A Tropical Storm Will Hit Hawaii

A weakening tropical storm is approaching Hawaii, where substantial precipitation and gusty winds are expected.  Tropical Storm Calvin now has maximum sustained winds of 45 mph.

The infrared satellite image this morning clearly shows the storm east of the big island.  You will note an absence of a distinct eye and rather amorphous rainbands.

The visible satellite imagery at the same time is also unimpressive.

According to the latest forecast from the National Hurricane Center, Calvin will pass south of Hawaii as it slowly weakens (see below)

Calvin's winds will interact with the substantial terrain of Hawaii to produce heavy rain.   

Below are the winds during Wednesday evening,  with yellow colors indicating the strongest winds (followed by green and blue).  You will also notice the wind "dead zone" to the lee (southwest) of the Big Island.

The precipitation totals for the entire event are shown below.  4-7 inches on the windward (eastern) side of the Big Island.  Wet on the eastern side of Maui as well.

Calvin has been rapidly weakening the past few days.  Why?

The answer is cool water.   Hurricane/tropical storms require warm water to strengthen, generally greater than 28C (82F).   

Below are the latest sea surface temperatures around Hawaii.  Temperatures east of Hawaii are now below 26C.  Not a healthy place for a tropical storm!  No wonder Calvin is declining.

Finally, the winds of Calvin are still strong enough to produce some impressive waves on the southwest side of the Big Island, with some waves getting to 12-18 ft.   Some good surfing opportunities!

Light Rain Coming During the Driest Time of the Year

We are now entering the driest time of the year in the Northwest.   A time when we are typically drier than the desert Southwest.

Below is a plot of the probability of receiving a trace (0.01 inch) or more of rain versus date for Seattle.  The last two weeks of July have extraordinarily low probabilities of precipitation.

Yet this year we may see some rain.

Tomorrow, an upper-level  trough of low pressure will move across British Columbia bringing rain to the WA coast and Northwest Washington (see upper level (500-hPa pressure, about 18,000 ft) map at 4 AM Monday below)

The total precipitation forecast through 7 PM Monday from the NOAA/NWS HRRR model is shown below. Wet coast and northern Cascades

British Columbia will enjoy a more thorough watering, with the UW modeling system showing up to an inch over coastal mountains and the Rockies through Tuesday.  This is good since it reduces wildfire threats in the province.

Temperatures will cool down on both sides of the Cascades on Monday.  Today, Sunday, is a bit cooler on the western side as some shallow marine air tried to push inland.  But much cooler on Monday morning

As in any cool-down in our region, there is the threat of temporarily enhanced winds on the eastern slopes of the Cascades and immediately downwind.   

Such a wind enhancement will occur during this event,  which will result in short-term enhanced fire danger in eastern Washington, as suggested by the Hot-Dry-Wind diagnostic for 8 PM tonight (below, orange and red colors indicate more problematic conditions).  Very good for wind energy generation, though.

So folks in eastern Washington need to ensure they don't start any grass fires.

After the upper trough moves by, temperatures in the west will warm into the lower 80s, while eastern Washington will move back to 100F.

The good news regarding wildfires during the next week is that there should be little lightning and winds will be weak.  So the wildfire threat is mainly from small grassfires, which are generally human-ignited.