243
FXUS21 KWNC 261903
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT July 26 2024

SYNOPSIS: A strengthening and westward shifting area of mid-level high pressure
supports an elevated risk of excessive heat conditions along with increased
chances for high winds for the western half of the contiguous U.S. (CONUS). A
secondary area of strong mid-level high pressure centered over New England is
expected to maintain elevated chances for excessive heat for much of the
Mid-Atlantic, eastern Great Lakes and the Northeast through the middle of
week-2. A healthier monsoon circulation, as well as possible tropical cyclone
activity in the eastern Pacific leads to increased chances for heavy
precipitation over parts of the Desert Southwest. Possible tropical cyclone
development in the Caribbean leads to increased risk for heavy precipitation
and high winds over portions of the Gulf Coast and Southeast.

HAZARDS

High risk of excessive heat for parts of the Northern intermountain, northern
Great Basin, Rockies, and Northern and Central High Plains, Sat-Mon, Aug 3-5.

Moderate risk of excessive heat for parts of California, Pacific Northwest,
Great Basin, Rockies and the Great Plains, Sat-Fri, Aug 3-9.

Moderate risk of excessive heat for the Northeast, eastern Great Lakes, Central
Appalachians, and Mid-Atlantic region, Sat-Wed, Aug 3-7.

Slight risk of excessive heat for much of the CONUS, except for parts of the
Southern Plains, Lower Mississippi and Tennessee Valleys, Southern
Appalachians, and Southeast, Sat-Fri, Aug 3-9.

Slight risk of heavy precipitation for parts of the Desert Southwest, Sat-Wed,
Aug 3-7.

Slight risk of heavy precipitation for parts of the Southeast and Lower
Mississippi Valley, Sat-Tue, Aug 3-6.

Slight risk of high winds for much of the western CONUS, and High Plains,
Sat-Wed, Aug 3-7

Slight risk of high winds for parts of the Southeast and Lower Mississippi
Valley, Sat-Wed, Aug 3-7.

Rapid Onset Drought risk for much of the Great Plains, and into western
Missouri.

DETAILED SUMMARY

FOR MONDAY JULY 29 - FRIDAY AUGUST 02:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY AUGUST 03 - FRIDAY AUGUST 09: Analysis of the mean week-2 500-hPa
forecast height anomaly patterns from the GEFS and ECMWF continues to feature
strongly amplified ridging overspreading nearly the entire CONUS, with near to
slightly below normal heights over the southern tier of the U.S. Since
yesterday, both model means have converged on the development of two separate
height anomaly maximas in the CONUS, one situated over the northern
Intermountain and the other centered over New England where the latest
temperature guidance favors the greatest odds for above-normal temperatures and
risks for excessive heat. As a result of the amplifying ridge center upstream
across the across the West, however, both the GEFS and ECMWF favor more
northwesterly flow across the Midwest, which is anticipated to lower the risk
of excessive heat conditions, particularly for parts of the Plains and
Mississippi Valley that remain favored to experience the strongest positive
temperature departures late in week-1.



Based on raw and calibrated temperature tools, a broad slight risk of excessive
heat remains posted for all of week-2, and is expanded to include nearly all of
the West Coast in today`s outlook.  Western Washington is excluded from the
slight risk due to the lack of support for temperatures reaching hazard
criteria throughout the period. Within the broad slight risk area, a moderate
risk of excessive heat is continued for all of week-2, covering the western
half of the CONUS, and is likewise expanded westward to include parts of the
Great Basin and the Central California Valley given shifts in the prevailing
height pattern, increased warm signals in the Probabilistic Extremes Tools
(PETs), and support in the National Blend of Models (NBM) indicating several
locations reaching near record breaking daytime temperatures within the
highlighted region. Relative to yesterday`s outlook, the high risk for
excessive heat is removed across the Central and Southern Plains due to the
aforementioned reduction of heat potential over this part of the country, but a
high risk is introduced over parts of the northern Great Basin, Northern
Intermountain, Rockies and the upper High Plains closest to the anomalous ridge
center. Within the high risk area, PETs depict 50-60% chances of maximum
daytime temperatures exceeding the 85th percentile and 95 (90) degrees F over
the lower (higher elevations).  Tied to the secondary height center over New
England, a moderate risk of excessive heat remains issued for through the
middle of week-2, where PETS show 30-50% chances of temperatures exceeding the
85th percentile. The addition of a high risk was considered based on the ECMWF
PET favoring >60% chances for exceeding the 85th percentile for parts of Maine,
however the GEFS PET maintains relatively lower chances and there are little to
no signals in the raw tools for maximum temperatures exceeding 90 degrees to
support a higher hazard designation.



With the amplifying ridging favored aloft over the northwestern CONUS, and
possible surface low development over the Northern High Plains early in the
period, strengthening pressure gradients to induce episodes of high winds are
favored in the region, as well as further south in Desert Southwest. A slight
risk of high winds is issued (Aug 3-7) for much of the western CONUS where
there are increased chances for wind speeds exceeding the 85th percentile in
the ECMWF PET.  The hot, dry and windy conditions are likely to lead to an
enhanced risk of wildfires in the highlighted region, and inhibit efforts to
contain any ongoing incidents.



The combination of continued above-normal temperatures and below-normal
precipitation also supports the continuation of a Rapid Onset Drought (ROD)
risk from most of the Northern and Central Plains into western Missouri. Dry
soils, stressed vegetation, and reduced water availability due to low ponds and
streams may lead to increased impacts to agriculture and livestock, with some
locations already experiencing impacts.



For precipitation, there is little support in the models for substantial
mid-latitude frontal forcing as any heavy precipitation risks look to be more
tropical in origin during week-2. Analysis of upper-level velocity potential
anomalies from the GEFS and ECMWF continue to feature an envelope of enhanced
divergence aloft likely tied to the passage of a convectively coupled Kelvin
wave over the tropical Americas. Although the ECMWF reveals more of an
Madden-Julian Oscillation (MJO) presence in these fields, regardless, the
large-scale environment appears to be more favorable for Tropical Cyclone (TC)
genesis in the Western Hemisphere, which is supported in the GEFS and ECMWF
ensembles featuring one or more areas of deepening low pressure to the south of
Mexico by next weekend.  Given a more favorable monsoonal ridge axis forecast
over the Four Corners, potential TC activity to trigger a Gulf of California
surge event (as supported in deterministic solutions with increased dewpoints),
as well as increasing signals in the PETs across the Desert Southwest, a slight
risk of heavy precipitation is issued for Aug 3-7.



Across the tropical Atlantic, the hazards perspective has become more
interesting. Over the past several runs, the ECMWF deterministic and ensemble
solutions have latched onto a tropical wave tracking westward from the western
Atlantic, with increased chances for development based on probabilistic TC
genesis tools by next weekend. As this potential disturbance moves closer to
the southeastern U.S., both raw and calibrated precipitation tools show an
increasing risk of heavy precipitation and high winds over many parts of the
Southeast and into the Lower Mississippi Valley through the middle of week-2.
However, both the GFS and GEFS remain largely unsupportive of this tropical
potential, with little to no precipitation signals in the raw and calibrated
PET guidance. Despite this, there is some support in the Canadian for this
potential, and the latest Tropical Weather Outlook from National Hurricane
Center shows at least a 20% chance of formation over the western Atlantic and
Caribbean during the next seven days tied to the tropical wave. Given this, and
good continuity in the models in regards to large scale conditions aloft
becoming more favorable for TC development, a slight risk of heavy
precipitation is highlighted for much of the Southeast and parts of the Lower
Mississippi Valley for Aug 3-6. In addition, a slight risk of high winds is
also posted (Aug 3-7) where the ECMWF PET shows increased chances for wind
speeds exceeding the 85th percentile.



No hazards are posted for Alaska during week-2. However, there is a significant
chance of accumulating snow for portions of the central and eastern Brooks
Range, which could have an impact on people within that area. Near Juneau,
localized flooding may be a concern associated with the glacial outburst flood
for the Mendenhall River.

FORECASTER: Nick Novella

$$