255
FXUS21 KWNC 292001
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 29 2024

SYNOPSIS: Strong and persistent mid-level high pressure has the potential to
bring an extended risk of excessive heat to much of the West, and to portions
of the Rockies and southern Plains. There is a slight risk of excessive heat in
this large area throughout week-2, and a moderate to high risk of excessive
heat over the interior West during the first half of the period. An excessive
heat risk also extends through all of week-2 on the Florida Peninsula, with the
greatest potential in the southern half of the state. Elsewhere, downsloping
may bring strong winds to parts of the northern High Plains early in the period
while a storm system in the Northeast brings a slight risk of heavy
precipitation.

HAZARDS

High risk for excessive heat for the Central Valley of California, Thu-Fri, Jun
6-7.

Moderate risk for excessive heat over much of the interior West, Thu-Sun, Jun
6-9.

Moderate risk for excessive heat over much of the southern Florida Peninsula,
Thu-Wed, Jun 6-12.

Slight risk for excessive heat throughout the West and in southern portions of
the Rockies and High Plains, Thu-Wed, Jun 6-12.

Slight risk for excessive heat over the Florida Peninsula, Thu-Wed, Jun 6-12.

Slight risk for heavy precipitation in the Northeast, Thu-Sat, Jun 6-8.

Slight risk for high winds for downwind areas of the northern High Plains, Thu,
Jun 6.

Rapid onset drought risk across parts of southern Texas.

DETAILED SUMMARY

FOR SATURDAY JUNE 01 - WEDNESDAY JUNE 05:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY JUNE 06 - WEDNESDAY JUNE 12: The GEFS, ECMWF ensemble mean (ECENS)
and the Canadian ensemble mean (CMCE) all show a moderate to strong mid-level
ridge over the western contiguous states (CONUS) as week-2 begins, with the
greatest 500-hPa heights (>590 dm) in the Four Corners region, and the largest
500-hPa height anomalies centered near the northern Great Basin. The ECENS is
strongest with the anomaly (>+18 dm anomaly at the start of week-2), but the
GEFS mean and CMCE are also fairly amplified (+12 to +18 dm). As week-2
progresses, this feature is expected to weaken only slightly and drift slowly
northward, which keeps mid-level ridging over the West during all of week-2.
This set-up is favorable for the development and persistence of excessive heat
in a large part of the western and south-central CONUS, potentially bringing
this broad area its first heat wave of 2024. Very warm air would already be in
place as the period begins, and the uncalibrated ECENS shows high chances for
temperatures to top 105 deg. F in the central California Valleys during the
first two days of the period. A broader area across the interior West,
especially the lower elevations, is also expected to feel the heat, with the
best chances during the first half of week-2. Temperatures in the broader
interior West are not expected to reach the temperatures forecast  in the
California Valleys or the climatologically-hot Desert Southwest, but the ECENS
shows substantial chances for some lower elevations across California, Nevada,
Utah, Idaho, Washington, and Oregon to reach or exceed 100 deg. F during the
first half of week-2. In addition, most models (particularly the ECENS) show
abnormally high humidity entrenched across the West during week-2, which could
generate higher heat indices than usual given the same temperatures. How high
temperatures will get is less clear in other parts of the West and far
south-central CONUS, as it is throughout the region later in the period with
increasing uncertainty regarding the exact evolution of the mid-level ridge.
However, there is at least a slight risk of excessive heat throughout this
large area during the entire week-2 period with the mid-level ridge wandering
nearby. The probabilistic extremes tools (PETs) show highly elevated chances
for anomalous temperatures in parts of the interior West, especially early in
the period. The PET derived from the ECENS shows greater than a 90 percent
chance for temperatures in the top 15th percentile as week-2 gets underway in
western Nevada while the GEFS PET shows odds exceeding 60 percent for similar
readings.These extreme deviations from climatological odds decline as week-2
progresses, but remain significantly elevated over the western CONUS throughout
the period, with the area of greatest probabilities drifting northward with
time.



In the Florida Peninsula, the GEFS and ECENS PETs continue to indicate a 20 to
40 percent chance for temperatures to exceed the 85th percentile each day
during the period. Abnormally high 850-hPa temperatures are forced into the
region on the backside of a mid-level ridge, and persist through the forecast
period. The ECENS shows daily high temperatures hitting at least 90 deg. F
every day at most sites, with temperatures remaining 1.5 to nearly 4 deg. F
above normal through almost the entire period. A few inland areas could reach
the upper 90s at times. The climatologically-high humidity will exacerbate the
effects of the heat, generating heat indices well into the 100s. Heat and
humidity has the best chance of bringing excessive heat to the southern half of
the Florida Peninsula, where a moderate risk of excessive heat has been posted
for the entirety of week-2. Farther north, where there is a little more
uncertainty, a slight risk of excessive heat is posted for the entire period. .



As for precipitation, the uncalibrated ECMWF and various PET guidance depict
the best chance for 3-day precipitation amounts to exceed 1-inch, and where
there is at least a 20% chance of the 85th historical percentile being
exceeded, is over the Northeast, Jun 6-8. This is associated with the expected
approach and passage of a low pressure system and trailing cold front moving
through the region. Showers and thunderstorms are also forecast to continue
west-southwestward from there across the central Appalachians and into the Ohio
and Tennessee Valleys and the south-central Plains near where the front is
expected to stall. However, there is not enough support between the various
ensemble means and derived tools to include this larger area on the map today.



There is a slight risk of high winds (20-25 mph) over areas downwind of the
Northern Rockies, Jun 6, reasonably well supported by the various PET wind
tools, especially the ECENS and the CMCE. This is associated with increased
west-southwest flow aloft, and with weak lee cyclogenesis possible in this area.



The greatest risk for rapid onset drought (ROD) during early to mid June exists
for southern Texas. The ROD hazard is based on 30 to 60-day precipitation
deficits, low soil moisture, and likelihood of above-normal temperatures along
with the lack of a strong wet signal during the next two weeks.



Multi-model ensemble means are in very good agreement that a deep 500-hPa
closed low will be located over the Gulf of Alaska by the start of week-2. This
anomalous mid-level trough is likely to enhance onshore flow to Southeast
Alaska where above-normal precipitation is favored. The GEFS, ECENS, and CMCE
PETs depict probabilities of about 20 percent for exceeding the 85th historical
percentile, but actual 3-day precipitation amounts remain below 2 inches
precluding the posting of a heavy precipitation hazard.

FORECASTER: Rich Tinker

$$