WEATHERAmerica Newsletter, Saturday, July 19, 2025; EXTENDED PERIOD FORECAST

[Larry Cosgrove]

EXTENDED PERIOD FORECAST

(Between Day 11 And Day 15)

 

Just How Long Will The Heat Ridge Complex Keep Going?

CIMSS (3)

GRAPES/WMO/Beijing

TropicalTidbits.Com (Dr. Levi Cowan) (3)

NOAA/PMEL

NOAA/CPC

IRI/Columbia University

HPRCC/University Of Nebraska (2)

Environment Canada

TropicalTidbits.Com (Dr. Levi Cowan) (5)

WeatherBell.Com (3) 

Heat ridge formation is actually one of the easier forecast chores in summer, since in theory hot air, subsidence and northward latitude shifts of cyclonic circulations in the polar jet stream are essentially a given. But questions arise as to how long these features can last, the expected position, the character (dirty vs. compressed) and the potential for trans-ride weaknesses that allow for tropical cyclone interaction. The peak heat day in the lower 48 states is July 23, but I have seen cases in a neutral-negative ENSO environment where heat waves can linger well into October as far north as 45 N Latitude. It makes sense to revisit my prediction methods now, as there is a lot of bad information floating around on the Internet as to what creates windows for hot air.

When I start reading remarks about a "heat dome high", which is right up there with "bomb cyclone" and "lake cutter" in the wannabe and hyped media discussions about upcoming weather, I move on and search for legitimate discussions as to causation and expression of subtropical highs. Same goes with "Atlantic Ninas", which simply do not exist and are a waste of time when tropical development threatens. One of the first parameters I check is synoptic climatology in the ENSO 3.4 sector character. If flat neutral, slightly negative (0 to -0.5 deviation from normal SST), or a marked La Nina signature (-0.5 or lower), your chance to see an ITCZ or "Cape Verde" storm grows, as could features in the western Atlantic Basin. Balance these with a check of winds aloft, which are easy to tell using water vapor imagery. If flow in the higher layers is opposite to direction (say westerly vs. a system coming from the east), odds decrease that a prominent storm or number of such impulses could organize. Water temperatures above 79 deg F will support a convective circulation, and the anomaly is not the issue. In short, you must see a "smooth sailing" scenario with everything lined up for a memorable event. I can recall Hurricane Maria (2017), which was treated by many sources as "no big deal", but in fact was easily recognizable for its danger to Puerto Rico from 8 days out. The prominent anticyclone to the north of the system, along with bathwater ocean values, practically screamed devastation. Conversely, large storms that seemed "likely" for major damage to urban areas, such as Rita (2005) and Laura (2020) ended up in a rightward curve into less populated areas after being targeted by the ECMWF series about 125 miles to the left of the eventual track.

All of these points are brought up for two reasons. With a monsoonal moisture fetch into the West, heat ridge presence over the central and eastern two-thirds of the lower 48 states seems probable as we move into August. The polar jet stream will probably be along or just above the Canadian border, with thunderstorms (some maybe reaching MCS or derecho status) a cooling mechanism in the Northeast. Most of the forecast models in the longer term show tendencies for weaknesses in Texas, Louisiana/Mississippi, and Florida. In fact, just such a risk is likely along the western Gulf Coast in the 11-15 day period. If a gap between heat ridges shows up, then the potential for a tropical cyclone strike exists (I think through October) which could temporarily disrupt this hot second half of summer. The 1995 analog may work here, where the mTw air masses get pushed out with a tropical feature that converts to a cold core storm, probably over the East.

 

Prepared by Meteorologist LARRY COSGROVE on 
Saturday, July 19, 2025 at 9:20 P.M. CT

Disclaimer: 
The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2025 by Larry Cosgrove

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