropospheric moisture
is favored to overspread the western and central CONUS, potentially bringing
heavy precipitation and possible flooding over portions of the Four Corners.

HAZARDS

Slight risk of much below normal temperatures for parts of the Northern Plains,
Upper Mississippi Valley, and eastern Great Lakes, Fri-Sun, Sep 18-20.

Slight risk of much below normal temperatures for parts of northern New
England, Sat-Tue, Sep 19-22.

Slight risk of heavy precipitation from the Four Corners region, Fri-Sun, Sep
18-20.

Flooding possible for portions of the Desert Southwest, Great Basin, and
Rockies.

DETAILED SUMMARY

FOR SUNDAY SEPTEMBER 13 - THURSDAY SEPTEMBER 17:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY SEPTEMBER 18 - THURSDAY SEPTEMBER 24: Consistent with previous
ensemble based guidance, both the GEFS and ECENS remain in good agreement in
regards to the evolution of the 500-hPa pattern late next week and into the
following weekend. As an amplified ridge shifts eastward from the Gulf of
Alaska into western Canada, a developing longwave trough is favored downstream,
where models favor a deeper solution of the trough axis over eastern North
America compared to yesterday. With more cyclonic flow developing over the
eastern CONUS early in week-2, much of the enhanced subtropical ridging that
has been established for quite some time is expected to ease and become more
shunted to the west, resulting in cooler and drier conditions for many areas
east of the Rockies. Such a cooldown is anticipated to be very much welcomed,
particularly for areas embattled by persistent heat conditions since August
across the Great Plains and Mississippi Valley, though the cold air advection
may possibly produce first frosts and/or freezes for portions of the northern
tier of the CONUS during the period. Underneath the amplified mid-level ridge
over the Gulf of Alaska, models continue to favor secondary mean troughing
encroaching the West Coast early in week-2. Although weak, this feature
combined with additional Tropical Cyclone (TC) activity in the eastern Pacific
may bring another round of enhanced moisture advection into the western CONUS
supportive of heavy precipitation and flood risks over the Four Corners.
Relative to yesterday, the eastern extent of this moisture intrusion looks to
become more concentrated over the Interior West due to aforementioned troughing
and the associated drier, northwesterly flow overspreading the east.



Tied to the deepening trough favored over the eastern CONUS, a slight risk of
much below normal temperatures remains posted over the Upper Midwest for Sep
18-20, with its coverage shifted eastward in the updated outlook to include
(exclude) more of the Great Lakes (Northern High Plains). A separate slight
risk area of much below normal temperatures remains designated for northern New
England valid for Sep 19-22 to capture the evolution of the cold air mass.
Within the highlighted areas, there continues to be agreement in uncalibrated
temperature tools showing increased chances for temperatures falling below 40
deg F, with the ECENS Probabilistic Extremes Tool (PET) favoring 20-30% chances
of minimum temperatures falling below the 15th climatological percentile. While
near freezing temperatures are not markedly unusual for this time of year
(climatological first freezes occur by late September for the highlighted
areas), any frost and/or freezes associated with the favored cold air mass may
be impactful to some vegetation and unsuspecting populations.



While cooler temperatures are favored further south associated with an
advancing frontal boundary, temperatures may be slow to moderate across
portions of the Southern Plains and Lower Mississippi Valley.  PETs maintain
increased chances for temperatures remaining above the 85th percentile over
this part of the country and along parts of the Southeast, however chances of
extreme heat are well diminished based on lower heat index values indicated by
the tools by the start of week-2, thus no corresponding hazard areas are
posted. Continued above-normal temperatures may exacerbate dry ground
conditions and sustain an increased wildfire risk, but given the relatively
cooler temperatures favored and decreasing sun angles with the start of autumn
soon approaching, the prospect of additional impacts associated with the
persistent warmth is lower overall. In addition, the latest U.S. Drought
Monitor indicates drought conditions now being registered over many parts of
the south-central CONUS, and with increased rainfall amounts favored over the
next week or so, this supports the removal of the Rapid Onset Drought (ROD)
risk in the updated outlook.



With troughing and onshore flow favored near the West Coast, with the potential
for added tropical moisture from the eastern Pacific, a slight risk of heavy
precipitation remains posted (Sep 18-20) over the Four Corners where
uncalibrated tools maintain elevated chances for 3-day precipitation amounts
exceeding a half an inch, and in some locations more than one inch. The slight
risk coverage is now omitted over the Great Plains, Mississippi and Ohio
Valleys, and lower Great Lakes in the updated outlook. This adjustment is based
on a reduction of signals in both the GEFS and ECMWF PETs, and an area of mean
surface high pressure descending from the north-central CONUS which is
anticipated to limit the eastern extent of enhanced moisture by the start of
week-2. In addition, given the continued indications of potentially heavy
precipitation during the next week or so, a possible flooding hazard remains
posted for portions of the Four Corners. This region is prone to flash flooding
and caution should be exercised near canyons and creek beds, and additional
rainfall may hamper recovery efforts from recent flooding in the Grand Canyon.



Across the eastern Pacific, the National Hurricane Center (NHC) is currently
monitoring newly formed TC Norbert as of 11am PDT, as well as a pair of watch
areas located near 150W and 110W with 30% and 70% chances of development,
respectively, during the next week.  While the latter higher likelihood area
looks to be more of a factor in regards to the enhancing moisture advection
into the CONUS, a continued westward track from Norbert beyond the official
forecast track from NHC, as well as the lower likelihood watch area in
proximity to Hawaii may pose an additional heavy precipitation threat to the
state during the next few weeks. Any additional TC related precipitation may
worsen already saturated grounds across parts of Hawaii, as there has not been
much time for recovery following the impacts associated with TCs Lowell, Moke,
and Lala this season.

FORECASTER: Nick Novella

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