The most unusual weather in anyone's memory may end this weekend with thunder, lightning and rain, and even snow on Hawai'i's highest mountains.

Yesterday's drenching may be the beginning of the end of a series of thunderstorms that have pounded the state for 40 days. A flash flood watch is in effect through this evening.

The forecast is for more showers today and tomorrow but the National Weather Service sees an "easing off" from current conditions heading into Sunday, the second day of April.

There's a good chance of snow today or tomorrow on the summits of Mauna Kea and Mauna Loa on the Big Island and, possibly, Haleakala on Maui.

"If the thunderstorm parks itself above the summits, there's a good chance for snow," forecaster Robert Ballard said.

The temperature on the Big Island summits was around 32 degrees and it could dip into the upper 20s, Ballard said. The forecast is for at least 4 to 6 inches of snow.

Ballard cautioned that anyone headed to the summit should prepare for icy roads and snow showers.

"They need to be very careful," he said.

Elsewhere, the main threat today will be flooding, Ballard said.

On O'ahu, yesterday's rain caused a landslide at about 1 p.m. on Round Top Drive above Kala'i'opua Place, the sixth landslide in eight days in that area. City Road Maintenance Division chief Larry Leopardi said that about 45 cubic yards of earth and a tree were removed Wednesday but that yesterday's slide was smaller.

"We've taken out close to 500 cubic yards," Leopardi said of the removal of material left by all the landslides. "We will be asking (the state Department of Land & Natural Resources) to do some erosion control to stabilize the area. Something needs to be done."

Kamehameha Highway near the Kipapa Gulch bridge was closed at about 7 p.m. after a small tree and mud fell onto the roadway. Traffic was diverted to H-2 Freeway or Kunia Road.

Earlier, lightning may have knocked out power at Aloha Tower.

Barry Kim, O'ahu Harbors District manager, said visitors were escorted safely off the observation deck.

Brendan Lane Larson, a private meteorologist in Hawai'i, has been surprised that no one is pointing to a possible connection between the abnormal rainfall pattern and the weak La Niña that was first predicted by the National Oceanic & Atmospheric Administration on Jan. 12. The World Meteorological Organization announced that it arrived on March 3.

NOAA says La Niña is "characterized by unusually cold ocean temperatures in the equatorial Pacific, as compared to El Niño, which is characterized by unusually warm ocean temperatures in the equatorial Pacific."

Larson said the La Niña affecting Hawai'i's weather is slightly off from the typical December-through-February time frame.

"It is affecting the rather unusual position of the jet stream, a branch of which has been swooping down over the Hawaiian Islands," Larson said. "Along the path of this jet stream branch, individual packages of energy have been traveling at Hawai'i almost once every day."

Gov. Linda Lingle yesterday extended an emergency proclamation through April 9 to offer immediate financial aid for damage, losses and suffering from the rain.

Reach Rod Ohira at rohira@honoluluadvertiser.com.