Category Archives: Weather Updates

WEATHER UPDATE- SAT AM

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Weather Update Sat 10:30 AM
Based on the latest NAM, showers have ended but widely scattered light sprinkles are possible. Low clouds lifting to brighter skies may be delayed until 2-3 PM today.

from earlier today:

The gusty windy conditions (Gusts to 35 mph) will continue and diminish later in the afternoon.

The rain ends by 11 AM.

Based on the 2 AM (06Z) NAM and GFS model runs, the low clouds should lift between noon and 1 PM, leaving brighter skies and some sun through a layer of higher cirrus clouds.   Continued improvement in sky cover later in the afternoon. High temp 74-76.

Currently, Sunday looks dry except some showers may develop in far western suburbs in the mid afternoon. High 67-69.

Forecast Update

The latest NAM model data has become available. Light scattered showers are forecast for Sunday, especially for the late morning and early afternoon hours. QPF values are low for these early showers  (0.05 inches)

It will be cloudy and increasingly windy.  There may be some brightening skies briefly late afternoon.  High 70-72 reached later in the afternoon.

Winds increase at night. A strong cold front moves through after midnight.  Heavy rain, thunderstorms and very strong winds will accompany this front.

WEEKEND FORECAST UPDATE -SUNDAY

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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.

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.

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. 

STORM UPDATE

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A quick review.  This has been an interesting storm.  As far as QPF predictions went, the NAM was superior to the GFS,  both with the previous snowfall Sunday night and with last night’s light accumulation.   We didn’t get the 0.35 inches of water forecast by the GFS last night.

Another bias this storm seemed to confirm:  the “off-hour” runs of the GFS and the NAM, (the model runs done at 1AM and 1PM eastern time),  should really be ignored.  They often complicate rather than clarify the forecast.    Some of the back and forth with the forecast could have been avoided by ignoring the off hour models.

The new National Blend of Models  (NBM) also did well, especially with forecasting precipitation type- snow vs sleet vs rain.  (PTYPE).

In my neck of the woods, we had less than 1 inch of mixed precipitation accumulate.  There’s a mix of light snow, rain and a bit of ice pellets right now and that should transition to sleet and rain by about noon.    Temperatures remain near 30 and below just outside of the city and should rise above 32 mid day.

The latest NBM shows a transition to all rain about noon to 1 PM here. (gridpoint- Blue Bell).  Later transition will occur further north and west.

Rain, heavy at times,  continues until after midnight tonight.