Monthly Archives: February 2019

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.

Winter Weather

WINTER WEATHER UPDATE- TUES 9:40 PM

The latest NAM data has become available.  According to the NAM, the snow starts between 9 and 11 AM.  Total QPF falling as snow  varies by location, but in and around the city, about 0.30 inches water or about 3 inches of snow before mixing with sleet and freezing rain.

North and west of the city, QPF values falling as snow increase to about 0. 40 inches water, with an additional delay in the changeover.   About 4-5 inches possible in areas north and west of the city.

Because it’s already the third week of February,  the sun angle is high; solar insolation through clouds may reduce accumulation on dark pavement and roadways.

The changeover to sleet is expected between 4:30 and 6 PM.  An hour or two of sleet and freezing rain before changing to rain.

Most of the heavy precipitation will fall as rain, leaving a soggy mess by morning.  Temperatures are expected to stay above freezing towards morning, so additional freezing of fallen precip is not expected with this storm.

These sort of warm air over-running cold air scenarios  with cold air damming are tough to predict accurately.  Let’s see how it unfolds.

Winter Storm Outlook

WINTER WEATHER UPDATE -TUESDAY

[su_box title=”Winter Weather Update: Tuesday 8 PM” box_color=”#defcdc” title_color=”#000000″]A quick review of the today’s models. We’re back to 4-5 inches of snow, changing to sleet about 5 pm. Tonight’s NAM data starts becoming available after 9 pm and I’ll update sometime after 9:40. [/su_box]

[su_box title=”Winter Weather Update: Tues 9:30 AM” box_color=”#defcdc” title_color=”#000000″]A change in the forecast, based on the latest NAM. Precipitation starts about 10 AM, earlier to the west and southwest. Snow until about 4-5 PM, then a changeover to sleet and then some freezing rain.

The revision in the forecast- most of the QPF will be arriving after the changeover to sleet.. about 3 inches of snow expected, more north and especially west, before a changeover to an extended period of heavy sleet and sleet mixed with rain, then rain.

I’ll be updating this evening. Likely update time is around 9:30 PM.[/su_box]

I’m waiting for the latest NAM data.  Last night’s NAM and GFS moved the precipitation start time later to 10-11 AM (with the exception of the FV3-GFS which still maintains an earlier start.)

NAM QPF values for the morning and afternoon increased to 0.80 inches water, but some of that will be falling as sleet.   The NAM maintains freezing temperatures into the evening.  At the current time, I’m still with 4-6 inches, leaning towards the higher amount, before a changeover to an extended period of sleet.   Snowfall will be heavy in the early afternoon.

I’ll  be updating with the NAM data this morning, before 10 AM.

A very busy schedule at work today, so I probably won’t get a chance to further update until this evening.  (It will be this evening’s data that will be most important.)