Wednesday, January 28, 2009

Dirty Ridge




A term often used in meteorological circles is "dirty ridge." No, this is not some kind of meteorological pornography. Rather its when there is a "ridge" or area of high pressure that is not strong enough to keep us dry and cloud free. Weather disturbances with sufficient amplitude can inject clouds and rain into the northern portions of the ridge...that is the dirty part. Take a look at the upper level pattern for Thursday at 4 PM (see graphic). This represents the heights of the 500-mb pressure surface about sea level and is roughly at 18000 ft (sea level pressure is typically around 1012 mb). The ridge is obvious.
I have included the 24h precipitation for the next two days...you see some precipitation over the northern portion of the domain...particularly over the mountains. Lots of rain shadowing. This is typical for dirty ridges since the flow tends to have a strong westerly component (from the west), which produces good rain shadowing (and strong orographic enhancement).
By the way, when do you think we typically get the lowest temperatures of the day? 9 PM, midnight, 3 AM, 6 AM, or 8 AM...or perhaps some other time? Will give the answer in the next blog.

30 comments:

Bryan said...

I say around 7:00am right now. For the rest of the year, pretty much right before the sun begins to rise. The temperatures dive until the sun influences our atmosphere the following day.

Although, can the temperatures really change too much if the clouds are thick enough, like today? It was 44F this morning in Sammamish, yet it was 42F this afternoon, so is this question based on clear days/when the sun has the most influence? Otherwise during this winter, it seems to be when the wind direction is just right, not how long the sun has been out.

Anonymous said...

I say 6:00am.

Tim Lawson said...

Just checked the local weather station in Port Townsend and it is clearly 8:00am. The bottom spike for the last three days is at 8:00am.

Anonymous said...

I can't say when the coldes time of day is because I heard you say it on the radio, like a couple years ago

andycottle said...

I say between 6 and 7am, but varies of course. :o)

Cliff, you mentioned about shadowing and about the showers. Though today was mainly dry, did run into a few spotty sprinkles when out riding my bike this afternoon and did also see some sun breaks.

Anonymous said...

It was mainly dry and sunny in Olympia but to the south, Centralia had a pretty good rainstorm early afternoon

Julia said...

I'm guessing 6am before the sun is up.

Ashley said...

I'm going out on a limb and saying 9pm, only because I just went out to roll the garbage containers in and froze my butt off ;)

Seashore Dream said...

I would take a guess of 7am...

Anonymous said...

Olalla: Sunny afternoon, good day for a walk. Dry all day.

Lowest temperature? Ordinarily I'd say just before sunrise.

- Pete

Joseph said...

between 4am and 6am typically (I am guessing).

Anonymous said...

My answer is just before sunrise when you are not taking into account any storms or winds which can change temps drastically throughout the day.

Anonymous said...

I'm going to say just after sunrise, which would be about 8 am at this time of the year.

Anonymous said...

Just after sunrise.

Bob Moore said...

I would say "a while before sunrise at my location", based upon the (probably naive?) models I: that the sun impacts the atmosphere (particularly when clouds or smog are present) at higher altitudes before the sunlight strikes the ground surface, and that although the primary effect of that will be to cause the warmed (hence expanded) air to rise, this will also stimulate circulation, which will probably raise the ground level temp.

II Also, at least in urban, suburban, and industrial areas, indoor structural heating and industrial heat-producing activities, and vehicle exhaust will often initiate at least an hour or two before sunrise, with some heat leakage even through well-insulated walls and roofs.

So, for example, since our Lake Forest Park programmable computer-controlled 20% biodeisel furnace turns on at 6AM, our outdoor thermometer 4" from a poorly-insulated basement office wall and 1' from an ancient wood-framed multi-panel single-pane window will probably rise 2-5 deg on a 32F morning even by 6:15, and the remote one in our non-insulated shed 15' from a (slightly?) better-insulated wall and large aluminum-framed thin double-paned (1980's vintage non-termopane) window set might experience slight pre-dawn heating too.

III. Similarly, our car is parked outside next to the intersection of 39th Ave NE and NE 148th where there is some (noisy!) pre-dawn [commute?] traffic up a steep switchback on a 100'(?) hill from Beach Dr. NE below the Burke Gilman Trail, so I would expect hot exhaust to raise its thermometer temp by several degrees some time before sunrise.

So, I'm fairly confident that the "before dawn" forecast is correct, but quite unsure of the timing and temp-rise effects from these models, and I'm betting that Cliff will point out other models and mechanisms, some of which might even have the effect of delaying min temp until after "sunrise" on cloudy days. (Certainly,new wind from colder regions can cause major temp drops, even at mid-day, and I suspect that there are near-dawn air circulation patterns that we non-meteorologists are unaware of, that could have major effects on the min-temp timing.) That's why Cliff's blog is such a major local asset: we don't have to pay tuition to learn from him!

BUT (or THEREFORE, as we Math types have to deduce...): We all SHOULD
(a) join the KUOW 94.9FM club and donate to KUOW and NPR to support Cliff's Friday Weekday forecasts;
(b) join Cliff at REI, Town Hall or NOAA, buy his book and get his autograph, and cheer him on...,
(c) lobby state govt to support UW and Cliff's dept; and
(d) lobby all govts to get the Doppler radar installed that we so desperately need for reliable forecasts. As the "Mayor Salt Nick" poem suggests, if we can afford solar panels for our stadia (a very good "green" idea!), why not good radar too?

Cliff fan and autographed-book owner/reader/promoter Bob Moore

Anonymous said...

I'm guessing 6AM...

Petr said...

I thought it was around 2AM-4AM. But then I looked at hourly weather graph for Seattle area and there is lowest temp around 7AM. It must be some effect of heat capacity of water Puget Sound or so. Inland (east WA) lowest temperatures happen much earlier (2-4AM).

Paul said...

Based on a month's worth of observations on Copalis Beach, 6 AM or just before sunrise is consistently coldest.

Jessica said...

I vote for just before sunrise - coldest.

Jessica said...

Oh wait - I just realized where I had that impression from. Some poetry we read in school..."it's always coldest just before the dawn". Or was that from Apocalypse Now? tee hee

Brendan Patrick said...

I say around 8:00, AFTER sunrise. When the early-morning sun is still low in the sky, I suspect the atmosphere continues to experience a net loss of heat. Eventually, of course, the sun becomes high enough -- and thus intense enough -- to reverse the net loss and start warming the atmosphere, but not until some AFTER sunrise. I like that Mr. Moore considers factors such as upper-atmosphere dynamics and early-morning human activity, but I suspect the impact of those factors is insignifcant. Just as the coldest day of the year is not the winter solstice, the coldest time of day is not the moment of sunrise.

Liembo said...

Unrelated., but last night I was driving back from Seattle at around 9:30pm and noticed that Elliot Bay, Lake Union, Portage Bay and Lake Washington were the calmest I've ever seen them collectively. Lake Union was like glass, being most sheltered, but even the larger bodies were just amazingly smooth. I wish I had a camera.

Anonymous said...

Cliff, you know that you jinxed yourself by asking that question. Since we`re dealing with the peculiarities of warm and cold fronts the next few days, you can bet that the minima for these days will occur anytime(like last yesterday`s mid-evening minimum)except the typical just-after- sunrise time!

andycottle said...
This comment has been removed by the author.
andycottle said...

Kinda unrelated to the topics about coldest time of day....but I have snow question for Cliff.

So Cliff... over recent days GFS/WRF-GFS models have been consistently showing that LATE Fri night into early Sat morning, that a bit of snow or rain/snow mix could fall inside the CZ that looks to develop around the north sound areas...mainly higher hill tops of say 300, 400ft and higher as 850mb temps drop to -8, -9c with good NW-WNW flow with 925mb temps around -4c.

So what are the chances of folks inside the CZ seeing a little snow out of the CZ, if it does take place? I`ve been seeing this possibility for last few days now, and looks like Seattle NWS is catching on to what I`m seeing/thinking.

Brian said...

I would say around 7:00 AM. Maybe 8:00. :]

Another thing that I am curious about is something that andycottle brought up. KOMO is mentioning a little, but not saying much about it. NWS is mentioning cold enough temps. but what about precipitation amounts?

I'm not looking for any huge snowfall, because I know it won't be. But I live on a high hill (roughly 600ft) in the CZ area, and so I was just a little curious to what you thought.

Thanks Cliff.

Anonymous said...

Check out the forums Andy. I just answered your question about the CZ. Dreary rain.

TT

Terry Schmidbauer said...

I'd like to hear your thoughts on how Redoubt volcano's eruption may effect our weather.

andycottle said...

Well see, Tim..

Anonymous said...

WOULD YOU PLEASE STOP CONTINUING TO TALK NEGATIVELY ABOUT THE WEATHER??????