Sunday, February 19, 2017

Weather It Is (Not Much Going On)

Good Afternoon:

It will remain on the cool side through the end of the week.

Moreover, there will be a reinforcing shot of cool air towards the end of the week, but at the moment it should pass by with at most local rain.

At the moment, it looks like next week will bring a moderate warm up.

The models have been hinting at the end of the month winter storm, but this is just a hint and at the moment not a very serious one.

Have a good evening,

Barry Lynn

Friday, February 17, 2017

Weather It Is (The Aftermath (And Dynamic Cooling))

Good Afternoon:

There was a lot of excitement the last week, and we certainly should be excited about almost 5 consecutive days of rain, in which we received more than 75 mm of rain (in Gush Etzion).

Some were relieved that the conditions were not strong enough for a significant snowstorm, while others, though were disappointed.

In a nutshell, the forecast models (especially our high resolution forecast) did very well in predicting that at most there would be some wet snow, and/or a very light accumulation in the highest elevations.  Moreover, as predicted, there were temperatures below freezing last night, and even ice (again) on our swimming pool.

The next week or so looks to be milder, and spring may have sprung by the start of the following week (after some tropical showers).  Yet, there is a 10% chance that it will turn cold and rainy at the end of the month.

So, why the hype? What did we all get so excited for?  There is something called "Dynamic Cooling," which can happen when there is very strong convective storms:

https://forecasterjack.com/glossary/glossary-of-cold-season-terms/dynamic-cooling/

Dynamic cooling is the cooling of the lower atmosphere through two processes: i) the mixing of relatively warmer air at the surface with comparatively (unstable) cold air aloft, and ii) the melting of snow/ice as it descends (which requires energy), lowering the temperature enough such that further falling snow/ice can actually reach the surface.

It is important to note here that air that descends in a stable atmosphere (where the lapse rate is adiabatic) does not cool the lower atmosphere. The reason is that air warms as it descends because of an increase in pressure. The result is that air parcels oscillate up and down, but never too far in either direction. Moreover, if air were to relatively cool as it descended, there would be quite a wind coming from above (and it doesn't --  so you intuitively know this to be correct).

Another way the atmosphere can cool is when liquid drops evaporate on the way down. It cools to the wet bulb temperatures (see: https://www.quora.com/What-is-the-physical-significance-of-wet-bulb-temperature).  This often happens in the northeastern United States when a storm advances northward up  the coast, and high pressure from the north blows cold, dry air into the storm.  The evaporation of droplets as they fall into this relatively dry air will occur until the wet bulb temperature is reached (or saturation), and the temperature will be colder than before (and perhaps even below freezing).

I don't think we have the latter situation to often -- and cold air often arrives from over the sea so it is near saturation anyway. Instead,  we must rely on dynamic cooling and the advection (arrival of cold air from the north), and it was this type of process that led meteorologists to be hopeful for an accumulating snow on Thursday.

However, our highest resolution forecast (which has a resolution high enough to predict dynamic cooling in strong storms) indicated only a mixture of wet snow/ice that would accumulate only small amounts at best, and only in the highest areas of the central mountains.

Looking at the big picture (especially when the later global model forecasts were a bit colder than previous) did lend some excitement to our winter (and even I was hopeful), but the fine scale details indicated:  no -- there wouldn't be a big snow.

Well, we're going to have to wait until the end of the month, or possibly next winter.

Shabbat Shalom,

Barry Lynn


Thursday, February 16, 2017

Weather It Is (Winter Weather Advisory (3))

Good Late Morning:

This early morning there was some mixed rain and snow in Efrat, but the snow seemed to melt as it passed my kneecaps.  As is typical in winter here, it is often just a tad warm for snow.  Still, we've had close to 100 mm of rain so far this week, which is a blessing.  Snow was reported on the Golan, where it accumulated at least several centimeters.

The coldest temperatures at 500 mb will occur within the next few hours as the axis of the trough moves through.

The passage of the upper trough should bring one last burst of precipitation.

Afterwards, temperatures in the middle and lower atmosphere will fall a couple of degrees.
While this might bring a bit of snow, it looks like the atmosphere will be drying out after the next burst of precipitation.  If there are any snow showers with the passage of the coldest air at 700 mb, it will be a surprise.

Our highest resolution forecast (1.0 km) shows a light accumulation of snow in the area of Hebron, and a very light snow accumulation in the area of Efrat and the western approaches to Jerusalem.

More importantly, some standing water may freeze on road surfaces this evening and night.  Moreover, tomorrow morning should have temperatures in the higher elevations near freezing, so slippery conditions may persist after dawn.

The weather in the next two weeks looks uneventful, so this may be our last chance to see any snowflakes this winter.

Barry Lynn

Wednesday, February 15, 2017

Weather It Is (Winter Advisory Update (2))

Good Evening:

The latest global data (0600 UTC) has trended colder, and our high resolution forecast now shows several centimeter of accumulation in the area of Hebron and Kiryat Area.

A fierce storm will arrive tonight with heavy rain that will spread from north to south.

The heaviest rain amounts will be in the central coastal and central mountain areas, where floods are possible.

Gale force winds are expected in the area of the central mountains.

Snow is expected to accumulate in the Hebron area tonight and tomorrow, while snow and ice will make for dangerous traveling conditions late tonight and tomorrow in the rest of Gush Etzion, possibly spreading to the area of Jerusalem later in the day as temperatures cool.

Because temperatures are forecast to be just above freezing, precipitating snow and ice (in thunderstorms) will probably cover the ground and roads, but then melt, which is why we're not expecting any snow to accumulate.

Still, the intensity of the precipitation will likely lead to travel problems.

The precipitation should be lighter in the north, but heavy precipitation may fall south into the area of the Arava.

We should have another update in the evening.

Barry Lynn

Weather It Is (Winter Weather Advisory)

Good Morning:

After a lot of talk, and even more than that, it comes down to a degree or two -- but for snow lovers a degree or two too warm.

Temperatures at 850 mb will fall until evening, but middle and upper level temperatures will warm.

Then, when the cold air and heavy precipitation arrives towards Thursday morning with an extremely cold upper level trough, temperatures will be steady or rise a bit in the middle and lower levels.

This is response to the arrival of a really moist layer (from the sea).  It has also been pointed out that the height of the 500 mb level is higher than usual in storms that bring snow.  This means that cold air descending in convective storms has more time to diabatically warm (with increasing pressure).  Hence, any snow/ice would just become wet snow or rain tomorrow morning as the cold air moves in.

The lower levels will start to cool down in the afternoon, but the upper trough will be moving through, reducing the air parcel ascent required for precipitation formation.

Yet, there is a middle atmospheric disturbance (at 700 mb) shown to arrive in the evening with the coldest air at lower levels, and our high resolution model shows some precipitation at this time.

For this reason, we'll issue a "Winter Weather Advisory" for the possibility of some snow/ice in the afternoon and evening on Thursday.

Very cold nighttime temperatures could lead to slippery conditions on Friday morning.

There are some lower level temperature differences between the different global models, but it is hard to see how the initial burst of very heavy precipitation tomorrow morning will be anything but rain.

If you like snow, then hope that your trusted weatherman will be wrong.

Barry Lynn

Tuesday, February 14, 2017

Weather It Is (Winter Weather Warning/Winter Storm Watch (5))

Good Afternoon:

There was a bit of excitement in the air this morning.

The reason?  The global forecast models are suggesting that it will be wetter and colder on Wednesday and Thursday than previously forecast.

The problem, it may not be cold enough to accumulate snow.

The cold air will arrive tonight in the lower and middle atmosphere (850/700 mb levels).  Our highest resolution model indicates that this will produce a very light snowfall in the area of Gush Etzion (and northern mountains), with some wet snow/rain possible in Jerusalem.

By mid-morning and afternoon, temperatures aloft will warm a bit, so any precipitation would just fall as rain.

Thursday, a very cold upper trough (500 mb) is expected to arrive in the morning and produce a period of much heavier precipitation than we've seen up to now into the evening hours.

The high resolution model indicates that snow is more likely during this period, but no accumulation is expected in Jerusalem, but perhaps on cars, while a light accumulation is expected in Gush Etzion (heavier amounts in the Golan).

The reason: surface temperatures are currently forecast to be too warm for a major accumulation.

Since one degree either way could make a big difference, we'll have to report back tomorrow with the latest high resolution forecast data.

Barry Lynn


Weather It Is (Quick Update)

The latest weather forecast indicates a greater likelihood of snow (accumulating) Wednesday and Thursday in the mountains of Jerusalem.

This means that the Winter Storm Watch will likely be changed to a Winter Storm Warning.

I hope to update the forecast with more specifics in a couple of hours.

Barry Lynn