Summary

Current ensemble and deterministic guidance indicates that Lake Havasu City sits on the western fringe of an approaching monsoon moisture surge valid Thursday 17 July. A weighted blend of ECMWF and GFS output supports a ~34 % probability of measurable rainfall (> 0.01 in) during the noon-to-sunset period. While convective coverage will be scattered, any cell that survives the dry low-levels could deliver a brief downpour and gusty, dust-laden outflow winds.

1. Synoptic & Mesoscale Setup

Parameter	ECMWF (06 z, Fig. 1)	GFS (12 z sounding, Fig. 2)	Implication
Column precipitable water (PW)	0.9 – 1.1 in over lower Colorado Valley; ≥ 1.25 in east of Prescott	Same magnitude, slightly drier west	Marginal but sufficient moisture for high-based convection
Mid-level steering flow (500 mb)	5 – 10 kt S–SE	5 kt S	Weak flow allows storms to drift NW toward the river
Surface temperature forecast	106 – 108 °F	104 – 106 °F	Strong heating helps erode convective inhibition by early afternoon
Mixed-layer CAPE	500 – 800 J kg-¹	400 – 700 J kg-¹	Modest instability supports pulse-type storms
0–6 km bulk shear	≤ 15 kt	≤ 15 kt	Limited storm organization; short-lived cells

2. Expected Timing

Local Time (MST)	Likely Evolution
10 am – Noon	Rapid surface heating; cumulus towers form over surrounding high terrain.
Noon – 3 pm	Isolated storms develop east of the Hualapai & Black Mountains; outflow boundaries may sweep west, producing 25 – 40 mph gusts and areas of blowing dust.
3 pm – 7 pm	Highest chance of a convective cell crossing Lake Havasu City. Rainfall in any one spot most likely 0.05 – 0.25 in, but many locations remain dry.
After 7 pm	Rapid stabilisation; lingering virga and weakening gust fronts.

3. Potential Impacts

• Rainfall: Scattered, brief downpours; basin-wide QPF consensus ~0.10 in or less.
• Wind/Dust: Collapsing storms may generate sudden outflow winds capable of lofting dust and reducing visibility on Highway 95.
• Lightning: Cloud-to-ground strikes possible even where rain does not reach the surface.
• Flash Flooding: Low risk for the city proper given limited moisture, but slot canyons east of Kingman could see locally higher rainfall rates.

4. Forecast Confidence & Key Uncertainties

Aspect	Confidence	Notes
Storm development over central AZ high terrain	High	Repeatedly indicated by both models.
Storm propagation to the lower Colorado River Valley	Moderate	Dependent on strength and penetration of outflow boundaries.
Measurable rain at Lake Havasu City	Low-Moderate (34 %)	A minor westward shift of PW axis or stronger outflows would raise odds; morning debris clouds would lower them.
Severe-level winds or flooding	Low	Limited CAPE and deep-layer shear keep threats isolated.