Yesterday I posted a write up on atmospheric wave and blocking patterns, check it out as this gives a description on the hows and whys of short and long term weather. It also shows a few of the many variables which models try to forecast global circulation patterns in the northern hemisphere. Today we look at pressure and fronts.
Constant Pressure Charts: Thickness
While typically plotted on surface charts, the ‘thickness’ contours are derived from constant pressure charts. Thickness is the measurement of the distance (in meters) between any two constant pressure surfaces.
One of the most common thickness charts used in meteorology is the 1000-500 mb thickness. This is the distance between the elevation of the 1,000 mb and 500 mb levels.
Typically, we use the 1,000 mb surface to represent sea level but that is a generalization. The 1,000 mb surface varies in elevation just like any other pressure elevation.
In fact, the 1,000 mb surface will often be below ground (or sea) level. The 1000-500 thickness values take that into consideration by estimating the elevation of the 1000 mb level.
As in the case of the 500 mb constant pressure chart, the last digit (zero) is truncated. So, a 1000-500 thickness chart value of 570 means the distance between the two surfaces is 5,700 meters. The contours are drawn in 60-meter increments.
Fronts
These thickness values are directly related to air density and temperature. The colder the air, the denser the air becomes and the distance between any two height levels also decreases. Conversely, warmer air is less dense and therefore the distance between any two height levels increases.
Because of this, thickness charts are used to help identify fronts and boundaries between air masses. Since air temperatures almost always fall behind cold fronts, thickness contours can become bunched together on the cold air side of the front. This is called packing, and a meteorologist may say the thickness lines are “packed”
A word of caution. There is no hard and fast thumb rule for determining fronts using thickness contours. Careful analysis, using multiple sources of information, is needed to identify their locations.
In the example below (bottom of page), there is a packing of thickness lines In Oklahoma and Texas. Turning the surface weather overlay on reveals precipitation in eastern Oklahoma and Texas well ahead of the cold front.
Based upon the isobars, the front is located along the Colorado/Kansas border extending south into the panhandle of Texas. The precipitation in east Oklahoma and Texas is from a dryline moving east ahead of the front.
A tighter packing of thickness lines is located in Canada south of Hudson’s Bay. The warm front’s location, indicated by the isobars, clearly demarks the colder air to the north of the warm front as opposed to the south side of the boundary.
Rain/Snow Line
The 1000-500 mb thickness value of 540 (5,400 meters) is the traditional line to determine rain verses snow. The 540-contour closely follows the surface freezing temperature (32°F/0°C).
If precipitation is expected for locations poleward of the 540-thickness line, the rule of thumb is to predict snow.
If precipitation is expected on the equator side of this line then the precipitation would be liquid. In the example below, the 540 (and lower) thickness lines are colored blue and 546 (and higher) thickness lines and greater are colored red.
As with everything else in meteorology there are always exceptions, especially with seasons and terrain. But the 540-thickness line is a good first guess for determining rain verses snow.
Gustiness of Wind
The wind speed is governed by pressure differences (pressure gradients) which are indicated on surface charts by isobars.
Wind speeds are higher when the distance between isobars decreases.
There is usually some variation in speed even in steady wind. That variation, or gustiness, increases based upon the angle at which thickness contours cross isobars.
Thickness lines that cross isobars at right angles indicate the gustiest winds. As the angle decreases so does the wind’s gustiness.
In the example at #1, the dashed thickness lines cross the solid isobars at right angles. At #2 the thickness lines and isobars cross each other at much more shallow angles. Therefore, while the wind’s speed is similar at both locations, it is gustier at #1 than at #2.
We have another lakeshore flood warning in place for tonight through Tuesday night and gale warnings out for Lake Michigan. We have chances of thundershowers, rain and winds for the next two days, even with temps predicted to rise into the 60s the weather will not be pleasant for outdoor activities.
Southeast gale force offshore winds to 35 mph will shift to an onshore direction out of the southwest this evening. Southwest winds of 35 to 45 mph with higher gusts are expected tonight, Tuesday and Tuesday night.
We could see rainfall amounts of a quarter to half an inch today with more overnight. We may not see the sun until Wednesday. Daytime temps will crash into the upper 40s to low 50s after today. Keep in mind it is fall and we tend to have cloudier conditions through the late fall and winter.
Currently 47* degrees out at thee YARDofBRICKS NE of GR with the furnace kicking on…Love it….INDY
9:08pm update: a few flashes in Berrien Springs…probably our final, little event.
Cold front through thee YARDofBRICKS NE of GR could this be the last time we feel 60’s till May or better…. Very Good Bet …INDY
Cold front moving closer, with a thin line of rain, and gusty winds by 10pm.
Very mild evening. The past hour the temp has gone up a couple to 63 now.
Beautiful Fall day currently RAIN temp of 58* degrees out at thee YARDofBRICKS NE of GR loving it ….INDY
59 here with light rain and at times some wind.
Slim
Must be for the lake shore but not sure at this time
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
252 PM EDT MON OCT 21 2019
LATEST UPDATE…
SYNOPSIS/DISCUSSION/MARINE
SYNOPSIS
ISSUED AT 252 PM EDT MON OCT 21 2019
– WINDS GUSTING OVER 50 MPH AT TIMES INTO THIS EVENING.
– BIG WAVES ON LAKE MICHIGAN THROUGH TUESDAY NIGHT WITH COASTAL
FLOODING AND BEACH EROSION.
– COLDER WEATHER FROM WEDNESDAY INTO EARLY NEXT WEEK.
Slim
Let’s revert the facts! GR has seen below average temps for 7 out of the last 9 months! What an incredible cool pattern we in and October will probably end up with below normal temps! Wow! What a trend!
Yeaaaa and what happened to the red maps the above temps outlook some kept saying that was on the way for around now?? Boy that sure changed….INDY
2019 has been very close to average temps overall so far. How can this be?
Here is a month by month break down of the departures from average this year so far.
January -2.6, February -1.2, March -3.4, April -0.3, May -1.7, June -1.7, July +2.9, August -0.1, September +3.4, and so far in October +0.1. for the year the mean temperature is at 51.6° with 51.9 being average at this time (for 10 months). This will change as we still have 10 more days to go in October yet.
Slim
So below normal for year! How can that be?
More rain moving into west Michigan with a big storm and another cold front boy them 60’s only last about 2 days now before it goes back to below normal shuuu the warmth is going away faster and faster as we get closer to what’s probably going to be another above average Winter with more cold and snow start preparing now …Have a good windy Tuesday…INDY
Well, after a warm summer and a warm September, GR is still above average for October. 68 degrees yesterday with lots of sun and calm wind almost felt like a summer day.
Lol …felt pretty good after the 2 week cool down we have had …. Nothing real hot this past Summer is right …INDY
Peak color this year will likely extend into November! Pretty crazy.
Sorry the trees will be bare come November we have 2 major storms on the way that will take the leafs will it …What a wet cool Fall we are having…INDY
Trees will be bare huh? So all the green leaves are just going to fall off in the next week? Better check my pic below. No way those trees will be bare next week unless a tornado comes through.
This will be a good year to test this old folklore saying
If the peak fall color is early, the winter will be mild. The later the peak, the colder the winter
Slim
Here is some Old weather folklore
If the peak fall color is early, the winter will be mild. The later the peak, the colder the winter
Keep an eye on the weather on October 9. If it’s sunny, the following winter will be very cold. If it’s cloudy, winter will be milder
The color is late this year and guess what? it was sunny on October 9th
Slim
Trees are turning fast, fall cold fronts are becoming more prevalent, winds are picking up, temps are dropping and soon we will be getting hammered with SNOW! Hopefully we will be having an early start to winter this year!
Still no snow in the forecast! Places out west have already seen several feet of snow. I love living in balmy southern Michigan.
I have not been outside yet this AM but looking out the window I can tell you that there has been a big uptick in the color in my area. Even with the clouds today the color is real good but there are a lot of leaves now falling and with the wind coming in that will only increase. At this time it is 53° both here at my house and at GRR. The H/L at Grand Rapids yesterday was 68/46 and October is now almost at average at +0.1° For today the average H/L is 58/40 the record high is 85 set in 1953 and the record low is 18 set in 1974.
Slim
Good reading again MV. You always have interesting stuff.
We have 1 tree out back that is the first tree to color up every year, and it finally flipped the other day, about 1 month later than normal. I snapped a pic yesterday and you can see how green all the other trees still are:
https://www.flickr.com/photos/184860862@N07/albums/72157711429618323
The color on the trees in my area have exploded in the last couple of days. But as you said it is later than in many past years.
Slim