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West U.S. Wildfire Risk Spreads North; Wet Southwest Monsoon Eases Drought

Executive summary: Ongoing raging wildfires continue in northern California in Redding. In this region Crop Moisture Index (CMI) and Palmer Drought Severity Index (PDSI) have grown more negative (dry) in recent weeks foreshadowing increased wildfire risk. The late summer climate pattern is forecast to shift the drier trend farther north throughout Oregon and Washington while dryness persists in northern California. The dry pattern strengthens beneath a mean upper level high pressure ridge. Meanwhile to the south a wet Southwest Monsoon evolves bringing periods of heavy rain, flooding and vastly easing the intense Southwest U.S. drought particularly in Arizona. The emerging Southwest U.S. wet pattern is likely to miss southern California where more excessive heat events affecting high population areas such as the Los Angeles basin continue.

Discussion: Measures of soil moisture include CMI and PDSI and while most commonly used for agriculture can also provide clues as to the influence on climate. Vast areas of dry-to-drought conditions during summer can act as a feedback mechanism to the atmosphere strengthening the initial conditions causing drought increasing intensity and duration of the event. In northern California in the region of the current Redding, CA fire both CMI and PDSI have accelerated their negative dry phase in recent weeks (Fig. 1). The driest trend in the Northwest U.S. is in north-central Oregon where a whopping 15.91 in. of rain is needed to end drought (Fig. 2).

Fig. 1: Crop moisture index, Palmer Drought Severity Index and rainfall needed to end drought in the Sacramento Drainage region of northern California where the Redding Fire is located. PDSI of -4 is considered extreme drought (worst possible).

Fig. 2: Crop moisture index, Palmer Drought Severity Index and rainfall needed to end drought in the North-Central Oregon region. PDSI of -4 is considered extreme drought (worst possible). This is the driest zone in the Northwest U.S. in late July.

During late summer to mid-autumn the NOAA Seasonal Drought Outlook (Fig. 3) indicates a pattern change in the Southwest U.S. A wet monsoon pattern evolves featuring excessive rainfall and elevated flash flood risk squashing what is currently an intense Southwest U.S. drought. The wet weather pattern is linked to warm sea surface temperatures (SST) in the southeast North Pacific (Fig. 4). Monsoon rains are convective in nature and release immense latest heat to the north warming the atmosphere and causing amplified high pressure zones. During the wet monsoon across the Southwest U.S. and upper ridge pattern crests over the Northwest U.S. causing drought expansion and increased wildfire risk. The drought expansion/wildfire risk will extend into Southwest Canada.  The Southwest wet monsoon is likely to avoid California where additional extreme heat (and humidity) is likely (Fig. 5).

Fig. 3: The most recent U.S. NOAA seasonal drought outlook indicates expanding Northwest U.S. drought (and wildfire risk) while the Southwest drought area is doused by heavy monsoon rains.

Fig. 4: Warmer than normal SSTA in the southeast North Pacific coupled with El Nino risk will enhance the Southwest wet monsoon for late summer.

Fig. 5: Burbank, CA 15-day max/min forecast by all models indicates (likely) risk of 100-104 during early-to-middle August.