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An Excessive Heat Warning continues in the Northwest U.S. stretching into parts of California and north to Calgary by tomorrow. Seattle, Portland and Spokane will break all-time records for the June 27-July 6 stretch.

Highlight: This week…Northwest super-heat and Northeast super-heat index.

Fig. 1: Daily maximum temperature forecasts for locations affected by the Northwest U.S. heatwave compared to normal and all-time record for the period.

Hot weather discussion: An Excessive Heat Warning continues in the Northwest U.S. stretching into parts of California and north to Calgary by tomorrow. Seattle, Portland and Spokane will break all-time records for the June 27-July 6 stretch (Fig. 1). Portland is hit strongest (of the cities shown) with 116F tomorrow. The extreme heat eases near the coast into midweek but remains “much above” normal. For inland areas, the heat rolls on through the 4th of July weekend. Meanwhile a Northeast Corridor heatwave is getting underway today. High temperatures are routinely into the 90’s through midweek. However, the accompanying high humidity drives excessive heat index peaking near 105F midweek (Fig. 2). Heat index between 105F and 110F is possible for some areas midweek. So far, Heat Advisories are issued for Philadelphia to Boston.

Fig. 2: Heat index forecast for the Northeast Corridor.

Fig. 3: NOAA/WPC 7-day rainfall forecast across the U.S.

Heavy rain forecast: Several heavy rain zones featuring gully-washer strong thunderstorms which can easily produce flooding are in this week’s forecast. Today through Tuesday heavy rains associated with strong thunderstorms are persistent across the southern Great Plains to Missouri Valley and into Illinois, New Mexico to Arizona and northern and the upper Texas Coast (Fig. 3). Currently, Flash Flood Watch areas are posted for New Mexico/western Texas to Missouri. The most prominent ongoing flooding is across central and eastern Oklahoma this morning. There are also heavy thunderstorms across southern Quebec the next 2 days. By mid-to-late week much of the eastern U.S. will receive a month’s worth of rainfall as the heat wave of the next few days breaks-down. A tropical wave well east of Florida is monitored for associated wet weather, likely to contribute to the eastern U.S. wet weather pattern later this week.

Medium-range/population weight CDD forecast: The most interesting medium-range forecast is produced by the GFS. The GFS is highly sensitive and prone to lower skill scores with medium-range forecasts. However, the model is frequently the first operational model to identify important pattern changes. In the 6-10 day forecast ALL models are in reasonable agreement on persistence of the Northwest U.S. heat wave while the Mid-south States (and into the East) are rain-cooled (Fig. 4). However, there is disagreement between the ensembles (which maintain a similar pattern to the 6-10-day outlook although less amplified) and the GFS which thrusts the Northwest heat into the Northeast States in the 11-15-day period (Fig. 5). Climate Impact Company prefers the GFS due to the tendency for the Northwest AND Northeast to observe the hottest conditions for summer of 2021. The CDD forecast for July 2-8 is cooler (near the 30-year normal) but hotter (lead by the GFS) for July 9-15 (Fig. 6).

Fig. 4-5: The GFS medium-range temperature anomaly forecast for the U.S.

Fig. 6: Population weight CDD forecast through July 15 compared to the 30-year/10-year climatology.