Category Archives: Winter Storm Outlook

PHILLY SNOWSTORM FORECAST 9:40 PM EDITION

The tonight’s NAM model data is becoming available.  Here are the trends:

Higher QPF values- precipitation falling as water has increased significantly to 1.80 inches water, in some areas, over 2.0 inches water!

The predicted vertical thermal profile is much COLDER; indeed, critical “thickness levels” and temperatures now support snow into New Jersey where previously it was thought to be mostly rain or rain mix.

With these changes, there’s been a significant increase in predicted snow totals!  Here are the NAM 10:1 snow totals by area:

NAM Final predicted snow totals 10:1 ratio

There are other algorithms that try to take into account other factors such as melting.  One is called the Kuchera Snow Algorithm.

Kuchera algorithm
NAM Kuchera Algorithm Snow Accumulation

I don’t generally find the predefined snow algorithms that useful or accurate.  I believe the 10:1 simple algorithm will do best here and may understate snowfall at times. An average of the two might also prove correct for this storm.

The reason I’m presenting both predefined snow algorithms is to show that significant snowfall is now expected in Philadelphia, NJ as well as in PA.

So we’re looking at 9-17 inches of snow, based on your location, as shown on the above maps.

Timing:  Precipitation as rain is starting as scheduled (9PM) and will mix with and turn to snow by 1-2 AM in western sections and a bit later along the I-95 corridor.   Heaviest snow during the morning and afternoon hours on Wednesday, ending about 7PM or so.

Temperatures are expected to fall to about 32-33 degrees by 2-3 AM with dynamic cooling that will occur with heavier precipitation on Wednesday

Winds will be 15 to 20 mph with gusts near 25 during the day on Wednesday.

The GFS model data becomes available about 10:45 PM.  I expect the GFS QPF values to be less than the NAM values; it always is.  I still think we’re talking about a major snowstorm for Philadelphia and the immediate areas.  This is a high confidence forecast.

I’ll amend this post with the GFS data if it presents a problem with this forecast.

10:45 PM – Tonight’s GFS model data is similar to the NAM. QPF values 1.43  inches water with distribution of precipitation maximum centered around I95, similar to the NAM.  I believe the NAM 10:1 snowfall map above  is a good estimate. 

QUICK STORM UPDATE- TUES 9:30 AM

At work…not much time, but a quick update for the snowstorm in Philly tomorrow.  The latest NAM has become available.  QPF values, somewhat less, still more than 1.09 inches water in PHL.  Band of heavier snow north and west from Valley Forge and Northern Montgomery county and north.

Temperatures at critical levels may allow some mixed precip in PHL during the morning, but a changeover back to all snow in the afternoon.  Timing still on track as earlier posts have mentioned. Still going with 12 inches in PHL, less along the adjacent counties SE of PHL, 18 + in western and northern suburbs.    (As was the case with the last storm, this is accumulation POTENTIAL.  March snowstorms have their own thermal issues from solar insolation effects, reducing totals.)

A busy day at work for me….won’t have time for elaborate posts or evalution of other models until this evening.   The all important data becomes available about 9:15 PM and the 3D temperature profiles about 9:45 PM.

 

 

PHILLY SNOWSTORM UPDATE – MON 10 PM EDITION

Here’s the latest on the snowstorm expected Wednesday. Tonight’s NAM data along with this afternoon’s model runs show high continuity suggesting that this is likely to be a high confidence forecast.

Latest NAM has a QPF of 1.34 inches water for PHL, higher amounts north and west of Philadelphia. (Expect those numbers to change with each model run.  It’s the nature of weather prediction.)

NAM Forecast Wednesday 1pm. A little less intense than this afternoon’s NAM

Still a significant snowstorm, with snow totals 12-18 inches as a current best guess.

Snow starts as a wet mix between 7-10 pm Tuesday night but changes to snow overnight. Snow falls moderate to heavy after daybreak Wednesday , tapering off about 6pm.

I’ll try to update tomorrow morning about 9:15 am.

Tonight’s GFS has an almost identical QPF of 1.33 inches water. GFS has heavier snow earlier on Wednesday 

PHILLY SNOWSTORM FORECAST UPDATE -6 PM MON

This afternoon’s model runs continue to show a major snowstorm for Philadelphia and its suburbs, especially north and west of I-95.

There isn’t much to add to my previous posts from earlier today.

The thermal profiles, especially the NAM, make this storm to be mostly snow.  Latest QPF values from the NAM is over 2.0 inches water! (24 inches of snow potential!). The GFS, as it almost always does, shows a lower QPF of about 1.0 inches ( 12 inches snow).

Usually in these situations, the NAM is overstating things, so for now, the average is 1.5 inches water.  At a 10-1 ratio (which may be too low), we’re still looking at a potential 18 inches.  With the thermal profiles getting colder Wednesday afternoon, the ratio will increase, so 18 inches might be conservative.  This looks like it could be a major snowstorm if these numbers hold up.

Timing: Snow or a mix of rain and snow starts about 9 PM Tuesday evening in PHL.  It will be light and may not accumulate much by morning, however, the storm will be forming before daybreak along the coast and snowfall will be increasing as temperatures in the upper atmosphere drops.  Snow ends early evening Wednesday.

So why haven’t the regular TV people been on the bandwagon sooner?  Not sure…I know they were heavily criticized for a poor forecast that impacted the Flower Show a few years ago.  So my guess is they’re treading lightly.