Category Archives: Winter Weather

Winter Weather

WINTER WEATHER OUTLOOK

[su_box title=” Forecast Update Wed 9 PM” box_color=”#defcdc” title_color=”#000000″]Tonight’s NAM has reduced the QPF to 0.13 inches water. It’s typical, as pointed out last night, for the NAM forecast QPF to reduce as it did between last night and tonight. So we’re down to about an inch of snow, falling late Thursday night and ending early Friday morning. Despite the minimal QPF, the snow falling during nighttime will allow for accumulation. Higher amounts near Delaware.  The GFS from this afternoon still showed no snow. Still a low confidence forecast. [/su_box]

[su_box title=” Forecast Update Wed 6 PM” box_color=”#defcdc” title_color=”#000000″]Today’s NAM continues with light snow for late Thursday night, ending daybreak Friday. QPF values have reduced to 0.17 inches water, or about 2 inches of snow. The GFS continues with no snow for us. Still a low confidence forecast.

Of increasing interest is a persistent GFS forecast for snow Sunday afternoon into Sunday night. The models have been quite inconsistent, as mentioned in previous posts this week. However, right now, the GFS is forecasting a 8-10 inch snowfall for us. Expect changes. Stay tuned. [/su_box]

As mentioned earlier today, there’s lack of continuity and agreement  with the model forecasts for this week, but the past several model runs of the NAM show a weak disturbance moving just to our south Thursday night into Friday.

Tonight’s NAM just available shows a strengthened low which brings snow late Thursday night, ending early Friday morning.  QPF values tonight approach 0.45 inches water, which would be a significant snowfall of 4-5 inches.

The NAM tends to overstate QPF early on and the GFS has been unimpressive with this low pressure development.  But this needs to be watched. Stay tuned.

Winter Weather

WINTER WEATHER UPDATE – WEDNESDAY

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[su_box title=”Winter Weather Update: Wednesday 3 PM” box_color=”#defcdc” title_color=”#000000″]The precipitation has changed over to sleet and freezing rain about 2:30 PM, about an hour earlier than this morning’s NBM had predicted.  The new NBM is proving to be the best predictor of “PTYPE”, or precipitation type. Interestingly, the hourly HRRR still showed snow for us. The afternoon run of the NAM, just becoming available, reflects the early changeover.

The question I had this morning was whether the upper atmosphere  warming would cause the changeover sooner.  Apparently it did.

It appears we will have several hours of freezing rain and sleet before surface temperatures rise above freezing. [/su_box]

from earlier this morning…

I’ve looked over the latest NAM data that has just become available.  The QPF during the time snow will fall has dropped to 0.25 inches water or about 2-3 inches of snow in the immediate PHL area, another inch or two north and west.

Transition time to sleet is about 3 -4 PM according to the higher resolution NAM nested grid.  The graphic below depicts the changeover to sleet, the transition to above freezing, at critical levels in the atmosphere at about 3:40 PM:

Snow to sleet transition (white shaded line) at 3:40 PM EST today.

So less snow expected.

An extended period of sleet and some freezing rain, as temperatures remain at or below freezing until 7 PM just outside the city.  Then heavy rain tonight, ending before daybreak. A heavy soggy mess tomorrow morning.

from earlier this morning…

Last night’s 1 AM (06 UTC) (“off-hour”) models show little change.  Current radar at 8 AM shows snow starting a bit earlier than predicted; it’s right at our doorstep.  Last night’s ECMWF (European) maintains about 3-4 inches in the immediate PHL area.

Of interest is the latest NBM, the National Blend of Models.  It has the changeover and the mix with sleet occurring earlier, about 3-4 PM.

The big question in my mind with this storm will be how much snow can we get when the upper atmosphere is expected to warm significantly in the afternoon, while the lower atmosphere (below 6000 feet) remains below freezing until 4-6 PM.

The new morning model runs data starts becoming available in the next hour (NAM) and the next two hours (GFS).  I’ll be downloading it and updating the forecast soon.