Even though Blanca was downgraded to a tropical storm by the National Hurricane Center Sunday afternoon, concern continues as it moves north just to the west of Cabo San Lucas and over Puerto San Carlos in the Baja Peninsula. Current forecasts show Blanca moving onshore near Puerto San Carlos, Monday morning.
Longer range models show that Blanca will continue to move north into the Desert Southwest early this week. The remnants of Blanca will then be incorporated into a broader system that is approaching California and will impact the central Rockies midweek. At this time widespread heavy rains are not anticipated. Rainfall forecasts issued by the NWS Weather Prediction Center through the week show that much of the western US, including parts of California, will receive between a quarter to around an inch of rain. Amounts increase later in the week to 1 to 1.75 inches across the Northern Plains states.
Even though we are not looking at widespread heavy rain, there is potential for localized intense rainfall, especially in the Desert Southwest US, as moisture levels will be well above normal for June. Precipitable Water (PW) values or essentially the water vapor that is available in the atmosphere to produce rainfall is very high with Blanca. Several atmospheric models, including the US GFS model, show that Blanca will bring PW values as high as 3 to 5 standard deviations above normal into California, Arizona and the Four Corners region .
To give a perspective, the graphic below shows the PW values for 2 standard deviations above normal. The greens and yellow show an area of 0.5 to around 1 inches of water. So even with 2 standard deviations rainfall rates approaching 0.75 to 1.0 inches an hour could lead to flash flooding.
For another comparison, the graphic below shows the highest PW values ever experienced in the month of June. Even with 2 standard deviations above normal PW values, the Southwestern US and Four Corners Region are approaching the highest recorded levels. If you consider the possible 3 to 5 standard deviations above normal that are forecast as Blanca moves into this region, the atmosphere will be latent with moisture and could lead to flash flooding due to impermeable desert soils.
The team at WxIntegrations continues to urge people that have interests in this part of the country to remain aware of the potential for isolated heavy rainfall and localized flash flooding this week. As you know, our goal is to help you better understand and prepare for the forecasted weather by incorporating weather impacts into your planning and response.