Philadelphia Weather Outlook

JUST A SNOW POSSIBILITY

Fri 07:37 AM Update — The next chance of snow for us (Monday evening into Tuesday) is about 96-108 hours in the future, still beyond the range of the short range, higher resolution  models.  (The shorter range, higher resolution models forecast 84-89 hours in advance at best. Many only forecast 48-60 hours in advance.)  

Current operational GFS snow totals by Tuesday afternoon—

GFS snow totals by Tuesday afternoon. (Click on image for a larger view.)

The above GFS snow totals are somewhat similar to the ECMWF (European), Canadian GEM-Gobal and German ICON.  Stay tuned.

Thank you to those of you who took part in my recent user feedback poll.  Here are the final results:
“Users, would you like  to see all  updates posted to Twitter or just the major new posts (as currently done)? “

With it so close, I’m going to stay with only posting major new posts to Twitter,  but with an occasional update also posting. (A slight change to the  current approach I use.)   The updates (such as those on this page) won’t be automatically posted to Twitter.

Thu 08:06 PM Update — Today’s GFS has joined other models (Canadian, European, ICON and GEFS)  in predicting about 1-3 inches of snow here Monday night into Tuesday morning. (Unfortunately, the newer GFS v16 has not been available today due to planned maintenance of NOAA’s servers.)
Current GFS snow totals for Tuesday 10 AM  (Click on image for a larger view.)

 

A new wrinkle for getting larger snow accumulation is going to be temperatures, which are trending somewhat warmer at certain levels of the atmosphere, especially near Philadelphia.  Many areas will have a sleet/rain mix at times during the event reducing snow totals and making an accurate snow total forecast difficult.   Since the storm is coming during the nighttime hours, a period of sleet or freezing rain is a possibility as temperatures near the ground may drop below freezing—

GFS Precipitation Type (PTYPE) Monday evening (Click on image for a larger view.)

A change back to snow is expected before ending Tuesday morning.

The fine structure of the thermal profile won’t be known until the day before the event.    Stay tuned.

 

From Wednesday …

Today, several people at work asked me, ‘so where’s the snow?’

The statistical ensemble (GEFS) model continues to forecast a minor snow possibility for the overnight period from Monday into Tuesday morning.  (see yesterday’s post for the precip forecast graphic; it hasn’t changed much with today’s model runs.)

The sort of thing I like to do is compare the soon-to-be-released GFS model version 16 and the current operational GFS model (version 15.2)’

(The GFS 16 is scheduled to become the operational model the first week in February.)

The soon-to-be-released GFS 16 has  significant snowfall for our area, 4-7 inches—

GFS v 16 precip rate forecast for Tuesday 7 AM   (Click on image for a larger view.)

 

The current operational GFS model has the storm a bust with little development and little snow—

GFS (current operational) forecast Tuesday 7 AM (Click on image for a larger view.)

 

The current Canadian Global and the European models are closer to the the low end with some snow 1-2 inches.

In the time period in question (~138 hours in the future), the statistical GEFS model is probably the one to consider most—

GEFS snow totals Tuesday morning  (Click on image for a larger view.)

 

If that’s the case, the soon-to-be-released GFS model v16 has a serious precipitation over-prediction bias. 

The fun of this is to see what happens.   Stay tuned.

 

Philadelphia Weather Outlook

REALLY QUIET WEATHER-WISE

This seems to be one of the dullest winter weather periods in recent years.  As shown in last Friday’s forecast and graphic, the jet flow is flat due to cold air that doesn’t seem to want to make its plunge into the US. 

For some time, the weather models haven’t shown any storm on the horizon for us, in contrast to last winter season, when the models showed numerous storms  (but they were usually false alarms).

Nonetheless, an upper trough will swing through early tomorrow (Wednesday) morning.  A strong area of vorticity is expected to pass through our area about 6-9 AM Wednesday.  There will be enough moisture for snow flurries or snow showers around daybreak Wednesday morning and it will become quite windy after it moves through—

NAM model vorticity 8 AM Wednesday  (Click on image for a larger view.)

While there are no major storms on the immediate horizon, over the past several days, the statistical ensemble GEFS model has been showing a low pressure system moving by next Monday into Tuesday morning with maybe an inch of snow for us—

GEFS model forecast for Tuesday Jan 26th.   (Click on image for a larger view.)

The models don’t show any coastal development of this low at this time. In the weather world, next Tuesday is a long ways off in the future.

Next week is also looking somewhat colder here and cold air is definitely building in Canada.    Stay tuned.

 

Philadelphia Area Weather Forecasts

WEEKEND WEATHER AND FORECAST TREND

Forecast Updated Sat 08:50 PM —

For Sunday, the upper air low pressure system moves off to our east.  The pressure gradient will cause it to be windy. An upper air wave approaches late in the day  Still above average temperatures for January.  (Seasonal average high is 40º and low is 23º  Blue Bell.)

Sunday —

• Sunny in the morning, clouds move in towards noon.
• High temperature 44.0  sd 1.1º  (NBM model) Blue Bell.

• Windy late morning and early afternoon  WSW 12-15  mph with gusts to 20- 25 mph

Sat 08:46 AM Update — All models last night showed a several hour “dry slot” of clear skies this morning about 10 AM. (In past weeks, the recently updated HRRR v 4 has been particularly good at cloud cover forecasts)—
HRRR forecast for 10 AM today from last night’s model run (00z) (Click on image for a larger view.)

 

This morning’s 12z (7 AM)  HRRR (High Resolution Rapid Refresh) model has reduced this area of clearing to a narrow area moving through about 10 AM—

HRRR model from 7 AM this morning (12z) showing narrow area of clearing. (Click on image for a larger view.)

I guess we’ll see if we get the brief clearing predicted. Cloud cover is one of the toughest weather parameters to get right.

Fri 9:10 PM- Updated Saturday and Sunday forecast below
What had been thought would be a persistent pattern change starting this weekend is turning out to be a just a temporary dip in the jet flow this weekend;  a return to non-eventful weather is expected for much of next week.

Regarding the dip in the jet flow this weekend, it’s best captured by the 1000-500mb “540 thickness line”  (RED),  which demarcates a colder, dense air mass from a milder air mass (often a dividing line for rain-snow)—

Canadian Global (GDPS) model surface forecast Saturday 7 AM showing 540 thickness line, upper atmosphere low pressure over Great Lakes and departing surface low pressure in the Northeast.  (Click on image for a larger view.)

Saturday —
• Showers end around daybreak.
• A few hours of sun early, then considerable cloudiness much of the afternoon.
• Widely scattered light sprinkles possible late afternoon or evening. Possible snow flurry mixes in.
• High temperature 46.4 sd 1.5º early in the day, then slowly falling temps  in the afternoon (NBM model) Blue Bell.
• Breezy, winds WSW 9 mph with some gusts to 15 mph

For Sunday, the upper air low pressure system moves off to our east.  The pressure gradient will cause it to be windy. An upper air wave approaches during the afternoon.  Still above average temperatures for January.  (Seasonal average high is 40º and low is 23º  Blue Bell.)

Sunday —
• Mostly sunny in the morning, cloudy in the afternoon.
• High temperature 45.1º sd 1.0º (NBM model) Blue Bell.
• WINDY,  WSW 12-15  mph with gusts to 25 mph mid-day.

A mostly flat, zonal upper air flow for next week.  Disturbances moving in this flow will bring cloudiness at times, despite the lack of any major storm development. In past winters, this might have been a setup for ice storms in the midwest at the air mass boundary, but a lack of extreme cold and a lack southern jet moisture seems to have ruled that out for the time being, but it’s something to watch out for—

Canadian Global for next Tuesday showing flat upper air flow. (Click on image for a larger view.)

I’ll update later this evening for any changes, but the forecast models have been fairly consistent over the past few days.

Weather… and Other Things 'Up in the Air'