Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Tue Jul 18 2023

Valid 12Z Tue Jul 18 2023 - 12Z Thu Jul 20 2023

...Lengthy & dangerous heat wave to continue in the Southwest, South
Central U.S., and South Florida...

...Flash flooding & severe weather in portions of the Nation's Heartland
and the East, headlined by a Moderate Risk for Excessive Rainfall in the
parts of the Lower Ohio & Tennessee Valleys...

...Tropical Storm Calvin weakening on approach to Hawaii, but heavy rains
and tropical storm force wind gusts still expected...

...Canadian wildfires/smoke responsible for Air Quality alerts over parts
of the Northern High Plains, Midwest, Great Lakes, and Northeast...

A prolonged heat wave continues through mid-week for much of the southern
tier of the Lower 48 thanks to an expansive dome of high pressure aloft.
Record breaking heat is expected in the Four Corners states, Texas to the
Lower Mississippi Valley, and South Florida each day. Daytime highs will
reside in the triple digits in the Desert Southwest and Texas. The Gulf
Coast and Mid-South can expect daytime highs in the upper 90s that
coincide with oppressively high dew points, resulting in sweltering heat
indices between 105-115F. Daily low temperatures will remain quite warm,
breaking record warm daily minimums in some areas, allowing for minimal
relief from the heat overnight. An expansive area of Excessive Heat
Warnings and Heat Advisories remains in the Southwest, Southern Plains,
western and central Gulf Coast, and even parts of South Florida where well
above normal sea surface temperatures and lighter than normal winds are
contributing to stifling heat. Please visit heat.gov for more information
on impacts from the heat wave. The Northeast and Mid-Atlantic will
continue to see hot and humid conditions through Tuesday, while a cold
frontal passage helps to make conditions a little less muggy by Wednesday.
To the west, a pair of cold fronts will provide the Midwest and Great
Lakes with cooler than normal temperatures. Some locations in the Upper
Midwest will struggle to get out of the 70s for highs on Tuesday.

A stormy couple of days are on tap from the Midwest to the Ohio and
Tennessee Valleys. Today and into Tuesday night, a stationary frontal
boundary over the Middle Mississippi Valley  will be the focus for
strong-to-severe storms. The Storm Prediction Center (SPC) has a Slight
Risk (threat level 2/5) that stretched from the Kansas City and St. Louis
metro areas to northwest Tennessee and western Kentucky. Damaging wind
gusts, large hail, and tornadoes are all possible in severe storms.
Meanwhile, developing thunderstorms will contain copious amounts of
moisture and could train over parts of the region, resulting possibly in
dangerous flash flooding. The Weather Prediction Center has issued a
Moderate Risk (threat level 3/4) of Excessive Rainfall from the
Mississippi-Ohio River Confluence to northwest Tennessee. There is also a
Slight Risk (threat level 2/4) of Excessive Rainfall in the Northeast due
to more rounds of thunderstorms in areas that feature highly saturated
soils. SPC has its own separate Slight Risk area today in North Dakota
where storms could produce very large hail. On Tuesday, more showers and
storms will form from the central Rockies and Midwest to the Mid-Atlantic.
SPC and WPC both have large Marginal Risk areas in place, while WPC does
have a Slight Risk in the Tennessee Valley and southern Appalachians where
the threat for flash flooding is greater. Further west, the Southwest
monsoon is showing signs of life with rounds of showers and storms
possible each day. The strongest storms could produce flash flooding and
damaging wind gusts.

In the north central Pacific Ocean, Tropical Storm Calvin is weakening as
it approaches Hawaii's Big Island where Tropical Storm Warnings have been
issued. The latest forecast from the Central Pacific Hurricane Center
shows the storm maintaining tropical storm status as it tracks south of
the Big Island Tuesday night. Calvin is likely to produce locally heavy
rainfall with as much as 4-8 inches and localized amounts up to 10 inches
along the windward slopes of the Big Island. A little farther north, 3-6
inches of rainfall are possible along the windward slopes of Maui. Calvin
is also likely to cause rough seas around "The Aloha State." See Central
Pacific Hurricane Center advisories on Calvin for more information on the
storm.

Smoke from the Canadian wildfires will remain in the picture through
mid-week as northwesterly winds aloft that originate over Canada continue
to direct smoke into the Lower 48. There are Air Quality alerts issued for
areas the include the northern High Plains, the Midwest, Great Lakes,
southern Mid-Atlantic, and into the Northeast where higher concentrations
of smoke will result in unhealthy air quality for sensitive groups.


Mullinax


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php