Monthly Archives: February 2019

Commentary, Weather Updates

WEEKEND FORECAST UPDATE -SUNDAY

[su_note note_color=”#d9f2da”]Wow, the clouds never broke today and temperatures remained in the 40s!  Yesterday’s NAM had predicted temperatures remaining in the 40s and I ignored it; I thought it was a modeling error because it was so far off.  So one model was an outlier, yet it turned out to be correct. [/su_note]

from earlier this morning:

The rain will move out later this morning. Some of the models are forecasting a break in the clouds after noontime for a few hours, so we may get some sun.  If not, certainly brighter skies.

I know you may have heard about a high of 61 today.  Only the GFS has been predicting this high.  Most of the models have a high in the low to mid 50s.  I guess we’ll find out.

A cold front will move through between 4 and 6 PM. Widely scattered showers and some cloudiness moves through at that time. Temperatures drop into the low 30s by daybreak Monday.

It will become very windy this afternoon and even more so this evening.

[su_note note_color=”#d9f2da”]

Contrary to an often-heard incorrect explanation on TV forecasts, high winds are NOT due to a “wind tunnel effect”.

High winds and wind gusts are principally due to 1. Rapid changes in atmospheric pressure, specifically, the rate of change of pressure over time.  (dp/dt).    2. Thermodynamic effects that cause a mixing down of normally high winds aloft down to the surface.

If you’re looking for “Wind tunnel effects”, they can be felt on the corner of Walnut and 15th streets in Center City on windy days.

Every time I hear a TV person attribute windy conditions to a wind tunnel effect, I wince.  [/su_note]

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST- EARLY EDITION

[su_box title=”Weekend Forecast Update 11PM” box_color=”#defcdc” title_color=”#000000″]Based on the latest models, the rain on Saturday evening begins between 6 and 9 PM. There’s some question whether the rain is over by noon (NAM), or lingers into the mid afternoon (GFS & NBM) on Sunday. The rest of the forecast below remains. [/su_box]

Earlier forecast…

The rain for this weekend in Philadelphia has been well-publicized.  But when will it start?   There’s good news: both this morning’s NAM and GFS have delayed the start of the rain until Saturday evening; in the case of the GFS, it’s delayed until late in the evening.

Saturday will start sunny,  but some mid to high cloudiness moves in late morning.  Increasing clouds during the afternoon, rain starts during the evening.  High 45.

High cloudiness (23,000Ft)  and surface wind direction at around 12 noon Saturday. (GFS 0.25)
NAM rain forecast and winds for  ~10:30 PM Saturday night

With the later start, it appears that the rain will end on Sunday either late morning or early afternoon.  Clouds break for some sunshine in the afternoon.   High 61.

[su_note note_color=”#d9f2da”]Perhaps my long time followers have noticed that my graphics have become a bit more detailed recently?

I used to depend on various college/university and NWS experimental sites for my data, which often was unreliable and/or required estimation from fixed grid points, such as Philadelpha airport (KPHL) or Wings Field (KLOM).

I’ve recently learned to download the NWS raw data direct from the NOAA servers, extracting the exact information I’ve always wanted.

I’m not sure if my forecasts will improve, but having access to the high resolutiuon data is quite an improvement. [/su_note]