I did have a little chuckle
A couple of days ago, I noted that all the weather apps (including Yr) were showing heavy rain and temperatures of up to 8°c for Bjorli for this Saturday. In fact, many updates went on to increase the rainfall further and to make things worse for the cold weather fan, the apps showed the rain to continue through Sunday, Monday and even into Tuesday. And that wasn't the end of the misery because all these apps had temperatures remaining above freezing continuously from Saturday through to Tuesday.
I remember looking at these apps and cross referencing them with the model output. Yr will use the deterministic run from ECM while many American apps will use data from the GFS operational run. I never once believed that the amount of rainfall and pegged high of 8°c would ever be likely based on what I was seeing in the weather models and the reason is simple. The SW wind was to be responsible for the high temperature but I know, as will most Bjorli folk, that we do not get a deluge of precipitation with a SW wind. If the wind had more of a W'ly component, then one can argue that the precipitation amounts could be high but the flip side will be that the temperature will be lower. ECM was showing a SW wind before veering it W'ly and in my opinion, was overdoing the rainfall. The UKMO was also in alignment with ECM. GFS was initially showing the same but at the time of the warmest temperature at 850hPA, the model had adjusted down the rainfall amounts under the SW wind. GFS was also keen in bringing the 2-meter temperature down to just below freezing during Saturday night and Sunday morning. Precipitation amounts on Sunday were also much less and with little wind, temperatures at ground level were being forecast to hover around freezing point. GFS was showing just light precipitation on Sunday with Monday expected to be quite similar to Sunday. An increasing W'ly wind would then bring wet snow by Monday night and Tuesday would see heavier snow with the wind swinging to the NW and with temperatures slowly beginning to fall. What about ECM (and therefore the European weather apps)? Well... as I alluded to at the start, they were showing heavy rainfall and with temperatures above freezing at all times. I did have a chuckle because I knew the apps were overdoing both the rainfall and the temperatures.
What do I expect?
The weather has been very settled and quite dry over the last couple of months in Bjorli. Temperatures have been below normal with an absolute low for December so far being -29.7°c while the average monthly temperature was running at -19.7°c up to December 10th. I wrote about a big change in the pattern being likely in my last post and details were always going to be difficult after such a long spell of dry and cold weather. Temperature inversions are quite common at this time of year in Bjorli and when a plume of mild upper air is trying to erode the dense and very cold air (which has been firmly established for weeks), the risk for both inversions and freezing rain goes up exponentially. Timing is also very difficult because we have seen many times before how computer models are all too eager to push weather fronts in from the Atlantic against a blocking high pressure... only for the model to either u-turn or to delay it's initial and overly progressive breakdown. All these factors can make winter forecasting particularly challenging and this is where local knowledge comes into play. Looking at the ensembles is also very important when attempting to make a forecast because these will show you alternative scenarios by tweaking the initial data which is fed into the computers.
The GFS T+120 chart (above) shows the prognosis for Tuesday and this is where I expect we'll end up. The weekend is proving to be a headache for forecasters because we have much milder air which will move across the country but as stated earlier, conditions on the ground can mean we end up having rain, sleet, freezing rain and snow.
If I were to stick my neck out, I will predict some light snow on Friday followed by a short spell of moderate precipitation on Saturday which could be of snow before turning to sleet and then to rain. I'd also say that a few light snow showers could later return by the end of Saturday evening. However, I do not think precipitation amounts will be as high as what the weather apps are showing and furthermore, I currently think that Saturday night will largely be dry with temperatures falling to just below freezing. This means any melting snow will at least take a pause overnight and Sunday will again only see light and patchy precipitation at most. Temperatures will be variable and will fluctuate from just below freezing to just above freezing and this continues into Monday. Monday night is when I begin to see the risk of heavier snow becoming the most likely outcome and this will then set the trend through next week. That's a long way off and so a further update will be posted in due course.
As always, these are just my views and it's always nice to sometimes have a bit of fun and to pit your wits against the apps! One thing is for certain and that is the weather pattern is changing and so stay tuned for the latest weather news as it unfolds...
Comments