Seattle.gov Home Page
Seattle Public Utilities Home page

Search Climate Change

October 2014
M T W T F S S
 12345
6789101112
13141516171819
20212223242526
2728293031  

Hubs, Spokes and Thunderstorms

Posted: October 14th, 2014

 

Broad offshore low pressure sending disturbances and showers our way

Broad offshore low pressure sending disturbances and showers our way

 

 

Like spokes rotating about a hub, little atmospheric disturbances, or shortwaves, will wrap around a broad area of low pressure currently hanging out offshore (image above). The shortwaves will kick off rounds of showers and gusty winds tonight through tomorrow or Thursday. That “parent” low will eventually dissipate and be replaced by another large system toward the weekend. At the moment, models only show continued periods of light rain in the long term.

 

Some of the showers this week, particularly those forecast for later this evening and again early tomorrow morning, may include thunder and an hour or two of intense rainfall. While the 2PM NWS forecast has no more than 0.50″ per day this week, the latest High Resolution Rapid Refresh (HRRR) run shows a 0.30″/hour period tonight, followed by a 0.50″/2-hour period ending a 4 AM tomorrow. Given continued leaf fall and recent drainage system performance, some impacts are likely overnight.

 

Over SPU’s mountain reservoirs about 50% more rainfall is expected, along with chances of thunderstorms. Mountain winds throughout the period also look to be stronger, gusting to 35-40mph, but from a favorable (SE) direction.

 

Excerpts from the 2:30 PM NWS Area Forecast Discussion (emphasis added):

 

A SHOWERY WEATHER PATTERN WILL BE OVER THE REGION THROUGH MIDWEEK AS AN UPPER TROUGH JUST OFFSHORE WEAKENS AND MOVES INLAND OVER THE PACIFIC NORTHWEST. THE TROUGH FILLS AND THE REMNANTS KICK THROUGH WRN WA WED FOLLOWED BY A BREAK IN THE SHOWERS WED NITE AND PERHAPS MOST OF THURSDAY AHEAD OF THE NEXT FRONT. ONLY THE HIGHEST MOUNTAIN PEAKS WILL SEE SNOW. THERE IS A CHANCE OF THUNDERSTORMS IN THE FORECAST AS THE COOL UPPER TROUGH MOVES INTO THE AREA AND THE BEST CHANCE OF A FEW TSTMS IS PROBABLY WED AFTERNOON.

A FRONT WILL SLOW AND GRADUALLY WEAKEN AS IT MOVES INTO THE AREA THU NIGHT AND FRI. WEAK HIGH PRESSURE WILL BUILD INTO THE AREA SATURDAY AND THE WEEKEND MAY TURN OUT TO BE AT LEAST PARTLY SUNNY AND DRY OVER WRN WA. ANOTHER FRONT WILL REACH THE AREA EARLY NEXT WEEK…AND THE 12Z GFS TAKES A 987MB LOW 12Z SUNDAY DOWN TO 987MB AS IT TRACKS NORTHWARD JUST WEST OF THE CHARLOTTES. THE WEATHER PATTERN LOOKS UNSETTLED BUT I DON`T SEE ANY GENUINELY IMPRESSIVE STORMS EXCEPT FOR THE 12Z ECMWF SOLUTION WHICH SHOWS A STRONG JETSTREAM HEADING INTO OUR AREA BY THE MIDDLE OF NEXT WEEK. THE EURO HAS A 969MB LOW AT 50N 130W 18Z WED. OVERALL…THERE DO NOT SEEM TO BE ANY STRONG SYSTEMS IN THE FORECAST OUT TO SEVEN DAYS BUT IT WILL LIKELY BE WET AND BREEZY AT TIMES AFTER A DRY WEEKEND.

 
– JRH
 
References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface