097
FXUS21 KWNC 211821
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT June 21 2024

SYNOPSIS: Mid-level low pressure across the northeastern contiguous U.S.
(CONUS) favors a brief cool down in the East late in week-1, but excessive heat
risk is forecast to increase during week-2 as mid-level ridging expands across
the CONUS. Another mid-level low over central Canada may limit the northward
extension of the heat across the Upper Mississippi Valley and Great Lakes.
However, frequent periods of thunderstorms with locally heavy rainfall and an
increased risk of flash flooding are expected to persist over these areas. The
Southwest monsoon is forecast to become more active, leading to increasing
chances of locally heavy rainfall and flooding across parts of the southwestern
U.S.

HAZARDS

Moderate risk for excessive heat for portions of the Ohio and Tennessee
Valleys, Sun-Mon, Jun 30-Jul 1.

Moderate risk for excessive heat for portions of the Mid-Atlantic, Sun-Mon, Jun
30-Jul 1.

Slight risk of excessive heat for portions of the eastern and central U.S.,
Sat-Fri, Jun 29-Jul 5.

Slight risk of excessive heat for portions of the Great Basin, Rockies,
Southwest, and California Central Valley, Mon-Fri, Jul 1-5.

Slight risk of heavy precipitation for portions of the Northern Plains, Upper
Mississippi Valley, and Great Lakes, Sat-Fri, Jun 29-Jul 5.

Slight risk of heavy precipitation for parts of the Southwest and Southern
Rockies, Sat-Fri, Jun 29-Jul 5.

Flooding possible for portions of the Northern Plains, Upper Mississippi Valley.

Flooding possible for portions of southern Arizona and New Mexico.

Rapid onset drought risk extending from New Jersey and Pennsylvania southward
through northern Georgia.

Rapid onset drought risk across the eastern Corn Belt.

Rapid onset drought risk across northern Oklahoma and adjacent areas.

DETAILED SUMMARY

FOR MONDAY JUNE 24 - FRIDAY JUNE 28:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY JUNE 29 - FRIDAY JULY 05: Late in week-1 mid-level troughing is
forecast to move across the Great Lakes and Northeast ushering in a reprieve
from the ongoing excessive heat. However, this break is predicted to be
short-lived as the 0z GEFS, ECENS, and CMCE quickly rebuild ridging across much
of the central and eastern CONUS early in week-2. The ECENS and GEFS
Probabilistic Extremes Tools (PETs) depict some parts of the Ohio Valley and
Mid-Atlantic having at least a 40 percent chance for maximum temperatures to
exceed the 85th climatological percentile over portions of the Ohio Valley and
Mid-Atlantic. Temperatures in the uncalibrated guidance have ticked upwards
compared to yesterday, with the 0z ECENS continuing to average a few degrees
warmer than the 0z GEFS. On day-8 (Jun 29), the warmer ECENS depicts high
temperatures in the mid-90s deg F across portions of the Ohio and Tennessee
Valleys (upper-80s to low-90s deg F in the GEFS). While the Mid-Atlantic may
remain under the influence of troughing initially, both models build higher
temperatures into the region by day-9 (Jun 30), with maximum temperatures in
the GEFS and ECENS ensembles above 90 deg F, and perhaps further increasing on
day-10 (Jul 1). Dewpoints are forecast to be relatively lower on day-8 (Jun 29)
in the wake of troughing, but increasing moisture thereafter favors heat
indices near or above 100 deg F across the Ohio Valley and 105 deg F across the
Mid-Atlantic, with the calibrated GEFS/ECENS skill-weighted heat tool depicting
at least a 40 percent chance heat indices exceed the 90th climatological
percentile across these regions. Given these signals, moderate risks for
excessive heat are posted across portions of the Ohio and Tennessee Valleys and
Mid-Atlantic Jun 30-Jul 1. A slight risk for excessive heat is posted across
portions of the eastern and central CONUS for all of week-2.



Early in the period, troughing is forecast to lift out of the western CONUS and
into central Canada. This favors an expansion of heat across the West, but may
also help limit the northward extent of the heat across the Upper Mississippi
Valley and Great Lakes. The GEFS and ECENS PETs depict expanding coverage of
probabilities of at least 20 percent for maximum temperatures exceeding the
85th climatological percentiles across portions of the Interior West and
California Central Valley beginning around day-10 (Jul 1) resulting in a slight
risk for excessive heat Jul 1-5. Maximum temperatures range from the 90s deg F
across the northern Great Basin and Rockies to triple digits over the
California Central Valley and Southwest. Increasing monsoon moisture may help
limit temperatures across parts of the Southwest and Four Corners.



The 0z GEFS and ECMWF ensembles depict multiple shortwave 500-hPa troughs
progressing east from the western to the north-central CONUS overtop the
expanding mid-level ridge. This longwave pattern along with uncalibrated
24-hour precipitation amounts from the GEFS and ECENS support a slight risk of
heavy precipitation across the Northern Plains, Upper Mississippi Valley, and
Great Lakes through the end of week-2. Thunderstorms could trigger flash
flooding. In addition, minor to moderate river flooding is expected to persist
into the beginning of July, especially across southern Minnesota.



The westward-expanding ridge favors the start of the Southwest monsoon, with
dewpoints increasing into the 60s deg F across the region in several recent
runs of the deterministic GFS. This is also aided by increasing southeasterly
moist flow related to Tropical Storm Alberto and potentially another tropical
disturbance later in week-1. Both the ECENS and GEFS PETs depict at least a 20
percent chance that 3-day rainfall totals exceed a half-inch across parts of
the Southwest and southern Rockies, with the ECENS being the most robust,
bringing these increased chances through much of Arizona later in the period,
and also having at least a 20 percent chance 3-day rainfall exceeds 1-inch
across extreme southern Arizona. This is also supported by the uncalibrated
tools which depict expanding coverage of week-2 rainfall totals of greater than
a half-inch into parts of Arizona and New Mexico. Given this support, a slight
risk for heavy precipitation is highlighted across portions of the Southwest
and Southern Rockies for all of week-2, with a flooding possible area covering
southern parts of Arizona and New Mexico where the stronger precipitation
signals are depicted. Precipitation is expected to be isolated and not all
locations will experience heavy rainfall. However, flash flooding is possible
both where precipitation occurs and in downstream valleys, particularly in and
around burn scars where debris flows can be triggered by heavy rainfall.



Rapid Onset Drought (ROD) risk extends from a region including New Jersey and
Pennsylvania southward through northern Georgia, as well as portions of the
eastern Corn Belt and northern Oklahoma. These areas are most vulnerable to the
rapid onset of drought due to: 30-day precipitation deficits of 2 to 4 inches,
low soil moisture, and above-normal temperatures with high evapotranspiration
rates forecast during the next two weeks.

FORECASTER: Thomas Collow

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