Yesterday’s cloud base did lift and thin, but we never saw the degree of clearing that was forecast by the HRRR or NAM-NEST models. Showers moved in, as expected, about 5 PM, but the northern-most extent and their duration into the evening hours was greater than predicted by any model.
For today, (similar to last Sunday), several upper air disturbances are causing the cloud deck right now. Sun should break out later this morning, but another upper air disturbance is expected about 4 PM (not talking about the Eagles here.) So, another period of cloudiness possible later.
The coastal storm for mid-week mentioned in last Friday’s post remains difficult to forecast. The NAM and Canadian GDPS have 1-2 inches of snow (less in the city) late Tuesday afternoon into evening. The GFS shows less development and a coating at most. The GFS has done the best this season with these storms. This storm is still beyond the range of the shorter range models. Stay tuned.
I got up this morning and saw the clouds and even some very light snow flurries. Last night’s models did not show these clouds or flurries. Nor did the higher resolution just-run hourly models like the HRRR or RAP show it.
Did the models capture anything that could explain the weather this morning?
They did, but apparently the model’s algorithms didn’t translate a strong upper air disturbance and moisture into the clouds and flurries that we have.
Last night’s GFS shows a strong area of vorticity (“an upper air disturbance”) passing through along with increased moisture in the upper and mid levels.
Add the humidity, vorticity and upper air cyclonic flow together and you get the weather we have this morning.
If the models were somewhat better, this combination of parameters should have triggered its cloud algorithm to show the clouds we have. But it didn’t.
Based on the same thinking and parameters, the GFS shows humidity and vorticity move off between 12 – 2:30 PM. We should see more sun by that time.
[su_note note_color=”#defcdc”]Update Wed 10 AM: This morning’s high resolution NAMNEST finally shows the light precip and clouds we already know about.
Well, I’m glad I had emphasized last night that all the major model’s built-in snow algorithms had predicted only 0.35 to 0.50 inches of snow and they didn’t support my old technique and the 1.5 to 2.0 inches which I had thought possible.
Indeed, this morning, the NAM QPF amounts look to be over-done, based on radar. (Perhaps this is why the NCEP has recently decided to stop further development efforts of the NAM.)
Looking back several days, the ‘new’ GFS probably did the best in advance, the Canadian not so good. I had not mentioned the new NBM (National Blend of Models) in my posts, but looking back, it also did well with the snow forecast in advance.